and SHORT-WAVE WORLD...119 P/ISIOA AND SHORT-WAVE WORLD SPECIFIED L A B co LAB controls are...

LARGE ,PICTURE PROJECTION By ZWORYKIN

and SHORT-WAVE WORLDJANUARY, 1938.

If-MONTHLY

DJUSTINGPERATINand

TESTING*TELEVISIO

A itECEIVER4 -stage

1 -to -10 Metre

ReceiverSCOPHONY

DEMONSTRATION

Britain's 1-10 Unique

No. 119. Vol. xi.

NEW EMITRON CAMERA

*MUNI) JONES PUBLICATIONS LTD.CNANSITCHlt HOUSE. CHANCERY LANE

LONDON W.C.2.

THE FIRST TELEVISION JOURNAL IN THE WORLD

119 P/ISIOAAND

SHORT-WAVE WORLD

SPECIFIED

L A Bco

LAB controls are

precision-built pro-

ducts,dspecially e

signed for amateur_,

constructorsa n

seengineer.

riceEa cvhtestedis

laboos-e

story for noselevel, contact e

eanrecy,.

long for

cutting to required

length.Without Switch 3

With Single -FSwitch .

With Double.Pole5i..Switch

in this issue for theFOUR -STAGE 1-10METRE RECEIVER

and theU.H.F. EMERGENCY

TRANSMITTER

Time and time again designersspecify Erie Resistors-proof of Eriereliability. Be sure YOU getErie Resistors. They are speciallyimpregnated to withstand extremesof heat and damp. In all values,

Is. per watt.AND ALSO THE

TONE & VOLUMECONTROL

Write for Erie Colour Code Chart, Post Free:-THE RADIO RESISTOR CO., LTD., I, Golden Sq., London, W.I.

Archimedes -like. Brown cries " Eureka! "

So that's why my set is a squeaker.I'll get some FLUXITEAnd this ve.ritsame nightRecreate it, (pto a LOUD speaker! "

See that FLU XITE is always by you-in the house-garage-work-shop-wherever speedy soldering is needed. Used for 30 years ingovernment works and by leading engineers and manufacturers.Of Ironmongers-in tins, 4d., 8d., 1/4 and 2,18.Ask to see the FLUXITE SMALL -SPACE SOLDERING SET-compact but substantial-complete with full instructions, 7/6.Write for Free Book on the art of " soft " soldering and ask forLeaflet on CASE -HARDENING STEEL and TEMPERING TOOLS

with FLUXITE.

To CYCLISTS ! Your wheels will NOT keep round and true unless the spokesare tied with fine wire at the crossings and SOLDERED. This makes a muchstronger wheel. It's simple-with FLUXITE-but IMPORTANT.

THE FLUXITE GUNIs always ready to put Fluxlte onthe soldering job instantly. Alittle pressure places the rightquantity on the right spot andone charging lasts for ages.

Price 1/6.

ALL MECHANICS

FLUXITEIT SIMPLIFIES ALL SOLDERING

FLUXITE LTD. (Dept. TN), DRAGON WORKS. BERMONDSEY ST., S E I.

JANUARY, 1938

TELEVISION AERIALS

This is a simple centre -fed half -

wave aerial with reflector whichshould be terminated by a

balanced input system at the receiver.Aerial and reflector consist of twobrass tubings of correct proportionsto give maximum reception of thevision channel at 45 mc. Mountedin cast aluminium clamp with in-sulating bush, the whole beingrigidly attached to a supportingmember led away at right angles,ending with a cast masthead fittingor flange for bolting to the wall ofa building, etc.

A dipole aerial is not directional byitself but may be made so by usinga reflector as illustrated. The advan-tage of a reflector is a likelihood ofconsiderable reduction of inter-ference other than that coming fromthe same direction as the signal.

These aerial assemblies are completewith mounting castings and insulators,but do not include feeder.

On the left is an illus-tration of a Belling -Lee masthead dipolesystem with reflector.Where Interference:issevere the pole shouldbe 'strapped or fixedco the chimney stackto give increasedheight.

List No.' 334Mast -head Tele-vision aerial.Price 45/-.

List No. 323As 334 but _com-plete with Re-flector. Price 65/-.

List No. 321Wall MountingTelevision aerial.Price 35 -.

List No. 1322Reflector for No.321. Price 35',

Feeder, 8:0ohms. List No.344, 65 ft. on reel,10s. 6d.List No. 366,same, but bulk,per yard, 6d.

BELLING & LEE LTDCAMBRIDGE ARTERIAL ROAD, ENFIELD, MIDDX

teigYISIOt1JANUARY, 1938

ORIGINAL PRODUCTSVOLUME CONTROLSA wide selection of efficientVolume Controls isjavailable inthe Bulgin range. Values from1011 to .1P4f1 Litt wire -woundtypes, 3 and ,5 watts rating; from.05 to 51s/112 in chemical element1 watt types. All are noiselesswith squashplate reliable action.With and withoutmains switches.Prices from 2/9 to 7,6

each.SHORT-WAVE CHOKES

There is a Bulgin, H.F. Choke for every;wave-band. Low -loss design and compactnesemakethem the choice ,of discriminating designersand constructors. List No. H.F .24-5-75 metres(shown on right) 1/- each. Other S.W. typesfrom 1/3 to 2/9 each.List No. H.F.15, screened all -wave type,5//- each.WRITE FOR THE 120 PAGE CATALOGUE

MEM

BULGINTo Messrs A. F. Bulgin & Co., Ltd., ;Abbey Road, Barking, Essex.Please send post free a copy of your new 120 page fully, ,illustratedCatalogue, for which I enclose 3d. stamps.

NAME

ADDRESS

MI (BLOCK LETTERS, PLEASE)

SEND THIS COUPON NOW!

SOUNDSALES

LTD.

MAKERS OFSOUND EQUIPMENTinvite applications for theirNew 56 - page Cataloguewhich includes many newproducts of special interestto readers of this Journal.You will be very interestedin the details of the " S.S.Fourband Super Six," the" D.X. Plus " Feeder Unitand the ," Sound FourbandTuner."

When writing for yourcopy, please enclose 6d.P.O. or Stamps.

SOUND SALES LIMITEDMarlborough Road, Upper Holloway,London, N.I9 Phone : Archway 1661-2-3 111

ANDSHORT-WAVE WORLD

Only precisioninstruments

accu-

ratelyenable you to test

and trace radio

efficiently."AV

instruments are outstand-

ing for precision.They

are the outcomeof

constanteffort to provide

amateur enthusiasts and

radio engineerswith instru-

merits of high accuracy

ia maximumutility at

a moderatecost.

THE D.C. AVOMINORThis accurate moving -coil instrument is13 meters in one. It has 13 ranges volt-age ranges for measuring H.T., L.T., GridBias, Mains and Eliminator Voltages ;Milliamp ranges for testing receivingvalves and apparatus ; Resistance rangesfor all resistance measuring. In case.complete with testing prods, crocodileclips, leads and instruction 45/.booklet.

UNIVERSAL

AVOMINORThis compact precision moving -coilinstrument provides facilities for allA.C. and D.C. testing. It has 22ranges covering A.C. voltage, D.C.voltage, current, and resistance. Allreadings are direct. Total resistanceof meter, 200,000 ohms, ensuringaccurate readings. Complete withtesting prods, crocodile clips andinstruction booklet. DeslOs.Leather Case. 10I-. alFoly

Deferred Terms ifdesired.

Write for illustratedliterature giving full *,

details of all " Avo"Testing Instruments

THE

AVODAPTERSimplifies valve testing. En-ables all valves to be testedunder actual working con-ditions without need for cuttingconnections and groping aboutinside the set. Instantly adapt-able for 4 -pin, 5 -pin and 07/.7 -pin valves. Price '9-Plri AVO-COUPLER

Attachment (not illustrated)renders AvoDapter suitablefor 9 -pin Alm,

New Edition /

RADIO SERVICINGSIMPLIFIED

A new and considerably enlarged editionof this popular test book. It has been entirely

re -written to conform with modern requirementsand in the light of present-day standards ofknowledge. It takes the reader by easy stagesthrough the whole routine of testing modernradio receivers, and describes in a clear andinteresting manner every test, and mv eh otheruseful information of value to the radio

enthusiast. 150 pages, 81 in. X 51 in.Numerous illustrations, diagrams and

graphs.

12/6

AUTOMATIC COIL WINDER &LTD.

Winder House, Douglas St., London,S.W.I. Telephone : Victoria 3404/7.

2/6 Putt

ELECTRICAL EQUIPMENT CO

ANDSHORT-WAVE WORLD JANUARY, 1938

FOR THE BEST RECEPTIONUNDER ALL CONDITIONS

ACISP.3A screened H.F. pentodewith an exceedingly highslope specially designedfor use in televisionreceivers.Heater Volts - - 4.0Anode Volts - - 250

Screen Volts - - 2 JO

Mt:tual Conductance - 7.5at Va 250 V5 100 Vg 0

Price 15!-

D.IA single diode in whichthe electrode capacity hasbeen reduced to a mini-mum by the use of pinssealed directly into theglass bulb.Heater Volts - - - - 4.0

Peak Anode Current 5omAAnode -Earth Capacity

1.8 mmfd.

Price 1016

VALVESfor

TELEVISIONThe requirements of modern televisionreception demand the utmost efficiencyfrom the valves employed in both thevideo stages and the scanning circuits.

The valves listed on this page aretypical of the care which has beengiven to development of special charac-teristics for the utmost efficiency andreliability in home built and commer-cial television receivers.

Full details of the other valves in theMazda range are given in the "MazdaManual" which will be sent to readersof "T. & S. W. W." on application.

T.3IA gas -filled thyratron foruse in scanning circuits.Heater Volts - - -

Peak Anode Current- 4.03oomA

Peak Anode Volts - 200

Voltage Drop- - 40Control Ratio - 20

Price 20/-

ACIP.4A special triode for usein scanning circuits withthe T.31 or similar thy-ratron. The anode is con-nected to the top of thebulb.Heater Volts - - - 4.0Max. Anode Voltage 600

Mutual Conductance 7.0Amplification - - 20

at Ea 100 Ea 0

Price 1716

CATHODE RAY TUBESEDISWAN CATHODE RAY TUBES are used by leading set makers and home constructorson account of their exceptional efficiency and brilliance.

The full range now includes 7", 9" and 12" tubes electrostatically and magnetically focussed.Write for full particulars to the Technical Service Department, 155 Charing Cross Road, W.C.2.

EDISWAN RADIO VALVESMazda Radio Valves are manufactured in Great Britain for The British Thomson -Houston Co. Ltd., London and Rugby, and distributed by

THE EDISON SWAN ELECTRIC CO. LTD., 155 CHARING CROSS ROAD, LONDON, W.C.2R.M.321

2

No. 119 VOL. XI JANUARY, 1938

and SHORT-WAVE WORLDSpecial Features

PAGEThe Low-cost Televisor ...A New Emitron Camera ... 11

Projection Tubes ... ... 13

Scophony Demonstration on B.B.C.Television 23

How Cossor Television Receivers areTested ... 24

A B C of Magnetic Scanning ... 26

4 -Stage 1.10 Metre Receiver ... 37

Aligning the RME69 40

5 -Band Mains -operated Monitor ... 41

A U.H.F. Emergency Transmitter ... 45

Automatic Gain Control ... 49

Considerations in Transmitter Design 50

Building the Precision Oscilloscope ... 58

TELEVISIONAND

SHORT-WAVE WORLDProprietors :

BERNARD JONES PUBLICATIONS, Lm.

Editor -in -Chief :BERNARD E. JONES.

Editor :H. CORBISHLEY, F.T.S.

Editorial, Advertising and PublishingOffices :

Chansitor House, 38, Chancery Lane,London, W.C.2.

Telephones : Holborn 6158, 6159.Telegrams : Beejapee, Holb., London.Subscription Rates : Post paid to anypart of the world -3 months, 3/6 ;

6 months, 6/9 ; 12 months, 13/6.Published Monthly-t/- net.

(On the first day of the month).Contributions are invited and will bepromptly considered. Correspondenceshould be addressed according to itsnature, to the Editor, the AdvertisementManager, or the Publisher, " Tele-vision and Short-wave World," ChansitorHouse, Chancery Lane, London, W.C.2.

IMPORTANT" Television and Short-wave World" is regis-tered at the General Post Office, London, fortransmission to Canada and Newfoundlandby Magazine Post. Entered as Second-classmail matter, Boston, Mass.

COMMENT OF THE MONTHTelevision for Cinemas.

HE recent demonstrations by the Baird and Scophon) companies ofT large -screen television of a quality sufficiently good to be used com-mercially in halls and cinemas create a problem which will have to receiveserious and considerate attention on the part of the Postmaster -General,The Television Advisory Committee and the B.B.C.

These demonstrations clearly indicate that sooner or later televiseditems must of necessity form part of practically every cinema programmeif the full advantages of the possibilities of television development are tobe secured. It is also clear that the type of entertainment that the B.B.C.is providing for the home is quite useless for public purposes, and againthere is the further matter of copyright. In regard to the latter, the presentposition is that the B.B.C. are waiving their rights in public demonstra-tions in order to enable the public to gain some idea of the possibilitiesof television, but it is certain that this attitude will not prevail if the B.B.C.transmissions were used for general public entertainment.

However strong an argument there may be for a B.B.C. monopoly oftelevision broadcasting, it does not seem to meet the problem which isnow arising and it seems impossible that the B.B.C. will be able to supplythe need which will come into existence. We understand that there is amove on the part of cinema interests to ask the Postmaster -General forpowers to start a second television service entirely for the benefit of cinema -goers and in our opinion they will be able to make out a strong case, butit is very doubtful whether the Government will allow private interests tooccupy space in the ether for entertainment purposes. It would appearthat if such entertainment is to be provided it must either be by cable orsome arrangement be made with the B.B.C. for public exhibition of eventsof national importance.

On the Ultra -shortsONSIDERABLY more light has been shed on to the possibilities of

ka ultra -short wave transmission and reception since the introduction oftelevision receivers. This may be a coincidence, but it so happens thatless than 12 months ago very few amateurs had any experience of recep-tion or transmission much below 8 metres. At the present time commer-cial and Government bodies are keenly interested in the working ofreceivers on wavelengths of around 1-5 metres, and are finding that theseultra -high frequencies are not quite so valueless as was at first supposed.

A commission has been sitting to determine the most suitable wave-length for general emergency use, and it is fairly safe to anticipate thatthe committee will recommend a wavelength of between r and 2 metres.It is claimed that these wavelengths will be reasonably free from motor-car ignition, and other interference problems, having at the same time amost reliable, although comparatively short range.

This fact, coupled with the growing interest and demand amongst ournormal experimental readers, and from our trade readers, has led us tointroduce in this issue a unique ultra -high frequency receiver plus thefirst section of an ultra -short wave transmitter for emergency use. Webelieve that the receiver is the first of its kind.

3

1ELEY1S1011AND

SHORT-WAVE WORLD JANUARY, 1938

Scophony, LimitedSir Maurice Bonham Carter and Mr. Solomon Sagall on Prospects of Televisionfrom speeches made at the Scophony Annual General Meeting on November 30, 1937

Substantial Technical Progress :: Co-operation with Cinema Television DevelopmentsFROM Sir Maurice Bonham Carter's

speech.The first year of the television

service supplied by the B.B.C. fromAlexandra Palace has now been com-pleted. A great deal of credit is un-doubtedly due to the corporation andto its technical and production staff fortheir achievement.

The provision of programmes of ade-quate entertainment value lies withinthe province of the B.B.C., and mustdepend largely on the financial re-sources which are made available forthe service. It is certain that a smallexpenditure will produce small re-sults. We must hope, therefore, thatthe B.B.C. and the Treasury will adopta generous policy in order to enablethis young service to maintain itsgrowth. Outdoor transmissions willprobably supply a very attractive sourceof entertainment.Larger Pictures

Other essential requirements of asuccessful service must be supplied bythe manufacturers. For home receivingsets a screen in size comparable to thatof the home cinema is necessary. Thisrequirement Scophony have achievedby the development of a home receivergiving a picture 24 in. by 22 in., andapproximating the home cinema in itsquality. The larger receiver which wehave developed for public viewing pur-poses measures approximately 6 ft. by5 ft., and we are at work on the develop-ment of a considerably larger screen.Picture Quality

Picture quality is dependent en twofactors-the adequacy of the transmis-sion and the adequacy of the reception.Scophony is satisfied with the capacityof its receivets adequately to convertinto a visual image the electrical wavestransmitted to it. To a large extentimprovement in picture quality is amatter of improvement of detail, andthis is a continuous process. I canfrom my own experience testify to theremarkable success of our technicalstaff in bringing about a continuousimprovement in this respect.Synchronisation Troubles

Scophony ascertained at the begin-ning of this year that there was a con-siderable amount of irregular timingand phase shifting in the synchronisa-tion signals radiated from AlexandraPalace, which made such signals un-suitable for reception by receivers usingscanning systems possessing inertia, asthe Scophony system does. Representa-tions were made in February last to theB.B.C. and later to the Television Advi-sory Committee, whilst at the same timewe were in a position to demonstrate bymeans of a new mechanical pulse

generator constructed by the Scophonyengineers that a perfectly satisfactorysignal for all types of television re-ceivers could be produced. I am happyto state that both bodies have acknow-ledged the fairness and justice of theScophony case and undertook to takesteps to remedy the trouble as soon aspossible.

In August of this year the B.B.C. in-stalled a completely new pulse generat-ing equipment for the generation ofstable synchronising signals.

On test the new B.B.C. pulse genera-tor it was found that some further im-provements were still required, and itis really only during the last few weeksthat the B.B.C. generator has reacheda stage to enable Scophony to havesatisfactory reception.The B.B.C. are also engaged now onexperiments with a new pulse generatorin their outdoor broadcast vans.Marketing Receivers

Scophony, Limited, are now makingarrangements for marketing receiversin the very near future. Such receiverswould in the first instance be manu-factured and marketed by Messrs. E. K.Cole, makers of the well-known Ekcowireless sets.Public Demonstrations

The company took part in the Tele-vision Exhibition at the ScienceMuseum, South Kensington, which weunderstand was visited by a quarter ofa million people. At the time, owingto the absence of suitable synchronisa-tion signals, the pictures shown by Sco-phony could only be on the 240 -linestandard transmitted by a low -powerradio transmitter from the Company'sCampden Hill laboratories. Thoughthis standard has necessarily beensuperseded by the higher AlexandraPalace standard, the pictures shown byScophony were the first large picturesof that definition ever to be shown inthis country or anywhere else in theworld.

I will now ask the managing directorto deal with certain other aspects of thetelevision situation and the company'sactivities.From Mr. S. Sagall's Speech

Mr. S. Sagall (managing director) :-We are now entering upon the secondyear of the world's first televisionservice.

Many are the difficulties still to beovercome in the way of making thepresent B.B.C. television service a suc-cess. Some very complicated andknotty problems have still to be solved.These problems are manifold. Theycover a variety of political, financial.technical, and entertainment aspects.I would like to use this occasion in order

to repeat a suggestion made by me lastyear to the Television Committee forintroducing the machinery of effectiveand constant consultation with thevarious television companies and inter-ests in this country in connection withall the problems affecting the furtherdevelopment of the television service.

The Television Committee, as con-stituted to -day, represents only the PostOffice, the B.B.C., and the Departmentof Industrial Research, all of which areGovernment or semi-official bodies, anda closer co-operation between the Tele-vision Committee and those interestsand concerns, without whom therewould have been no television to -day,appears to be highly desirable.Large Screens

You will recollect that at one time orother there were many people whodoubted the possibility of achievinghigh -definition pictures with any butthe cathode-ray tube method. How-ever, we have actually achieved in thecourse of the last two years, by meansof the specific Scophony methods, pic-tures so far unequalled in publiclyavailable results. We are fully awareof attempts to achieve projected largescreen pictures by means of the cathode-ray tube, but we can confidently claimthat as things stand to -day no othermethod has succeeded in giving pic-tures with the same brightness as theScophony methods.

I would like to say-without prema-turely divulging research secrets-thatthe above achievements do not in anyway represent the limit of Scophonypossibilities.

In this connection I feel it also neces-sary to state that though the presentB.B.C. standard is 405 lines-and, bythe way, some further improvementsin the transmission apparatus andtransmission technique are required tomake full use of such a standard up toits roo per cent. efficiency-Scophony,Ltd., is in a position to design and con-struct apparatus for higher standardsand higher numbers of lines, shouldsuch higher standards be adopted at alater stage.CinemaTelevision

By virtue of our large screen at-tainments, we are already actively en-gaged in investigating the conditionsfor an early establishment of a tele-vision service for cinemas.

One may immediately disperse a veryridiculous conception, fortunately notto be found in serious cinema circles',that television would kill the cinema.Just the reverse : the cinema is goingto benefit materially from the introduc-tion of television.

4

JANUARY, 1938AND

SHORT-WAVE WORLD

THE LOW-COST TELEVISORTHE COMPLETEUNITS FORI2 -in., 10 -in.

AND 7 -in.CATHODE-RAYTUBES

The photographs on this page show the complete unitsrequired for building the low-cost Televisor which hasbeen described in the three preceding issues (October,November and December, 1937). The complete equipmentcomprises four units-the vision receiver, the time baseand tube mount, the power pack for the vision receiverand the power unit for supplying high and low tensionfor the cathode-ray tube and time base. All these unitsembody ordinary wireless constructional practice andare well within the ability of any amateur to build.The vision receiver is a super -het, so designed that eitherone, two or three R.F. stages can be incorporated at will,thus making the receiver suitable for long, medium or

local range. This receiver using three R.F. stages hasprovided excellent pictures at a distance of approximatelyseventy miles.The time base and tube mount are incorporated in oneunit and the arrangement is such that either a twelve, tenor seven-inch tube can be used as desired. The smaller

5

The receiver chassis and time baseand tube mount for 12-, 10- and7 -in. tubes.

size, of course, permits of economy in tube cost, otherwisethe conditions applicable to the different sizes are thesame.Two power packs are used, one for supplying highand low tension to the vision receiver, and the other forthe time base and cathode-ray tube. The latter, as will

These two photographs show the vision receiver power pack angthecathode-ray tube and time base power pack respectively. Adequateprecautions have been taken to render the latter safe in operation.

be seen from the photograph, is contained in a metalgauze case and is so constructed that it cannot be openedwithout first disconnecting the power supply. The unit,therefore, is quite safe in use under all conditions.Notes on the adjustment and operation of these unitsare given in the following pages.

'MS/510BAND

SHORT-WAVE WORLD

Excellent pictures are obtainable on a four -inch tubeand the definition is so good that they will allow ofmagnification. The advantage of using a small tube arethat considerable economies can be effected in severalways, for, apart from the cost of the tube, lower operatingvoltages are possible with a consequent saving in costof components.The same vision receiver is employed as for the largertubes. As with the larger tubes three ranges are availableaccording to the number of R.F. stages incorporated. Inaddition to the units shown the vision receiver powerpack on the preceding page is required so that the com-plete equipment consists as before of the vision receiver,the vision receiver power pack, the time base and tubemount and the cathode-ray tube and time base powerpack, two of which are of simplified construction.A maximum of 1,200 volts are used with this receiver andthis enables a considerable saving to be made in the con-struction of the power pack which is shown by thephotograph on the right. With the comparatively lowvoltage employed no special precautions are requiredand the entire assembly is quite simple.Important features of these units are that they have beenspecifically designed for easy home construction, entail-ing no more difficulty than would an average wireless set.

JANUARY, 1938

HALVINGTHE

COST

THE UNITSEMPLOYED FOR

A 4 -IN. TUBE

This is a photograph of the time base andcathode-ray tube mount. The chassis is ofsheet aluminium and the construction verysimple.

FULL CONSTRUC-

TIONAL DETAILS

OF THESE UNITS

WERE GIVEN LAST

MONTH

The same receiver is employedas for the larger tubes. Thisreceiver is particularly sensitiveand will give excellent results atany distance up to seventymiles from the transmitter.

The power unit employed for the 4 -in. tube is of particularly simpleconstruction with a maximum output of 1,200 volts.

6

IELEYISIOAAND

JANUARY, 1938 SHORT-WAVE WORLD

ADJUSTING AND. OPERATINGLast month the final constructional article of this series appeared, which gave instructions for building theTime Base and C.R. Tube Power Supply Unit. This article completes the series. It gives full operatingand connecting instructions. It also includes a few notes on some refinements that may be added when the

very best picture quality is desired.

By S. WESTTHE adjustments required to a vision receiver are

similar to those for a normal broadcast receiver.Similar principles apply. The main difference is

that the required frequency response of the vision re-ceiver is necessarily broad.

For the complete outfit there is, in addition, thetime base to adjust and the tube operating conditionswill require to be correct.

It is preferable to undertake these adjustments inthe same order as written, that is, the vision receiverunit is first correctly tuned, then the time bases andthe tube's electrodes voltages are adjusted. Of course,the final exact adjustments to the vision receiver arebest made while observing the received picture.

In accordance with the above remarks, we shall firstdeal with the initial adjustments required to the visionunit.

Headphones, or a fairly sensitive speaker may beused for these adjustments. IThey are connected inthe first instance to the M and E terminals on the ter-minal strip. A o.i-mfd. or similar value condenser isincluded in series to exclude the V.F. valve's D.C.voltage.

The plug is inserted into the power supply unit. Thisunit can be connected direct to the mains for this pre-liminary test and not to its normal junction on the timebase power pack. It is assumed that a vertical half -wave aerial will be used, and the feeder of this is con-nected to the two aerial terminals.

When the Alexandra Palace transmitter is radiating

the preliminary adjustments can be undertaken in thisorder. It may be pointed out that the transmitterusually commences radiating about 3o minutes beforethe programme is scheduled to commence. The re-quired adjustments can conveniently be carried outduring this period.

There is a definite advantage derived by making theadjustments at this time as, for about half this period,the carrier is modulated for a cruciform pattern. Thismodulation has a very distinctive sound and a closeadjustment to the required one is obtained by tuningfor a signal that has a characteristic as here described.

ale first tuning is accomplished with the oscillatorcondenser. C19. The signal when tuned will have aconfused sound, and the frame synchronising pulseswill predominate as a rapid heavy beat. As the adjust-ments become more correct the signal will be fullerand the line sync. pulses component will become audibleas a shrill whistle. The adjustment is approximatelycorrect when this point is reached. The signal will nowhave a clear cut ring that is similar to that obtainedwith a mains receiver that has an open circuited aerialcoil.

No excuse is made for describing this signal noisein simple language.. By observation the writer hasfound that a novice can complete the required prelimin-ary adjustments more simply by listenirg to this signalthan by observation of the C.R. tube's screen. Thefinal exact adjustment, as has been earlier remarked,is made while observing the received picture.

SF M SL

VISIONUNIT

LT.

TIME. BASE MD TUBE UNIT

+4Doe.y.

A

SL

LTE.

LT.HT+HT-

VISIONPOWER PACK"/M1

Diagram showing the method of connecting the various units together.

C.R.TUBE &TIME BASEPOWER PACK

7

VILLIY1S1.04SHORT-WAANVDE WORLD JANUARY, 1938

TRIMMING THEgi VISION RECEIVERActually,. as has been pointed out in the original

article of this series, due to the coil's specifications,the tuning is very simple.

CI is the aerial tuning condenser and is uncritical.C19 is the oscillator tuning condenser and is critical,furthermore there is a very definite optimum point forthe best picture definition and freedom from I.F. har-monics feed back. This feed back is revealed as awatered silk effect on the screen or, as it is explana-torily termed, I.F. spots.

i'fhis is an inherent quality of all super -heterodynevision receivers and is unavoidable without extremelyelaborate screening. However, no difficulty is experi-enced in finding the correct setting, for the adjustmentis quite definite.

Of the trimmers C4, C8, and C12 are the R.F. trim-mers. They are adjusted for the best picture. As aguide, in the original, C4 and C8 are screwed in abouthalfway. Cx2 about two-thirds. C16, C24, and C29are the I.F. trimmers. They are, in the first instance,adjusted to resonance. They may be more finely setwhile observing the picture. Due to the heavy circuitdamping, they will be found quite uncritical.

It may be pointed out that the I.F. trimmers have acomparatively high capacity, consequently their settingwill vary the I.F. frequency over fairly wide limits.The optimum I.F. frequency will thus require to befound by trial. This is, however, quite a simple task.

Having completed these adjustments there remainsthe correct setting for R38 to be found. (This is thesynchronising control near to the MSP4 valve. Removethe phones or speaker lead from the terminal M andattach to the terminal S.F. R38 is now rotated slowlyuntil the noise due to the picture is lost, leaving thatdue to the sync. pulses only. This completes the ad-justments to the vision unit.

Adjustingthe Time Base

Adjustment of the time bases may now be under-taken. The potentiometers R4, R8, R2I and R23 areset about halfway round their full travel. Rr is turnedfull anti -clockwise.

The Belling -Lee five -pin plug is inserted in its ap-propriate socket on the time base and C.R. tube powerunit, as is also the six -way high -voltage plug. TheDLSx delay switch may be removed and the variousheater voltages can be checked. Alternatively if allthe various valves' heaters, including the C.R. tube'sheater are seen to be glowing, it can be assumed thatall is probably in order.

The remaining connections should now be made.Short leads are taken from the junctions on the visionunit's terminal strip to the appropriate points on thetime base unit. (These terminals are respectively M(this connects to the centre terminal of the small net-work panel), E (this connects to the earth terminal ofthe time base) and the S.F. and S.L. terminals. (Theseare connected "to the left- and right-hand terminals ofthe small network board in this order.)

Before re-inserting the DLSI delay switch, the poten-tiometer R4.1 is turned full anti -clockwise.

The DLSI switch is now inserted. After a short

interval its contacts will close and the time base adjust-ments are commenced.

The potentiometer R41 is slowly rotated clockwise.The raster will then become visible on the screen. Ifthis raster is small it may be enlarged with an altera-tion to the time base controls.

It is desirable also at this stage to centre the rasteron the screen. This operation is performed with thepotentiometers R26 and R3o. (These two controls aresituated on top of the time base.

The focus potentiometer R37 is now adjusted. Rota-tion of this control affects the quality of the screenillumination.

A setting is found where the raster edges are clearand distinct. It will also be possible to see the frameflybacks. When this is so the focus is approximatelycorrect. Set the potentiometer R8 for the least rasterwidth, i.e., full anti -clockwise. With R4 adjust thewidth to approximately half of that required for thepicture. Rotate R8 until the picture is the correctwidth.

With R33 reduce the raster to its minimum height,then with R2I adjust the height to equal approximatelyhalf the screen diameter measurement. With R23 in-crease the height so that the two extremes are withinabout i in. of the screen edge. This completes thepreliminary rough adjustments to the time base.

The vision unit power pack plug may now be con-nected to its socket on the time base power unit. Whenthe various valves' heaters have acquired operatingtemperature, black and white patterns, due to the pic-ture modulation, will appear on the C.R. tube screen.Also the raster will have locked"into synchronism forthe frame dimension.

Rotate R4 slowly until a single picture is formed,then lock this into position with a slight adjustment ofRr.

The correct picture dimensions may be restored withadjustment of R8 and R2I.

There only remains the correct picture contrast tobe found with adjustment of R4.1 and adjustment toR37 to secure correct light spot focus. This latter con-trol is rotated slowly until each individual line cgposing the picture is plainly visible.

The final adjustment of the vision unit trimmers cannow be carried out to obtain the best picture definition.

These time -base adjustments, although described infair detail, are simple. No difficulties are encounteredif the correct order of operation is observed.

When some operating experience is acquired, thereare one or two refinements that can be added with aview to improvement of picture quality. These notesare included as it is felt that a number of constructorswill like to incorporate some or all of these refinements.

If the theoretical circuit of the 'vision unit is referredto 'it is seen that the gain is controlled with the variableresistance R3. This resistance is in the cathode leadof the R.F. valve V r. While such an arrangementis perfectly satisfactory for most conditions, there isthe possibility, as the Mazda AC/SP3 valve has notvar-mu. characteristics, of modulation distortion occur-ring when a strong signal appears across the coil.

An alternative arrangement is to control two or allthree of the R.F. valves.

8

JANUARY, 1938 SHORT-WAANVDE WORLD

ADDING REFINEMENTSQuite a simple alteration will arrange this. The

cathode resistances R7 and RI' are returned to theslider of R3 and not direct to chassis.

Alternatively, the screen volts may be varied. Thisis simply arranged by returning the resistances RI,R6, Rio to a potentiometer across the vision H.T.supply. This potentiometer may consist of the originalpotentiometer R3 in series with a fixed resistance ofsuitable value.

Reference to the theoretical circuit will reveal that

MO I

C21

OSRAM

D.42.

GRID OF C.R.TUBE.

To 4.v. ;-3.0winding an,Mains Trans-'

Circuit' diagram showing the method of restoring the D.C.component.

the I.F. coils are damped with the resistances R21,R24 and R29. When fewer R.F. stages are used orif it is desired to increase the receiver's sensitivity,this damping may be considerably reduced withoutaffecting picture quality. Greater care will, however,be necessary with the " lining up " of the I.F. stages.As a guide the damping may be reduced to that of a5,000 -ohm resistance.

The anode load resistance R35, of the V.F. valveV9 is given as 7,000 ohms in the original specification.Picture quality is improved by reducing this resistancevalue.

ReducingAttenuation

A further improvement is effected by inclusion of asmall inductance in series with the load resistance.This inductance has no effect at low frequencies, butat high frequencies its effect is sufficient to reduce at-tenuation and phase shift.

Choice of a suitable inductance is important for acircuit resonant to a certain frequency is formed. Alsoto ensure that the calculated constants are not upsetby stray capacitances it is preferable to screen theinductance from the remainder of the circuit.

In the design space is left for the coil between thetwo valves V8 and V9. A Bulgin " coil can " enclosesthe inductance.

The value of this inductance is chosen so that withthe capacitances of the valve, etc., a resonant circuitis formed whose natural frequency is higher than theworking range of the magnifier stage.

The reactance of this inductance at the highest work-ing frequency is made approximately equal to half thevalue of the load resistance when this load is equal tothe reactance of the valve at this frequency.

Although having an inductance somewhat higherthan is required, a Bulgin H.F. choke, type H.F.24,serves quite well. A few turns may be removed toarrive at the best value.

This choke is connected at the top end of the loadresistance which is reduced in value to about 3,500ohms. A 2 -watt type will suffice.

For obvious reasons a re -adjustment to the potentio-meter R38 is entailed due to the alteration in value ofthe load resistance.

The smoothing of the C.R. tube exciter volts is, inthe interests of economy minimal.

If any hum remains, this is revealed as dark hori-zontal bands on the screen, an additional condensermay be included between the 3rd and 1st anodes. Ao.r-mfd. will be suitable and it must be capable ofwithstanding the full 3rd anode voltage of the tube.

Similarly, it may be found that the smoothing of thetime base high-tension is insufficient, although this isextremely improbable. Hum on the time bases is re-vealed as a wavy vertical screen edge or as unequalspacing of the lines. The remedy is to increase thesmoothing capacity.

The D.C. component is not retained with this re-ceiver. Absence of the D.C. component causes unevenillumination of large uniform picture areas. Further-more, the picture has a set illumination value level andis not instantaneously controlled by the transmitter.This has a deleterious effect on certain classes of trans-mission. Particularly news reels and any items havingsudden transitions in lighting value.

RestoringD.C. Component

The D.C. component is simply restored with a diodevalve. A winding is included on the heater supplytransformer of the power pack to permit this. The

Diagram showing inclusion of aninductance L to reduce attenuationand phase shift at high frequencies.R35 is reduced in value to 3,500

ohms.

method of connection is shown in an article by thewriter in the December number, entitled " High Volt-age Power Supply Units for Television," page 726.

If it is not possible correctly to centre the picture,the connections of each set of plates to the shift poten-tiometers may be reversed, i.e., DX2 is returned toR3o and DVI to R26. This permits an additive move-

B

11.111

V.ISNYISJOhAND


CENTERING THE PICTUREment in opposition to the stray field causing unbalance.

No alteration to any of the connections is entailedif it is desired to use a 12 in. tube. The picture withthis large tube is extremely good and can plainly beseen by fair-sized audiences.

In the article on the time base it was mentioned thatthe resistances R12 and R14 might be replaced with apotentiometer. A finer control is imparted to the framerelay with a variable feed.

A suitable value for this potentiometer is ,5o,000ohms. The adjustment is made so that correct inter -

the complete receiver, a variety of layouts is permis-sible. This is a pleasing feature when an existingcabinet is to be utilised.

In the original' the vision unit was mounted reversed,i.e., with the output terminal strip uppermost. Thetime base and tube were placed on top of this and thetwo power units underneath. An extremely compactarrangement results when this scheme is adopted. Con-structors can decide their own layout, however. Theonly point to be observed is that the M., S.L. and S.F.leads are preferably kept as short as possible.

II

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lace is achieved. Close examination of the picture willreveal when this is so. No rolling of the lines existsand each is equally spaced from its neighbour.

Three incorrect adjustments are possible. The first istermed " pairing " of the lines. When this happensthe lines comprising the picture are grouped in adja-cent pairs.

The second when no interlace exists. The picturethen has a coarse appearance, the lines being visibleeven at large distances. Lastly poor frame lock. Thisis revealed as an apparent weaving in and out of thelines.

Due to the design of the various units comprising

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As some readers have experienced a little difficulty infollowing the diagrams showing the vision receiver con-struction and the synchronising application network andmodulation feed, additional diagrams are given here.The top left diagram shows the wiring for sections N, 0and IF and that on the right (above) section S. To theleft is the synchronisation application network.

Baird Cinema TelevisionTHE Baird Company, last month, staged a sur-

prise by showing the B.B.C. television programmeon a full-size cinema screen at the Palais Theatre,

Bromley, Kent. The approxifnate size of the picturewas 8 ft. by 6 ft. and the definition and illuminationwere so good as to enable those present clearly to seethe picture from any part of the theatre.

The picture was produced by projection from a cath-ode-ray tube of special design and about two inches indiameter and it is understood that 25,000 volts wereemployed.

Mr. J. L. Baird was present, and he said that he hadbeen working on the idea since 1930 and now felt con-fident of putting the results before the public.

He said it was his intention very soon to equip largegroups of London cinemas with the necessary appara-tus for showing television programmes as adjuncts tothe ordinary news reels. He hoped to transmit hisown programmes from the Crystal Palace.

10

JANUARY, 1938TELLIVISTA

ANDSHORT-WAVE WORLD

A NEW EMITRON CAMERAWITH GREATLY INCREASED SENSITIVITY

Here are details of a new pick-up camera with approximately ten times the sensitivity of thetype formerly in use. This increased sensitivity will enlarge the possible scope of television to

a very considerable extent.

FOR the first time, for the tele-vising of the Lord Mayor's Showand again for the Cenotaph cere-

mony transmission, a new type ofEmitron camera was used which hasan efficiency of almost tenfold thatof the type that has been in normaluse for the past twelve months.

Secondary EmissionThe new camera, which has been

developed by engineers of the Mar-coni-E.M.I. Company, depends forits increased sensitivity upon second-ary emission. It will be rememberedthat in the case of the originalcamera the actual picture was

ELECTRON FOCUSING COIL

AC C ELERATOPHOTO -ELECTRIC

. SCREEN

SIGNAL.MOSAIC ELECTRODE PLATE

SCANNINGBEAM

Schematic drawing of the new Emitron camera employing secondary emission.

The efficiency of a camera dependsupon the amount of light with whichit can be operated, and photographersare aware that in the case of an ordin-ary camera under poor lighting con-ditions it is necessary to use a largelens aperture with the attendant lossof focusing range. This difficulty hasalso been experienced with the Emi-tron camera with the result that ithas been necessary to follow veryclosely any movements of the objectnearer to, or further away from, thecamera; also it has been the experi-ence that whilst an object close tothe camera may be in focus, othersquite a small distance further awaywould be out and appear blurred.

Obviously one solution of the diffi-culty would be to have a more sensi-tive surface upon which the lens pro-jects the picture so as to enable alens with a smaller aperture to be em-ployed which would provide greaterdepth of focus, or enable it to beoperated under adverse lighting con-ditions with somewhat increasedaperture.

focused upon a mosaic composed ofa number of photo -cells and that thecharges due to the liberation ofphoto -electrons were discharged bythe scanning beam. In the newcamera there is also a mosaic, butthe light image does not fall on thisand neither does this mosaic requireto be photo -electric. Instead thelight image falls upon a photo -elec-tric screen at the front end of thecamera and it is the photo -electricimage which falls on the mosaicscreen, the latter being scanned bythe cathode beam. This has the ad-vantage that secondary emissiontakes place With greatly increasedefficiency.

The drawing shows the generalarrangement of the camera and itwill be seen that it consists of a glassvessel with a cathode gun, capableof producing a fine beam of electrons,placed so that the mosaic screen canbe scanned in the usual manner,means, of course, being provided fordeflecting the beam. The mosaicelectrode consists of a multiplicity of

elements which are insulated fromeach other and from a backing platewhich is made of metal and is calledthe signal plate. This assembly ofelements may take several formsand, for instance, the elements maybe of silver disposed upon a sheet ofmica which may in turn be backedwith a sheet of aluminium.

At the other end of the glass tubeor vessel and parallel to the mosaicelectrode there is a photo -electricallyactive screen of non -mosaic charac-ter which may, for example, consistof glass having a coating of silveroxide on which is deposited a layerof caesium. It is necessary that thiscoating be so thin as to be semi-transparent.

Surrounding a part of the spacebetween the photo -electric screenand the mosaic is a coil which whensuitably energised constitutes anelectronic lens of large aperture, theobject of this being to form an elec-tron image of the screen upon themosaic. Between the coil and themosaic electrode there is a metalcylinder placed coaxially with thetube, the purpose of which is to ac-celerate the photo -electrons' from thephoto -electric screen and to collectthe secondary electrons which areliberated from the elements of themosaic when, the tube is in operation.This electrode is given a suitablepositive potential with respect to thephoto -electric screen.

As will be seen, an ordinary opti-cal lens system is provided outsidethe tube for the purpose of focusingthe image on to the photo -electricscreen.

How the Camera FunctionsThe operation of the tube is as fol-

lows : When a light image falls uponthe photo -electric screen, electronsare emitted from various parts of thisto an extent proportional to the lightfalling upon any particular part.These electrons are accelerated to-wards the mosaic electrode by meansof the accelerator and the electronlens produces a magnetic field whichhas the effect of focusing electronsfrom any point of the photo -electric

(Continued at foot of next page.)

II

ANDSHORT-WAVE WORLD

BRITISH TELEVISIONAS AMERICA SEES IT

6 6 NGLAND is far ahead of theUnited States in television,and should anyone doubt this

statement, let him take a trip and seefor himself," said Mr. William Grim -ditch, Chief Engineer of Philco Radio& Television Corporation, upon hisreturn from an extended stay abroadduring which he studied the televisionsituation closely.

" The British Broadcasting Cor-poration rightly assumed the respon-sibility of programme and transmis-sion experiments, thus permitting thetelevision receiver manufacturers towork out their side of the problem.I am convinced that in America theproblems of programmes and trans-mission, in television, must bedivorced from the manufacturing ofreceiver sets. After seeing the re-sults obtained in England by theB.B.C. method, I am firmly con-vinced that, in America, no manufac-turer of receivers in television shouldever be granted anything but an ex-perimental transmission licence. Few,if any, manufacturers in Americacould afford to assume the financialburden and responsibility of doing thepioneer research in programmes andtransmission

When asked what, in his opinion.,was the comparison between the pre-

sent status of television in Americaand England, in view of his recentobservations, Mr. Grimditch said," England has behind it a few yearsof intensive research which we, inAmerica, have still before us, beforewe can have television such as I sawin London. We can learn much fromthe experiences and experiments inEngland, but then again we havepeculiar problems of our own whichwe must work out here in America forourselves.

Mr. Grimditch was profoundly im-pressed by the television which hesaw in England, and said that Ameri-can engineers might well with profitlook into such things as positivemodulation, the types of waves usedin England, and the types of syn-chronising impulses. He was alsoimpressed by the fact that many satis-factory television receivers were madeby comparatively small manufactur-ers who could not have spent greatsums on experimenting. This, in hismind, was a great compliment tothe B.B.C.'s transmissions.

" I would like to put at rest, if pos-sible, a false notion held by manypeople in America. iThese people feelthat television in this country is beingheld back by radio set manufacturerswho fear that, should television come

JANUARY, 1938

out, their sales of sound radios wouldbe hurt. It is my honest opinion thatmost radio receiver manufacturerswould far rather make and sell thehigher priced television receiver thanthe great number of small radio re-ceivers now made, in which field com-petition is exceedingly keen.

" In America we have manyproblems to solve before we canhave the television which Lon-don enjoys to -day. In the firstplace there are the engineeringproblems, problems that must beworked out in the field, problemswhich the B.B.C. has gone a longway towards solving, and which wemust yet do. England is well throughthis, we are facing it.

" Then there are the difficult prob-lems of patents, licences, and such.These are indeed involved. Butabove all else we must have stationsconcerned chiefly in transmission andprogrammes. Transmission andprogrammes, I am now more con-vinced than ever, are not the problemof the receiver set manufacturer, norshould they be.

" We cannot disregard the fact,"said Mr. Grimditch, in conclusion," that in London to -day a man canbuy a television receiver, have it in-stalled in his home, and view sche-duled programmes. That speaks foritself when the question arises as tothe status of television in England ascompared to that in America."

" KNEW EMITRON CAMERA "(Continued from preceding page

screen on to a corresponding posi-tion on the mosaic.

The bombardment of the mosaicresults in the production of second-ary electrons, the number of second-ary electrons greatly exceeding thenumber of primary. Each elementof the mosaic forms a small conden-ser with the common signal plate andin between successive scans each ofthese small condensers is charged toan extent dependent upon the photo -current striking the element. Thevelocity of the scanning beam is soadjusted in relation to the nature ofthe mosaic surface that the potentialof each element when scanned ischanged to an equilibrium value. Ifthe effect of the photo -electron is toraise the potential of an element, thevelocity of the beam is so adjustedthat it reduces the potential. Thecondenser formed between an element

and the signal plate is thus discharged)"----'11714-...tke value of the discharge cur-

rent depends upon the charge whichthe condenser has acquired since thelast scan. This charge is, of course,dependent upon the intensity of thephoto -current striking the element,which in turn is dependent upon theamount of light falling upon a corre-sponding point of the photo -electricscreen.

As the scanning beam, therefore,sweeps over the multiplicity of ele-ments of the mosaic there are de-'veloped across the resistance in thelead to the signal plate voltageswhich depend for their value on thelight intensity of correspondingpoints in the object and these " pic-ture " voltages are amplified in theusual way.

Alternative constructions of thistype of camera are possible. For ex-ample, instead of the photo -electricscreen consisting of a continuous sur-

face it may take the form of a wiremesh with a photo -electric coating.With suitable arrangements, trans-parency of the photo -screen is not es-sential for the light image can bethrown upon it on the side facing themosaic.

In addition to increased efficiencywhich will permit of transmissionsunder adverse lighting conditions, as,for instance, in theatres, etc., the dis-position of the elements which com-prise the camera makes it possible toobtain a better optical layout thanwas formerly the case, and this willpermit of better use being made oftelephoto and wide-angle lenses whichwill enable the camera to be operatedunder many conditions which hither-to have presented insuperable prob-lems. For studio work small aper-ture working will be possible enablingthat momentary loss of focus after achange of distance which has beenapparent in the past to be avoided.

4

12

TEIVIS1011JANUARY, 1938

PROJECTION TUBES

The Latest R.C.A. projection tube with which pictures four feet wide have been obtained.

ANDSHORT-WAVE WORLD

AN ACCOUNT OF EXPERIMENTAL DEVELOPMENTBy V. K. Zworykin and W. A. Painter, R.C.A. Manufacturing Company Inc.

1N ordei- to increase the size of image on a directlyviewed tube it is necessary merely to increase thephysical dimensions of the tube. Within reasonable

limits this can be done without serious sacrifice of bril-

o

0*°°4Vol*

2

0 inn Ina *0

SECOND ANODE CURRENT- MICROAMPERES

so

2.000 4eop &we Sam 104>>2

SECOND ANODE VOLTS

Fig. 1. Top curveshows variation oflight output offluorescent mater-ial with beamcurrent. Bottomcurve shows lightoutput V.S. secondanode potential.Both curves arefor RCA phosphor

No. 1.

liance. However, if we wish to make the picture size18 in. by 24 in., the physical bulk of the tube becomesenormous, and the glass face would have to withstanda pressure of over five tons.

An alternative method of obtaining an enlarged pic-ture is by the use of a projection tube. The projectiontube is similar to the directly viewed tube in principle,but produces a small, very bright image. By the useof a suitable lens the picture may be projected to anydesired size on a screen. The principal problem in-volved in this method is that of obtaining sufficientillumination on the screen.

Let us consider for a moment the ordinary 12 in.tube. The tube operates at an overall potential of4,000 to 6,000 volts with a beam current of 25o micro-amperes for the high lights. Thus, about 1.5 watts ofelectrical energy are available for conversion into lightin the high lights. This energy is supplied to a tinyspot about 0.5 millimetre in diameter which is made toscan the fluorescent screen. The luminous output ofthe fluorescent material is a function of both current

Reprinted from the Proceedings of the

and voltage, as can be seen from Fig. I. Under theconditions specified the fluorescent material has a lumin-ous output of about two candles per watt. The bright -

1.5 x2ness of the screen, therefore, will he - 5.8

75 x -144

candles per sq. ft., or 18.2 foot -lamberts.Motion picture engineers have devoted considerable

attention to the subject of brightness in relation tomoving picture projection, but the subject is full ofcontroversy. Experience has shown that the highlights of a 35 -millimetre moving picture should have abrightness of about II foot -lamberts if eye: fatigue isto be completely avoided and it is estimated that theactual level attained probably ranges from to 9 foot -lamberts. As a temporary measure it has been recom-mended that 3.7 foot -lamberts be adopted as a standard,with limits of from 2.7 to 5.2 foot -lamberts. Thisrecommendation was actually made in terms of lightfrom a projector running with no film in the gate, but

Fig. 2. Brilliancedistribution of twotypes of transmis-

sion screens.

CO,IMERC i AL5 C R EEm

we have converted them to terms of highlight brilliancefor convenience in this discussion. The suggestionhas also been made that a high -light brilliance of 2.7foot -lamberts be considered standard for 16 -millimetreprojection. In this case the recommendation wascouched in terms of intensity of light falling on the

Institute of Radio`tngineers, August, 1937.

13

TELEVISIgilAND

SHORT-WAVE WORLD

OPTICAL CONSIDERATIONS

JANUARY, 1938

screen; in converting, we assumed a diffuse screen witha reflection factor of 75 per cent.

For the sake of direct comparison, the brightness ofthese and other familiar objects is shown in the follow-ing table :-Lighted page (minimum recommended brightness for

reading fine print), ro foot -lamberts.High -light brilliance on screen of moving picture

theatre, 2.7 to 5.2 foot -lamberts.

Fig. 3. Schematic diagram of optical system forprojection tube.

High -light brilliance in i6 -millimetre movie, 2.7 foot -lamberts.

Outdoor scene-bright day, 300 to boo foot -lamberts.High -light brilliance of picture on is -in. television re-

ceiver, 18.2 foot -lamberts.These figures, particularly those referring to the i6 -

millimetre movie and the r2 -in. tube, should be kept inmind as we investigate the elements constituting ameans of projecting television images.

The most efficient viewing screen to use for our pur-pose is a highly directional transmission screen. Thetransmission characteristics of two such screens areshown in Fig. 2. The solid line represents a commer-cial type of screen made from a rubberised material,while the dotted curve was obtained from a piece ofordinary tracing paper. Screens of this type will trans-mit in a direction normal to their surface several timesas much light as would a perfect diffusing screen. Forthe commercial screen the ratio is 480 per cent. whilefor the tracing paper it is 36o per cent. In the formercase the picture can be viewed without too serious lossof light within an angle of twenty degrees on eitherside of the normal, while the tracing paper allows anangle of about fifteen degrees. In both cases, thelight is distributed with sufficient uniformity to avoidthe bright area due to direct transparency of the screenknown as " hot spot."

For the purpose of computation, let us consider theprojection arrangement shown in Fig. 3. Before wecan arrive at any conclusions as to the light outputrequired from the tube it is necessary to make certainrestrictions on the lens that is to be used. The lensmust be one that can be manufactured in quantities andmust be relatively inexpensive. The correction of thelens does not have to be as perfect as in the case of aphotographic objective. From our present under-standing of costs, it seems that the lens diametershould not be much greater than three inches nor the f

value much smaller than f x.5. A lens of this type inuse at present has an angle of field of approximately35 degrees, and the focal length is 120 millimetres.

From these lens specifications, and using our 18 in.by 24 in. standard, it can be shown that the distancefrom lens to viewing screen is about 4.6 feet. Theimage on the projection tube screen would measure1.66 by 2.22 in.

The brightness of the projection tube screen can becalculated from the brightness of the image on theviewing screen, the lens aperture, the magnificationand the losses in the system. Assuming fifty per cent.transmission through the lens and applying the distri-bution of our transmitting screen to the result, we findthat the image on the projection tube must be 48o timesas bright as that on the viewing screen. To attain abrilliance equal to that on our t2 -in. tube we must havea brightness of about 2.800 candles per square foot.

At first glance this brightness seems very large in-deed, but when it is remembered that we are using asmall area and that we are referring only to the picturehighlights it will be seen that the light output is notexcessive. The equivalent light output will be

2,800 x i.66 x 2.2271.4 candle power.

144There are, unfortunately, practically no data avail-

able on the efficiency of fluorescent materials at highinput levels. However, if we assume that it may someday be possible to produce a fluorescent material hav-ing an efficiency of 1.5 candles per watt at such levels,then we find that the peak input required to producethis light in the high lights would be 47.6 watts. Atio,000 volts this would require a peak beam current of4.76 milliamperes; if the voltages were raised to 20,000

Fig. 4. " Flat -surface " type ofprojection tube.Use is made of thegreater amount oflight emanatingfrom the side ofthe screen which

is scanned.

ELECTRON GUN,

DEFLECTING YOKE

PROJECTION LENS

FLUORESCENT SCREEN

volts, the required peak current would be only 2.4 milli-amperes. It should be remembered that the requiredbrilliance has been figured on the basis of equallingthe brilliance of present directly viewed tubes. If,however, we are satisfied with the brilliance attainedwith a reasonably priced home movie projector, thebrightness of the high lights need be only 427 candlesper square foot, or 'the light output only i i candlepower. At ro,000 volts the current needed to producethis light, even with present screen materials, is only0.73 milliampere. Thus, while the type of tube des-cribed will not yet compete with the directly viewed tubein brilliance, it does not fall so very far below the mini-mum requirements.

Active development of the projection tube was under-taken at the R.C.A. laboratories. The original pro -

(Continued on page 17.)

14

JANUARY, 1938 ANDSHORT-WAVE WORLD

WORLD PIONEERS & MANUFACTURERS OFALL TYPES OF TELEVISION EQUIPMENT

FIRST IN 1926 FINEST IN 1937One of the factors contributing to the outstanding successof all Baird television receivers has been the consistentlygood quality of the " Cathovisor " Cathode Ray Tube.Baird Television Ltd. as pioneers, have developed thisCathode Ray Tube to adesign which fulfils all

the conditions for excellentpicture resolution.For modern television, therequirements of the Cath-ode Ray Tube are that abright, well-defined pic-ture be formed on thescreen by the electronbeam, that the picture bein sharp focus all over thescreen, that the focus re-main always equally sharpwith the variations of grad-ations from light to dark,and that these variationsbe a faithful reproductionof the original gradations of the scene being televised.The developments in Cathode Ray Tube technique under-taken by Baird Television Ltd. have ensured that thesefactors are complied with by their latest type " Cathovisor "Cathode Ray Tubes, which are the most satisfactory on the

Heater volts .. ..Heater amps. .. .. .. .. .. ..Peak to peak volts, between black and highlightsMaximum electro-magnetic sensitivity ..Modulator/earth capacity ..Modulation sensitivity (slope)Anode volts .. .. .. ..Maximum input power to the screenScreen colourPRICE

market. Not only is the electrode system extremely simpleand robust but, due to the type of cathode employed andthe magnetic focusing, a high intensity cathode ray beamis produced which results in a very brilliant picture on the

screen. Screen grain doesnot impose a limit on spotsize due to the special grad-

ing of the material used,and picture definition is

outstandingly good.

"Cathovisor " Cathode Ray Tube Type 12M1171, as arranged in one unitfor the Baird TH Receiver.

Every tube receives strin-gent tests for total cathode

emission, modulation range

and illumination character-istics, filament rating andscreen quality, and follow-ing normal picture recon-stitution under service

conditions, the completedCathode Ray Tube issubjected to a further

high external air pressure test.A special feature is that each tube is completely electro-magnetic in operation. Full details supplied on requesttogether with information concerning scanning andfocusing equipment.

TECHNICAL DATATYPE 15MW2. TYPE 12MW1.2.2 volts approx. ..

. 2.5 amps. .. ..56.5 volts. ..

: : 22mmlitiF/AT...

(approx.) .. ..'

17 µA/V (approx.) .. ..6,500 volts (working) ..3.5 milliwatts/sq. cm. ..

. Black and white .. ..

2.2 volts approx.2.5 amps,14 volts.2mm/AT.4 titiF (approx.).17 µA/V (approx.).4,90o volts (working)4 milliwatts /sq. cm.Black and white.

15 gns. 12 gns.

BAIRD TELEVISION LTD.Head Office : Laboratories :

Greener House, 66, Haymarket, Crystal Palace, Anerley Road,LONDON, S.W.I LONDON, S.E. 19.

'Phone: Whitehall 5454 'Phone : Sydenham 6030

15

tagYISLOt1AND

SHORT-WAVE WORLD

OF TECHN:CAL :NTEREST

MADE INENGLAND

U12

JANUARY, 1938

TO ALL USERS OFELECTRICAL GRAMOPHONE

REPRODUCERS

Here is the way to bring realisminto Orchestral Recording

It is well known that in making gramophone disc recordings it is often essential

to compress the range of sound in order to accommodate the size of the disc.Thus it is difficult to reproduce music at its natural contrast of full volume andquiet tones unless means are taken in the reproduction to compensate for therecording.

A practical means of doing this is to employ an Automatic Contrast Expansioncircuit between the pick-up and the main amplifier. Such a circuit has the dual

benefit of restoring the natural intensity of the loud passages and, at the sametime, by a suitable adjustment, greatly reducing the surface noise or needlescratch on the softer passages.

A typical circuit for linear and distortionless expansion, employing three to four

OSRAM valves and an H.T. source, can easily be constructed by the amateur, and

some practical details with component values of such a circuit are available on

application to :-

THE OSRAM VALVE TECHNICAL DEPARTMENT.

The General Electric Co. Ltd., Magnet House, Kingsway, London, W.C.2

valvesOSRAM VALVES DESIGNED TO ASSIST THE DESIGNER

16

',W1\1011JANUARY, 1938

PROJECTION TUBE CONSTRUCTION

ANDSHORT-WAVE WORLD

(Continued from page 14.)jection tube involved simply a scaling down of dimen-sions of a standard 9 in. tube. The electron gun andscreen were assembled in a common Ehrlenmeyerchemical flask. The usual vacuum technique did notpermit of very high voltage operation, yet a picture ofreasonable detail, though lacking in brilliance, was ob-tained. The tendency of the soft glass to crack underthe heat generated by the beam spelled the doom of amajority of these tubes.

One of the first major steps forward was the realisa-tion that the projection tube deserved recognition as aseparate problem. A realisation of the need for farhigher beam currents concentrated in much smallerareas than had hitherto been considered feasible guidedfurther experiments on the electron gun.

GunDevelopment

While the principle of electron optics had not yetbeen widely espoused at that time, the gun developmentproceeded along quite logical lines. Attention firstcentred on the focusing field between first and secondAnodes until the optimum conditions, within the res-trictions of the bulb, were determined. It was soonrealised that a major source of trouble was the fieldadjacent to the cathode and effort was concentrated onimprovements there. The necessity of decreasing thearea of the beam near the crossover was recognised ;a wide range of means for accomplishing this wastried, including a study of preconcentrating cylindersattached to the grid and the shaping of fields by theintroduction of various sizes and shapes of electrodes.

Two circuit considerations served to handicap thework. The first was the need of maintaining a fairlyrestricted modulating range, since a high signal volt-age covering the necessary range of frequencies wasimpractical. The other was the need of maintaining asmall constriction in the tube neck, in order that mag-netic poles might be placed close enough together toallow full deflection of the beam. This in turn set sucha limit upon the size of electrodes which could be usedthat considerable aberration was always present. Thefirst of these difficulties is being overcome by refine-ments in tube design; the latter has been greatly modi-fied by improvements in deflecting circuits.

The advent of electron optics allowed a theoreticalanalysis of the remaining faults to be made and pointedthe way to refinements which are so necessary.

While the major effort was spent on tubes of thetype described, several investigations of interest weremade along somewhat different lines. One of theseparticularly worthy of mention is the so-called " frontsurface " type of projection tube. It has been deter-mined that the light on the surface of the screen adja-cent to the scanning beam may be double that appear-ing on the outer surface. To utilise this fact seemedan easy way of improving the brilliance of the image.

Attempts to accomplish this were made by depositingthe fluorescent material upon a metal plate within thetube. To enable the lens to be placed directly in frontof the screen, it was necessary to seal the neck con-taining the electron gun on to the bulb at an angle, asshown in Fig. 4. This imposes several difficulties upon

the problem of focusing, first of all, because of inter-ference with the optical lens there is little clearance ofthe magnetic deflecting yoke. The electron gun must,therefore, be placed well back in the neck and the dis-tance between gun and screen is greater than in thedirect type. This, of course, limits the focus obtain-able. Since the beam strikes the screen at an anglethe fluorescent spot is no longer round but is elliptical.To make the long axis of the ellipse equal to the dia-meter of the corresponding round spot requires betterfocusing than in the direct viewing type of tube ifthe same resolution is to be maintained. The keystoneshape of the scanned pattern can be corrected by suit-able changes in the scanning circuits. While somefairly good pictures with low definition have been ob-tained, this design has not yet worked successfully ina high -definition system.

In a model of one of the projection tubes developed inour laboratories the picture size was 2.25 x 3 in., thehigh -light brightness under operating conditions about28o candles per square foot. Since the image is largerthan in our example the lens suitable for use with itmust have a slightly longer focal length (assuming thefield is limited to 35 degrees). If this lens has the samediameter as the one considered in the previous exam-ple, the brightness of the picture on a viewing screen1.5 X 2 ft. in size is about o.6 candle per square foot,or about 1.9 foot -lamberts. This illumination is notquite great enough to allow comfortable viewing forany length of time.

Electron -opticalArrangements

The electron gun used in this tube is shown in Fig. 5.It operates at an overall voltage of 15,000 volts anddelivers beam current of about qoo microamperes, thusgenerating six watts in the high lights at the fluor-escent screen. (The spot size for this condition is abouto.005 in. In principle this gun is similar to the gunused in the directly viewed tube. It consists in essenceof a cathode, a control grid, and a two -lens electronoptical system. The first lens in this system causesthe electrons from the cathode to converge into a nar-row bundle known as the crossover. This crossoverhas a diameter much smaller than that of the emittingarea of the cathode. As well as forming the crossover,the first lens produces a virtual image of the crossoverlying slightly behind the cathode itself. This virtualimage serves as a virtual object for the second lensand is imaged on the fluorescent screen in the form ofa small electron " spot." Fig. 6 shows the electrontrajectory through the gun.

The control grid, as shown, consists of an apertureddisc near the cathode, whose potential is controlledby the television signal. The potential of this elementcontrols the size of the area on the cathode over whichthere is a positive field allowing the escape of electrons.Fig. 7 shows the control characteristics of this typeof grid and the variation of spot size with bias.

The cathode is of the indirectly heated oxide -coatedtype. The emissive coating consists of a mixture ofbarium and strontium oxides and covers an area ofabout 6 x to' square inches. This material is oper-ated at a brightness temperature of 1,050 degrees Kel-

17

TVAVIS1011AND


OPERATING VOLTAGES AND TUBE DESIGNyin. Although the entire coated' surface is capable ofemission, only an area slightly smaller than the gridaperture is utilised. From this portion a current ofi.o to 1.5 milliamperes is drawn, about 0.4 milliamperebeing delivered to the beam and the remainder beingcollected in the first anode.

The fluorescent materials commonly used in the pro-jection tube are zinc or zinc -beryllium orthosilicates.These give a green or greenish -yellow fluorescence.Another class of fluorescent materials are the zinc sul-phides, some of which produce a nearly white light.In general, the sulphides have higher initial efficiencies

GRID IV ANODE.

2P ANODENORMAL PROPORTIONS - TOTAL SEAM PATH IN 2's' ANODE 15

TWICE THAT IN 15rANODE

Fig. 5. Sectional diagram of electron gun used in projectiontube. The second anode is formed by a conductive coating

on the bulb wall.

than the silicate materials; however, they are charac-terised by instability, and their use to date has there-fore been restricted.

It is interesting to note, however, that use of a mate-rial giving white light under normal conditions doesnot seem as important in the case of the projection tubeas it does with larger tubes. When viewing the imageprojected from tubes having green fluorescent screens,several observers have commented on the apparentblack and white appearance of the picture. This mightpossibly be due to a broadening of the spectral charac-teristic of the fluorescent material at high intensities,although further study is necessary before reliable con-clusions can be drawn.

Turning now to the question of bringing the per-formance of this tube up to the standard set by ourearlier considerations, the most important requirementis found to be that of increasing the brightness. Inaddition, it would be desirable to increase the contrastof the picture and to improve the resolution.

IncreasingBrightness

An increase in brightness of the projected pictureinvolves (i) increasing the power output from the gunwhile maintaining the present spot size, or, if possible,reducing its dimensions; (2) improving the fluorescentmaterial and screen design; and (3) designing a moreefficient optical system. These points will be taken upin the order mentioned.

The improvement of the electron gun can take placealong one or more of the following lines : (i) Construc-tion of the gun so that it can be operated at a higherpotential; (2) improvement in the electron optical sys-tem so that less current is lost to the first anode; (3)increase of specific emissivity .of the cathode; and (4)increase of the usable area of the cathode by alteringthe electron optical system.

The problem of constructing a tube which will with-

stand voltages of 20,000 volts does not seem particu-larly difficult. The problem is concerned chiefly withthe elimination of sharp points or edges where excep-tionally high gradients might build up and with propershielding to insure that the electrons are confinedwithin their designated path. One outstanding diffi-culty is the problem of applying a black conductivecoating to the inside of the bulb. This coating is usedto reduce reflection from the walls of the bulb. Con-ventional coatings consist chiefly of carbon. We haveexperienced some difficulty in preventing minute parti-cles from shaking off and causing arcs. In high -volt-age oscillograph tubes we substituted a coating ofplatinum which adheres tightly to the glass.

ImprovingElectron -Optical System

The matter of improving the performance of the elec-tron optical system is much more difficult. In order toapproach the question in a systematic manner, it willbe necessary to examine in greater detail the way inwhich this system works. As was described above, thesystem consists of two focusing fields or lenses. Thefirst of these lenses produces a small virtual objectwhich is imaged by the second lens on the fluorescentscreen. (This second lens, like the lens which is usedin ordinary optics, produces an image which is subjectto the same aberrations which are met in the Seideltheory. Assuming good alignment of the gun parts,we are concerned only with axial aberration; that is tosay, chromatic and spherical aberration. As far asthis lens is concerned, chromatic aberration, that is,the aberration produced by the fact that the electronsdo not all have the same velocity, may be assumed tobe negligible. This is because the electrons enteringthis lens have attained a velocity, due to the accelerat-ing field of the first lens, which is a different order ofmagnitude from the initial velocity of the electrons.

Spherical aberration, however, is by no means negli-gible. In order to reduce this aberration, it is neces-sary (i) to shape the electrodes in such a way that thefields they produce give a minimum of aberration, or(2) to limit by means of apertures the portion of the

(VERTICAL DIMENSION - 4TIMES NORMAL)

Fig. 6. Diagram of electron trajectories through the electrongun of Fig. 5. The Electron lenses are represented by lines

of equal potential.

lens used, or conversely (3) to increase the diameter ofthe electron lens, which involves necessary improve-ments in the deflecting system.

The spherical aberration of such a lens has been thesubject of considerable investigation. It is possible toshow mathematically that spherical aberration Lannot

18

JANUARY, 1938

TUBE LIFE

ANDSHORT-WAVE WORLD

be entirely eliminated in any electron lens, However,the aberration can be reduced and guns which are usedto -day are superior in this respect to earlier guns. Asour knowledge of the properties of specific lens fieldsincreases, it will undoubtedly be possible to make betterelectron lenses.

With the type of lens available at present about fif-teen per cent. of the lens aperture (i.e., fifteen per cent.

400

350

500

250

200

150

100

SO

Epi = 2800 VOLTS

Ep2.10000 VOLTS

-160 -140 -120 -100 -8o -6o -4o -2o 0

CONTROL -GRID VOLTS

Fig. 7. Control characteristic of a projectiontube. The upper curve represents the variationof focused spot size with control grid bias.

of the diameter of the first anode) can be used. Thecourse of electron rays coming from the crossover, ifextended to the second lens, would occupy slightlymore than twenty-five per cent. of its diameter. Theaperture, therefore, limits the beam current to aboutthirty per cent. of the current from the crossover. Thisnot only reduces the beam current, but also representsconsiderable power loss and causes undesirable heatingat the first anode. As the spherical aberration inherentin the lens is reduced by betterlens design, it will per-mit the use of larger stopping apertures and, therefore,a greater beam current.

The configuration of the first lens and the cathodediameter determine the current in the crossover andalso the angle subtended by the electron rays as theyenter the second lens system. From a theoreticalstandpoint the first focusing lens is very complicated.The current density is very high in this region andcannot be neglected. Furthermore, the electrons enterthe lens system at a low velocity so that the initial velo-cities produce considerable chromatic aberration.Finally, the analysis is further complicated by thevarying action of the control grid as modulation isapplied. In spite of its complicated nature, consider-able progress has been made in analysing this region.As this study has progressed, it has been possible todecrease the angle of the beam leaving the crossover

and to increase the ratio of the area of cathode used tothe area of the crossover.

Analysis of the electron optical properties of thecathode-ray has now made it possible to design a gunin which there is very little change in spot size withcontrol -grid voltage, which utilises an area of the cath-ode several times greater than that of the crossover,and in which thirty to fifty per cent. of the current leav-ing the cathode is delivered into the beam.

The cathode materials used at present give fairlysatisfactory performance but it is quite possible that amaterial may be developed which will give even higheremission and have greater stability. One of the moreserious problems is the destruction of the emittingsurface due to bombardment by positive ions originat-ing in the residual gas always present even in a highvacuum. The cathode, of course, lies directly at oneend of the beam path and, except for the control grid,is the most negative element in the tube. The ionsgenerated by the passage of the beam through the resi-dual gas strike the surface with tremendous impact.Numerous cathodes have been examined from whichthe emitting material opposite the grid aperture hasbeen knocked off completely. The solution of thisdifficulty is not yet at hand; still, it is believed that arigorous processing will yield good enough vacuum toat least minimise the effect. This hope is borne out bylife tests in which tubes have run at an anode potentialof fo,000 volts for well over 500 hours with little traceof the bombardment effect.

Also connected with the general problem of cathodesis the characteristic dark spot which appears on thefluorescent screen directly opposite the apertures of theelectron gun. The exact nature and cause of this dis-coloration is still the subject of considerable study, butmany indications point to the fact that it may be dueto bombardment of the screen by negative ions origin-ating at the cathode upon bombardment by the posi-tive ions mentioned above.

The type of gun just described, though quite practi-

FLUORESCENTMATERIAL

Li Qui°FILLED

LENS

PROJECTIONKINESCOPE

PROJECTIONPROJECTIONLENS

Fig. 8. A hollow shell filled with liquid and sealed to theface of a projection tube will increase the amount of lightentering the projection lens. Aberration is a serious defect

in the system.

cal, is not the only one possible. Work is being doneon different types which make use of a higher ratio ofworking area of cathode to crossover in order to in-crease the beam current.

While most of our attention has been devoted toquestions concerning the electron gun, the problems

(Continued at foot of next page.)

19

Tart VISJOtiAND

SHORT-WAVE WORLD

TELEVISION IN COLOURSFURTHER progress has been

achieved by Mr. J. L. Baird inthe development of colour tele-

vision. The pictures are produced byoptical -mechanical methods andthree light sources are employed-blue, green and red.

" I have been experimenting foryears, and succeeded a few monthsago," Mr. Baird said in speaking ofthis development.

" Experiments were carried out inmy private laboratory under condi-tions of strict secrecy.

" This is the first time that wire-less television in colours has beenachieved, although I demonstratedthe principles of it in 1928 beforethe British Association.

" The invention has nothing to dowith films. A Union Jack can, forexample, be held in front of the tele-vision camera and be reproduced innatural colours."

It is hoped to give a public demon-stration at the Dominion Theatre,Tottenham Court Road, shortly.

INCREASED SALESIt is calculated that within the past

four months nearly three thousand

television receivers have been soldwithin the Greater London area.This upward rise in sales is generallyattributed to the Lord Mayor's Showand Cenotaph broadcasts with anoriginal impetus given by the Corona-tion transmission. The promise oflonger hours of transmission and thelikelihood of Sunday broadcasts nodoubt have also influenced pur-chasers.

TELEVISION IN CLACTONA television demonstration was a

special feature of the ChristmasTrade Fair and Exhibition at Clac-ton. Some doubt was felt at firstwhether it would be possible to pickup the Alexandra Palace programmesin Clacton for the distance is close onsixty miles, and therefore well out-side the normally accepted range.Day -time programmes were at firstspoiled by some unknown interfer-ence but excellent pictures were re-ceived in the evening, and the demon-strations attracted large numbers ofpeople.

Experiments have also been car-ried out at Norwich, which is morethan ninety miles from London, anda picture received. A great deal of

JANUARY, 1938

interference was experienced, but thetests showed that reasonably goodreception at this distance was pos-sible, particularly in view of the factthat the aerial height was only 4.0 ft.In all cases of long-distance receptionit appears that the evening transmis-sions are received better than those inthe afternoons and mornings.

TELEVISION - DIARY FOR 1937 "Television will look back to the

early days of Radiolyinpia, 1936, andrecall all that has happened since thenin " Diary for 1937," an hour's pro-gramme to be presented on the lastnight of the Old Year, This trans-mission will be a combination of filmrecords and " live " studio presenta-tion which will tell the story of theworld's first high -definition televisionservice. Guest artists, films takenside by side with the televisioncamera at events such as the Corona-tion Procession, sporting contests,shooting in the film studios, andmemories of the high spots in " Pic-ture Page "-all these will be com-bined in a high speed programmewhich will open in the Announcers'Room at the Alexandra Palace, where

(Continued on next page)

" PROJECTION TUBES "(Continued from preceding page)

involved in the fluorescent material can in no wise beneglected. At the current densities and voltages usedin the directly viewed tube, present fluorescent mate-rials yield fairly satisfactory light output and havedemonstrated their ability to withstand bombardmentfor extended periods of time without undue deteriora-tion. However, as the beam current and operatingvoltages are increased, a saturation effect becomes evi-dent. The exact point at which the effect becomesobjectionable varies with different materials, and notvery much information concerning- the behaviour offluorescent materials at high input levels is yet avail-able for discussion.

Extension of the point where voltage saturation setsin seems to depend upon a better understanding of thesecondary emissive properties of the screen material,which place a limit upon the effective bombarding volt-age as distinguished from the final acceleratingpotential.

The question of the useful life of the material imme-diately comes to mind when voltages and currents ofthe order contemplated are mentioned. The picture ismore encouraging than might be imagined; life tests

run with a steady beam current of 200 microamperes ata potential of to,000 volts have shown an efficiencydrop of only twenty-seven per cent. in 1,200 hours.

There is some possibility of improving the perform-ance of the projection system by improving the opticalsystem. An obvious way of doing this is by increasingthe diameter of the projection lens; however, there weare restricted by the increasing cost of ruch lenses.There is, however, the possibility of increasing thelight -gathering power of the system by a factor of twoor three times by the use of a liquid lens in contactwith the face of the projection tube. Such a lens isshown in Fig. 8. The arrangement consists of a hol-low shell sealed to the front of the projection tube.This shell is filled with a liquid whose index of refrac-tion is about equal to that of the glass of the projectiontube. Aberration is a serious problem. but properdesign of the shell can minimise this defect. Such anarrangement increases the effective aperture by a factorequal approximately to the square of the index of re-fraction of the liquid in the shell. With water as theliquid, output is increased by a factor of about seventyper cent., while if a liquid such as paraffin oil or cedaroil is used the gain in light is in the neighbourhood oftwo -and -one-fourth times.

20

TE114.Y1.2.10nJANUARY, 1938

MORE SCANNINGS

ANDSHORT-WAVE WORLD

Jasmine Bligh, Elizabeth Cowell andLeslie Mitchell will ask each other," Do you remember . . .?"

Viewers, as they watch the proces-sion of events, will, it is believed, bestruck by the steady development inpresentation since that first Radi-olympia programme. Diary for1937 " will be repeated in the after-noon programme on January 4.

SOME FUTURE O.B.'sThe " Rugger " International at

Twickenham, the Boat Race, Croy-don Airport and the Derby are pros-pective objectives for future outsidebroadcasts. Tests of the possibilityof transmitting from Epsom Downshave already been made, :and it isunderstood that they have provedsatisfactory. There is little doubt,therefore, but that the Derby will betelevised and the dream of televisionpioneers become an actual fact. Itis not expected that there will be anydifficulty at either Hainmersmith,Croydon or Twickenham for the othertransmissions, but careful tests areto be made with regard to the possi-bility of interference which ruined thebroadcasts from Hatfield and KingGeorge V Dock a few months ago.

It is noteworthy thbt of all the pro-grammes that have been transmittedduring the past twelve months onlythose for which the B.B.C. were notresponsible have created any con-siderable amount of interest. Thesewere all outside broadcasts.

TWO-WAY TELEVISION(The televised two-way conversa-

tion between Mr. Gerald Cock, theTelevision Director, at AlexandraPalace, and Mr. Walter Mycroft atElstree, was an interesting experi-ment in the future possibilities oftelevision. It was made possible bythe arrangements that had been madefor the transmissions from the Els-tree studios.

LORD HIRST TO BE R.M.A.PRESIDENT

The Rt. Hon. Lord Hirst of Wittonhas succeeded Lord Gainford as Pre-sident of the Radio Manufacturers'Association. The announcement wasmade at the eleventh annual banquetof the Association held on November22. In the course of a speech at thisbanquet, Mr. M. M. Macqueen,chairman of the R.M.A., said " Theindustry believed more than ever in

the future of television. So far, thiscountry had led the world in tele-vision development.

" We in the industry believe thatthe efforts of the Government will bevalueless unless it takes a bold policyto increase television broadcastinghours and to raise the general levelof the entertainment provided." Hesuggested that a children's hour fea-ture should be introduced in the tele-vision programmes at once.

TELEVISION IN ITALYPlans are being made for the erec-

tion of a television transmitter on thesummit of Mount Mario, which isnorth of Rome. Few details are asyet available, but it is understood thata wavelength of 7 metres will be usedand that the system will he very simi-lar to that employed in this country.It is expected that the station will beready in the early part of 1938.

SUNDAY TRANSMISSIONSIt is practically certain that Sunday

television programmes will be intro-duced early in the New Year irrespec-tive of an increased Governmentgrant for the television service. Theprovision of a Sunday service is oneof the first matters which is to haveconsideration in the development ofthe service.

THE TELEVISION SOCIETY'STENTH ANNIVERSARY

The Television Society has now at-tained its tenth anniversary and tocelebrate the event a dinner was heldat the Grafton Hotel, Tottenham CourtRoad, on Wednesday, November 24.

For this event collaboration with theB.B.C. was arranged and the hair -man of the Society, Dr. Tierny, afterthe dinner concluded, went to theAlexandra Palace and gave a talk onthe history and objects of the Society.Dr. Tierny was seen and heard bythose present at the dinner, who ad-journed to the Marconiphone pre-mises a short distance away. Fivereceivers were in use and the recep-tion was excellent and entirely freefrom interference.

"MUSIC HALL PARTY "Veterans of the Music Hall will

hold a New Year party in the tele-vision studio on the afternoon ofDecember 31. Gathered round a tableloaded with cakes and Christmascrackers will be seen Sam Mayo, who

will entertain the company with" I've Only Come up for the Day " ;May. More Duprez, who will sing herfamous number " By the Sideof the Zuyder Zee " ; CharlieLee, the seventy -two -year -old highkicker; Tom Leamore, the original" Percy from Pimlico " ; GeorgeMozart, who is still famous on " theboards " ; and that music -hall favour-ite, Daisy Dormer. The Chairmanwill be a " youngster," George Ben-son, who is well known to viewersfor his appearances in musicalcomedy and in the " Television Fol-lies." There will be an audience inthe studio, which will be televised atintervals while the party is in pro-gress.

MR. LESLIE MITCHELL RESIGNS

Mr. Leslie Mitchell, the first maletelevision announcer, has resignedfrom the B.B.C. in order to acceptan appointment with a news reel com-pany. His resignation will takeeffect towards the end of February,until when he will continue to appearregularly in the television pro-grammes. Mr. Mitchell joined theB.B.C. as an announcer in 1934.

AN R.M.A. ASSURANCEIn order to allay any doubt that

might exist in the public mind thattelevision receivers as manufacturedto -day will become obsolete in a shorttime, or that there will be any con-siderable reduction in price, theRadio Manufacturers' Associationmake the following announcement :

The success which has attended thereception of the Alexandra Palacetransmissions over the metropolitanand a large part of the home countiesareas, and particularly those of spe-cial events of national importance andinterest, has shown that televisionhas now reached a stage such as towarrant its installation in a largenumber of homes.

With this in view, the Radio Manu-facturers' Association, after confer-ence with the authorities concerned,takes this opportunity of advising thepublic that the television receiversavailable are of a type that will con-tinue to receive the transmissionsfrom Alexandra Palace for a longtime to come.

It has been stated in authoritativecircles that the present system oftransmission will continue for a defi-nite period of a length which will

21


AND MORE REFLECTIONS.avoid any question of obsolescencein present-day equipment.

The prices of present-day receiversare such that there is practically nolikelihood of better value for moneybeing offered for some considerabletime.

EXTENSION OF HOURS LIKELYThe Radio Manufacturers's Asso-

ciation has reason to believe that ex-tensions of the present hours of trans-mission may be expected shortly.

The parties concerned with theproduction, distribution and recep-tion of television are all agreed thatthe development of this new branchof the radio art has reached a definiteand stable position where the utilityvalue of the service is now fullyavailable to the public as a homeentertainment of great merit.

AMATEUR BOXING -TELEVISIONBAN

The Amateur Boxing Associationhas circularised club secretaries in-forming them that no television willbe allowed at any competitive boxing" until conditions are improved."

MORE AUSTRALIAN BROADCASTSThere is to be a very considerable

increase in the number of broadcastson short -waves from Australia.Starting on January 26 and finishingup the beginning of May, 1938, Aus-tralia is celebrating its rsoth anni-versary. The short-wave stationsare to be brought into more generaluse, and a considerable number ofthe events scheduled are to be relayedvia short -waves. The first transmis-sion is to take place on Tuesday,January 18, with a commemorationof Capt. Phillips' arrival at BotanyBay, which has been organised byCapt. Cook's Landing Place Trust.

TRANSMISSIONS FROM LABRADORAn amateur short-wave station

isolated in the north of Labrador,and unable to receive any mail fornearly four months, will shortly beheard in all parts of the world. Thisstation, call -sign, VO6D, or theVoice of Six Ducks, as he is fami-liarly known, is going to broadcasta programme of his own which willinclude some native trappers whowill face the microphone for the firsttime. It is also hoped that some ofthe local inhabitants will come alongand say a few words, and so makethis programme of general interest.

A 5 -METRE TEST

The International 5 -metre testsorganised by the Radio Society ofGreat Britain, will have considerableinterest to designers of television re-ceivers. As amateurs in all coun-tries are taking part in the tests, theranges covered by their simple ap-paratus will be a guide to the possiblerange of television signals. It isexpected that consistent transmissionand reception will be possible overquite long distances, but this ismerely a matter of conjecture.

BRITISH AMATEUR CALL -SIGNSThere has been a startling increase

in the number of licences issued toamateurs wishing to use transmit -

GERALD COCK, Television Director,sending an exclusive message to " Tele-vision and Short Wave World," says :

" Television has already shown whatit can do, and is in a sufficiently advancedstage of development to provide anintimate and altogether delightful homeentertainment, for which alone it isdevised and produced : but I am surethat what has been accomplished in

months will prove to beas nothing when compared with thefuture. I believe we are at the beginningof a new era in entertainment in thewidest sense of that much abused word,though it is bound to be a bit of an uphillfight against inertia and non -co-operation. If the public could be madeto realise what this amazing medium iscapable of giving them, their encourage-ment would be invaluable in these stillearly days of disappointments anddifficulties. To the staff at AlexandraPalace their jobs are an exciting adventurein a world without precedents.

" The field that can be covered bytelevision has proved wider than mostpeople thought possible even a year ago.To have a set in one's own home is aunique pleasure."

ting apparatus. This is accountedfor in official quarters by the largenumber of all -wave receivers soldthat cover the amateur waveband.Ordinary listeners merely interestedin short -waves have been hearing thetests carried out by amateur trans-mitters and a percentage of themhave taken up this fascinating hobbyfor themselves. Owing to the num-ber of licences issued the G2, Gs, G6and G8 numbers have all been ex-hausted, so that G3's are now beingused. Also the British Isles havebeen split into sections with Englishstations having the prefix G, Welshstations GW, Scottish stations GM,and those in Northern Ireland GI.

BRITISH COMMUNICATIONRECEIVERS

America has for the past few yearsscooped the amateur market for com-munication receivers. (The majorityof experimenters in this country areusing receivers of American design,particularly with ultra -high frequen-cies. It seems now as if Britishmanufacturers have realised thatthere is a good amateur market inthis country. In the next fewmonths there will be quite a num-ber of British built communicationreceivers of an entirely new type.

The British makers are capable ofproducing high-grade communica-tion sets, which has made it all themore annoying that they should haveignored this market for so long. Thestandard British domestic receiverhas a degree of efficiency higher thanthat of the American receivers witha similar number of valves, and manyamateurs have hoped that before longthey could obtain British communica-tion sets embodying a similar degreeof efficiency. This wish is now aboutto be realised.

CIRCUS TELEVISIONBertram Mills' Circus will be tele-

vised daily from Olynipia from Janu-ary 4 to January 8 next. This will bethe first occasion on which the mobiletelevision unit has transmitted pic-tures direct from a place of enter-tainment in London.

The items which it is hoped to tele-vise will include Albert Schumannwith his twenty horses and ponies;Harry and Merkey, the crazy contor-tionists; the Five Vikings-Sweden'sSilly Sailors; Crocker"s Teddy Bears,in climbing and balancing feats; theThree Codonas in their flying trapezeact; Koringa, the only female fakirin the world; the Chinese LuckyGirls; Gindl and his Elephants; theArthur Family of acrobatic andcomedy cyclists; the Karpi troupe ofacrobats; the Four Maniacs inComedy Ladder acts; and Mr. Ber-tram Mills' world-famous beam ofhorses.

The television cameras will workat varying distances from the ring togive close-ups of the clowns andcomedy turns, as well as full views ofthe arena and audience. Telephotolenses will he used. Vieivers will be" visiting " the Circus in the com-pany of Frederick Grisewood, whowill comment on the. items.

22

Tar/ISIOilJANUARY, 1938

ANDSHORT-WAVE WORLD

SCOPHONY DEMONSTRATIONON B.B.C. TELEVISION

SYNCHRONISING DIFFICULTIES OVERCOMELAST month Scophony demon-

strated their receivers for thefirst time on transmissions from

Alexandra Palace. Accounts of theperformance of these receivers onScophony transmissions have beengiven previously in this journal and

Right : Scophonytelevision projectorfor public halls pro-ducing a 5 ft. by 4 ft.picture.

Below : Interiorview of the projectorshowing amplifierwhich drives the lightcontrol. Above theamplifier is the high-s pe e d synchronousmotor.

it has been explained that owing toirregular timing and phase shiftingin the Alexandra Palace transmis-sions, previously it has not been pos-sible to synchronise mechanical re-ceivers. This fault has now been re-medied and the recent demonstration

showed that synchron-ism by the Scophonysystem is just as good aswhen a cathode-ray tubeis used.

Two receivers weredemonstrated-the hometype with a screen 22 in.by 24 in. and the cinematype with a screen 5 ft.by q ft. Both receiversemploy " split focus "and " supersonic " lightcontrol which are funda-mental to the Scophonysystem.

" Split focus," is anoptical arrangement of

23

cylindrical lenses with their axescrossed, so that a beam oflight is focused in two separateplanes. An advantage of the splitfocus is that where scanners are em-ployed (as is essential in opticalmechanical television systems) theycan be of a considerably smaller sizethan would be necessary with ordin-ary spherical lens systems.

Synchronising DifficultiesOvercome

The Scophony supersonic light -control consists of a container filledwith a liquid, at one end of which isa quartz crystal. When the quartzis actuated by a modulated carrierfrequency, supersonic waves are setup at a speed corresponding to thevelocity of the sound waves in thatparticular liquid.

The container has on either side ofit a lens, and when light is passedthrough the container and focused onto a scanner, and from the scanneron to a screen, an image of the lightcontrol itself is, by means of suitablelenses, formed on the screen. If themodulated carrier frequency is nowapplied to the quartz crystal nothingwill be seen on the screen until thescanner, which is betm een the screenand the light control, is rotated at aspeed that follows exactly the speedof the waves in the liquid.

The modulation then becomes vis-ible on the screen as an image. litthe liquid, the waves produced by theelement frequency on the quartzcrystal, are equivalent to one scan-ning spot on the screen. A largenumber of scanning spots- are there-fore used simultaneously. In the ap-paratus demonstrated 15o pictureelements were shown simultaneouslyon the screen, increasing the appar-ent brightness of the picture 15otimes.

A further advantage of the Sco-phony light control is that compara-tively low power (5 to To watts) isrequired to operate it.

In the Home Receiver the lightsource used is a high-pressure mer-cury lamp which operates from aD.C. source. Internally, the appara-

(Cant/trued on page 25.)

TELEn.11.011AND


HOW COSSOR TELEVISIONRECEIVERS ARE TESTED

A special technique is necessary in the testing of a television receiver. The methodemployed is described in this article.

BEFORE a television receiver iscompletely ready to install inthe home of the viewer it has,

of course, to undergo a series oftests. It will be interesting to des-cribe some of these tests as carriedout by Cossors, since they are quitepeculiar to television as distinct fromradio.

A television receiver can he con-veniently considered as three distinctsections. Firstly, theme is the radio

This pattern Is a photographtest raster.

set, which amplifies the signals fromthe dipole aerial and delivers thevision information, consisting of thepicture and synchronising signals, tothe scanning chassis, and the accom-panying sound to the loud -speaker.Then there is the scanning chassis,which has the task of reconstitutingthe picture from the vision signal,separating the picture modulationfrom the synchronising pulses, andsupplying the scanning motions tothe light spot on the cathode-ray tubein exact time with the scanning atthe transmitter. Thirdly, there is thepower unit which delivers the variousvoltages to drive the radio receiver,the scanning chassis and the cathode-ray tube.Trimming theVision Receiver

The radio receiver is trimmed upon a special signal generator designedfor the ultra -short wavelengths. TheCossor vision receiver is of the super-heterodyne type, and great care hasto be taken when trimming the inter-mediate frequency amplifiers to seethat both sound and vision signals aretuned in simultaneously. There must

of a complete

also be no hint of cross -talk betweenthe two outputs; sound modulationappearing super -imposed on the pic-ture is one fault N\ iliCh is carefullywatched for on test.

The power unit has, among otherthings, to deliver a voltage of 4,500to the cathode-ray tube. In order tohandle this voltage safely, particularcare has to be given to insulation.The transformer is tested for break-down at fo,00c volts and the smooth-ing condensers are tested at doubletheir working voltage. When check-ing these high potentials, the meterused must not add an appreciableload, or a false reading will be given,so electrostatic meters are used.

ScanningTests

It is, however, in connection withthe scanning chassis that the mostinteresting tests are made. Thispart of the apparatus has no parallelin normal broadcast receivers, andeven the valves have ,work to dowhich would have -broker: the heartsof valve designers' a few years ago.The four triodes that are used togive the vertical and horizontal scan-ning deflections have to work underconditions that allow the anodes torun to many hundreds of volts. Ofthese, the two thne-base valves whichgenerate a saw -tooth waveform, haveto simulate an infinite resistance oneinstant and as nearly as possible, ashort-circuit the next. This meansvalves which can pass a heavy cur-rent for a short time, with a sharpcut-off when the grid is a few voltsnegative.

The other two scanning valves areamplifiers, and it is imperative thatthey should have characteristics thatfollow very closely to a particularcurve. If their characteristics falloutside the very close limits set, thenthe scanning spot will not travel uni-formly, but will speed up in certainparts of the picture and so makepeople appear to get fatter as theywalk across the screen, or give thema small head and very long legs; inother words, the scanning would benon-linear. Slight variations incurvature of characteristic in valves

intended for broadcast receivers areof no importance at all, but in a tele-vision receiver the effects of suchvariation can be such as to completelyruin the picture. The same thingapplies to the valve that sorts out thesynchronising impulses from the com-posite vision signal; a valveslightly off specification could destroythe picture in a different way, bycausing lines to be scanned too shortwhen there is a black area near theright-hand edge of the raster. Thevalve could not differentiate betweena " black " and synchronising signal.ValveChecks

To examine every valve by plottingthe complete characteristic curve ona valve -testing board would be an im-possibility. It would be much tooslow and much too expensive. So apiece of apparatus was devised con-sisting of a time -base valve which .

caused a horizontal trace on a cath-ode-ray oscillograph. Part of thissweep voltage was taken and appliedto the grid of the valve under test.A resistance was put into the anodecircuit of this valve and the voltagedeveloped across this resistance wasapplied to the vertical deflector plateof the same oscillograph. So the hori-zontal position of the spot gave ameasure of the grid volts on the valveand the vertical position lave

A portion of the test raster shown aboveenlarged to show the 2 -megacycle dots.

cation of the anode current corres-ponding to that grid voltage. Thuswith the time -base valve working atabout fifty cycles per second, a con-tinuous line was drawn giving thedynamic characteristic of the valvetested.,

24

IiILL'YIS1011JANUARY, 1938

Selected valves representing theupper and lower limits were thenplugged into the apparatus and theircurves copied on to the screen of theoscillograph. 'Thus, by making eachvalve draw its own curve, it is a sim-ple matter to go through a batch ofvalves and select quickly and accu-rately those that lie within the limits.

An ArtificialSignal

When testing the scanning chassisin conjunction with its cathode-raytube previous to putting the completeset into its cabinet, one finds that asuitable testing signal presents aproblem. (The B.B.C. transmits afilm for one hour each morning and,of course, there is the usual hour inthe afternoon, but two hours out ofeight does not provide sufficient timeto do justice to a day's production ofsets. A transmitter at the factory istherefore available, providing a sig-nal on tap at several points in theworks. It has been found, however,as the result of experience, that test-ing and adjusting a receiver on actualpictures does not always give satis-factory results. One needs some-thing analogous to the signal inbroadcast receiver testing, or the beatfrequency oscillator in checking am-plifiers.

A special form of signal which con-tains the essential information re-quired to make all the necessary ad-justments has been developed, there-fore, and this is always available inthe testing department. Photo-graphs taken off the end of a cathode-ray tube when this signal was beingused are shown.

The signal is generated by a com-bination of sinusoid and saw -toothoscillators, and the whole is lockedto the synchronising signals so thata stationary pattern is obtained onthe receiver under test. The screenis covered by minute dots of lightwhich are produced by a two -mega-cycle beat, and at regular intervals inthe horizontal or line direction, bandsof maximum and minimum amplitudeof oscillation occur. Also at regularintervals in the vertical or framedirection appears a dark line. Thefine two -megacycle pattern is a goodsignal to check up the performanceof the cathode-ray tube, since anywoolly focusing of the scanning spotimmediately makes the points of lightrun into one another.

A different kind of bad focusing,known as astigmatism, which causesthe focus to be good in a given direc-

tion and poor at right -angles to it,also shows up quickly, and the pre-set controls which are provided on theset to eliminate this error can bequickly adjusted to their correct set-ting. The fact that the pattern isstationary makes it easy to see thatinterlacing of the raster is good, thisbeing difficult on a moving picture,since the eye tends to follow anymovement and the resulting strobo-scopic effect apparently throws theinterlacing out.

The vertical bars and the horizon-tal lines give accurate time markingsand show up immediately any non -linearity in either direction, whichwould cause the intervals to be spacedout at unequal distances. Non-linearscanning is an error that is not alwaysimmediately obvious on a picture, butcan be very annoying when there ismuch motion from side to side or upand down.

ANDSHORT-WAVE WORLD

Any departure from the parallel orfrom straightness of the time -mark-ing bars shows as a non -rectangularraster or poor synchronising.While still on this signal, the rasteris centered on the screen of the tubeby means of the shift controls, andthe horizontal and vertical sweeps areset so that the picture is correct asregards size and shape, and so willcomfortably fill up the mask in thecabinet.

When everything is satisfactory,the set is then put into its cabinet,the chassis are bolted down and thevarious connecting leads are pluggedin and anchored down.

The set is finally given a long runon the B.B.C. programmes as a finaltest under normal working condi-tions, attention being given to con-stancy of performance of the pictureand quality of the accompanyingsound.

" SCOPHONY DEMONSTRATION"(Continued from page 23)

tus is arranged so that the lightsource is first focused on to the lightcontrol, from the light control to thehigh speed scanner (a stainless steelpolygon) and from there on to thelow speed scanner which gives thepicture repetition frequency, andthrough a projection lens on to thetwo -foot screen.

The large receiver is very similarto the Home Receiver except that thelight source is a standard cinema arcconsuming moo amperes and that thelow speed scanner is of a larger dia-meter. (The picture is rear projectedon to a sand -blasted glass screen.

The high speed polygons of bothreceivers are rotated by special syn-chronous motors and synchronisedby the line frequency transmitted bythe B.B.C.ExcellentReproduction

At the recent demonstration thetwo receivers were shown to at leasta hundred people, all of whom wereable to see quite clearly even on thesmaller model.

The definition on the small modelwas, as far as could be comparedfrom memory, equal in every way tothe best type of cathode-ray receiver,while the synchronisation was per-fect in every way. There was aslight lateral movement of the entirepicture twice during the transmis-sion but this was apparently due tofaults at the transmitter end.

On this domestic receiver light

values were very good althoughthere appeared to be rather more lightin the centre of the picture with aslight drop towards the sides. How-ever, the instrument gave a very welldefined picture.

A stage was fitted up on the linesof a cinema complete with stage anda large screen 6 ft. by 5 ft. Thecommercial type receiver provided apicture that completely filled thisscreen.

The definition was not so good aswith the smaller receiver, but it wasas good as the present standard ofdefinition will allow. Compared withthe normal cinema screen the defini-tion was poor, but at this stage defi-nition is not the most importantfeature,

The demonstration clearly indi-cated that the Scophony big screensystem has considerable scope witha promise of coping with higherstandards. At the present time thetransmissions are not good enoughfor big screen television both as re-gards the number of lines and studioarrangements. Lighting is poor,there being far too many shadows,while the poor scenery shows up verybadly. This defect is not noticeableon the smaller television receivers.

The noise which might be expectedfrom mechanical units was quite un-noticeable-a most important feature.

A delivery date for the smaller re-ceiver was not fixed although it isexpected that the price will beand £80. Orders for the big screenreceiver can be taken with deliverywithin three months.

D 25

tagYISIONAND

SHORT-WAVE WORLD

FOR THE BEGINNER

JANUARY, 1938

ABC OF MAGNETIC SCANNINGIn view of the increasing use of magnetic scanning, this article, byG. Parr, explaining the elements of the system, is of particular interest

THE use of magnetic fields forcontrolling the beam in thecathode-ray tube is by no means

a new development, and is in fact a

Fig. 1. How the beam is deflected by amagnetic field.

reversion to the original means bywhich the beam was deflected andfocused. The use of a coil for focus-ing the beam was described in 1905,and in illustrations of that period thetube is seen with two coils strappedround the neck to provide the deflect-ing force on the beam.

There is, however, a wide differ-ence between the simple deflection ofthe beam by a magnetic field and theuse of this field to produce a regular

ties involved it is convenient to revisebriefly the principles of magnetic de-flection of the beam. The beam canbe considered as an exceedingly flexi-ble conductor carrying a current. Ifthis conductor passes through a mag-netic field it will experience a forcedue to the interaction of the magneticfield surrounding the conductor andthe field through which it passes.

Fig. i shows a cross-section ofthe beam as it passes up 1 he tube andthe deflecting field is represented bythe lines passing transversely acrossits path. The interaction of the cir-cular field surrounding the conductor(i.e., the beam) and this field areshown by the merging of the lines onone side of the conductor and thebulge produced in the transverse linesof the field. if we consider these asin the nature of elastic threads theiraction will be to force the conductorout of the field in the direction shownby the arrow. 'l'he direction of move-ment of the conductor depends on thedirection of flow of the current andthe polarity of the magnetic field.In the case of the cathode-ray tubethe direction of flow of the current isnon -reversible and alteration of move -

angles to the direction of the field.This means that if w e mount twocoils so that the axis of each is hori-zontal the beam will be deflected atright angles to this axis, i.e., verti-cally.

The amount of movement of thebeam can be calculated from thestrength of the field and the accelerat-ing voltage applied to the beam, butthe theoretical deflection is seldomrealised in practice owing to the diffi-culty of finding the exact extent ofthe field. A certain number of linesof force spread out in a curved pathat each end of the magnet poles, asshown in Fig. i, and this "fringing"field also influences the beam, al-though its effect can only be estimatedapproximately in any calculation.

Whether the beam is deflected byan electrostatic or by an electromag-netic field the same requirementshold-a deflecting force which in-creases uniformly with time and thenceases momentarily to allow the beamto return to its original position. Inelectrostatic deflection we have thefamiliar saw -tooth wave of potentialwhich is applied to the deflectorplates. In magnetic deflection thesaw -tooth represents change of cur -

Fig 2. Circuit suitable for use when iron -coredcoils are employed.

Fig. 3. Ticking -Grid oscillator scanning circuit.

series of scanning lines when thespeed of scan is one line in f / to,000thsec. At very high frequencies the de-sign of both the coil and the circuithave to be carefully investigated, andit is no longer a matter of windinga number of turns of wire and fixingthem to the neck of the tube.

To appreciate some of the difficul-

ment of the beam is done by reversingthe magnetic field in polarity.

Note from the diagram that themovement of the beam is at right

" Television and Short-wave World "circulates in all parts of the world.

rent through the deflecting coil,which must increase at the same rateto produce the same scanning move-ment.

It is this change of current whichgives rise to some of the difficultiesin the design of magnetic scanningcircuits as, owing to the inductanceof the coil, the rapid change of cur -

26

TELVISIOtiJANUARY, 1938 AND

SHORT-WAVE WORLD

ADVANTAGES OF MAGNETIC SCANNINGrent produces undesired effects. Forexample, in a highly inductive circuitthe abrupt cessation of the currentat the completion of the scanningmovement will give rise to a highvoltage induced in the coil, which maybe sufficient to break down the insula-tion. The inductance of the coil willalso tend to retard the fall of currentto zero at the end of the line and itmay even be necessary to introduce apotential of opposite polarity to assistin the reduction of the current.

Fig. 4. Typical yoke for iron -cored deflectingcoils.

The problem is a little simplifiedin the case of the scanning circuit forpicture frequency as the rate ofchange of the current is slower andcoils with iron cores can be used. Thepresence of the iron core serves toincrease the density of the field andlocalises it over a more sharply de-fined area. This enables a lowervalue of current to be used and thescanning circuit can be made up onthe lines of Fig. 2. The saw -toothwave is generated by the thyratronand condenser shown. R is the vari-able speed control and the chargingcondenser is divided into two parts Cand C' to reduce the potential appliedto the grid of the pentode amplifier.In the output of the pentode the de-flecting coils DD are connected. Thecoupling condenser C, needs to be oflarge capacity at the low frequencyof the scan-about 8 -to mfds. Asimilar circuit could be used for theline scanning frequency although thecoils would not be iron -cored owingto the increased impedance whichwould be produced at Io,000 cycles.

A more complicated scanning cir-cuit was described in an issue of theI.R.E. Journal* some years ago inwhich a negative pulse of potentialwas applied to the scanning amplifierduring the flyback of the beam. Thismaterially improved the efficiency of

Holmes, Carson and Tolson, Proc. I.R.E. Nov2934.

the scanning circuit as the followingbrief explanation will show:

The scanning voltage is producedby a " ticking grid " oscillator, theanode and grid circuits of the left-hand triode being coupled and therate of oscillation being controlled bythe time constant of the leak andcondenser R, and C1. The valve V,is normally biased so that no anodecurrent is flowing, but each pulse ofthe oscillator applies a positive pulseto the grid, causing the anode cur-rent to flow and charge the conden-ser C2. This produces a negativepotential across the grid of V,, whichis followed by a positive one as theanode current in V, ceases and thecondenser discharges through Rs,.With this arrangement a sharpnegative pulse is followed by a posi-tive saw -tooth wave of voltage.

When this pulse is applied to thegrid of V, the negative portion cutsoff the anode current by increasingthe bias. Each saw tooth is, there-fore, followed by a short period dur-ing which the impedance of the valveV3 is high and this ensures a rapiddecay of current through the scan-ning coils. The anode load of theoutput valve is the choke L, which islarge in value compared with the in-ductance of thescanning coils.

Scanning CoilDesign

As said before, it is not possible todesign scanning coils accurately fora given tube as so much of the theo-retical values are modified in prac-tice. The values of ampere -turns anddimensions of coils are largely theresult of experience and are deter-mined empirically, the principal dif-ference between the line and picturecoils being in the inductance. Forlow scanning frequencies it is per-missible to use a relatively high in-ductance coil with a low value of de-flecting current and for the line fre-quency the coil is of few turns witha high value of current.

This is one of the advantages ofthe magnetic deflecting system overthe electrostatic system. In the lat-ter the scanning voltage must be highin order to produce sufficient move-ment of the beam in a large tube andthe value of voltage is not affected bythe frequency of scan. In the mag-netic system the operating voltagecan be kept low provided that it is

sufficient to enable the valves to beoperated on the straight portion oftheir characteristic and a consider-able saving in components is theresult.

From the point of view of distor-tion the all -magnetic system hasseveral advantages over the electro-static focusing and deflecting system.The various distortions which arepresent in the electrostatic tube havealready been described in this journaland it is understood that troubleswhich arise from the presence ofhigh -voltage deflecting fields do notexist in the magnetic sy sten,. At thesame time the presence of the deflect-ing coils may introduce a certainamount of distortion, particularly if

MOT IiOF S

Fig. 5. Showing how a beam passing througha curved field is influenced by two forces.

the magnetic field is spread over awide area by leakage.

In some cases a form of " cross -modulation '' is produced by theinteraction of the fields due to the twopairs of deflecting coils and the posi-tion of the coils has to be carefullychosen to avoid this. In some casesthis effect is produced by want ofsymmetry in the deflecting system,the coils either being displaced rela-tive to the axis of the tube or themagnetic circuit (in the case of iron -cored coils) being of unequal cross-section.

A typical magnetic circuit is shownin Fig. 4, which is specially designedto keep the field uniform and to mini-mise leakage.

When the deflecting field is curved,such as occurs at the edges of themain field (see Fig. I), a certainamount of distortion of the spot mayoccur, which is due to the fact thatthe beam has a definite area, althoughsmall. In Fig. 5 the beam is shownas a circle passing across a curved

(Continued at foot of page 29)

27

TELIVISICMIAND


RECENTTELEVISIONDEVELOPMENTS

A RECORDOF

PATENTS AND PROGRESSSpecially Compiled for this Journal

Patentees :- Marconi's Wireless Telegraph Co., Ltd., and L. M. Myers :: Marconi's WirelessTelegraph Co., Ltd. :: Baird Television Ltd., C. Szegho and D. M. Johnstone :: TelefunkenGes. fur Drahtlose Telegraphic M.b. h. :: E. Michaelis :: Baird Television Ltd. and

G. Dovaston :: Baird Television Ltd. and T. M. C. LanceLight-sensitive Amplifiers

(Patent No. 470,102.)LIGHT is focused on to a sensi-tised cathode, and the, electrons -so liberated are controlled first

by an open -mesh grid and then by aseries of close -meshed electrodes,coated with sensitive materials of lowwork -function. Secondary emissionoccurs at each electrode stage, suc-cessive electrodes being biased in anascending order of positive voltagesfrom a common potentiometer. Theamplified current is drawn off froman outer circular anode.

Suppressor grids may be arrangedin front of the secondary emissionelectrodes in order to reduce back -coupling. No electron focusing de-vices are necessary.--Marconi'sWireless Telegraph Co.. Ltd., andL. M. Myers.

Making Fluorescent Screens(Patent No. 471,190.)

When making a fluorescent screenfrom willemite (which contains zinc,silicon and manganese) the mineralis usually ground down into a veryfine powder, preparatory to laying iton the glass face of the cathode-raytube. It has now been discoveredthat the grinding process injures thecrystalline surface of each of thesmall particles, and so causes thefluorescent light given off by themto be less intense than it should be.

In order to remedy this defect, thewillemite, after it has been powdered,is immersed for some hours in an al-kaline solution. This appears tohave the property of restoring theoriginal crystalline formation, eitherby dissolving the injured face and ex-posing a fresh one, or else by dis-solving a part of the powder and re-depositing it as a new crystalline faceton each particle. The result is ascreen which emits more light and isless liable to be burnt out.-Mar.coni's Wireless Telegraph Co., Ltd.

Cathode-ray Receivers(Patent No. 471,539.)

A certain amount of " flicker "

may be traced to the fact that the flu-orescent light from the screen hasno perceptible after -glow. In orderto remedy this, the picture is pro-jected in two stages. It is firstthrown on to a fluorescent screenmade of materials selected to give anappreciable " after glow." The lightfrom this first screen is then passedthrough a transparent sheet on to athin layer of photo -sensitive material.

Cathode-ray Tubes(Patent No. 471,913.)

A sensitised electrode S is fitted inthe centre of a cathode-ray tubewhere it is scanned on both sides byseparate electron streams from thetwo ends of the tube. The sensitisedelectrode may consist of a "mosaic"of small photo -electric cells, or it mayconsist of insulated " squares " ofmetal which liberate secondary elec-

trons under the impact of

S

Kt

MINr.

Transmitting tube with double scanningarrangements. Patent No. 472 913.

Here it liberates a stream of elec-trons, which are focused by an ex-ternal winding on to a second fluor-escent screen, which is mounted onthe bulb end of the C.R. tube, in theordinary way. The second screen ismade of material selected to producea white or bluish -white light of highbrilliance.-Baird Television, Ltd.,and C. Szegho, and D. M. Johnstone.

the scanning streams.The secondary electrons

are collected on the metalwall coatings W, WI, andpass through a resistance Rto produce signalling cur-rents. The arrangement canbe used for interlaced scan-ning by applying differentcontrol voltages to the de-flecting coils L, Li and L2,L3, respectively.

Or the tube may be usedfor transmitting secret tele-phony. In this case speechcurrents from a microphoneM are applied to the two con-trol electrodes K, Kt to varythe amount of secondaryemission from the electrodeS. By causing the scanningstream to move in oppositedirections over the two facesof the electrode S, the speechis " distorted " or madesecret to any listener not pro-vided with a correspondingform of receiver.-Telefun-hen Ges. fur draghtloseTelegraphie m.b.h.

Low -voltage Tubes(Patent No. 472,073.)

A cathode-ray tube is designed tobe run on low operating voltages andto produce, in the first place, only afaint picture on a small -sized fluor-escent screen. The dim picture isthen focused by a lens on to a photo-sensitive surface, from which elec-

The information and illustrations on this page are given with permission of the Controller of H.M. Stationery Office.

28

TELIP/ISIOtlJANUARY, 1938

trons are emitted. These are passedthrough an electrostatic focusingsystem, which intensifies the streamand projects it on to a second fluor-escent screen where it produces abright picture of normal size.

The use of low operating voltagesresults in a considerable saving in thecost of the component parts of theset.-E. Michaelis.

Driving Motors(Patent No. 472,274.)

The drawing shows an arrange-ment used for coupling the drivingmotor M to a film to be televised. Itis found that if the motor is first runup to the proper speed before beingclutched to the film, the sudden ap-plication of the load tends to makethe motor drop out of synchronisa-tion. On the other hand, if it is" revved " up to synchronous speed

set at an angle to each other, some-thing like the slats in a venetianblind. In each case the primary elec-trons strike against the strips at themost favourable angle to producesecondary emission, and the overallamplification is correspondingly in-creased.-Baird Television, Ltd.,and T. M. C. Lance.

SuMmary of other jelevisionPatents

(Patent No. 471,185.)Time -base circuit for generating

saw-toothed oscillations with a veryrapid " fly -back period.-StandardTelephones and Cables, Ltd., andR. M. Barnard.

(Patent No. 471,250.)Cathode-ray tube adapted to pro-

duce continuous oscillations by auto-matic deflection of the electronstream.-Farnsworth Television Inc.

F

Driving motor for film whichremains in synchronism on appli-cation of load. Patent No. 472,474.

whilst coupled to the film, the firstfew pictures of the Film are not pro-perly radiated.

The motor is accordingly providedwith an artificial " load " K to whichit is clutched whilst it is being speededup. Once it has reached the syn-chronous speed, it is switched overto the film apparatus F. Wheneverit is necessary to disconnect the filmF, the artificial load K automaticallycomes into play, so that the motorcan be left running at the properspeed, ready to drive the film whenrequired.-Baird Television, Ltd.,and G. Dovaston.

Electron Multipliers(Patent No. 472,485.)

The electrons liberated by lightfrom a photo -electric cathode are am-plified by making them strike againstmetal electrodes which liberateseveral secondary electrons for eachprimary impact.

It is found that the number ofsecondary electrons can be increasedif the primary stream is made tostrike the metal surfaces at an angleof abort 300.

As shown in the drawing, the elec-trons produced by light focused on asensitive cathode C are directedagainst a series of strips S which are

The information and illustrations on this page are given with permission of the Controller of H.M. Stationery Office.

29

(Patent No. +71,365.)Method of amplifying light by

back -coupling a photo -electric cath-ode to a fluorescent screen containedin the same tube.-Telefunhen Ges.fur drahtlose Telegraphie m.b.h.

(Patent No. 471,696.)Preventing mutual reaction be-

tween the scanning voltages appliedto the deflecting plates of a cathode -

/C

Electron multiplier. Patent No. 472,485.

ANDSHORT-WAVE WORLD

(Patent No. 472,064.)Photo -electric cathode for use in a

light-sensitive cell.-The BritishThomson -Houston Co., Ltd.

(Patent No. 472,162.)Cathode-ray television receiver or

transmitter fitted with a screen cap-able of giving " abnormal " second-ary emission. --H. G. T.iibszynski.

(Patent No. 472,284.)Improvements in the construction

and support of the deflecting platesin a cathode-ray tube. --F. H. Nicoll.

'ABC OF MAGNETIC SCANNING"(Continued from page 27.)

magnetic field represented by thelines FF. At the edge of the beamthe field line may be considered ashaving two components of deflectingforce, one in the direction of deflec-tion and the other at right angles toit as shown in the figure. The effectof the component in the direction ofmovement of the beam will be to de-flect it at right angles, and as a resultthe circular section of the beam willbe stretched into an ellipse. Thismainly occurs at the extremities oftravel of the spot, w ith the resultthat the screen at the edges of thepicture may be slightly blurred. Inmost cases this effect is minimisedby careful adjustment of the focus ofthe spot when the line screen is actu-ally formed on the screen.

On the whole the magnetic scan-ning system suffers from less majordefects than the electrostatic systemand with careful design is capable ofgiving results in some respectssuperior to those of the electrostatictube. With the added advantages oflower operating voltage it is probablethat the next few years may see thedisplacement of the electrostatic tubeby the magnetic in commercial tele-vision receivers. For the amateurconstructor the former still possessescertain advantages which commendit in home -built receivers and the pur-chaser of an electrostatic tube neednot be deterred by the thought thatit will become obsolete in a shorttime.ray tube.-The Plessy Co., Ltd., and

C. E. G. Bailey.(Patent No. 47 t,747.)

Arrangement for polarising light,suitable for use in telel..ision.-Radio-Akt. D. S. Loewe.

(Patent No. 471,825.)Rotating -disc system of interlaced

scanning.-Cie pour la fabricationdes Compteurs et nzateriel d'Usinesa Gaz.

Philips' Cathode-ray TubesWith regard to the particillars of

Philips' cathode-ray tubes given onpage 730 of last month's issuer itshould be noted that these tubes areonly supplied for the continental mar-ket and are not available in thiscountry.

EELEVISICAANL

SHORT-WAVE WORLD

NO one would be rash enough topredict what new developmentsin television we are likely to see

in 1938, but if the remarkable rateof progress witnessed in 1937 ismaintained, we can certainly expecta great deal.

After all, the first regular tele-vision service in the world startedproviding daily programmes as re-cently as November, 1936. The tele-vision fare of to -day already seemsa long step from those first pro-grammes which ushered in the newservice. Those early programmes,produced at the Alexandra Palacemostly in a hectic rush and often withmuch improvisation, were consideredwonderful enough at the rime. Yetcompare them with what is seen onthe home screen to -day, and theybecome crude experiments in amedium hardly understood by thosehandling it.

The great television developmentsof the past year have been not somuch in the quality of material orartists, but in the methods and studiotechnique used to present materialand artists. The early programmeswere all based on the first wimpleprinciple of television-placing ob-jects and people right before thecamera and transmitting. Settingswere scarcely thought of. The an-nouncers came to view between eachitem, destroying any continuity ofprogramme or any attempt at pro-viding a continued effect with theprogramme as a whole.

How this has altered in the spaceof a year !

To -day it is difficult to enumeratethe many settings and methods ofpresentation used in a week's pro-grammes. The use of two-or eventhree and four-cameras to obtainsuperimposition effects, movementand a sense of spaciousness, hascome to the fore in a startling man-ner, and offers inestimable possi-bilities for the year ahead.

An art has sprung up, which mightbe termed the art of getting the bestout of faces. Producers have learntfrom which angles to shoot artistsand speakers to their best advantage,

JANUARY, 1938

A MONTHLY CAUSERIEon

Television Personalitiesand Topics

by K. P. HUNTEditor of "Radio Pictorial"

not merely to flatter their beauty orhandsomeness, but to get over theirpersonality. 1937 saw the birth of" television personality " making.

In the matter of settings, theAlexandra Palace property room isas busy and packed with scenic de-vices to -day as that of a Hollywoodfilm studio which has been turningout " mammoth spectacles " foryears. As many as eight differentsets a week are frequently used,turned out by a staff which in theyear has grown from a man and a boyto fourteen skilled craftsmen andartistic designers.

Among sets which have been land-marks were undoubtedly those for" ,The Mizzen Cross _Trees " (oldworld sea -port settings); " The Eveof St. Agnes " (a monastery);" Journey's End " (with its .lug -outrealistically collapsing in the final ex-plosion); and " Once in a Lifetime."

" Once in a Lifetime " epitomisedthe great strides made all round. Itwas the first full-length play at-tempted. Youngest producer in theservice, 23 -year -old Eric Crozier,proved with this production that tele-vision enters 1938 as a home enter-tainment of high staudari. Therewere five different sets, ranging froma railway coach to a Hollywoodstudio. (The whole thing was amiracle-if one remembered to com-pare it with what was being put onthe screen but a year ago.

There is little doubt that settingswill be still further developed during1938.

A plan is afoot, I hear, to convert adisused theatre at the AlexandraPalace into a multi -set studio. Itwould have perhaps a dozen differentsets around its walls, the cameras inthe centre of them all. Thus it wouldbe possible to televise a productionwith as many as twelve quick -changescenes. Something of the slicknessand range of setting of the bestcinema films would thus be obtained-and will be obtained, the expertshave no doubt.

I must not give the impression,however, that the glory of television's

first year has been entirely in presen-tation and technique. Good methodssometimes can make a good showout of poor material, but there hasbeen none of that about the pro-grammes. In fact, one of the biggestsurprises has been the ease withwhich varied programmes containinghundreds of stars and celebrities havebeen arranged.

* *

All the early talk about entertain-ment interests threatening to do theirworst to strangle television by res-trictive clauses in artists' contractswas soon drowned in the shouts ofjoy with which most of the entertain-ment moguls welcomed the sight oftheir artists on the screen.

A typical example was the greatwillingness of the film companies toallow the B.B.C. to televise work intheir studios. Only one West Endmusic -hall manager remains to -daywho forbids his stars to televise.Only a glance is needed at the list offamous names in the A .P.'s visitor'sbook to convince you that the enter-tainment profession has completelyaccepted television. Gracie Fieldsand George Robey have starred invariety; Laurence Olivier and DianaWynyard have come from the legiti-mate stage; G. B. Shaw and J. B.Priestley have represented the worldof letters; and the celebrities fromthe sporting, scientific and medicalspheres who have demonstrated theirwork before the camera already areinnumerable.

Television of events outside thestudio came late in the first monthsof the service owing to the wait forthe O.B. vans. But when this fly-ing squad of eighteen men once gotout and about, the prophecy thattelevision would revolutionise thecommunication of news was soonfound to be no exaggeration. Thetelevising of the Coronation proces-sion, the Cenotaph Service, Wimble-don tennis, motor racing at CrystalPalace and the like all demonstratedthe unique value of a medium whichshows people what is going on as itis going on.

30

JANUARY, 1938lagYISIOn

ANDSHORT-WAVE WORLD

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CLIX SPECIFIEDIN THIS ISSUE FOR THE :-1-10 METRE RECEIVER2. "Acorn" typeI. 5 -pin type, V.I.I. 7 -pin type V.2.

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I. The Elements of aSystem of Radio Com-munication

2. Circuit Constants3. Properties of Resonant

Circuits4. Fundamental Pro-

perties of VacuumTubes.

5. Vacuum -tube Ampli-fiers

6. Vacuum -tube Ampli-fiers (Cont.)

7. Power Amplifiers8. Vacuum -tube Oscil-

lators9. Modulation

to. Vacuum -tube De-tectors

t. Sources of Power forOperating VacuumTubes

12. Radio Transmitters

13. Radio Receivers

14. Propagation of Waves

15. Antennas

16. Radio Aids toNavigation

17. Television

18. Sound and SoundEquipment

Appendix. Formulas forCalculating Induc-tance, Mutual Induc-tance, and Capacity

Index

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31

7.ELEVISIOnAND


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32

TELVISIOt1JANUARY, 1938

Outside televising has been themost significant development, simplybecause what it has accomplishedleaves no doubt of the great thingsit will do in 1938.

(Tests already. have been made, Ihear, for the televising of the BoatRace, the Derby, the Olympia Circus,cricket at the Oval, and the arrivalof celebrities at the great London sta-tions. It is perhaps most surprisingthat of these tests, those taken be-tween Epsom and the AlexandraPalace have given the best O.B. pic-tures. From these successful inves-tigations it will be recognised at oncethat there can be no telling wherethe mobile television camera will getto in 1938. The possibility of tele-vising a " Music Hall " from St.George's Hall, though at the momentdropped, opens up the other greatavenue of progress anticipated thisyear, that of television from theatresand halls away from the station. Astart will be made, in fact, with thetelevising of the Olympia Circus, be-ginning January 4.

The inauguration of a floatinghour " supplementary to The presenttwo which will be used for O.B.'swill be the first expansion of the year.Sunday programmes will also be in-troduced, and will consist largely ofplays. Although the production staffat the Palace has grown from six pro-ducers to a round dozen, still morerecruits are expected within the nextfew months. The full television ser-vice staff now numbers 25o, and bythe middle of the year, when summerweather provides an expected spateof outside events, the staff list islikely to reach the 30o mark.

Extension is also expected of thefilm unit, which has been much indemand going out at short notice toshoot backgrounds and incidentalmaterial for variety and features. Agreater use, in the sense of more ef-fective not necessarily a greateramount, of film in pure television pro-grammes, written for television, isexpected.

* '

When I visited Alexandra Palacethe other day I was rather impressedby the way in w hich everything seemsto have settled down to a definiteroutine. 'In the early days at thePalace there was not the same atmo-sphere as there is now. AlexandraPalace has become very " B.B.C."

'The additional space now availableas a result of the one system of trans-

mission is a great help to the produc-ers. For instance, they used the oldBaird studio (now known as No. 2studio) for a complete variety pro-gramme not long ago, and it is awonderful standby. At present thisstudio is used mostly for announce-ments and talks, and it has enabledthe producers to bring about a newdevelopment in talks, which in theirpresentation are distinct from mereannouncements. The fact that aseparate studio can be devoted to anannouncement makes it possible toprovide a setting for the announcer.For example, we have recently seenElizabeth Cowell opening a door orsitting at a table prior to making an-nouncements which are now regardedalmost as miniature programmes inthemselves.

Then, in producing big pro-grammes such as the 'Vic Wells bal-let, by the use of the two studios oneprogramme can be set up beforetransmission and not disturbed,which, of course, is a great advan-tage.

* * *

Referring to the Vic Wells balletreminds me that the production of" Le Lac des Cygnes " was one ofthe most beautiful that televiewershave seen. Balletomanes will recallthat the ballet begins with asight of the swan moving slowlyacross the stage, and this is againseen at the end of the ballet. In thetelevision version an improvementwas effected because a shot was madeof an actual flight of swans on thelake of Alexandra Palace.

I think that Mr. D. H. Munro,productions manager at AlexandraPalace, deserves a special word ofcommendation for the capable andartistic way in which he is develop-ing television ballet, for which Iunderstand he is mainly responsible.I have not yet had the pleasure ofattending in the studio for one of theballet programmes or rehearsals, butI hope to do so when opportunityoffers. I believe that Munro does thething with meticulous care, all therehearsals being timed with a stopwatch and great attention paid togrouping and the general artisticeffect. In fact, ballet looks like be-coming a main line in television, andin the early New Year we are to seethe Vic Wells company again onJanuary 3 in "Nocturne" (Delius),while on the following day the Polishballet, which has just come to CoventGarden, is to make a television

ANDSHORT-WAVE WORLD

appearance. The art director isNijinska.

Leslie Mitchell, the television an-nouncer, is not leaving until Febru-ary, when he takes up his new posi-tion with British Movietone News.I gather that the cause of his leavingis merely that he wants more scope,and it is ridiculous to suggest, assome newspapers have done, that asa consequence he will be lost to tele-viewers. On the contrary, he will befrequently seen in the television pro-grammes in 1938, as I am told he willcome back on a contract basis andthe B.B.C. is quite anxious to retainand utilise as far as possible hisexperience.

Exactly who is to be Mr. Mitchell'ssuccessor is, at the moment of writ-ing these notes, a mystery, but Iheard the other day that several can-didates already have been observedat Alexandra Palace.

Although it is rather late in theday, I must not forget to mentionJoan Miller's recent party at theLangham Hotel, which was a sort ofbirthday celebration in honour of" Picture Page," which passed itstooth performance. About seventypeople were present, including mostof the important folk in television.Mrs. Westhead, of Brighton, who issaid to be the B.B.C.'s most distantregular television viewer, also wasthere.

Baird Cathovisor Tubes.In our December issue, pages 736-

737, we have given details of theBaird Cathovisor tube, type 12MWI,having a screen diameter of 12 in.This data was incomplete insomuchas we omitted to state that the 12 in.tube has a cut-off voltage of minus 18and operates with an anode voltageof 4,900. This tube, which also hasa heater voltage of 2.2 and takes acurrent of 2.5 amperes is priced at12 guineas.

German Amateur Radio.In the November issue we mentioned

that German amateur stations were nowvirtually off the air. We are glad tosay that now, however, the situationhas rather improved and that contactscan now be made with German amateurshort-wave stations. The present posi-tion is that although no new licencesare being issued for the time being, theoriginally licensed stations are stilloperating:

33

1110510t1AND


Our Readers' ViewsCorrespondence is invited. The Editor does not necessarily agree with views

expressed by readers which are published on this page.

The Eiffel Tower TransmissionsSIR,

I was interested to read your sug-gestion in a recent issue of TELE-VISION AND SHORT-WAVE WORLD thatlisteners on the South Coast shouldattempt to receive the Eiffel Towertelevision transmissions.

During the latter part of Augustand September, the sound channel on42 mc. and the vision channel on 46mc. were audible here daily, the sig-nal strength approaching that of theLondon tranmissions.

While the transmissions were re-ceived at fair loudspeaker strength,except when fading, ignition inter-ference from traffic on a main roadioo yards away rendered them oflittle programme value. A plain in-verted " L " aerial was employed,but the use of a simple " V " or dia-mond system, correctly matched tothe receiver input would probablyhave brought the French signals toquite strong loudspeaker volume,above the For theBritish signals, the directive systemwould merely increase the interfer-ence level as well as the required sig-nals, as the road lies to the north ofthe aerial.

Transmissions were made at inter-vals from 16.0o to ci.00 B.S.T. (onSundays till T.00 B.S.T.), but I donot know the present schedule.

It would be interesting to hear howthese French signals are received inother parts of the country, as thoughreceived daily, they were the onlyones audible on many occasionsabout 25 mc.

B. W. F. MAINPRISE(Hythe, Kent).

SIR,You may remember :ny writing to

you re reception of Eiffel Tower tele-vision a few months ago. I haveagain heard their transmitter at R7on a recent Sunday. (The itemsheard consisted of songs by a ladyand gentleman, accordion solo, pianosolo, Spanish music, and a dancerwith castanets, a lady announcer giv-ing out the items broadcast. Astrange thing occurred when a trainpassed ; it completely cut out signalsfrom this station. Does this suggestthat 7 m. wireless waves travel overthe face of the earth. My set is in

an upper room with the lead in wireat the top of aerial, which is 17 feetlong, the lower end being below thelevel of the railway coaches.

J. TAYLOR

(Lake, Isle of Wight).

Miniature Television --A NoteSIR,

In the description of the miniaturetelevision receiver on p. 668 of theNovember issue it is remarked thatthe connections of the tube are such asto give a reversed line scan and it isquestioned whether the plate connec-tions can be re -arranged to give thecorrect scan in both directions.

Mr. L. C. Jesty has pointed out tothe writer that it is possible to

" wangle " the connections to theplates to obtain any direction of scanif the plates themselves are crossedover from picture to line frequency.

The diagram above explains this,giving four positions of the tubewhen rotated in the clockwise direc-tion.

If we assume that the direction ofthe scan is always towards the "free"plates 1 and 2, then on turning thetube and at the same time alteringthe picture and line scanning connec-tions it is possible to obtain any com-bination of spot movements, as shownby the arrows.

Unfortunately, in the receiver inquestion, this will mean drilling freshholes for the tube socket, and underthe circumstances it may not beworth while. I might add that I

have obtained excellent pictures fromthe 1 in. tube using a straight re-ceiver and the definition is surpris-ingly good considering the limitationsof the tube and circuit.

G. PARR (London, N.21).

Correction.We regret that the diagram of the

valve socket given in Fig. 3 on p.669 of the November issue was in-correctly drawn.

One or two readers have pointedout that the heater was shown joinedto the anode. The correct connec-tions are given below.

AvometersAVO apparatus, made by the

Automatic Coil Winder andElectrical Equipment Com-

pany, Limited, is becoming in-creasingly popular, as shown bythe fact that no less than ten in-struments were stolen from a repre-sentative's car in Islington onNovember 2. The company has theserial numbers of these instruments,which we hope someone can identify,in which case information should besent to the above company at WinderHouse, Douglas Street, London,S.W.I. The instruments are:-Universal "Avometer," No. 66-5454.D.C. " Avometer," No. 6594.Universal " Avominor," No. U.-

23257-46.D.C. " Avominor," No." Avo " Oscillator, No." Avodapter."" Avocoupler."" Avo " Exposure Meter No. 14015-

107.Smethurst High -light Meter, No.

H.1012-67." Avo " Light Meter, No. 11.1020-

37.i-"P" type Zeva Iron, No. H.I6.1-"P.O." type &Ara Iron, No.

G.109.All Avometers are numbered and

a record is kept of any instrumentsthat are lost or stolen. In the eventof any such instrument being re-turned for repair the original ownersare advised and in many instanceslost instruments have been traced inthis way. This is a very good ser-vice and yet another point that shouldbe borne in mind when readers arebuying test equipment.

41931-46.3150.

34

JANUARY, 1938TELVISIOPI

IN the calculation and design oftransformers for saw -tooth oscil-lators it is often required to know

the frequency components whichform the wave and a knowledge ofthese enables ordinary A.C. wave-form theory to be applied to thedesign.

Assuming a standard saw -toothwaveform in which the return stroketime is zero, the Fourier analysis ofthe wave can be written :

Akf(t)=- T (-T/2 <t< T/2)

Twhich gives rise to the expression :

cc

f(t)-= 2 An sin. nwt.n = 1

An= (___ /) n-1. Ak/n..Ak is the amplitude of the saw -

tooth wave.The amplitudes A of the partials

decrease with r/n, and for a final re-turn stroke time Tr = p % of T, thevalue of A. becomes :

(_1)n-1. Ak- . Ioo/p . sin (n.p.r)n2.772 ( Too )

The amplitudes of the partial wavesdecrease more rapidly than 'in andperiodically become zero, the firstzero position lying at the value of n,

THE TELEVISION ENGINEER

ANDSHORT-WAVE WORLD

THE FREQUENCY SPECTRUMOF SAW -TOOTH WAVES

By Manfred von ArdenneThe following article is published from notes kindly supplied by the author. The

" Television Reception," Dec. 12th, 1936.which equals roo/p. Fig. 1 showsthe frequency spectrum of a saw -tooth wave with various return stroketimes.

While the degree of distortion ofsaw -tooth wave with a return stroketime of zero will reach a very highvalue when all the harmonics are cutoff above a certain value (e.g.,n = io), a saw -tooth wave with areasonable return time will be free

original appeared in a monograph,

from distortion when all the har-monics above the value of the firstzero point are suppressed. The onlydifference will be a slight roundingoff of the corners of the wave.

In practice the above considera-tions give two alternatives for thetransformation of a saw -tooth im-pulse :

(a) By suitable measurement of thereturn stroke time, saw -tooth waves

Fig. 1. The additionof harmonics to afundamental waveproduces the saw -

tooth shown.

T

xf(t)-S A sin nutnut

.410.g A S117 nut

Anq-1)n:Tii I' -If2

fw(t) flAll-%II-ill

p= 1 0

1 IWAMMIllMIIMVATAIIMAIMAII

illinivaerdiRKWAll

ILVIIWAIMMIIIIIIIMEIII=1111111MM1111KM

ArtIAI

WOO.;

MINIMT P1-70

-3

Q

t

T'2

Fig. 2. The frequency spec-trum of a saw -tooth wavewith various return stroke

times.

Rlicklaufzeil7;,-pfr

p -0

5

1

as

Z 0

Imo-- Tom.tr

P=10

0

1

0 , 5

03 4 5 6 7 8 9101121114151617189120

t

still2 3 4 5 6 7 8 9101112131415M 17 1819 25

n-Ordnawszahl der Tr/haw:17mgal

tiPi. -,!4, 12. -Ti

EnA Schwerpurkl= =E An

I

till2,3 4 5 67 8 9 47 fl 121310151117t7N20

A

I I

are produced with a relatively rapiddecrease in harmonic and in the sub-sequent circuit the partial oscillationsare suppressed after the first zeropoint.

(b) Initial saw -tooth oscillationswith a short return stroke time(slight decrease of harmonic) are ap-plied to the circuit and the responseof the following circuit is so adjustedthat the harmonic amplitude decreaseis proportional to the law of decreasefor the final return stroke time.

In the case of change of phase ofthe fundamental oscillation as com-pared with the higher harmonic, thephase displacement for a degree ofdistortion < 1. r and Tr 5 per cent.must not exceed 1.4°. The effect ofthe change in amplitude may beneglected.

(Continued at foot of next page)

35

..ravisionAND


TelegossipBy LUMEN

MirrorsICAME across an amusing question

in a book on optics the otherday which sounds almost para-

doxical. Here it is : Prove that if aman walks towards a mirror hisimage moves at the same rate, butthat if the mirror moves towardsthe man the image will move at twicethe rate. It might bear explainingon paper, but how does one prove it?Anyhow, you can amuse yourselfwith a bit of paper and pencil sketch-ing little men in front of mirrors.

For reflecting television images itis generally agreed that surface sil-vered mirrors are essential. If plateglass is used with the back silveredthe double reflection from the silverand from the front surface of theglass produces a nasty ghost imageto the observer. As a matter of factif the plate is thin enough the ghostshould not be too distinct, but in.plate is difficult. On the other hand,surface silvered mirrors are verydear and there is always the risk ofscratching them accidentally.

For a mirror measuring about18 in. square the average figure isLi, and a slip in mounting it maytouch the pocket heavily.

The majority of commercial mir-rors are surface silvered, and I be-lieve' that some firms use a specialaluminium surfaced mirror made byMetropolitan Vickers. All surfacemirrors need to be handled carefullyand it is generally fatal to use a clean-ing fluid as it dissolves the protectivetransparent coating. The best thingis to wipe with soft cotton wool, al-though most manufacturers havetheir own special recommendations.Another tip-don't poke your headin the cabinet if your hair has bril-liantine on it. A splodge of brillian-tine takes a lot of wiping from themirror !

It is a clever trick of MurphyRadio to tilt their tube slightly in thecabinet. It is inclined slightly offthe vertical facing towards the backof the cabinet and the angle of themirror in the lid is much improved sofar as viewing is concerned.

I believe there is a patent by Mr.Jesty (G.E.C..) for a cabinet in which

the mirror is mounted at the bottomand the tube is upside down. Theidea is to enable a better view to beobtained by a large audience al-though it doesn't appear very muchdifferent from the standard arrange-ment on casual inspection.

The G.E.C. staff are usually verysmart in ,their ideas-witness thesame gentleman's patent safetyswitch for tubes which operates ifthe tube bursts-so there is no doubtsomething in it.

Phone SeeiiigLinking television and telephone.

which has been so successful in Ger-many is to be tried by the G.P.O.early next year. It will be for experi-mental and not public use, but nodoubt before very long the generalpublic will be able to have the benefitof these experiments. In Germany aphone -television service already existsbetween Berlin and Leipzig, and it isshortly being extended to Nurem-burg and Munich. The cost per callis approximately 3 shillings for 3minutes, which is not very high forsuch a novelty. A screen, about 8 ins.wide, is attached to the telephone.

Looking BackIn examining the progress made by

Gerald Cock's staff at the Palace dur-ing the past year, there naturallyfirst comes to mind the great widen-ing of scope which resulted from thecoming of the " O.B." televisionvans.

The Coronation Procession gavethe B.B.C. a chance in a million totake the television medium out of thestudio in a big and significant way.The success of the vision programmetransmitted from Apsley Gate openedup wide vistas of the possible uses towhich television can be put in the wayof bringing national events toviewers' own homes simultaneouslywith their occurrence-a possibilityhitherto mostly dreamed of but neverrealised so completely. The vanshave also given the B.B.C. the meansof putting sport on the home screens.Also the televising of matches onthe Centre Court at Wimbledon, forinstance, was another significantmilestone reached with brilliantsuccess.

" Television and Short-wave World "

will be sent post paid to any part of the

world for twelve months for 13/6.

Book ReviewsAutomatic Frequency Control

Systems-By J. F. Rider(Holiday and Hemmerding-er, 74-78

Hardman Street, Deansgate, Man-chester, price 5s. plus 5d. postage.)

Automatic and semi -automatic tun-ing systems with circuits as used inAmerican receivers are dealt with reallycomprehensively in J. F. Rider's latestbook on this subject.

The details and circuits given can beapplied to British -built receivers whenin the original stages of design withoutvery much difficulty, and as automatictuning has not been very extensivelyused in this country, engineers in de-sign work will find this book of verygreat value.

Chapter II, devoted to a general re-view of automatic frequency controlfrom all angles, gives a very conciseidea as to the general principles in-volved.

The difficulties in aligning and test-ing A.F.C. circuits have been carefullybrought to light, and suggested methodsof counteracting these difficulties arefully given. The final chapter on ser-vicing A.F.C. systems is most interest-ing, and also those who are at presentusing an automatically tuned receiverwill find some interesting lips

"THE FREQUENCY SPECTRUM OFSAW -TOOTH WAVES"

(Continued from preceding page)

The effect of the circuit can bederived from the ordinary theoreticalconsiderations of a highly dampedoscillatory circuit. In Fig. 2 the am-plitude and phase conditions of ahighly damped series resonant circuitare shown for various values R/2L.

For the effective transmission ofsaw -tooth oscillations the resonantfrequency of the circuit should lieat the centre of the frequency spec-trum to be transmitted, so that

En Ann -

Z AnThe majority of defects in the saw -

tooth generator may be attributed toover controlling or faulty synchron-isation.

(a) Breakdown of lines.(b) Visible return stroke both in

line and picture.(c) Wandering of the picture.

The regularity of the synchronisingimpulses for a 400 -line picture withan independent saw -tooth oscillationshould be within 2-5 per cent. accord-ing to the strength of impulse avail-able, but with dependent oscillations.5 per cent. is required.

36

TELEVISIOtiJANUARY, 1938

An efficient layout, with the panel controls symmetrical,as can be seen from the illustration.

WE have great pleasure in pre-senting to constructors andultra -high frequency experimen-

ters generally a new type of receiverthat has a guaranteed performance, andis, to the best of our belief, one of thefirst of its kind to be otlered to con-structors in this country.

The normal method of presentation is

ANDSHORT-WAVE WORLD

Britain's1-10

UniqueA New -style 4 -valve Receiver

covering1-10 Metres

of how to describe the receiver in sucha way that constructors would not haveany difficulty in obtaining results equalto those given by the original model.

It was finally decided that it wasmost unlikely that the constructor couldbuild the H.F. and detector stages in themost .efficient manner. For this reasonit is not suggested that the amateur try

to wire. In this way there is very littlepossibility of any difficulties occurring.

This enabled us to utilise one or twocomponents which are not normallyavailable to constructors. We refermainly to the special ceramic conden-sers which are only available fromDubiliers by special ordering. Thesecomponents are being obtained by

This four -stage ultra -high frequency receiver, using a most efficient super -regenerative circuit withsingle high -frequency stage, has been designed by Kenneth Jowers in collaboration with E. van Rheeand H. R. Adams, G21VO. It is intended for communication use of the ultra -high frequencies and not

primarily for programme reception.

quite useless with a receiver of thiskind, as we do not feel that constructorswould be able to build a receiver togo down to one metre without ex-periencing a considerable amount ofdifficulty.

In the circumstances, therefore, evenafter the original receiver had beenbuilt and passed as perfectly satisfac-tory, we then had the problem to solve

and build the first two stages eventhough he may do his best to follow theoriginal design.

Ready -builtUnits

Messrs. A.P.A., Ltd., have offered tobuild the H.F. and detector units withthe components in the correct positions,so that they are ready for the constructot

Messrs. A.P.A. and will be included inthe R.F. and detector units whenordered This scheme solves all outproblems, for the constructor can nowconfidently begin the construction of aprecision receiver and not experi-ence any more difficulties than wouldnormally occur when building a two -stage amplifier.

Also, as the receiver is going to be

O

0T,2

-ItC5

C4 RFC 2. H FC

T

0

Rs

CIO11-

LF C.1

L.S.

R3

+2ooo

This is the comparatively simple circuit of the 1-10 receiver, having an Acorn high -frequency stage and an Acorn triodeas a super -regenerative self -quenching detector. The two low -frequency stages are quite conventional.

37

lailYIS*10J1AND

SHORT-WAVE WORLD

Construction S. Short LeadsJANUARY, 1938

purchased by commercial companies,Messrs. A.P.A. are building completereceivers ready for operation. Thesereceivers, and also complete kits forthose who want them, will be marketedby Messrs. Webb's Radio, who alsohave a full stock of individual com-ponents.

The receiver utilises four valves ina more or less conventional circuit,being made up of a pentode H.F. am-

ary of L3. In this way there is nowire at all coupling the high -frequencystage on to L3, as the anode lead to thepentode goes directly to the coil.

As can be seen from the construc-tional photographs, the coils are of aspecial type mounted on ceramic basesand are of the plug-in type. The basesare arranged so that the grid side of thecoil in each case solders directly to thefixed plates of the appropriate tuning

Providing the longer leads under the chassis are groupedtogether in the manner suggested the general constructionunderneath the baseplate remains clean and straightforward.

plifier, using an Osram ZAI acorn,transformer coupled to an HAI triodedetector operating as a self -quenchingsuper -regenerator. This valve is coupledby means of a transformer to an MH4triode, followed by a KT42 tetrode inthe output stage.

The whole of the high -frequencystage up to the anode of the acorn pen-tode is included within the first screenbox. The anode lead, however, of VIcomes through the box on to the prim -

which satisfactory reception was pos-sible under 2+ metres.

It was, in fact, the very high frequen-cies that proved most difficult to getperfectly satisfactory, but a combina-tion of several developments enabled usto obtain a high performance below 2-1.metres. For example, the grid coil L2,is tapped differently for each coil soas to adjust the load imposed on thegrid of the acorn pentode by the aerial.

This back view gives a good idea as to the clean lines of thereceiver. Power supply is connected by means of a 5 -way

cable with a two -contact strip for the loudspeaker.

condenser. In the detector stage thegrid contact on the acorn valve holderalso solders directly on to L4 via thespecial ceramic grid condenser, Cri.In the high -frequency stage the anodelead joins to the coil, L2, by a lead only5/I6th in. in length.

By carefully using special compon-ents and pruning down all waste spacewe have eliminated every possible ex-cess wiring in both H.F. and detectorcircuits. This was the only way in

Left. This is thedetector unit withthe HAI triode super -regenerator. The coilis mounted on aspecial ceramic base,almost on top of thetuning condenser.

Right. In the high -frequency stage thevalve is mounted inbetween two screensand is so arrangedthat all leads areabsolutely to a

minimum.

38

Correct H.F.Choke

Then the choke, HFC2, is only usedon the higher frequency bands whenthe choke, HFC1, begins to fall off inefficiency. For this reason HFC2 is anintegral part of the three higher fre-quency coils so that it is only broughtinto action on definite frequencies.These are but two of the features thathave helped to make the receiver sosatisfactory.

illin/ISIOt!JANUARY, 1938

Coil Ranges It must be admitted that the circuit

is fundamentally quite simple, but itis the layout of the components whichis one of the most important parts ofthis receiver. There must not be anyvariation in layout, and to satisfy our-selves that this cannot occur the twofirst stages are assembled by the manu-facturers.

The low -frequency section is rather

Power Supplyto the chassis. In this way wiring isquite simply carried out and muchmore neatly than if the componentswere bolted directly to the chassis.

Another point to remember is that theslow-motion drive fixed to the two tun-ing condensers is at earth potential,whereas the condensers are isolatedfrom the chassis. Therefore the drivemust be insulated from the spindle of

The three screening cans are mounted in this way.Notice that the two L.F. amplifiers are in the left-hand screen, while the bolts that can be seen at thebottom are the negative contacts of the cathode -

bias shunt condensers.

unconventional owing to the fact thatthe transformer stage conies before theresistance -capacity coupled stage, soreversing the usual procedure. Itwas discovered that with resistance -capacity coupling following the super -regenerative detector the regenera-tion could not be smoothly ^ontrolled.Reversing the couplings cured this de-fect without causing any other difficul-ties.

Bias is obtained automatically in thelow -frequency stages, but a bias of 3 -volts negative is essential in the high -frequency stage if the ZAi pentode isto function satisfactorily. Automaticbias in this circuit was not desirable, sofor that reason a small 3 -volt grid cellhas been included which should have alife of at least 12 months. This gridcell is by-passed by the condenser, C3,having a capacity of .002-mfd.

The three screening cans are gappedby approximately in. and it is im-portant when mounting these cans tomake quite sure that when the lids arein position they do not touch, for other-wise the efficiency is greatly impaired.Wiring the H.F.and Detector

The H.F. and detector stages arewired before the actual calls are bolted

ANDSHORT-WAVE WORLD

the condenser by means of an ebonitebush.

It can also be seen from the plan viewthat the condensers and valve holdersin the first two stages are mounted onpaxolin strip with strip in turn boltedto the side of the screening can.

The two coil holders for Li and L2,and L3 and L4, are fixed to the side ofthe screening can by two steel brackets

(Continued on page 64)

The finished receiver presents quite a professional appearance,being built on a steel chassis and housed in a steel cabinet.

Components forA FOUR -STAGE 1-10 METRE RECEIVER

CABINET.s-Steel, ix in. by 8 in. by 91 in. with lid,

finished black (A.P.A.).CHASSIS AND PANEL.1-Chassis with brackets 91 in. by ro in. by

24 in. finished black (A.P.A.).1-Aluminium panel * in. finished black II in.

by 7f in. (A.P.A.).COIL HOLDERS.2-Special ceramic 5 -pin (A.P.A.).COILS.ro-To cover i-rx metres (A.P.A.).CONDENSERS, FIXED AND VARIABLE.r-20-mmfd. type Apex (Cr) (Webb's Radio).1-2o-mmfd. type Apex (Ca) (Webb's Radio).r-.002-mfd. type 62o (C3) (Dubilier).5-.0003-infd. type 620 (C4) (Dubilier).I-.0005 mfd. type 4802 (C5) (Dubilier).I-.003-mfd. type 62o (C6) (Dubilier).5-.5-mfd. type 4608/S (C7) (Dubilier).i-lo-mfd. type 401, 50 -volt working (C8)

(Dubilier).r-so-mfd. type 401, 50 -volt working (C9)

(Dubilier).1-.1-mfd. type 46o3/S (Cxo) (Dubilier).1-.00005-mfd. type CDSr (Cir) (Dubilier).I-5-mmfd. type Apex (Cr2) (Webb's Radio).r-.5-mfd. type 4608/S (C13) (Dubilier).

HOLDERS, VALVE.2-Type ceramic Acorn (Clix).r -5 -pin type Vr, less terminals (Clix).I -7 -pin type VI, less terminals (Clix).PLUGS, TERMINALS.x-Ceramic terminal block, type 1046 (Eddy-

stone).1 -5 -way socket type I26o with plug (Belling -

Lee).I-a-way terminal strip type P54 (Bulgin).RESISTANCES, FIXED AND VARIABLE.5 -35,00o -ohm type I watt (RI) (Erie).I-5o,(oRoeoi-ohm wire wound (R2)

1 -2o,000 -ohm type x watt (R3) (Erie).1 -700 -ohm type i watt (R4) (Erie).1-Ioo,000-ohm type I watt (R5) (Erie).r-.5-rnegohm type watt (R6) (Erie).5 -420 -ohm type r watt (R7) (Erie).1-5oo,000-ohm potentiometer (R8) (Relince).1-20-megolim type watt (R9) (Erie).SUNDRIES.2 -Coil mounting brackets (A.P.A.).2-Extension outfits type r (Eddystone).3-Special r -in. knobs (Eddystone).2-Coils Quickwyre (Bulgin).2-Dozen 4 B.A. roundhead bolts, nuts with

washers (Webb's Radio).TRANSFORMER, LOW -FREQUENCY.

CHOKE, HIGH -FREQUENCY. 1-Special type 1/4 ratio (Tr) (Keston Mann-r-Type roil (RFCr; (Eddystone). facturing Co.).CHOKE, LOW -FREQUENCY. VALVES.1-Type 2o -henry to specification (LFCr) i-ZAi Acorn R.F. Pentode (VI) (Osram).

(Keston Manufacturing Co.). 1-HAI Acorn triode (V2) (Osram).DIAL, SLOW-MOTION. I-MH4 (V3) (Osram).1-Type 1085 (Eddystone). x -XT42 (V4) (Osram).A Complete Kit of Components and Completely Assembled Receivers obtainable from :

WEBB'S RADIO, 14, SOHO STREET, W.I.Assembled Radio -Frequency and Detector Units and wired receivers

obtainable from :MESSRS. A.P.A., LTD., 171C, BATTERSEA PARK ROAD, S.W.8.

39

11111VISICMAND


Aligning the 690 NE of the most popular receivers

in the country at the present timeis the RME69, but despite its

multiplicity of stages in older to obtainmaximum performance the intermediatefrequency stages must be very carefullylined up.

The manufacturers of this receiverdo not place very much reliance on theaverage commercial oscillator which issupposed to have a frequency of 465 kc.,and they advise users of the RME69not to use a commercial oscillator butto rely upon a system which they haveevolved specially for this receiver.

If any reader should find that thegain on their RME69 is not quite whatit might he, try checking the 1.F. trans-formers in the following way. Theseremarks will also be applicable to othertypes of communication receivers em-ploying a crystal filter.

It is for the purpose of re -alignmentof the intermediate frequency transfor-mers that the following procedure isoutlined. It is essential that the 465kc. intermediate signal which is usedfor re -alignment of the intermediatefrequency amplifier is not set accord-ing to any arbitrary calibration on thetest oscillator itself, since it has beenfound that many oscillators vary to suchan extent that they will not permit ofproper alignment in a receiver using acrystal filter.No Test Oscillator

In the circumstances it is better thatno test oscillator be used since a broad-cast station of constant signal strengthwill furnish an adequate test signal forchecking up the 1.F'. amplifier usingthe quartz filter for stabilising the pro-per I.F. frequency in the followingway.

The meter on the RMEG9 receiveraffords an excellent method of indicat-ing the peak alignment of each of thetransformers, which are designed for afrequency of 465 kc. Since the receiveris always supplied with a quartz crystalfilter it is essential that the intermediatefrequency tnansformers be accuratelytuned to that frequency. Crystals aresupplied in frequencies slightly atvariance from 465, but never more thanplus or minus one kilocycle. It is,therefore, advisable to align the I.F.amplifier to a set frequency governedby the crystal rather than the 465 kc.In this way the I.F. amplifiers are al-ways perfectly in tune at crystal fre-quency.

The first step in the alignment proce-dure is to tune in a broadcast station,preferably towards the low -frequencyend of the broadcast band. The signalshould be one of medium strength, sothat the R meter indicates the level of

Those who operate the R.M.E. 69 re-ceiver will find this article of help whenchecking up the I.F. stages for maximum

gain.

R9 or slightly less. If no station ofthis amplitude is available, but astronger station can be received, a re-duction of the efficiency of the aerialcan be made by using only a very shortlength of wire sufficient to give a sig-nal of the required level.

When the station has been chosen,assuming for the purpose of examplethat the frequency is 7oc, kc., the nextstep is to slightly de -tune the maintuning control, so that the frequencyreads approximately 715 or 720 kc.This, of course, will tune the stationout. It does not necessarily have tobe the frequency mentioned, or theexact frequency of de -tune, but thegeneral procedure is to tune the maincontrol slightly higher than the chosenstation so that it may be brought backto resonance by decreasing the scalereading of the band -spread control.This is done merely to provide verniertuning.

With the station chosen and reson-ated on the band -spread scale, thecrystal filter is switched to the seriesposition, which is the middle positionof the three available. The hand -spread scale is then adjusted with res-

should be used. Then the control, E,should be set so that the condenser isadjusted to be 5o per cent. in mesh.Then without particular attention to acourse of procedure, any I.F. trans-former can be adjusted to resonance bymerely noting a rise or fall in the Rmeter. It is, however, advisable tostart with No. 1 transformer, followingwith No. z and No. 3.

From time to time it is advisable tomake sure that the signal is still ad-justed to peak resonance of the crystalby slightly varying the adjustment ofthe band -spread control. When this pro-cedure has been completed and alltransformers adjusted and left at maxi-mum meter reading, the intermediate -frequency amplifier of the receiver is inpeak adjustment and the crystal alignedwith it for maximum effectiveness infilter action.

With an RME69, including a typeLSI noise -silencer, one or two pointsshould be noted. 1 he general proce-dure is the same as for the standardRME69, but preliminary adjustments,as previously mentioned, should bemade with the silencer threshold con-trol set at maximum clockwise posi-tion. When the intermediate -frequencytransformers have been aligned, thesilencer transformer may be peaked byturning the band switch to No. 6 bandon the receiver and tuning in and re -

This is the R.M.E. 69 receiver with DB20 pre -selector. This combination is about thebest possible for both amateur use and general broadcast reception.

pect to the signal so that a maximummeter reading is obtained. The pro-cedure is one that requires patience andaccuracy in adjustment, since the re-ceiver is ultra -sharp with the crystalfilter in circuit, but there will be onedefinitely sharp peak indicating crystalresonance. The receiver should betuned to this peak and left on it duringall adjustments that have to be madeto the I.F. amplifier.

When the peak has been tuned in andthe meter is showing maximum read-ing, a small standard trimmer toolof the insulated screw -driver type

sonating the frequency band around tometres so that the receiver is sensitiveat this point. Then, under conditionsof motor -car ignition interference, thesilencer control should be set to maxi-mum counter clock -wise position and asmall screw accessible to the hole in thenoise rectifier transformer adjusted forminimum response. of the interferingnoise.

This ensures accurate alignment ofthe noise amplifying system with thatof the interfediate frequency, a condi-tion which must necessarily exist forefficient silencer action.

40

JANUARY, 1938

TELL,Y1SWIIAND

SHORT-WAVE WORLD

A 5 -band Mains operatedWITH the increasing use of phone

transmitters in this countryreaders are more frequently ask-

ing for designs of monitors suitable forchecking speech quality, and also atthe same time, without waste of appara-tus, being able to have some sort ofradiation check. At the moment weare producing a special receiver forgeneral amateur use which includes asan integral part a radiation meter -cum -

phone monitor, alsosignal strength meter.

In view, however, of the number ofamateurs who have asked specificiallyfor a monitor, we are publishing thisunit rather before we had anticipated,and as a unit on its own rather than asa part of a communication receiver.

This accounts for the unusual shapeof the chassis and panel, which is 12 in.by 4 in., and made from .1 in. thick alu-minium. The panel is supplied cutready to take the flush mounting meter,special tuning dial, rotary switch andother panel accessories.

The layout of the components, whichare very few, can be seen from the ilus-tration in these pages, but for those whowould prefer a different type of chassisthere is no reason why componentsshould not be differently arranged tosuit individual requirements. It shouldbe remembered that there are no H.F.or L.F. amplifiers in a monitor of thiskind, so that the possibility of oscilla-tion or instability does not arise.

As regards component values, theseshould be strictly adhered to, otherwisethe tuning range of each band may

a multi -voltage

MonitorTransmissions on any of 5 wavebandscan be checked with this double -diodemonitor. Details are also given forincluding the 5 -metre band if required.

differ so that instead of covering the to -and zo-metre bands, constructors mayfind that the tuning range falls short

The bottom knobis for the wave -change switch withthe headphone plugand jack to the leftand the cut-outswitch to the right.Although a ver-nier is fitted to theCrowe dial this isonly used as an

indicator.

of their requirements. All componentsare of a standard type with the excep-tion of the 5 -way coil, which must behome -constructed, and this should bethe first job when building the monitor.Making theCoil

Obtain a z -in. diameter coil form 6 in.long from Webb's Radio and mountthis on two small stand-off insulators,so that the bottom of the coil form isat least 'On. away from the metal

" C2

By Kenneth Jowerschassis. Drill two holes in the end ofthe coil form for use as fixing holes forthe wire, and wound on three turns ofzo-gauge enamel -covered wire to .covera space of i in. Then drill two moreholes to terminate this coil and take theend down to one contact on the rotaryswitch.

Such a coil will cover approximatelythe to -metre amateur band. A similarcoil must then be wound for zo metres,again having the same spacing and ter-minated in the same manner. Thiscoil is joined in series, with the to -metrecoil, so that there are six turns in use

zo metres.The 40 -metre band requires 14 turns

of zo-gauge wire wound in solenoid -fashion without any gap between turns.This is in turn connected in series with.the 20- and to -metre coils, so that the40 -metre coil actually consists of zoturns.

On 8o metres the coil is wound ex-actly as on 40 metres except that 16turns are required of 26 -gauge enamel -covered wire, so giving a total of 36 -turns to cover the So -metre band.The LargestCoil

The hardest job of all is windingthe tho-metre coil, for 32 -gauge wireshould be used with a total of 56 turns.As the surface of the former is veryslippery the wire should be held in avice and kept perfectly taut when wind-ing, while it is advisable to fix a sol--dering tag rather than to depend onthe two holes, for when fixing in thisway the wires are inclined to becomeslack. How this coil is used can clearlybe seen from the theoretical circuit.The coils are switched in series to in-crease inductance so that on the 16o -metre band there is a total of 02 turns.On the other hand, when tuned to to,metres the only coil in circuit is thethree -turn one which, with the largecondenser specified, more than coversthe to -metre band.

With high power there is no need tohave any aerial at all connected to thecoil, but with low -wattage stations itwill be advisable to connect a shortlength of wire of about 2 or 3 ft. total

I length to the fixed vanes of the tuning--

soC3

C

cs

C1. 6

Very few compo-nents are requiredin this circuit whilethe coil must be

home wound.

. 41

liaittevislOt1AND

SHORT-WAVE WORLD

Operating Field strength MeterJANUARY, 1938

condenser. A .000l mfd. fixed conden-ser should advisedly be connected inseries wth the aerial lead in order toprevent damping on the higher -fre-quency bands. Refer a moment to thetheoretical circuit. It will be seen

-that a V914 double -diode is used as aconventional rectifier with both anodesstrapped together. In this way a recti-fied current of at least 5 mA. can beobtained, should only a high scale read-ing meter be available, but as a matterof interest, .5 to .75 of a mA. currentreading coincides with ample head-phone strength.

In order to obtain maximum sensi--tivity there should be connected inseries with the cathode of the V914 aresistor of 2,000 ohms shunted by a.00t-mfd. condenser. Also in series

-with the cathode is the o-i milliam-meter, which is in turn shunted toearth by another .00i-mfd. condenser.The headphones in the same circuit

_lave in parallel with them a two -point

toggle switch, so that the phones canbe cut out of circuit as required.This is most important otherwise witha strong signal feed -back is caused be-tween the headphones and the micro-phone circuit.

The filament trans-former is mountedbeneath the coil andterminated at twosoldering lugs.Five holes are re-quired to take thefive leads from thecoil to the switch.

Another point to remember is thata monitor of this kind can give a verydistorted idea as to the percentage ofhum level on the carrier unless thereare shunt condensers across the filamentwinding which are connected to earthin the manner shown. In a circuitsimilar to the one of this monitor,which was described in February, 1937,these two condensers were not includedand it took two months before we real-ised that the rather high hum level ex-perienced was due to this omission.

The mains transformer, Ti, ismounted underneath the chassis, so thatit is screened from the coil, and has itsprimary terminated in two double -endedsoldering tags which are fitted to a

This is how the components are arranged beneath the chassis.mounted on a paxolin strip.

The valve holder is

strip of bake] ite mounted on the top ofthe chassis.

If the unit is completely screened, asit will be in the finally designed re-ceiver to be published very shortly,then there is absolutely no pick-up onthe monitor coil when the unit is beingused as a field strength meter. Whenused as a monitor, however, the head-phone leads provide sufficient indirectpick-up when there is any sort of radia-tion directly from the tank coil. How-ever, at the present time with a correctlydesigned P.A. stage running with aninput of so watts link coupled to theaerial there is insufficient pick-up onthe monitor unless an aerial is con-nected.

This point is mentioned because con-structors might be in doubt as to theamount of pick-up they should expect.It should, however, be remembered thatexcessive current in the monitor indi-cates that the P.A. stage is not puttingall the power it might actually in theaerial. To quote an example, with atransmitter correctly tuned, providinga current of .5 mA. in the monitor, thiscurrent rose to over 5 mA. when an un-tuned line was used for a feeder whichwas not correctly terminated at theantenna end. A point to watch whenusing this monitor as a radiation meteris that maximum current in the moni-tor does not mean maximum radiationunless the meter is actually within thefield of the antenna. If it should be

(Continued on page 6o)

Components forA 5 -BAND MAINS OPERATED

MONITORCHASSIS AND PANEL.1-Steel panel finished black to specification

(A.P.A. Ltd.).1-Steel chassis finished dull black to specification

(A.P.A. Ltd.).COIL FORMER.1-2 in. by 6 in. bakelite former (Webb's Radio).CONDENSERS, FIXED.2-.00r-mfd. type 69oW (Dubilier).2-.or-mfd. type 691 (Dubilier).r-.00ca-mfd. type 69oW (Dubilier).CONDENSER, VARIABLE.r-Type 942/18o (Eddystone).DIAL.I-Type Crowe (A.C.S. Ltd.)HOLDER, VALVE.I -5 -pin type Vx less terminals (Clix).METER.1-0-2 M/a flush mounting (Ferranti).PLUGS TERMINALS, ETC.6-Insulated sockets type 12 (Clix).6-Coil pins type 36 (Clix).1-Terminal type B marked Aerial (Belling -Lee).RESISTANCE, FIXED.1---2,000-ohms, I -watt (Erie).SUNDRIES.l-lb. 26 gauge enamelled covered wire (Webb's

Radio).2 4-6BA roundhead bolts, nuts and washers

(Webb's Radio).z-J2 jack (Bulgin).x-PIS plug (Bulgin).SWITCHES.2-S8oT (Bulgin).1-Srig (Bulgin).TRANSFORMER, FILAMENT.1-4 volt .3 amp with wire ends (Premier Supply

Stores). ACCESSORIESVALVE.x-V94 double -diode (Mazda).HEADPHONES.x-Pair type F (S. G. Brown, Ltd.).

42

JANUARY, 1938AND

SHORT-WAVE WORLD

The Short-wave Radio WorldIN view of the extra interest in the

W8JK flat top beam it is interestingto note that VK2NO, the well-known

Australian authority on 5 metres, hasbeen using a type of aerial of this kindfor DX transmission and reception. Thearrangement used is shown in Fig. iwhere the main support is a length ofi4 in. bamboo pole 17 ft. overall carry-ing three cross -pieces for the radiator

Fig. 1. Two British amateurs have heard5 -metre signals transmitted by VK2NO fromAustralia when the transmitting aerial has

been this beam array.

spacing, and also a right-angled sup-port at the centre for the sub -line. Asvertical polarisation appears to be ofmost use-although tests may proveotherwise-while it is a very simplematter vertically to erect an array suchas this.

Being only 2 ft. 2 in. in width thesystem offers very little wind resistanceeven when erected on top of a highmast. As it is not always possible toobtain a 17 ft. bamboo centre piece, twolong poles can be used with the thinends cut off. These can then be fittedinto a base and pegged and finallysecured with stout twine covered withclear lacquer.

This system is directionally end -fire.It consists of four half -waves, two perside, spaced wave and fed out ofphase. The dimensions given in Fig.are correct for approximately the centreof the 5 -metre amateur band. At thecentre of the array the inner ends ofthe radiators are crossed over as shown,

A Review of the Most Impor-tant Features of the World's

Short-wave Developments

and at the centre a a wave stub is fitted.In reckoning the length of this stub, theincluded length of i ft. i in. to thecrossover from each aerial length mustbe considered, so that the actual pro-jecting stub is only 3 ft. 3 in. in lengthfrom the crossover.

Spacing of the stub and crossoverleads is 4a in. for No. 14 gauge wire, ofwhich gauge the aerials are also made.Incidentally the length of each aerial is7 ft. 64 in.

The method of resonating is quitesimple. The transmitter is switched onwith reduced power and a 600 -ohm lineis connected somewhere about i ft. fromthe crossover on the stub. A radio -fre-quency meter is used as the jumper fordetermining resonance, or if a meter isnot available, a low -wattage lamp canbe used equally well. This is adjustedalong the stub until maximum light orcurrent is shown. When this is reachedthe array is resonant at the frequencyof the transmitter, so a shorting bar isthen soldered in position at this setting.

Then increase the power and checkthe feeder line for standing waves byusing an indicator device such as anabsorption wave -meter plus lamp. Thisindicator is moved uniformly along theline from the aerial to the transmittingposition.

If large variations in R.F. currentare noticed at intervals of about 4 ft.the line has standing waves on it. Theseare reduced by moving the feeders alongon the stub -line until the R.F. shown ismore or less uniform along the wholelength of the line. It is unlikely thatstanding waves will be completelyeliminated, but they can always bematerially reduced.

If it is desired to use a twisted pairline in preference to an open wirefeeder, the feeding of the array isgreatly simplified. A twisted pair linesuch as the Belling -Lee 8o -ohm is at-tached at the end of the stub and noshorting bar is used. This method offeed may not be so efficient as the spaceline, but it is very effective.

Alternately the array can be fed by aconventional tuned Zepp feeder con-nected directly at the crossover without,of course, any stub section.

Hauling the array to the top of thepole is quite simple. A length of woodcarrying two hinge bolt sockets is fixedto the rope halyard. Two hinge boltsare fitted to the bamboo at the correctdistance apart and near the centre. Thetwo arrangements are fastened together,and the whole is pulled to the top of thepole. As the halyard is fastenedstraight up and down the array is kept

vertical. It is arranged for rotationsimply by attaching a rope to the ex-treme end of the projecting woodencentre piece holding the stub line. Bypulling on this rope the array can beswung from the ground through 18odegrees.

In practice the aerial gives excep-tionally good results with low -angleradiation, and so far tests have com-pared very favourably with the morecomplicated Bruce aerial. We hope thatmany more experimenters interested inultra -high frequency working will con-sider the use of an aerial such as this,for as long as the conventional half -wave di -poles are so consistently usedwe cannot see any possibility of moreDX records being broken on the ultra-high frequencies.

This beam aerial can be used equallywell for reception and can be modifiedfor use on other wavelengths, includingtelevision. The fact that it includes areflector and can be used in any direc-tion through 18o degrees makes it par-ticularly useful for long-distance recep-tion of ultra -high frequency signals.RegenerativeDoubler

In the December issue of Q.S.T. wasdescribed a very simple regenerativefrequency doubler using a very popularvalve, the 46. W8CSE is using this cir-cuit and claims that the output from theregenerative doubler can be materiallyincreased over the normal circuit whichis an ideal arrangement when a littlemore drive is required on the higherfrequencies. The circuit arrangement isshown in Fig. 2 and it will be seen thatthe modification over the normaldoubler is very slight.

Fig. 2. To make a doubler stage regenerativeonly requires these simple modifications.

All that is required is a small trim-ming condenser and a few turns of 26gauge double cotton covered wire. Byusing a separate neutralising coil in themanner shown, the neutralising con-denser CN need not be of the high vol-tage type, since there is only a low D.C.potential across it.

It is claimed that this circuit will givean increase of ioo per cent. in the gridcurrent of a type 10 when used as a

43

TELY1S1011AND


Signal Generator . . Frequency Meterdoubler. At the same time the anodecurrent of the preceding 46 was loweredby approximately one-third.

The addition of the neutralising coildoes not mean any changes in the exist-ing connections. The coil Ln has thesame number of turns as L2 and is

cuited position when working into theaerial circuit of a sensitive receiver.

A sheet of aluminium is mounteddirectly behind the dial so as to elimini-nate possible radiation directly fromthe tuned circuit, but it is also recom-mended that when checking high gain

Fig. 3. An all -wavesignal generator ofthis type is mostuseful for generaltest work. Its manyadvantages are ex-plained in the text.

wound in. away from the low poten-tial end of Lz. No. 26 gauge wire isquite large enough, while the circuit isneutralised in the normal way. The cir-cuit is in Fig. 2.

An All -waveSignal Generator

We were very interested in the designof an all -wave signal generator whichwas described in detail in the newA.R.R.L. Handbook. When lining upmulti -stage receivers some type ofsignal generator is a necessity. Thegenerator must also have a wide fre-quency range, give stable output and,at the same time, be mains operated.The circuit for an oscillator havingthese requirements is shown in Fig. 3,where it will be seen that three valvesare used in a more or less conventionalcircuit. It has a continuous frequencyrange of from 45o Kc. to 6o Ms. A type6K7 pentode is used as an electron -coupled oscillator with a 6C5 as a vari-able audio frequency generator andnormal type 8o rectifier.

Although the original coil switchingarrangements called for a special turretmade by Communications Products,Inc., suitable switching gear can be ob-tained from Messrs. A. F. Bulgin. Allunused coils are shorted to earth so asto prevent any dead-end or absorptionlosses, while the high capacity in thetuned circuits gives increased stability.The attenuator consists of a potentio-meter connected so as to present a con-stant impedance to the receiver inputterminals. A three -position switch giveshigh and low output plus a short cir-

receivers at maximum sensitivity, theentire unit should be well screened.

A separate L.F. oscillator employinga triode with cathode feed -back and biasprovides 20 different L.F. frequenciesbetween Too and io,000 cycles. The fre-quency is controlled by means of tapson the oscillator inductances and alsofour tuned capacities selected by meansof two rotary switches. The four -posi-tion capacity switch brings into circuitthe major ranges, while the inductanceswitch provides intermediate steps.

The output of this generator is usedto suppressor grid modulate the oscil-

HEATERS

M.07/HP

per cent., so to this end the volume con-trol must be retarded to provide the re-quired 8-15 volts output.

Terminals on the panel enable ex-perimenters to use this output from theaudio -frequency generator for externalpurposes, such as checking modulationin transmitters.

Although the harmonic content hasbeen reduced to a very low level, thewaveform is not absolutely perfectowing to the use of iron -core induct-ances, so this point must be remem-bered in checking amplifier distortion.

If trouble is experienced in makingthe audio frequency oscillator oscillate,increase the value of R5 or decreaseR6.

When used with single signal supershaving a crystal filter it is a good planto use the crystal itself to control thetest oscillator when lining up the I.F.transformers.

With this end in mind it is advisableto provide a holder for a crystal filter,so it can be installed in the oscillatorin place of the grid coil when makingchecks on single signal receivers. It ismost unlikely that the average test os-cillator will provide a sufficiently stablesignal to test a receiver using a crystalfilter, so that by using the actual filterfrom the crystal itself perfect align-ment is possible.A Combined Frequency Meter,Monitor and Keying Oscillator

The Canadian amateur, VE2EE, hasdesigned a complete frequency meter,monitor and audio oscillator using tworeceiving type valves. This unit shouldbe almost indispensable to a well -opera-ted amateur station despite the almostgeneral use of crystal control. It mustbe remembered that the best of crystalsare inclined to peak and that a trans-mitter does not always function satis-

1;4. c.t. ofKEYED., TAGE

Fig. 4. This is thecircuit for a com-bined frequencymeter, monitor andoscillator designed bythe Canadian

amateur VE2EE.

lator with transformer coupling and anoutput volume regulator. In the originalmodel designed by A.R.R.L. a modula-tion percentage of Too was obtainedwith this control approximately half -on.It is suggested that the modulationlevel be restricted between 3o and 5o

factorily whenever it is switched on.VE2EE has used a 56 triode as a de-tector and by re -arranging the gird cir-cuit to function as an audio oscillatorfor listening to C.W.

A D.P.D.T. switch is connected in(Continued on page 63) '

44

JANUARY, 1938AND

SHORT-WAVE WORLD

A Emergency TransmitterBuilding the Modulator

This modulator is the first half of a complete emergency ultra short-wave telephony transmitter. Theentire equipment runs from a 12 -volt accumulator.

RANSMITTERS for ultra -highfrequency use are most suited foremergency and portable work. It

is anticipated that before very long Gov-ernment bodies and local borough andmunicipal councils will be instructedto have installed some type of trans-mitter suitable for maintaining short -

TheModulator

First of all is the modulator section.The requirements are an amplifier ofgreat simplicity that does not requireany dry batteries, but at the same timewill give an output sufficiently high

The finished amplifier with crystal microphone and built-in converter. The output isbetween 5 and 7 watts depending on the percentage of distortion allowed.

range contact should there be any fail-ure of the normal telephone service.

No MainsUsed

Such transmitters, in fact, the entireemergency equipment would have to beindependent of public supply mains,while dry batteries could not be usedin view of the comparatively short stor-age life. This limits the power supplyto something run from a chargeablelow-tension accumulator, which meanseither a rotary converter or vibratorunit.

In anticipation of the general re-quirements of an emergency transmit-ter this equipment has been produced,but it must not be taken for grantedthat it can only be used for emergencyuse. During the coming year there willbe a considerable number of amateurfield days on ultra -high frequencieswhere phone transmitters powered byan accumulator will be in great demand.

Obtaining a sufficiently high inputwithout overloading the accumulatorwas rather a problem, but. two suitablepower units were obtained which givemore than sufficient output for themodulator and transmitter with quitea reasonable low-tension drain.

fully to modulate a io-watt carrier.After considerable experimental worka suitable circuit was designed, asshown in this page. It consists of anA.C./D.C. pentode valve, resistance -capacity coupled to a pair of high -slopepentodes in parallel. This arrangementgives a comfortable 5-6 watts of audiofrom a low-level input. A specialmicrophone was also designed for usewith this amplifier and is of the handset type, but with a crystal unit.

R

3

VR

A CrystalMicrophone

This crystal microphone is suitablefor feeding into a soo,000-ohm load andhas a sufficient output to enable a con-siderable gain to be obtained in thefirst L.F. stage by means of the SP r3Cpentode. However, unless the first pen-tode valve is correctly used with theoptimum anode impedance the gaindrops quite rapidly. For this reason itis essential that the anode resistance inthe first valve be ioo,000 ohms followedby a decoupling resistor of 50,000 ohms.The screen voltage is quite easily ob-tained by means of a series resistortapped into the junction of the anodeimpedance resistance and decouplingresistance.

A resistance of 250,000 ohms is re-quired to drop the main H.T. supplyto the correct voltage to the screenowing to the fact that the screen cur-rent is extremely low when the SPI3Cis used as a straight L.F. amplifier.

The 4-mfd. decoupling condenser inthe first L.F. stage is part of a doubleunit and the second condenser in thisunit is for smoothing and follows theX32B smoothing choke.

With the SP i3C used in the mannersuggested sufficient output is developedfully to load the 7D8's in parallel. Asatisfactory value for the coupling con-denser between stages is .or-mfd. fol-lowed by a grid resistor having a maxi-mum value of 500,000 ohms. This gridresistor is actually a variable potentio-meter per forming the dual function ofgrid impedance and volume control.

Owing to the high slope of the 7D8'sthe auto -bias resistor has the low valueof ioo ohms, but this is fairly critical

128

000

L.F.0

O12.v.

The circuit is very simple consisting of a pentode amplifier followed by twopentodesin parallel.

45

tes.vgionAND

SHORT-WAVE WORLD

Building Connecting UpJANUARY, 1938

for a lower value causes the total anode mA. from the low D.C. 2 -volt input.current to rise to a point where it is I As can be seen from the illustrations,

This plain view clearly shows the lay -out of the components and the constructionof the converter.

in excess of the output given by thepower supply.

In order to prevent L.F. instabilityand oscillation a 5,000 -ohm resistorshould be connected in series with thearm of the volume control potentio-meter, while in addition there shouldbe a resistance of too ohms in the anodeof each 7D8. A point to remember isthat the too -ohm resistors should besoldered directly to the anode pin onthe pentode valve holders.

A special output transformer has beendesigned so that the 7D8's are accu-rately matched to the working imped-ance of the transmitter. Actually, al-though the transmitter is to be fullydescribed in our February issue, it is inorder to state at this point that it hasbeen designed to have an operating im-pedance of 6,000 ohms. For this reasonthe output transformer was speciallydesigned to have a secondary of thisimpedance.

ToneCorrector

Across the primary of the outputtransformer is a resistance and conden-ser network in order to attenuate thetop notes which are rather accentuatedwith these pentode valves. In theoriginal amplifier a resistance of 20,000ohms in series with a condenser of .oi-mfd. gave a sufficient top note attenua-tion for average use. This networkmay at first glance seem to be a littledrastic but it must be remembered thata crystal microphone unit is to be usedwhich is rather prone to boost the upperfrequencies.

All valve heaters are wired across ar2-yolt accumulator, which in turn isconnected to a rotary converter whichprovides an output of 25o volts at So

this converter is housed in a metal con-tainer, complete with lid, and bolted tothe chassis on the extreme end so thatwith the exception of the r2 -volt accu-mulator the amplifier is entirely self-contained and self -powered.Construction

Just how the components are laid outcan be seen from the plan view. Thethree valves are, on the left, SP 13Cpentode, followed by the two 7D8's. Thecomposition of the converter is quiteeasily understandable after referenceto the photograph of it, but care shouldbe taken in the way it is mounted.

Two large holes on the side of theconverter box are bushed with rubberbushes through which come the L.T.and H.T. leads. These two leads godown to the chassis on the extreme edgethrough bushed holes in order to pre-vent any possibility of breakdown onthe metal chassis.

As regards the two iron -core compon-ents which will be noticed are mounted

(Continued on page 6z)

Keep as many components as possible anchored to the 5 -way strip in the mannerillustrated.

Components forA U.H.F. EMERGENCY TRANSMITTER

MODULATOR SECTIONCHASSIS.I-Special steel finished black 134 by 74 by 3 ins.

(Bryan Savage).CHOKE, LOW -FREQUENCY.x-Shrouded type X32B (Bryan Savage).CONDENSERS, FIXED.3-25-mfd. type 3016, 25 -volt working (Dubilier).1-.5 -mid. type 4608/S (Dubilier).2-.01-mfd. type 4601/S (Dubilier).1-4 plus 4 mid. zoo -volt working type BE355

(Dubilier).HOLDERS, VALVE.3 -7 -pin less terminals type V2 (Clix).PLUGS, TERMINALS, ETC.2-Terminals type B marked Output (Belling Lee)x-Shrouded top cap connector type 1224 (Belling -

Lee).I-Two-way connecting block marked plus minus

(Andrew Bryce).I-Microphone socket (Bryan Savage).RESISTANCES, FIXED.I-5oo,000 ohms type r watt (Erie).x-x,000-ohms 1 watt (Erie).

1-m0,00o-ohms x watt (Erie).1--25,000-ohms r watt (Erie).x -5o,000 -ohms x watt (Erie).4 -200 -ohms I watt (Erie).x -2o,000 ohms x watt (Erie).RESISTANCE, VARIABLE1-5oo,000 ohms potentiometer type L.A.B.(Erie).SUNDRIES.2-Coils Quickwyre (Bulgin).I -5 -way group board (Bulgin).I-IP7 dial (Bulgin).2-Dozen 4BA roundhead bolts, nuts and washers

(Webb's Radio).TRANSFORMER OUTPUT.1-Type 4250/6000 (Bryan Savage).ACCESSORIESMICROPHONE.t-Special crystal unit (Bryan Savage).TRANSFORMER, ROTARY.x-Type x2 volt input 250 -volt 8o M/a output

(Electro Dynamic Construction Co.)VALVES.x-SPI3C (Mullard).2-7DB (Brimar).

46

IELEVISIORJANUARY, 1938

If any amateur wishes to make the most of hisaerial system a globe of this kind is essential.

IF reception conditions remain asthey have been for the past few weeks,listeners will be in for a special treat

on New Year's Eve in view of the newschedules for the General Electric In-ternational broadcasters, W2XAD andW2XAF, located in Schenectady. Thesestations will be on the ail continuouslyand simultaneously for 40 hours from2 p.m. Friday, December 31, until 6a.m. Sunday, January 2. Dance bandsfrom coast to coast will be in almostcontinuous use, particularly aroundmidnight on December 31.

W2XAD operates on 19.56 metres,or 15.3 megacycles, while W2XAF uses31.48 metres, or 9.5 megacycles. Thelower wavelength station is suitable forreception up until 9 p.m. or so, afterwhich W2XAF will probably maintaina stronger signal.

I have just received some good newsfrom VK2NO in Australia. It must beremembered that he is the first Austra-lian amateur to transmit a signal intoEngland on 5 metres. He has now,however, been heard in ;Wellington,New Zealand, by Mr. P. A. Morrison,who gave him a QSA4, R5 report.

At the time of this transmissionVK2NO was using an 8JK beam north-west/south-east, and was on automaticC.W. This is the first time that Austra-lian signals have been heard in NewZealand, and it is hoped that beforelong there will be a two-way QSObetween these two countries.

F3CX has written a long letter tome about his W8JK beam. It seemsto be gaining in popularity in France,for just recently several of the stationsI have worked have been loud in itspraises. W8JK, incidentally, has writ-ten me a special article on the beamantenna specially for the benefit of

ANDSHORT-WAVE WORLD

With the AmateursBy G5ZJ

This feature is intended for the transmitting amateur, and dealswith current topics of general interest. Some details regarding

the W8JK beam are given in these notes.

European amateurs, and this includesa number of new type end -fed beamsof original design. All the data isbeing published in the February issue.

To get back to F3CX, since usingthe beam he had had more QSO's withAmerica in a month than he had pre-viously in two years when using a133 ft. Zepp or a 66 ft. Windom. Thebeam in use is the one described in theNovember, 1937, issue, and althoughhis power is only 55 watts, the averagereports are about R8.

The same aerial is being used byF3CX on io metres without any altera-tions whatsoever, and, as on 20 metres,results are highly satisfactory. A sin-gle unit flat -top beam is also in use forZL, VK, PY and LU contacts. Thisaerial works well, for in one month 15ZK's were contacted as compared withfive in the previous year.

G2HK is doing extremely well con-sidering the maximum input averages8 watts, for during the two years hehas been licensed he has worked allcontinents half -a -dozen times on phone,and has just worked his 6oth countryon phone. This is a very good answerto those who say consistent DX is im-possible with low power.

The transmitter consists of a 47 crys-tal oscillator, a 210 doubler and a 210P.A. For a modulator system G2HKuses a single stage pre -amplifier fol-lowed by an ML4 driving a pair ofPX25A's in Class A push-pull. In thisway more than sufficient audio is ob-tained without spoiling quality, andthis probably accounts for the con-sistent DX worked.

This station is mainly used for ex-perimenting with aerial arrays and

during the past two years or so, morethan a dozen different aerials have been.erected. At the moment a horizontal.Johnson "Q" is performing very satis-factorily.

A photograph of the station is in thispage, and from left to right can be seenon the top shelf a 12 -watt modulator,5 -metre transmitter and a 5 -metre re-ceiver. On the bottom shelf, a headamplifier, transverse current micro-phone, 6 -valve communication receiver,pre -selector and monitor. The trans-mitter is link coupled to the aerial coilwhich can be seen on the wall, for thiscompletely eliminates B.C.L. interfer-ence and has proved most effective.

Dorothy Hall, whose station W2IXY,was described in our December issue,has sent me some further informationregarding her activities. In a periodof 12 months operating on 20 -metrephone she has worked 61 countries withan input of 25o watts. Amongst hercontacts has been Australia 156, Africa48, Europe 751, South America 232,North America 466, Asia 3r, making atotal of 686, of which 1,390 were out-side the United States.

She tells me that VO6D, located atNorth West River, Labrador, operates -on a frequency of 14,276 kc., and has aregular schedule with W2IXY everyMonday at 21.45 G.M.T. and Thursdayat 02.30 G.M.T. If any G stationwiould like to contact VO6D pleasecontact W2IXY, who will be glad to,arrange it.

VE2JK is the operator at VO6D,while the power is 500 watts. A petrol -driven engine supplies the neceissaryA.C., and so far this station has doneextremely well despite its limitations._

This is G2HK whooperates mainly on20 -metre phone withan input of 8-10 watts.In this illustrationcan be seen the com-munication receiverand 5 -and 20 -metre

transmitters.

47

_...swiN111111111MMIFEIL

ANDSHORT-WAVE WORLD

ic Metres Australian Schedules

JANUARY, 1938

Incidentally, if any listening station;has sent a QSL card to VO6D theyneed not expect confirmation for ap-,proximately six months, for at the pre-sent time VO6D is isolated from the.rest of the world except for radio.

Here is another note for listeningstations. VK2ME, Sydney, operating

-on 31.28 metres, is on the air from o6.00-to o8.00 G.M.T. and ro.00-14.00 hourseach Sunday. VK3MF, Melbourne,.are radiating every weekday on 31.5'metres from 09.00 to r2.00 G.M.T.,Awhile VK6ME, located at Perth on

For those who still prefer the type 10 triodevalve, this P28/500 Tungsram triode isinterchangeable with the American 10, and

gives amazingly high R.F. output.

31.28 metres, radiates from o9.0o tor.00 G.M.T. every week -day.Although conditions on the ro-metre

band have been rather poor during thepast few weeks, I hear from W6 thatBritish stations are being received quitewell in California around to p.m. al-though so far it has not been consist--ently possible to maintain two-way con-tacts. W6 amateurs are queryingwhether British receivers are at fault orwhether it really is conditions. They.also tell me that the Hungarian sta-tion, HA4A, is being consistently heard'out in America, so I have got in touch

with this station and obtained a wholeload of data on the transmitter andaerial systems used. I intend to pub-lish these details in the February issue,but meanwhile, for those who knowthis station, here are one or two briefnotes.

The transmitter consists of a APP4Ccrystal oscillator, an APP4C sub -am-plifier, two 0-15/400's in push-pulldriving a pair of 0Q71 /i,000's as finalamplifiers with an input of 75o watts.The modulating system consists of acrystal microphone fed into an HP212,followed by an HR2io with twoAPP4C's in push-pull driving a pair of0-75/1.000's in Class AB.

Short-waveClubs

The Edgware Short Wave Societyhave applied for a full transmittinglicence, and to enable their membersto become actual operators, are holdingmorse lessons each week. Full detailsof this Society can be obtained fromthe Hon. Sec., Mr. G. Yale, 4o RaeburnRoad, Edgware.

A new television society Las beenformed at Cambridge, and the firstmeeting had an attendance of over 4o.Full details can be obtained from theHon. Secretary, W. Jones, 115 MiltonRoad, Cambridge.

Michael Hedgeland, the Hon. Secre-tary of the Maidstone Amateur RadioSociety, tells me that the society has amembership of 22, including G8UC andfive A.A. licence holders. They have alarge clubroom, which can be openedon request almost every evening, alsoWednesday, Saturday' and Sundayafternoons. I strongly advise anyreader within a reasonable distance ofMaidstone to get in touch with the Hon.Secretary of this enterprising Societyat 8 Hayle Road, Maidstone.

The Sheffield Short-wave Club,which is now in its fifth year, hasmoved to much larger premises. Quitea considerable amount of equipmenthas been installed so that members canobtain good working knowledge of

48

radio equipment and test gear, whenperhaps otherwise cost would preventthem from owning such equipment. Thesubscription is ten shillings a year, butfull details can be obtained from theHon. Secretary, D. H. Tomlin, 32Moorsyde Avenue, Sheffield, ro.

A receiving contest is being organ-ised by the Secretary of the WirralAmateur Transmitting and Short-waveClub, the President of which is G20A.This contest is, of course, mainly in-tended for club members, but all therules and regulations can be obtainedfrom the Secretary, J. R. Williamson,49 Neville Road, Bromborough, Bir-kenhead.

I notice that several new valves havebeen introduced just recently, in parti-

These new Valpey crystals should make itpossible for every amateur to have a crystalcontrolled transmitter for they cost only15/6d. in a dust -proof holder, or 10/68

unmounted.

cular an RCA -8o9. This is similar tothe popular Teo, being rated for 25watts anode dissipation and is fittedwith ceramic base and top cap anodeconnection. It differs, however, in thefact that it requires a 6.3 -volt heatersupply with a current of 2.5 amps. As aClass C amplifier it requires 750 voltson the anode, minus 200 on the grid,takes a maximum plate anode currentof too mA., and is designed for an anodeinput of 75 watts with a dissipation of25 watts.

The B.E.R.U. Contests takeplace next month, and here aresome of the trophies to bepresented to the winners later

on in the year.

TEUVISIOtiJANUARY, 1938

ANDSHORT-WAVE WORLD

Automatic Gain Control F. E. HendersonByWe consider these notes on automatic volume control as applied to low -frequency amplifiers tobe of the utmost interest to those undertaking public address work and amateur transmission.

CASES often arise in microphonework in which considerable varia-tion in the microphone sound in-

put, such as occasioned by a speaker'svoice, may introduce considerable diffi-culties in intelligibility of reproduction.

In the normal way the gain of themicrophone amplifier is proportionalto the intensity of sound picked up bythe microphone up to the limit ofvolume which the amplifier will handle.With moving coil, or condenser micro-phones, as distinct from the carbontype, in all probability the amplifierwill " blast," that is overloading willoccur before distortion is set up in themicrophone itself, such microphonesbeing capable of handling a wide rangeof input without distress. A suddenloud noise, or sudden increase in inten-sity of the speaker's voice, may, there-fore, give rise to serious " blasting,"and in the opposite sense the droppingof a speaker's voice, or turning away ofthe head from the microphone, maycause remarks at that moment to belost.

Microphone Feed -backThere is another serious difficulty

which often occurs in microphone work.That is the tendency for howl -backwhen operating the microphone andloudspeaker within the same enclosedspace, this usually necessitating a re-duction of amplification to such an ex-tent as to prevent howl -back shouldthe speaker suddenly raise his voice orsome extraneous loud noise be impingedupon the microphone.

A means of compensating for boththe above disabilities is therefore oftenof considerable use, and this can be

flicao-PHONE

effected by means of an Automatic GainControl circuit.

Thel simplest method of automaticgain control is by utilising the sameprinciples as are now commonly em-ployed in radio frequency circuits forwireless reception and known as A.V.C.

An adaptation of such a circuit isquite satisfactory for low frequency am-plification in conjunction with a micro-phone providing the input to the con-trolled valve is of a very small order,which it normally is with a condenseror moving coil microphone.The A.V.C. Circuit

A typical circuit is shown hich usesa variable -mu H.F. pentode as the con-trolled valve, coupled by two stagesof amplification to a diode. The recti-fied voltage from the diode serves tobias back the control grid of the vari-able -mu pentode, the increased biasbeing dependent upon the strength ofthe signal. As increased bias naturallyintroduces some non -linearity into theamplification, the method must only beused with a small input to the valve,but it is found that for ordinary micro-phone work the very slight distortionintroduced is not noticeable in practice,the advantages of the Automatic GainControl completely offsetting thisslight theoretical distortion.

The amount of gain control intro-duced can be varied by means of apotentiometer input to the second stageof amplification.

The output from the complete unit isapplied to the main power amplifier,and the volume control should be ap-plied between the unit and the poweramplifier.

Hi+285.v

0.5.v.Rt15OUTPUT TO MAIN

AMPLIFIER

D4I

Mn

AUTO -GAIN 7cIRCONTROLREGULATOR.

0

HT -

This is the circuit of the A.V.C. low -frequency amplifier. The fact that it 13 mainsoperated is another feature to be considered.

It will normally be found that byincreasing the gain control slight ad-justment requires to be made to themain volume control at the same time.Once set, however, at any level the gaincontrol and volume control should notbe altered for any given set of condi-tions.

It will be found that a small time lagis experienced, depending upon thetime constant of the resistance andcapacity of R, C,. This lag can beadjusted to suit individual circum-stances and the values given are thosesatisfactory for all normal conditions.

When using the unit with 'he com-ponents and valves shown, it will befound that the gain control potentio-meter can be so set as to maintain theoutput from the amplifier sensibly con-stant and that, if the microphone isemployed close to the speaker, the over-all gain can be at a higher level with-out howl -back when using the micro-phone amplifier and loudspeaker in thesame room than would be possible with-out the gain control.

Such a unit can easily be constructedfrom simple components, and will befound very useful for many applica-tions of public address.

A New A.R.R.L. HandbookWE have just received a copy ofthe new 1938 A.R.R.L. Hand-book, which is handled in this

country by F. L. Postlethwaite, G5KA,of 41 Kinfauns Road, Goodmayes, andalso available from Webb's Radio, of14 Soho Street, W.I. This interestingpublication is not primarily Intendedfor beginners but is a complete referencebook for any matter in connection withshort-wave transmission or reception.

Circuits covering almost every type oftransmitter are shown in detail with fullconstructional matter, including suit-able modulating systems. Several chap-ters are devoted to aerials of all kinds,both reception and transmission, andshort-wave and ultra -short wave types.The chapters devoted to receiving andtransmitting valves are the most com-plete guide we have ever seen forAmerican valves, and give every pos-sible detail likely to be required by theconstructing amateur.

The ultra -high frequency section,dealing both with transmitters and re-ceivers, is absolutely up-to-date, includ-ing designs of the very latest type. Wewere also pleased to notice that manyof the designs were inexpensive for theconstructor to build and did not in-clude specialised components that werenot obtainable in this country.

Stocks are available and the price is5s. 6d.

49


Considerations in TransmitterDesign

In this article supplied by the well-known American amateur, WgLIP, is a wealth of valuable informa-tion suitable for both the novice and the experienced amateur.

A'TRANSMITTER which presentsa neat and workmanlike appear-ance is a great source of satisfac-

tion when visitors drop in. Not onlyis neat appearance desired, but stability,in the sense of reliability, dependabilityand consistency, is a very importantrequisite.

be avoided since they usually involvecritical adjustments.

In general, for transmitter use, tri-odes combine low cost, efficient opera-tion, ease of adjustment There is oneexception :- For crystal oscillatorstages, pentodes or valves with pentodecharacteristics have advantages which

A good example of efficient P.A. design using the T-125 triode amplifier.leads and the lay -out chosen.

Transmitters which fail to functionproperly when mains voltage drops andwhose adjustments change as the stageswarm up, are provoking. The maxi-mum amount of operating pleasure canonly be obtained from a transmitter thatworks perfectly every time the switchis thrown.

A dependable and stable transmitteris not difficult to construct ; the use ofgood components and good design arethe major requirements. Each stagemust function properly with no inter-action between it and other stages andshould be free of spurious or parasiticoscillations or regeneration except inthe case of a frequency doubler whereregeneration is important and almostindispensable. The transmitter musthave a sufficient number of stages sothat more excitation is present at thegrid of each valve than is actually re-quired. This point is of particular im-portance yet there should not be morestages than are necessary to accomplisha reasonable surplus of excitation. Itis almost mandatory that trick circuits

Notice the short

dictate their being used for this purposein preference to triodes.

CircuitsBasically, the requirements for a tri-

ode amplifier are, an input circuitcapable of being tuned to the desiredoperating frequency, an output circuitcapable of being tuned to the same fre-quency, a source of R.F. voltage ofopposite phase for neutralisation, andsuitable sources of filament, anode, gridand excitation voltages.

Starting with the grid circuit, theanode circuit of the preceding stage mayalso act as the grid coil for the ampli-fier under consideration. See Fig. i.This is an instance when capacity coup-ling is used. The advantages of thissystem are its low cost and simplicity.If the impedance of the grid circuit isapproximately the same as that of thedriver anode circuit this coupling sys-tem is equally as efficient as any other,at least on the three lowest frequencybands.

On zo metres and higher frequencies

the combined input and output capaci-ties plus the capacities of associatedequipment to ground may be highenough to prohibit the use of enoughinductance in the circuit and the anodeefficiency of the driver stage may be-come poor. If the grid impedance ofthe driven stage is lower than the anodeimpedance of the driver stage maximumefficiency may be obtained by tappingdown on the anode tank at a point whichgives maximum grid drive at minimumanode current to the driver stage. Suchan arrangement will provide maximumpower efficiency, but it is not commonlyused since it generally creates new cir-cuits which invite oscillation at parasiticfrequencies.

When the grid impedance of thedriver stage is sufficiently different fromthe anode impedance of the driven stageto prohibit using the same tuned circuitfor both or where the capacities result-ing from this connection would be toogreat it is common practice to use aseparate tuned circuit for the anode ofthe driver stage and the grid of thedriven stage. The coil may be placedin the field of the driver anode coil orit may be placed out of the direct fieldof the anode coil and coupled througha low impedance line. This lattermethod is usually called link coupling.See Fig. 2.

InductiveCoupling

Inductive coupling in one of its formsis usually of greatest advantage at thehigher frequencies and when workingfrom a single ended stage to a push-pull stage and horn a push-pull stageto a single ended one and when thedriven grid impedance is widely differ-ent from the driver anode impedance.A further advantage is the eliminationof or minimisation of the importance ofR.F. chokes. The disadvantages arehigher cost because of more parts,greater space requirements, and moretuning adjustments and coils to changewhen shifting bands.

Choice of an input circuit will dependupon all of the considerations previouslymentioned and when not duplicating acomplete unit already designed thevarious factors should be weighed care-fully that an intelligent selection maybe made. The present trend towardlink coupling is probably due to thefact that with it best results may beobtained under most any conditionseven though the cost is higher and thecomplexities greater.

1

50

IELEYISJOilJANUARY, 1938

Neutralising Bias Troubles

ANDSHORT-WAVE WORLD

Anode circuit considerations can bewell determined in advance. The im-portant considerations are the L/C ratioto provide maximum unloaded tank im-pedance consistent with minimum har-monic content and if phone is used tohave sufficient capacity in the circuitto provide for linear action under modu-lation.

One of the best low -capacity triodes available.It is the Taylor T-55 with a 55 -watt anode

dissipation.

Neutralisation also ties in with gridcircuit and anode circuit design, exceptin the case of push-pull. With push-pull the fundamental circuit arrange-ment is identical whether the stage is atuned grid, tuned anode oscillator, ora neutralised amplifier. The onlypractical method of neutralising a push-pull stage is the so-called cross neutral-ising method employing two neutral-ising condensers, each connected fromone grid to the anode of the other valve.See Fig. 3.

However, in a single ended stageeither of two methods may be used. Withgrid neutralisation the centre of the gridcoil is at ground R.F. potential. SeeFig. 4. One end of the coil goes tothe grid of the driver valve and theother end to one side of the neutralisingcondenser. Grid neutralisation willusually operate satisfactorily and maybe of advantage in an unbalanced out-put stage.

However, the grid of the valve putsa resistive load across half of the inputcoil resulting in the opposite end of thecoil being other than 18o0 out of phaseif the coupling in the grid coil is lessthan unity. Unity coupling is neverrealised in practice at radio frequenciesbut satisfactory neutralisation mayusually be realised if the turns of thegrid coil are wound as close togetheras possible. If difficulty is experienced

it will usually be at the higherquencies.

Anode neutralisation of a single endedamplifier necessitates operating thecentre of the anode coil at ground R.F.potential. One side of the neutralisingcondenser is connected to the oppositeend of the anode coil to which the plateof the tube is connected and the otherside of the neutralising condenser goesto grid. See Fig. 5.

For maximum power gain with rea-sonable efficiency the bias should beapproximately cut-off and the grid cur-rent the normal recommended value.An amplifier of this type is suited foruse as an intermediate or buffer stageor as the final stage in a C.W. trans-mitter.

Class C operation requires somewhatgreater than twice cut-off bias with nor-mal recommended grid current. Thisstage would be characterised by slightlybetter anode efficiency than the onepreviously described and would be suit-able for intermediate or buffer stage,final stage C.W. or anode modulatedphone.

Another type of Class C operationwould require bias of several times cut-off with normal grid current flowing.With the anode circuit tuned to the samefrequency as the grid circuit and witha high impedance anode tank, highervoltages may be applied and higher

fre-

Fig. 1-The basic anode and following gridcircuit.

anode efficiencies, over 75 per cent, ob-tained. However, the harmonic con-tent will be high and unusual precau-tions to prevent harmonics being radi-ated by the antenna must be takep.This grid bias and excitation conditionis also ideal for the operation of adoubler stage with the anode circuittuned to twice the frequency of the gridcircuit. Reasonable efficiency may beobtained from a doubler if the bias andexcitation are high.Class BLinear

The Class -B linear of grid modu-lated amplifier requires approximatelycut-off bias and little or no grid cur-rent. The unniodulated efficiency willbe low, varying between approximately25 per cent. and 35 per cent. This typeof operation necessitates the use of fixedvoltage bias sources. The voltage must

remain consistent regardless of cur-rent variations. Batteries are probablythe most satisfactory source because thesmaller types may be used and the lifewill be long.

For all other types of operation itmakes little or no difference how thenecessary bias is obtained. The sim-plest and cheapest method is the useof a resistor in the grid return circuit.This arrangement is entirely satisfac-tory in every respect except that no pro-tection is afforded the valves in theevent the excitation fails or is removedwhile the anode voltage is on. Bat-teries are frequently used and they makean excellent bias source from everystandpoint except that of cost, life, andin some cases, size. The same amountof power which would be dissipated ina resistor in the same circuit is dissi-pated in the batteries. This heat driesout the batteries so rapidly where anygreat amount of grid current is presentthat the use of batteries can becomevery expensive.

BiasPacks

Bias supplies of one type or anotherare becoming more common. A dis-cussion of the units would be too in-volved and lengthy for the space avail-able. Let it suffice to say that even the .simplest types should pi ove entirelysatisfactory for R.P. stage bias sinceregulation, except in the case of effi-ciency modulated amplifiers, is not afactor.

Cathode bias also is used. It con-sists of a resistor between filament cen-tre tap and ground. It must be capa-ble of carrying the total grid and anodecurrent and as the anode current isincreased the bias also increases. Ifthe stage is modulated the resistor mustby by-passed with a large condenser inaddition to the R.F. by-pass which isrequired anyway. The principal usefor this type of bias is to protect thevalves in case of excitation failure.Without excitation cut-off bias cannever be reached. The drop across theresistor must be subtracted from thesupply voltage to give the actual anodevoltage.

Fig. 2-A link coupled circuit.

The most common arrangement whichtakes advantage of the best features ofeach type of bias supply is usuallyenough pack or battery bias to provide

5I

"TELEYEJOLIAND

SHORT-WAVE WORLD

Crystal Oscillators BuffersJANUARY, 1938

cut-off with no excitation and the bal-ance by resistors. It is particularlyimportant in a C.W. transmitter to haveall stages following the keyed stagebiased to cut-off. In the phone trans-mitter where the anode voltage is offduring standby periods enough cathode

' bias to limit the anode current to areasonable value plus resistor bias forthe balance makes an excellent arrange-ment.

Grid current should never be per-

mitted to exceed the maximum ratedvalue. If high drive is required ordesired the bias should be increased tothe maximum which will allow normalrecommended grid current to flow. Inthis manner you may increase grid ex-citation without greatly affecting valvelife.

If any type or combination of typesof bias are used they may be measuredby connecting a voltmeter from the fila-ment centre tap to the grid side of thelast source of bias in the circuit. Themeasurement should, of course, be madewith anode voltage applied to the stageand everything operating under normalconditions. If the rectified grid currentand the value of the resistance in thecircuit are known the bias will be theproduct of the two. If any cathodebias is used due allowance should bemade for the anode current through theamount of cathode resistance used.

CrystalOscillator

No triode can provide the versatilecrystal oscillator opet ation obtainable_,with tetrodes and pentodes. In select-ing a crystal oscillator circuit there arefive requirements to be remembered.r. High output on fundamental and

second harmonic frequencies.

2. Minimum number of coils io windand change.

3. Minimum number oftrols.

4. Lowest current through crystal forbest frequency stability and crystalsafety.

5. Lowest cost consistent with maxi-mum performance.

The 6L6G is a very popular tetrodefor a C.O. It fulfils the first, third andfifth requirements perfectly and to a

tuning con -

This is about themosteillelent mannerin which to builda low -loss outputstage. It is suitablefor low and highpower use with anyof the modern low

capacity valves.

greater degree than any other circuit.It has only one tuning adjustment andone coil to change. The tritet, its near-est competitor, has two coils to changeand two tuning adjustments. The onlybad feature, which is the fault of the6L6 and not of the circuit, is that whenworking straight through on the funda-mental frequency with a zo- or 40 -metrecrystal the R.F. current through thecrystal is too high. When doubling inthe oscillator the crystal R.F. currentis extremely low-about to to zo mA.A type 6F6G in the same circuit pro-vides satisfactory fundamental opera-tion with safe R.F. crystal current withall crystals including :0- and 40 -metreones, but when doubling the output isonly a small fraction of that obtainedwith a 6L6G. As a result we use a6L6G with an 8 -prong ceramic basewhen doubling in the oscillator and re-place it with a 6F6G when workingstraight through. Both valves fit thesame socket and no circuit changes arenecessary. Merely place the 6L6G inthe socket when the output of the oscil-lator stage is twice the crystal frequencyand the 6F6G when the oscillator stageoutput frequency is the same as thecrystal. With a '40, 8o, or t6o-metrecrystal working straight through theoscillator output will 5e about 15 watts.

52

With a 20 -metre crystal it will be aboutto to 12 watts. The 40 -metre outputfrom an 8o -metre crystal will be about12 watts and the 20 -metre output froma 4o -metre crystal will be 6 to 9 watts.

While even badly cracked crystalswill operate on their fundamental inthis circuit a good crystal is requiredfor harmonic output. It must not onlybe active but must have only one fre-quency. zo-metre crystals will be verysatisfactory in this circuit for straightthrough operation, but do not providesatisfactory Jo -metre output. Most 20 -metre crystals are 6o -metre crystalsoperating on their 3rd harmonic. Inthis circuit the io-metre output is ap-parently the 6th harmonic of 6o latherthan the second harmonic of 20.

When not oscillating the 6L6 willdraw about 140 mA. With the crystaloscillating and the anode circuit offresonance the current will be about toomA. When the anode circuit is broughtinto resonance on the fundamental orharmonic frequency a very pronounceddip in anode current will be noticed.With the 6F6G, the non -oscillatinganode current will be about 8o mA. andwill dip in the conventional mannernear resonance.

The fact that this crystal circuit is tobe preferred does not mean that it mustbe used. There is probably more dif-ference of opinion concerning which isthe " best " crystal oscillator arrange-ment than there is concerning any otherunit in the transmitter.

The circuit as shown delivers zo to25 mA. of grid current to the followingstage even on 20 from a 40 -metre crys-tal and the circuit used should doequally as well if equivalent results areexpected.

BufferDoublers

A triode such as the Tzo or T55 makesa fine buffer. At the maximum ratingsof 75o volts and 75 mA. input, a T20

Fig. 3-How to neutralise a push-pull stage

will deliver about 40 watts of output.At frequencies lower than 15 mc. wherecircuit losses may be kept at a eason-able value and practically the whole4o watts delivered to the final stage, aT20 will furnish sufficient *grid drivefor an anode modulated final stage withan input of approximately 50o watts or

7:11 oVISION4444olis

JANUARY, 1938AND

SHORT-WAVE WORLD

NOW READYPremier 1938 New enlarged Illustrated p - F- - lijCatalogue, Handbook and Valve Manual !

miERSend 6d. in stamps for 90 pages of Valve Data,Technical Articles, Circuits and Premier 1938 SUPPLY STORESRadio.

A lanufacturers of

POST ORDERS CALLERS TRANSMITTERS, RECEIVERS,Jubilee Works,

167, LowerClayton Rd.,

London, E.5.Amherst 4723

5C, High Street,Clapham, S.W.4

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Central 2933.

MODULATORS,

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10 -watt 'PhoneTransmitter (as illus-

6L6G

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Complete for

"00,

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ready use.

To charge 2 volts at

SHORT

Low -loss interchangeablestandard and other

12-26 metres22-47 metresOther wavebands

S.W. Coil Formers24 in. long 11 in.fitting. Plain or

.

1

WAVE COILS

coils, 4- or 6 -pinfittings.

119 41- 94 metreseach 78-170 metres

up to 2.000 metres from

as used in above coils,diameter, 4-, 6- or 7 -pinthreaded, 1/- each.

Oscillator Doubler.902 Pentode Amplifierwith Suppressor GridModulation. Pricecomplete with 4 Tubes

and Crystal. £15.

25 -watt 'PhoneTransmitter. 6L6Gexciter RegenerativeCrystal OscillatorDoubler. 801 TriodeAmplifier. Grid neu-tralised arid and anodemeters. Price completewith 6 Tubes and

Crystal, £21.

10 watt A.C./D C C.W.rans.' 'mitter. Pentode Crystal Oscillator.Low crystal current. Minimumfrequency drift. Price, completewith Valves, Crystal and Key, £4 4s

.1 amp., 10/. ; 6 wigsat '., amp., 16'6 ; 6

volts at 1 amp., 19'6 ;12 volts at 1 amp.,'41 - ; 6 volts at 2

amps., 32 6.

'TWO NEW MODELScompletely enclosed.To charge 6 volts at 4amps., 13,1916 Tocharge 6 or 12 volts at2 amps., £3/19/6.

v.

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TROLITUL insulation. Certified superiorto ceramic. All -brass construction. Easilyganged.

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AMERICAN VALVESWe hold the largest stocks of U.S.A. tubes in this country andare sole British Distributors for TRIAD High -Grade Ameri-can Pelves. All Conceivable typestypes in stock. Standard types,

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53

TagYS/011AND

SHORT-WAVE WORLD JANUARY. 1938

Doublers S. L/C Ratios

less. For C.W. only the Tzo will prob-ably drive a 70o -watt stage. No defi-nite statement on this subject can bemade because the amount of grid driverequired can vary over wide limits indifferent transmitters.

A T20 as a straight buffer on tometres may be expected to adequatelydrive a 25o to goo -watt final stage.

As a doubler to to from zo the effi-ciency will vary greatly with the ex-citation, the bias and the L/C -ratio, but15 watts of output should be obtainedunder average conditions without anycolour showing on the anode. This

Fig. 4-The grid neutralising system.

excitation would permit an input ofabout t so watts to the final stage.

For those applications where the Tzodoes not have sufficient output a T55may be substituted. The grid drivefrom the oscillatorfor full rated input to the T55. 150 to170 watts of output may be obtainedwhich will be far more excitation thanis required for 1 -kW final stage. Nochanges need be made in the circuitwhen replacing the Tzo with a T55except to use components with adequatevoltage ratings so that breakdowns willnot be experienced if higher voltagesare applied to the T55 than to the Tzo.

The first two stages, a 6L6 and Tzoor T55, will make an excellent z -stageall -band transmitter, and would beideal for portable operation. The ex-citation on all bands from 40 to 160 issufficient for anode modulation withfull input. On zo the excitation is ade-quate for anode modulation with tooto 150 watts of input. If operating 20 -

metre phone with two stages a 40 -metrecrystal should be used. The 6L6 oper-ates as an oscillator and doubler andapparently the isolation on zo is ade-quate to keep frequency modulation toa negligible amount.

DoublersA doubler is used to obtain an output

frequency of twice the input frequency,and one or more of these stages areusually necessary when working on thehigher frequency bands. Because theanode circuit is not tuned to the samefrequency as the grid circuit, neutralisa-tion is not necessary to prevent oscilla-tion. However, the efficiency of adoubler may be improved greatly ifregeneration is added. The circuit for

a regenerative doubler is exactly thesame as for a neutralised amplifier sothe same stage may be used for eitherpurpose by merely changing the anodecoil. The neutralising circuit providesregeneration when doubling.

As a doubler the anode efficiency ofa stage will be much less than whenworking as a straight amplifier. Con-sequently for a given anode dissipationthe output must he much less. Moregrid drive is required also so the powergain will be less. The grid bias shouldbe several times that for straightthrough operation. The L/C ratioshould be as high as possible.

If a stage is used for both straightthrough and doubler operation it maybe neutralised and the neutralising con-denser setting left the same for doubleroperation. if it is used for doubleroperation only, the neutralising con-denser adjustment should be for maxi-mum efficiency without oscillation.

InsulationWhen operating on the lower fre-

quencies only insulation is no greatproblem. Below about 8-mc. bakeliteor most any of the various commonlyused types of insulation are sufficientlygood. About the only important consideration is that it be able to stand thevoltages involved without breakingdown.

Above 8-mc. the situation is muchdifferent and at 8- and 56-mc. only thevery best insulation is good enough.(At these higher frequencies the amountof grid drive required by a valve doesnot increase greatly but circuit lossesincrease tremendously making valvesappear to require more grid drive.) Ifsufficient grid drive is to be had at thehigher frequencies and )if reasonableefficiency is to be experienced insula-tion should be used only where neces-sary and it should be the best available.

Sockets and condenser insulationshould be of ceramic or Mycalex.Wherever possible the inductancesshould be self-supporting with noforeign objects in their field.

AntennaCoupling

The method of coupling the trans-mitter to the antenna will depend uponthe type of feeder system used and thecharacteristics of the antenna coil willdepend upon the impedance of thefeeders at the transmitter. Becausethere are so many variables I cannotprovide any quantitive data. In thecase of untuned transmission lines thecorrect number of turns in the pick-upcoil will be the number which will loadthe final to the desired input. Withtuned feeders the characteristics of theantenna coil will depend upon theamount of inductance necessary to tunethe antenna system to resonance. With

single -wire fed or end -fed antenna sys-tems, a separately tuned circuit coupledto the final tank is probably advisablebecause of the lack of harmonic dis-crimination with these antennas. If theL/C ratio in the final stage is lowenough for reasonable harmonic sup-pression the L/C ratio of the separ-ately tuned coil may be very high.

Any pick-up coil coupled to a tankcircuit should be placed at the point ofminimum R.F. voltage to minimisecapacity coupling. In the case of abalanced output tank as with push-pullor an anode neutralised single endedstage the coupling coil should be at thecentre of the output tank. With a gridneutralised single ended final stage thecoupling coil should be at the cold endof the tank coil.

A Faraday screen between tank andpick-up coil is usually very helpful inpreventing even harmonic radiation,but sometimes presents mechanical diffi-culties if the coils are changed formultiband operation. Grounding thecentre of the pick-up coil to the final

Fig. 5-The usual anode neutralising method.

stage filament or ground circuit isusually equally effective and far moresimple mechanically.

. With transmission lines it is legal totap the feeders, single cr double wire,directly on to the final tank, but this islikely to result in high mutual imped-ance between anode circuit and radiat-ing system at the harmonic frequency.It is almost impossible for this condi-tion to exist with a two -wire iine anda pick-up coil whose centre is groundedso it would be wise to avoid direct coup-ling and to use a two -wire line to keepharmonic radiation at a minimum.

L/CRatios

The tank circuits are worth carefulconsideration for they greatly influencethe operation of the transmitter. Asfar as valves are concerned the induct-ance and capacity in the circuit whentuned to resonance are a resistance, asshown in Fig. 6. With no load coupledto the tank, the impedance (A.C. re-sistance) should be high and its value

Lwill be proportional to - where L is

CRthe amount of inductance, C the amountof capacity, and R the resistance. When

54

Filament Transformers, singlewinding. Up to 30 watts. 12/6Twin Winding, up to 30 watts.

15/6Triple Winding, up to 40 watts.

I9/ -High Voltage Transformers.750-0-750 at 200 mils, Inter-leaved. a 13s. Od.1,000-0-1,000 at 200 mils.

E3 I5s. Od.1,000-0-1,000 at 350 mils.

a 10s. Od.1,000-0.1,000 at 500 mils.

f5 Os. Od.Swinging Chokes.5 to 25H at 100 m.a. 15/65 to 25 H at 200 m.a. 21/.Filter Chokes.20 Henry at 120 m.a. 15/620 Henry at 200 MA. 28/ -Other sizes to order.

CLEARANCE OFFERSMu -Metal Microphone Trans-formers (cost 55/- each). 7/6Mu -Metal Smoothing Chokes.5.2 H, 120 m.a. 3/64 -In. Transmitter Bakelite Dials.}-In. spindle. 6d. each

EDDYSTONE ALL -WORLD TWO, completewith valves, calibrationchart, ready for use.

Price E3 17s. 6d.Aerovox 2 mfd. 2,000 v.WKG. 9/6

G6VA

E. J. PICKARD

JANUARY, 1938

G2NO

H. R. ADAMS

SELECTED ITEMS FROM

OUR LIST

MICROPHONESShure. 701A. Grill type. 84/-Astatic D104 75/-Webb's Transverse Carbon,with table stand. 25/ -With floor stand. 35/-

VALPEY CRYSTALS7 Mc. mounted.Calibrated to + or - .1 percent. 15/6Unmounted, as above. 10/6

DUMONT CATHODERAY OSCILLOSCOPE.Model 164. 3 -In. tube.Vertical and Horizontal Ampli-fiers. Amplified sweep.Internal Power Pack.

PrIce complete 18 guineas913 I -In. Cathode Ray Tube.

37/6

ACORNS. Price reduced.Type 955 Triode. 25/6Types 954, 956 Pentodes. 36/.

RAYTHEON TUBESRK-34 Duplex Transmit-ting Triode. 22/6RK-39 Beam Power TX 22/6RK-47 (Beam Power RK-20).

105/-100-TH, New Elmac. 95/-RK-20A replacing RK-20. 95/-RK-25 and RK-23. 27/6

TAYLORTaylor T.I25. L4 Ss. OdTaylor T.20. 17/6Taylor TZ.20. 17/6Taylor 866 Junior. 7/6Taylor 866 11/6Taylor T.55 45/- TED McELROY, WORLD'S CHAMPION TELEGRAPHIST, RELEASES

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MAC OSCILLATOR.FOR PRACTICE. PRODUCES A 1,000 CYCLE NOTE 10/KEY AND OSCILLATOR COMBINED - 20/-

1938 McELROY BUG KEY 38/.

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ANDSHORT-WAVE WORLD

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COMMUNICATIONTYPERECEIVERS

RME-69. 1938.

The Amateur's finest receiverPrice E38 Os. Od.

RME-69-LS1.

Incorporating the famous Lamb.Noise Silencer unit.Price £41 10s. Od.

RME-DB.20.Two -Stage H.F. Pre -Selector.9 to 550 metres.Built-in Power Pack (230 v.).

Price E12 10s. Od.

1938 HALLICRAFTER.The New Super Sky RiderCrystal Gate " R " Meter.1,000 deg. Band Spread.5 to 550 metres.Price 230-v. model f32 Os. Od.

1938 SKY CHALLENGER9 tubes.Fully calibrated.9 to 550 metres.Crystal Gate.Price 230-v. model f25 Os. Od.

1938 SKY CHIEF.7 Tubes (I R.F.).Built -In Speaker.

Price complete E12 10s. Od.

THE NEW SKY BUDDY.5 Tubes.17 to 550 metres.Built-in Speaker.

Price complete £9 Os. Od.

NATIONAL.National " One -Tee "The finest U.H.F. Receiver.NOTE REDUCED PRICE.Complete with Tubes.

E18 10s. Od.Power Pack a 10s. Od.National H.R.O. Senior

Price complete £49 I5s. Od.National H.R.O. Junior.

Price complete E39 I5s. Od.

NEW NATIONAL 80XDelivery Is expected JanuaryI st.A.C./D.C. Operation.

Price £23 Os. Od.

HARVEY TRANSMITTERSUHX-1010 to 20 watts R.F.5 to 160 metres.Crystal Control.Price Transmitter complete2 Band Operation E19 Os. Od.Power Pack f6 10s. Od.

HARVEY 80TComplete for two -band opera-tion. £63 17s. Od.

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Price complete £110 Os. Od.

WRITE FOR LIST ANDDETAILED HIRE-PURCHASE FACILITIES

WIJYN

TED McELROY

55

T LgYIS:1011AND

SHORT-WAVEWORLD

Efficient Valve Workinga load is coupled to the tank the situa-tion, as shown in Fig. 7, is obtained.Rp, is the unloaded impedance of thetank circuit. Rp, is the load impedancereflected across Rp and the power de-veloped across Rp, will be delivered tothe load or antenna. It is useful out-put. However, the power developedacross Rp, is power wasted in the tankcircuit and shows up in the form ofheat. It may easily be seen that as theratio of Rp, to Rp, is raised less powerwill be wasted and more will be de-livered to the load. Because Rp, is theunloaded tank impedance it can be seenwhy, as the impedance of the tank cir-cuit is raised, less power will be wastedin the tank and more delivered to theload. As shown from the formulaL/CR the impedance may be increased

HT 0-Fig. 6-The anode inductance and capacitycan be considered as a pure resistance in this

form.

By increasing the amount of L and re-ducing the amount of C.

If efficient coils and condensers areused it probably would not be practicalto attempt to reduce R, but if C. werereduced to half its former value, Lwould have to be doubled to hit reson-ance at the same frequency so the im-pedance would have increased to fourtimes its former value. Actually, itwould not be four times because Rwould increase slightly with an in-crease in L, *but the gain in impedancewould approach four.

From the foregoing it would seemadvisable to use as much inductanceand as little condenser as possible.From an anode efficiency standpointonly this is more or less true, however,after a certain value of tank impedanceis reached the efficiency increases onlyvery slightly, yet the driver power re-quired continues to increase and from%n over-all efficiency standpoint theremight be no improvement.

However, increasing the L/C ratioleads to one disadvantage for C.Woperation and an additional disadvan-tage for phone operation.

As the L/C ratio is increased the har-monic content is also increased. Ifthese harmonics reach the antenna, andit is sometimes difficult to prevent themfrom doing so, they will be radiatedand may cause interference to otherservices. Consequently it is necessaryto select an L/C ratio which must neces-sarily be a compromise between anodeefficiency and harmonic content. For-

tunately, a ratio may be selected whichdoes not result in appreciably lowerefficiency yet the harmonic content iskept at a reasonably low value. Infact measurements seem to indicate thatexcessively high L/C ratios do not re-sult in increased fundamental or usefuloutput. The increase seems to be com-posed entirely of harmonics which arenot useful and are to be avoided.

For phone operation a certain amountof capacity is necessary if the amplifieris to be linear. Less than this mini-mum of capacity will result in distor-tion and carrier shift. This nmount ofcapacity is usually somewhat greaterthan the amount required to reduceharmonics to a satisfactory value andfor this reason different L/C ratios havesometimes been specified for C.W. andphone operation.

However, the permissible L/C ratiofor phone is high enough that increas-ing it to permissible C.W. value forthat type of operation results in so smallan increase in efficiency that it seemsdesirable to specify L/C ratios for allforms of operation which are capableof linear phone operation to permitmodulation if desired and to obtaingreater harmonic suppression. On theother hand, if only C.W. operation iscontemplated the L/C ratio may bequadrupled by using about half as muchcapacity and twice as much inductanceas would be required for phone. Twiceas much indurtance may be obtainedwith about 41 per cent. more turns..

Correct L/C ratios become the great-est problems cn the highest and lowestfrequencies when all -band operation isdesired. For example, any condenserwith enough capacity to provide a rea-sonable ratio on 16o metres would un-doubtedly have a minimum capacity sohigh that efficient operation would beimpossible en metres, whereas a con-denser with suitable capacities for tometres would have so low a maximumcapacity that poor linearity and highharmonic content must necessarily bepresent on 16o metres even though theefficiency would be good on both bands.Probably the most satisfactory answerto this problem would be to build thecircuit for the highest frequency tobe used and connect another condenserin parallel with the H.F.tuning con-denser for low frequency operation.

If the L/C ratios are correct it ispossible to obtain maximum funda-mental frequency output with minimumharmonic content and minimum distor-tion if modulated. If the L/C ratiosare not correct one or more of thesedesirable characteristics will be lost.

In general, if the L/C ratios are rightthe minimum anode current with noload coupled to the output circuit willbe about to to 20 per cent. of the loadedvalue. Minimum anode currents inexcess of 25 per cent. of- the loaded

56

JANUARY, 1938

value are usually an indication thatcircuit losses are higher than theyshould be. However, tank circuit lossesdrop rapidly as the loading is in-creased so minimum anode currents,unless greatly excessive, need not betaken too seriously.

Optimum L/C ratios are not directlya function of the type of valve or valvesused. The factors used in the calcula-tions are anode voltage, anode current,frequency, and the type of operation.

Valve InstallationTo obtain efficient performance from

any radio circuits where valves areused great care should be exercised inthe installation of valves in these cir-cuits. The heart of a vacuum valve isits filameint. Improper operation ofthe filament will shorten its life.

Although small variations in filamentvoltage are compensated for in valvedesign, most satisfactory results areobtained when filaments are operatedat their rated voltage. Lower voltagelimits the electron emission of the fila-ment and generally results in the over-heating of the valve, while higher volt-age will rapidly dissipate the supply ofthorium in the filament.

Use sockets with large sweeping con-tacts. Poor contact between socketsprings and valve prongs will cause adrop in filament voltage. Heavy, well -soldered leads are very essential. Lightvalve at rated filament voltage for tenminutes before applying anode voltagefor the first time. Preheating of fila-ment after first installation is notnecessary.

The ground return should be con-nected to centre tap of the filament.Where the ground is returned to oneside of the filament, connections shouldbe reversed at intervals of too hours.Using one side of the filament for

0 HT

Fig. 7-A loaded anode circuit should be con-sidered in this way.

ground return causes the opposite sideof the filament to function harder.Where D.C. is used on filaments, con-nect grid and anode returns to nega-tive side of the filament.

All connections in the anode tank cir-cuit should be heavy enough to standthe circulating R.F. current. At fre-quencies higher than 14,000 kc. boltedconnections are recommended as theheat at these frequencies will meltordinary solder.

'TELE YK1011JANUARY, 1938

We

know

you won't

believe us!

There's nothing easier than for us to sayour condensers and resistances are thebest in the world and the only ones youshould consider. The trouble is-wouldyou believe it

Only two things can possibly convince you.You can either try Dubilier componentsand discover for yourself how reliablethey are-or you can ask any other goodconstructors for their own experience ofDubilier. We have confidence in the resultof either test.

DUBILIERCONDENSERS

DUBILIER CONDENSER CO. (1925) LTD.,Ducon Works, Victoria Rd., North Acton, London, W.3

. R. Casson 19

ANDSHORT-WAVE WORLD

STANDARD BOOKS ONTELEVISION AND WIRELESSTELEVISION : THEORY AND PRACTICEByJ. H. REYNER, B.Sc. (Hons.), A.M.I.E.E., M.Inst. R.E.CONTENTS . . . Parc I. RECEIVING TECHNIQUE. What is Tele-vision f Simple Mechanical Systems. The Eye in Television. TheCathode Ray Tube. Time Base Circuits. Cathode Ray Reception. HighDefinition Mechanical Systems. The Television Receiver. Ultra -ShortWaves. Part II. TRANSMITTING TECHNIQUE. Photo Cells. TheTelevision Transmitter. Film Transmission. Velocity Modulation.Colour Television. Appendix.Second Edition, Revised and enlarged. Demy 8vo., 235 pages, 126illustrations, including 24 half -tone plates. 12s. 6d. net.

TELEVISION RECEPTIONConstruction and Operation of a Cathode Ray Tube Receiver forthe Reception of Ultra Short -Wave Television Broadcasting

By MANFRED Von ARDEN NE.Translated by 0. S. PUCKLE, A.M.I.E.E.,

Research Department A.G. Cossor, Ltd.

" Coming at the present time and from the pen of such a well-knownauthority as Von Ardenne, this book supplies a real need, particularlyas ithas been adapted to English requirements to a very large extent."-Television and Short -Wave World.Demy 8vo., 140 pages, 96 illustrations, including 44 half -tone plates.

10s. 6d. net,

TELEVISION CYCLOPAEDIABy ALFRED T. WITTS, A.M.I.E.E.

" As its title Implies, this book contains definitions of terms commonlyused in television practice and thus naturally includes expressions whichoriginated in the fields of wireless and optics. The author wisely does notconfine himself merely to definitions, and except in the simplest casesthey are amplified by some description of the mode of operation of thecircuit or apparatus."-The Wireless World.

Demy 8vo., 151 pages, 97 illustrations. 7s. 6d. net.

THE LOW VOLTAGE CATHODE RAYTUBE AND ITS APPLICATIONS

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Dewy 8vo., 187 pages, 76 plates and figures 10s. 6d. net.

SHORT - WAVE WIRELESSCOMMUNICATION

By A. W. LADNER, A.M.Inst.C.E.Superintendent of Instruction, Marconi's Wireless Telegraph Co., Lcd.,

andC. R. STONER, B.Sc. (Eng.), A.M.I.E.E.,

Lecturer at Queen Mary College, London." This book is the best yet published on short-wave working, and shouldhave a wide welcome from all interested in the technical aspects ofwireless communication."-The Electrical Review.Third edition, revised and enlarged. Demy 8vo., 469 pages, II plates and

248 figures. 21s. net.

RADIO INTERFERENCE AND ITS SUP-PRESSION

By J. H. REYNER, B.Sc. (Hons.), A.M I.E.E., M.Inst.R.E.Dewy 8vo., 138 pages, 10 plates and 55 figures. 9s. 6d. net.

CATHODE RAY OSCILLOGRAPHYBy J. T. MacGREGOR-M ORRIS, M.I.E.E.,

University Professor of Electrical Engineering, Queen Mary College,London,

and J. A. HENLEY, M.Sc.(Eng.).Demy 8vo., 262 pages, 151 figures, including 18 plates, 21s. net

A List of Standard Books on Wireless and Television,may be had post free, on application

CHAPMAN & HALL LTD.I I, HENRIETTA STREET, LONDON, W.C.2

57


A Precision Oscilloscopefor

ExperimentersN0 matter how carefully the wir-

ing has been carried out, no at-tempt should be made to connect

up the cathode-ray tube to the chassisuntil careful tests have shown thateverything is in order. No transmit-ting amateur would think of switchingon until he was certain that all connec-tions were properly made and the samecaution applies to any high voltage testequipment although the actual poweremployed is small.

ConstructionalChecks

Before fitting the tube in place,therefore, the chassis should be turnedupside down and the whole of the wir-ing carefully inspected to make surethat there are no loose bits of solderor wire snippings left under the groupboards. Each wire, whether well pro-tected by systoflex or not, 'should standclear of its neighbour and of thechassis. Where the wires pass throughthe chassis they should be insulatedwith a double thickness of sleeving andeven if the voltage is low it is best tobe on the safe side. An inspection ofthe wiring diagram of Fig. i will showthe high voltage points and particularattention should be paid to these. Forexample: R 1 1, where the lead passesthrough the chassis; R16, ditto; thelead from L2 through the chassis tothe group boards.

These precautions may seem elemen-tary to the experienced amateur, butit is surprising how even the old hand

This month's article contains instructionsfor the preliminary tests to be applied tothe oscilloscope before it is finally finishedoff. It is regretted that space does notallow the completion of the instrument tobe described this month, but readers' whohave any difficulty are invited to write

to us.

may overlook an obvious fault, espe-cially if his attention has been dis-tracted during wiring up !

For final checking an electrostaticvoltmeter is almost essential. If thisis not available a high resistance volt-meter can be used with sufficient seriesresistance to enable it to read 1,500volts. The H.T. supply to the tubemay be taken on trust if no electrostaticvoltmeter is available, as the load ofeven a high resistance voltmeter willalter the voltage across the tappings.

To commence the detailed tests, re-move all the valves from their sockets,remove all connections to the tube tem-porarily, and switch on the transform-ers by themselves. If there is any faultin the wiring to the rectifiers, or anaccidental short-circuit, a loud humwill lie heard which should be the sig-nal for immediate switching off and acareful search for the trouble.

Timed DelaySwitching

If the transformers are working cor-rectly, replace the tube H.T. rectifierin its socket and switch on again. Note

that the switching on the chassis is soarranged that the rectifier filaments canbe run up to operating temperaturebefore the H.T. is applied.

This is particularly desirable in thecase of mercury vapour rectifiers feed-ing an appreciable current, althoughin the case of tube supply units it cansometimes he dispensed with.

On switching on the H.T. the rectifier should flash blue as the "ondensercharges and then the glow should diedown to a faint haze. Excessive glowindicates abnormal current due toleaky condensers or a fault in the poten-tiometer chain.

For those with electrostatic volt-meters the following are the approxi-mate readings across the potentiometertappings : With one end of the meterconnected to the cathode,

Between A, and cathode: 350-400volts in max. position.

Between A, and cathode : 3,30o volts.Bias voltage between grid and cath-

ode : 70 max.The above values are, of course, ap-

proximate as they depend cn the trans-former voltage, but they will serve asa guide to any abnormal conditions. Ifno electrostatic voltmeter is availablea milliammeter connected in the mainH.T. lead to the chain (remember thecondensers may be charged !) shouldread between e.7 and 0.9 mA. If at alater date the H.T. voltage requiresreducing to obtain, greater sensitivitythe resistance R6 in the circuit can beincreased.

R

L2

C30030MEI

Cs

v3

3

R Tc6Rio

0 Xi

ii---0 Yi

C20

Constructors making this oscilloscope will have a ready means always at handbf conducting tests of all kinds with the highest accuracy.

58

JANUARY, 1938AND

SHORT-WAVE WORLD

Selected and Specified for the

VrownSUPERIOR HEADPHONES

ANY RESISTANCE

All instruments of quality must bear a price in proportion to the genius andtime attending their invention, and the craftsmanship and labour involvedin their manufacture.

Such instruments do not come into being by chance, neither can they be manu-factured at small expense, and although the price may appear heavy at first,they are, when their efficiency and value is justly estimated, the cheapest.Instruments constructed for cheapness, and not for efficiency and excellenceof workmanship, are the frequent and certain cause of inferior Radioreception.

Will YOU weigh the certainty of the highest degree of SENSITIVITY, FRE-QUENCY RESPONSE, QUALITY of REPRODUCTION and absolute RELIA-BILITY for many years, against the outlay of a few shillings more ?

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Engineers' GuidEShows how to qualify in Television, SoundRecording, Radio Engineering and Ser-vicing, Wireless Communications, etc.,by studying at home with The T.I.G.B.Write to -day for this Great Guide containingworld's widest choice of engineering coursesand which alone gives the Regulations forA.M.I.E.E., A.M.I.R.E., A.M.I.T.E., etc.

THE TECHNOLOGICALINSTITUTE OF GREAT BRITAIN160 Temple Bar House, London, E.C,4

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Explains over a thousand terms

TELEVISIONTECHNICAL TERMSAND DEFINITIONSBy E. J. G. LEWIS-108 pages 5,"- net (by post 5/3)'Here in one volume is easily acces-

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-WIRELESS WORLD

" The definitions are concise and

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59

Vag YISTOnAND

SHORT-WAVE WORLD

Assuming the H.T chain to be satis-factory, switch off and discharge thecondensers by shorting them withioo,000 ohms held in insulated pliers.It is bad practice to short them "dead"and it is safer to use a medium resist-ance rather than to rely on the H.T.resistance chain to discharge them.Actually, if this is used and the con-

densers are allowed to discharge natur-ally, the potential at the end of 5 secsis still 1,200 volts !

Before testing out the time basevalves it is convenient to disconnectthe H.T. feed to the group boards GIand G2 (last month's diagram, p. 76o).Should there be a fault present, it iseasier to localise it if the amplifiers

This is the sub-ehassis completelywired with allcomponents inposition. Carefullynotice wires have

been screened.

Components forA PRECISION OSCILLOSCOPE FOR EXPERIMENTERS

CHASSIS.r-Double Steel Chassis to specification finished

black (A.P.A ).Expanded metal side shields (optional).CONDENSERS, FIXED.Cz--a-mfd. r.000 volt working type 95oA (Du-

bilier).C2-4-Mfd. ;000 volt working type 951B (Du-

bilier).C3-4-mfd. r,000 volt working type 95IB (Du -

biller).C4-a-mfd. 4,000 volt working type 951C (Du-

bilier).4,000 volt working type 951C (Du-

bilier).C6-.5-mfd. type 46o8/S (Dubilier).C7--.x-mfd. 4,000 volt working type 951C

(Dubilier).C8-.x-mfd. B77o goo volt working, or-mfd.

.005 mfd..00x-mfd. oo5-mfd. type 6^o (Du-bilier).

C9-.o05-mfd. .00x-mfd. .0002-mfd. .000z-mfd..00005-mfd. type 67o (Dubilier).

Cro-.5-mfd. type 460/S (Dubilier).Cxx-a-mfd. x,000 volt 95o working (Dubilier).C22-4-mfd. type 95o x,000 volt (Dubilier).Cx3-.x-mfd. Loon volt working type H.V.T.P.

(Dubilier).Cr4-50-mfd. I2 volt working type 3016 (Du-

bilier).

Cx5-x-mfd. x,000 volt working type 95oA(Dubilier).

C15-i-mfd. x,000 volt working type 95oA(Dubilier).

Cx6-.1-mfd. r,000 volt working type H.V.T.P.(Dubilier).

C17--.x-mfd. x,000 volt working type H.V.T.P.(Dubilier).

Cx8-50-mfd. r2 volt working type 3016 (Du-bilier).

C19-.2-mfd. r,000 volt working type H.V.T.P.(Dubilier).

CONDENSERS, PRE-SET.C2o-14.5-mmfd. type SW54 (Bulgin).CHOICES, LOW -FREQUENCY.2-3o h.5o M!A. (Lx, L2), type 3o H (Sound Sales)

HOLDERS, FUSE.1-Type 1054 (Bulgin).HOLDERS, VALVE.4 -7 -pin ceramic chassis type less terminals

(CliX).2 -4 -pin ceramic chassis type less terminals

(Clix).

PLUGS AND TERMINALS6-Top connectors type P92 (Bulgin).2-Terminals type B marked " Mod." (Belling -

Lee).

1-Terminal, type B marked " Earth." (Belling -Lee).

2-Terminals type B marked " Input." (Belling -Lee).

2-Terminals type B:nsarked ".,output " (Belling -Lee).

2-Terminals type B marked " XI," " l'i,"(Belling -Lee).

2-Terminals type B unmarked (Belling -Lee).RESISTANCES, FIXED AND VARIABLE.Rr-roo,000-ohm x watt (Erie).R2-xoo,000-ohm z -watt (Erie).R3-2-megohm and x.2-megohm 2 -watt (Erie).R4-.5-megohm potentiometer (Reliance).R5-.I-megohm potentiometer (Reliance).R6-2oo,000-ohm type x -watt (Erie).R7-.5-megohm potentiometer (Dubilier).R8-.5-megohm potentiometer (Dubilier).R9-loo-000-ohms 1 -watt (Erie).Rio-2-megohm 1 -watt (Erie).Rrx-.5-megohm I -watt (Erie).Rx2-2-megohm potentiometer (Reliance).R13 -5o,000 -ohm type r watt (Erie).12/4 -5o,000 -ohm type 1 -watt (Erie).Rx5-2oo,000-ohm 2 -watt Erie).R16 -25o,000 -ohm 2 -watt ( rie).R17-xoo,000-ohm potentiometer (Reliance).RI8-2-megohm 1 -watt (Erie).Rx9-2-megohm 1 -watt (Erie).Rao-a-megohm 1 -watt (Erie).R2I-tooOhras r -watt (Erie).R22-.5-megohm 2 -watt (Erie).R23 -5o,000 -ohm 2 -watt (Erie).R24 -5o,000 -ohm 2 -watt (Erie).R25-2-megohm 1 -watt (Erie).R26--2oo-ohm I -watt (Erie).R27-.5 megohm 2 -watt (Erie).R28 -5o,000 -ohm 2 -watt (Erie).R29 -5o,000 -ohm 2 -watt (Erie).SUNDRIES.5 -5 -way group boards type C3x (Bulgin).Paxolin Panel to Specification (Bulgin).SWITCHES.1-Ganged 5 -way Rotary switch 1039K (F. W.

Lechner).i-Ganged 3 -way Rotary switch 1039K (F. W.

Lechner.)2-S8oT switches (Bulgin).TRANSFORMERS.5-4 V. 4a.-2 v. 2a to specification (Tr) (Premier).-2 V. 2a.

1-1000-0-3500 v. to specification (T2) (Premier)1-2 V. 2a. to specification (T3) (Premier).VALVES.z-M.U.2. (Mazda).2-AC/S2PEN (Mazda).2-AC/S2P3 (Mazda).a-Type 5H C.R. Tube with socket (Ediswan).

JANUARY, 1938

are temporarily out of service. Do notinsert the time base valves, but put inthe H.T. rectifier and after switchingon note the current taken.

By a printer's error in the issue ofNovember, 1937, the value of R16, thescreen resistance was given as 25,00oohms. This should have been 250,000ohms, and if the alteration is not madeexcessive current will be taken by thepotential chain. Normally, this shouldbe between 5 and 8 m.1. At this stagethe resistances and condensers can bechecked for leakage. No currentshould flow thiough RI 1, R12 and C8,nor should there be any signs of spark-ing or " sizzling " when the full H.T.is applied to the condensers..

If all is in order, plug in the valvesand note the current taken by both,which should not exceed 25 mA. Thecurrent may be altered by changingR13 and R15, but these are best leftuntil the final running of the time base.

The amplifier valves are the last tobe checked and these should consume5 mA. max. with an anode voltage of300 approximately. When checkingthese the grid leaks should be con-nected (.R2o and R25 in the diagram)as a free grid will upset the readings.

When using the amplifier, it mustbe remembered that the value of theleak is the effective resistance of theinput circuit and this is 2 megs. in thecircuit given. Under certain circum-stances this may be increased, but notif there is any tendency to grid currentin the valve. A simple test for gridcurrent is to short-circuit the leak andnote if the anode current changes ap-preciably. The final adjustments tothe time base and the calibration ofthe amplifier will be described nextmonth.

" A 5 -band Maine -operatedMonitor."

(Continued from page 42.)mounted near to the transmitter maxi-mum radiation is more often indicatedby minimum current reading, showingthat the R.F. is actually going into theaerial instead of being radiated fromthe tank circuit.

With the meter shunted and with along aerial connected, we have beenable to work a small loudspeaker fromthis monitor, but the suggestion is notto be recommended in view of the factthat feed -back is almost bound to occurwith the average type of microphoneused in amateur radio.

Operation on 5 metres is possible butnot with the coil former specified. Forthose who are more interested in 5 -

metre than in some of the other amateurbands covered by this meter, shouldobtain a i in. length of f in. former andwind three turns on it and connect it inseries with the 10 -metre winding, andearth. On the other hand, this arrange-ment could be used to cover six bandsif a 6 -point multi -switch is obtainable.

60

JANUARY, 1938IgloVISIOfi

ANDSHORT-WAVE WORLD

Choosing the Link BetweenCrystal and Amplifier

The problem of connecting a crystal unit to a main amplifier is fully discussed inthis article by C. K. Gravley of The Brush Development Co.

B- EF ORE the announcement of thefirst commercial crystal micro-phone in 1931, the radio or elec-

tronic engineer had little reason to con-cern himself with the transmission ofelectrical energy from relatively highimpedance sources, aside from two pos-sible exceptions, the condenser micro-phone and the photo -cell.

In any case their problems were notentirely similar, and as a great dealhas been accomplished since then, it isthe purpose of this article to presentthe existing information in its simplestform.

There has been and continues to be acertain degree of mysticism connectedwith the name crystal, while actually apiezo crystal is about the simplest formof device for converting electrical tomechanical energy or the reverse. Forall practical purposes a crystal whenused in a microphone, gramophone pick-up, vibration pickup or a similar device,can be considered as a capacitancegenerator having negligible internalresistance, but high internal capacitiveimpedance. Since the device is a capa-citance and has effectively no seriesresistance, a capacitance connected inparallel with it will only reduce thevoltage output and no frequency distor-tion will occur since this reduction willbe the same for all frequencies.

The expression for computing the volt-age loss in db. caused by a capacitanceload across a crystal generator deviceis as follows : db. loss = 20 log (t + C,C,) when C, represents the capacitancein microfarads of the load and C, thecapacitance in microfarads of the crys-tal device. Since the impedance of ashielded cable is effectively capacita-tive reactance, a cable can be consideredas a capacitive load on the crystal de-vice. Thus it is a simpler matter tocompute the loss that can be expectedfrom a length of cable of known capa-citance.

Another point that might be con-sidered here is the proper input resist-ance to use in the amplifier. The inputimpedance of an amplifier is essentiallyresistive when connections are madedirectly to the grid, which is the usualcase if crystal generators are being used,and since the internal impedance ofcrystal generators is capacitive the pro-blem can he considered as similar tothat of choosing a grid resistor for acondenser -resistance coupled amplifier.Neglecting valve capacitances, in thelatter case, the low frequency cutoff ofthe stage is determined by the relation-ship between the coupling condenserand the grid resistor. The higher thevalue of the grid resistor, the lower

the frequency which the amplifier stagewill pass for a given coupling conden-ser. The same is true of a crystal de-vice, the crystal capacitance representsthe coupling condenser. The valvemeasures the voltage drop in the gridresistor and this is vectorially at rightangles to the reactance drop in thecrystal generator. The total impedanceof the crystal generator circuit is there-for the vectorial sum of the reactance ofthe crystal and the grid resistance, i.e.,the square root of the sum of the squaresof these values. The useful voltage istherefore proportional to the resistanceR divided by the impedance, and theloss in decibels for a resistance R isgiven by the expression:

R' + X'db. loss = 20 log

RWhere R = the grid resistance in ohms

I 59,000X crystal reactance =

fCC = capacitance of crystal in

microfaradsf = frequency in cycles per sec.

For example, let us assume we havea grid resistor with a resistance of500,000 ohms and a crystal device witha capacitance of 0.005 microfarads andwe wish to determine the voltage lossat 6o cycles per second.

1K9,000The Xc = ---- = 530,000 ohms

bo x 0.005approximately and

500,0002 4.530,000'zo log =20 log 1.4572

500,000= 3.26 db. loss at 6o cycles per sec.It should be pointed out that for a

combined parallel capacitive and resis-tive load, the capacitance to be con-sidered when determining the size ofresistor to use is the sum of the crystalcapacitance and cable or load capaci-tance.

The New Super Skyrider.The new Super Skyrider, built by

Ha]licrafters, is on demonstration atWebb's Radio, and we were surprisedto find that this rr-valve rtceiver coversall frequencies from 545 kc., right upto 62 mc., with separate coils on eachband. It is one of the most popularreceivers of its kind, and is one of thefew to include efficient reception be-tween 5 and TO metres. Other refine-ments on this set are carrier level meter,a crystal filter, and, of course, switch -able A.V.C. and a beat note oscillator.The receiver is priced at L32, and issuitable for 110 to 250-yolt input.

RADIO CLEARANCEThe reliable Mail Order House offer

the following

BRAND NEW GOODSSPECIAL OFFER. Huge purchase 7 ValveUniversal All -Wave Superhet Chassis, covering4 Wave Bands, 2 Short, Medium and Longtotal range 15 to 2,000 metres covered by the 4Bands. Fitted 7 Mullard Valves, Pre H.F.Stage, etc.. Pentode Out -put. Chassis com-plete with Valves, less Speaker, £3 17s. 6d. each.

Aerodyne 5 Valve Battery Superhet, fitted inmagnificent Walnut Cabinet, Upright Design,Brand New in sealed cartons as from the makers.Listed 8 gns., our price £5 2s. 6d. with Valves,less Batteries.

Ideal Christmas Gift, Electric Trouser Presses,made for a Pall Mall firm to sell at 3 gns.,Brand New, Boxed. The Press that steams andpresses your trousers in one operation, price12/11, plus 1/6 carriage.

BRYCE MAINS TRANSFORMERS, stand-ard for the season. These transformers areBritish made and are fully guaranteed for 6months.

350-0-350, 120 m/A., 2-0-2 volts, 2.5 amp.,2-0-2 volts, 4 amp., 10/6. 350-0-350, 150 in/A.,20-2 volts, 2.5 amp., 2-0-2 volts, 4 amp., 2-0-2volts, 2 amp., 11/6. 500-0-500, 150 m/A., 2-0-2volts, 2.5 amp., 2-0-2 volts, 6 amp., 2-0-2 volts,2 amp., 2-0-2 volts, 2 amp., 17/6.Huge Purchase of Wet Electrolytic Condensers,by well-known manufacturer. Following typesavailable : 8 mfd., Metal Case, Single holefixing, Wet Electrolytic complete with LockingNut, 450 volt working, 2/0 each.16 plus 16 ditto 2/3 each.16 mfd. 320 volt working, ditto 1/10 each25 mfd. ditto

ditto1/10 each

32 mfd. 2/3 each50 mfd., 35 volt working, one hole fixing, metalcan 1/6 each.

Plessey 3 -gang Straight Condensers, semiscreened, each section .0005, geared type, 3/: -each.

Plessey 2 -gang Midget Piano type Condenser,fitted top trimmers, each section .0005, fullyscreened, 2/6 each.Lissen Universal Iron Cored Coils, completewith Reaction, Brand New, Boxed, 2/6 each.Lissen Class B Driver Transformer, Brand New,Boxed ratio 1-1, 1/-.Set Lissen Aerial and H.F. Coils, Air Cored,fully screened 2/6 pair.

Centralab Volume Controls, Long Spindle,fitted with Switch ; 1,000, 5,000, 10,000,25,000, 50,000, 250,000, 500,000, 2/6 each.Special offer Plessey 3 -gang Straight MidgetCondensers, approximately 2k" by 2" wide.An ideal condenser for midgets or any type ofSet where a precision job is required, 2,!3 each.Porcelain formers, 3i" long, 1" in diameter,2d. each.

All orders 5/- or over, post free. Orders under5/- must be accompanied by a reasonableamount for postage. C.O.D. orders under 5/-cannot be accepted.

RADIO CLEARANCE63, High Holborn, W.C.I

Holborn 4631

61

TaLEYISIOt1AND


" The U.H.F. EmergencyTransmitter."

(Continued from page 46.)

with the cores at right -angles, the left-hand component is the X32B smoothingchoke, while on the right is the specialoutput transformer.

TheSub -chassisjtiow refer to the photograph showing

the layout of components underneaththe chassis. The double 4-mfd. con-denser is mounted straight on to the

GSNIANNOUNCES

NEW 1938 MAN UALReady this month-three times enlarged-the greatest Manual publishedin this country.

Full constructional details of a 10 -meter superhetconverter, single to multi tube CW and 'phone trans-mitters, TRF and superhet receivers, short-waveadaptors, the most practical American valve chart,full details, including tables on all the popular trans-mitting antenna, meter/KC conversion charts, ClassC operating data on all the latest transmitting tubes-triodes, pentodes and beam power-hints and tipsgalore, International amateur prefixes, lists of short-wave broadcast stations, beam power and Class Bmodulators-in fact, the whole of this page wouldnot allow us to describe, even briefly, a tenth of thecontents of this Manual.

Still the old price71d POST FREE

CRAFT A CREEDG 5 N I (BIRMINGHAM) LTD.44, HOLLOWAY HEAD, BIRMINGHAM

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lip of the chassis and has four termin-als, two of which are joined togetherand are connected to the nearest earthpoint. On the front lip of the chassisare mounted the special holder for thecrystal microphone, the volume control,output terminals and two-way block forthe low-tension input.

As regards volume control, this isbushed off the panel by means of a cir-cular disc of bakelite iu order to makequite sure that there is no possibility ofleakage from the moving arm to earth.This would cause a, big' drop in volumeowing to the input to the pentode beingalmost short-circuited.

Similarly with the output terminals,which must be carefully bushed, but inthis instance manufacturers supplybushes with the terminals. It must beremembered, however, that there willbe a potential of 25o volts between theseterminals and chassis so that good insu-lation is essential.

A special 5 -way group board shouldbe mounted in the centre of the chassis,as can he seen from the illustration.This is mounted on two lengths of2 B.A. studding and enables some ofthe resistors which would otherwise beleft floating to be carefully anchoreddown. Make the most possible use ofthis group hoard in order to have atleast one end of resistors or condensersanchored.

In normal designs of this type it isquite in order for small resistors andcondensers to be connected in the wir-ing without further fixing, but whenthe equipment is to be roughly handledit is advisable to firmly fix every com-ponent.

Another point in construction is themethod of fixing the cathode by-passcondensers. It will be noticed that thereare three such condensers; one acrossthe cathode of the SP r3C, and oneacross each cathode in the 7D8 circuit.

These condensers should be mountedalongside the cathode pin of the tespec-

TELEVISIONA NEW COURSE OF INSTRUCTION,

e have pleasure in announcing that our new " Television " Coursehas met with remarkable success. The enormous demand for theCourse has shown, beyond any shadow of doubt, that it fills that long -felt need which we anticipated when we went to such trouble andexpense in its preparation.We shall be happy to send full details of this special " Television "Course on request. Particulars are also available of other Coursesin all branches of Wireless, Television, Talking Picture Engineering,etc., and of the easiest way In which to prepare for the A.M.I.E.E.,A.M.I.W.T., A.M.! .R.E., etc., Examinations.

We teach by correspondence, and guarantee " NOPASS-NO FEE." Our Technical Consultant Is

Professor A. M. Low.Send for full particulars to -day. Free and without obligation.

THE BRITISH INSTITUTE OF ENGINEERING TECHNOLOGY337, SHAKESPEARE HOUSE, 17/19, STRATFORD PLACE,

LONDON, W.I.

Read

" Television

and

Short -wave

World "

Regularly

tive valve -holders. They are fixeddown by means of a length of thinpresspahn folded around it with the twoends bolted down underneath the nutand bolt used primarily to hold thevalve -holder in position.

Another component that should becarefully watched is the .oi-mfd. coup-ling condenser between the input andoutput valves. The components havebeen arranged so that one side of thiscondenser can be soldered actually tothe anode leg on the valve -holder withthe opposite end of the condenser sol-dered to the top end of the potentio-meter. In this way the condenser,which would normally be left floating,is properly fixed.

ConnectingUp

It is suggested that all filament leadsbe wired with heavy gauge twin flex inorder to prevent any possibility of volt-age drop. The remainder of the wiringcan be satisfactorily carried out withordinary push -back or Quickwyre, usingred wire for anode connections andblack wire for all negative connections.Again, it must be emphasised that theequipment will probably bel used byoperators not fully conversant with theapparatus, so that in case of breakdownthey must be given every assistance inorder to locate the source of the trouble.By differentiating between negative andpositive wiring this should help veryconsiderably.

For those who wish to use this am-plifier for gramophone or speech work,a special output transformer will haveto be obtained from Messrs. BryanSavage. The secondary of this trans-former will have to be designed to havean impedance equal to that of the loud-speaker it will work. Those who arewithout any source of mains supply andwho need an amplifier giving compara-tively high output will find this equip-ment ideal for the purpose. It will alsobe suitable for outdoor work, and infact for any usage where an accumulatorwill be the only source of supply avail-able.

All valves are of the 7 -pin type withBritish bases and are designed for a 12 -volt accumulator only.

It has been arranged that a completekit of parts be obtainable from Messrs.Webb's Radio, of 14 Soho Street, W.',while the complete amplifier, ready -wired and tested with microphone, canbe obtained from Messrs. Bryan Savage.

In our next issue will be given fulldetails of an ultra -high frequency trans-mitter also running front a 72 -volt accu-mulator, while to complete the U.H.F.installation, elsewhere in this issue willbe found details of a special receivertuning from t to to metres.

62

JANUARY, 1938

TaVISIOtiAND

SHORT-WAVE WORLD

4

"The Short-wave Radio World."(Continued from page 44.)

circuit as shown in Fig. 4, which earthsone side of the cathode resistor, whilethe grid is connected to L2. This coilconsists of five turns of No. 20 gaugewire on a valve base, and without anytuning condenser it resonates at ap-proximately 20 metres well enough toprovide an R9 tone signal with theswitch in the oscillator anode to the offposition.

With the oscillator anode switchclosed the carrier or key transmitter canbe tuned in by means of CI, so function-ing. as a frequency meter. With theD.P.D.T. switch in the opposite posi-tion the grid of the 56 triode is con-nected through the grid leak and con-denser, R5, C7, and also the L.F. trans-former secondary to earth, whilecathode is connected through one wireto the centre tap of the keyed stage inthe transmitter.

The L.F. oscillator is therefore keyedsimultaneously with the transmitter. R6is a panel control for L.F. output andenables the pitch of the note to be variedbetween 30o and 5,000 cycles. Thevolume control, RA, provides a meansof controlling the volume from theoscillator without affecting the fre-quency. This is an essential point as

Subscription rates for " Television and

Short-wave World " post paid to any

part of the world are : 3 months, 3/6 ;

6 months, 6/9 ; 12 months, 13/6.

the level of the audio oscillator isabout 15 db over the output from thefrequency meter. The toggle switch inthe anode circuit of the 2A7 is importantaS" it prevents the R.F. oscillator signalfrom damping the audio oscillator be-cause C6 is always in circuit.

The 2A7 functions as a negative re-sistance oscillator of the retarding fieldtype with electron coupling to the out-put stage. In the circuit shown, gridNo. 2 functions as a screen, with gridsNos. 3 and 5 coupled together as ananode. Grid No. 4 is the conventionalsuppressor. The screen and suppressorare at the same R.F. potential, andwhen the potentials of both vary to-gether increase in screen potential willbe accompanied by decreasing screencurrent .and vice versa, so giving thenegative resistance effect.

Grid No. i is connected to cathodebut as there is no feed -back required LIis an untapped solenoid coil.

GALPINSELECTRICAL STORES

75, LEE HIGH ROAD, LEWISHAMLONDON, S.E.13. Telephone: LEE GREEN 5240

TERMS CASH WITH ORDER OR C.O.D.CHARGES EXTRA

All Goods sent on 3 days'approval against Cash

LARGE MAINS TRANSFORMERS fully guaranteedby Foster, Philips, etc., 200-250 volt. 50 cycles. I phaseinput, output 2 windings of 3,200 volts at 350 mA , also 2windings of 20 volts at 10 amps., £5, c.f. Another 3 kWVoltage Changer, 200-250 volts to 100-120 volts, or viceversa, double wound, 65/-.BURGOYNE FURY FOUR -VALVE UNIVERSALCHASSIS for 200-250 volts A.C.-D.C. mains in workingorder, but sold cheap enough for spares, 10/6 each. Post1/-. Valve used S.P.13, HP13, PEN26, UR3.ELECTRIC LIGHT CHECK METERS, for 200-250volts, 50 cycles, I phase, 5, 10 and 20 amps. for garages,workshops, subletting, etc., 6/- each. Post I/-. Fullyguaranteed.A.C. INDUCTION MOTORS, approx. 1 h.p., 1.425r.p.m., 200-250 volts, 50 cycles, I phase, suitable forpumps, sewing machines or any small power work.Condition new, 22/6. Post free.CHARGING DYNAMO, 50-75 volts, 10 amps., shuntwound, condition as new, 60/,CHARGING OR LIGHTING DYNAMOS, all shuntwound, fully guaranteed, 15 volts, 12 amps.. 21/-. 12 volts,8 amps., 17/6. 25 volts, 8 amps., 32/6. Regulators to suitthese and other dynamos from 10/- to 15/-.MAINS TRANSFORMERS, input 200-250 volts, output330-0-350 volts, 80-100 mA.,4 volts, 4 amps.,4 volts,2 amps.7/6 each.220 volts in 4-,14- and 36 -volts at 15/7 amps., out-put. 15/. Another, 20 and 35 volts at 5-8 amps., output,15/.. Ditto, 20 volts, 15 amps., 12/6. Ditto, 6, 15 and 30at 50 amps., 20/-. Ditto, 3, 4k, 8 and 15 volts at 50-75amps., 25/.. Ditto, 2f -0-2f and 31-0-3f volts at 50 amps.,17/6EX-R.A.F. WAVEMETERS, 200 to 550 metres, visualtype, 3/.. 300 to 2,500 metres, valve type, 7/6 each, withvalve. Post freeMOVING COIL METER MOVEMENTS, 21 in. dial,by E. Turner E. Edgcumbe, etc. Approx. 4 to 7 mA., fullscale for recalibrating into multirange all test meters, 5/.,each. Larger size ditto, 4.6, 8 in. dial', 6/6. Post 9d.MOVING COIL METERS, by Weston, E Turner, etc0 to f amp , 10/, 3 in. dial, 0 to 100 mA., 15/.. 0 to 4amps., 12/6. 4 in. dial, 0 to 400 mA., 15/.. 5 in. dial, 2readings, 5 and 25 mA.. 20/-. Three readings, 5, 50 and250 mA., 25/.. 61 in. dial, 2 readings, 6k and 65 mA.,27/6. 21 in. dial, hot wire, 0 to If amps., 5/. 6,1 in. dial,0 to 1,000 mA., 25/-.ELECTROSTATIC

15VOLTMETERS, 2+ in. dial, 0 to

2,500 volts for /..SWITCHBOARD VOLT AND AMP. METERS, 4, 6,8 in. dial, by well-known makers, 0 to 120 volts, 12'6. 0 to250 volts, 17/6. 0 to 40 volts, 8/6. 0 to 10, 20, 50, 100,150amps., 12/6. All m/coil hot wire for A.C., 0 to 120 volts12/6. 0 to 250 volts, 17/6 Amp -meters as above for A.C.or D.C., 12/6 each.SILVERTOWN HIGHLY SENSITIVE GALVANO-METERS, flat type, with calibrated dials, 7/6.EX-R.A.F. 3 -VALVE RECEIVERS, 150 to 2,500metres, less valves, 6/6. Useful to the experimenter.LARGE SLIDER TYPE RESISTANCES, with covers,suitable for dimmers, etc., 10 ohms. to carry 10 amps.,17/6 ; 30 ohms. 5 amps., 17/6 ; 100 ohms. 21 amps., 17/6 ;350 ohms., lf amps., 20/. ; 1,000 ohms.. I amp., 20/-.

ith worm wheel control, 2/6 extra.2/INCH SPARK COILS, complete with condenser andhigh-speed contact breaker, input volt. 6 to 12 volts., 19/6each. 16/inch SPARK COIL, with Condenser, 65/,10 -inch ditto, 55/-. 8 -inch ditto, 50/-. All in goodcondition, c.f.FLUORESCENT SCREENS, " Platinum " 12 by 9inches in frame. Good condition. 35/-.X-RAY TUBES, with Tungsten Targets, 17/6. With7P5Ia/t-incu.mf. Targets, 25/.. Packing free, c,f.

Discharge Rods, etc., in polished case, in new condition,12.inch SPARK COIL, complete with Condensers,

X-RAY BARGAIN FOR THE EXPERIMENTER,a 12 -inch Coil with Condenser, Discharge Rods, Tubeand meter holder, complete with brand new X -Ray Tubeand Moving Coil 2 Reading rn/amp. meter 0 to 5 and 50m/amp., the lot, 100/. c.f. Packing free.EX-G.P.O. GLASS TOP HIGHLY SENSITIVERELAYS. Type B, operating current approx. I m/amp.,7/6 each. Post 6d.EX-R.A.F. MORSE KEYS, heavy cur.ent type, 3/., each.X.RAY TRANSFORMERS, input 200-250 volt, 50cycles, 1 phase, output 100,000 volt, 30 mA., £12 10s.Another, 100 volts input 40,000 volts at 30 mA. output,£7 10s.WESTINGHOUSE RECTIFIERS, output 500 volts at250 mA. voltage, doubler circuit, 22/6 each. T.C.C., 1

m.f. Condensers, 2,000 volt working, 2/6 each, post 6d.Cossor, General-purpose Valves, 2 volts. 3 for 2/6, post 6d.Microphone Buttons, 9d., p.f.

New Componentsfor Constructors

jUST recently we have been testingsome of the new Shure crystal micro-phone units which are being stocked

in this country by Webb's Radio, Ltd..of 14 Soho Street, W.I. We have cometo the conclusion that whenever possibleamateurs should use microphones ofthis type2 for their advantages are toonumerous to mention in the smallamount of space available. However,the fact that they do not require anyenergising battery or input transfor-mer, and at the same time are entirelyfree from feed -back troubles, even whenused on the ultra -high frequencies, isa very big point in their favour.

High -gain amplifiers can be built touse a crystal microphone that are en-tirely free from hum, which would nor-mally be picked up in the microphonetransformer circuit. There are twomodels in particular that we recom-mend to amateurs, and these are the7o1A. and the 7o2A. The 7oIA at foulguineas represents excellent value formoney and is about the best type ofmicrophone that we have so far tried.

We have previously mentioned thenew Valpey crystals which Webb's arestocking, but we did not mention thatthere is also in this range a crystal suit-able for use in the intermediate fre-quency stages of a super -het :eceiver.Crystals for this purpose are availablein frequencies of 465, 500 and 525 kc.

A new crystal for zo-metre funda-mental operation is also available andis capable of withstanding a grid cur-rent of is() mA. It therefore, once andfor all, overcomes the objections tocrystal operation on zo metres with afundamental crystal owing to the pos-sibility of crystal fracture. Actually,the LW2o Valpey zo-metre unit is a6o -metre crystal cut to oscillate at a14 mc. frequency.

Johnson feed -through insulators havea lot more uses than merely to carryR.F. or high -voltage D.C. from onechassis to another. We have used themon which to mount variable condensersby means of a small bracket so that therotor plates can be joined to a coilunderneath the chassis without addi-tional wiring other than the zBA boltthrough the insulator. These Johnsoninsulators are made in two types, cost-ing tod. or Sd. each, according to size,and it is absolutely impossible for thereto be any breakdown to metal chassisowing to the use of a long insulatingcollar inside the insulator.

It is possible to obtain no less than245 watts of audio from a pair of ZBi2oAmpere Class -B triodes. These valvesdo not require a grid bias and can bedriven by quite a small pre -amplifier.These valves cost 65s. each and areabout the cheapest way of obtaininghigh audio output.and it has been arranged that the grid

63

TrIVISMIAND

SHORT-WAVE WORLD

" Britain's 1.10 Unique."(Continued from page 39.)

pills in each case conic directly abovethe fixed plates of the tuning conden-ser, so that they can be soldered directlyon without additional wiring.

Plug-in coils fitted with handles,which also serve as protective covers,plug in to these holders, and are soarranged that when the lid of the canis in position there is at least a / in.gap between the top of the coil andthe bottom of the lid.

In the L.F. section the only compon-ent above the chassis is the volumecontrol, R8, which is mounted close tothe grid of V3. The remaining L.F.components are kept as near as possibleto the L.F. end of the chassis, with theoutput choke and condenser in the filtercircuit next to the 5 -pin power socket.

Resistance R9 is of a special type,having a D.C. resistance of 20 meg-ohms and with a i-watt rating. Thiscomponent is rather important, other-wise the lead between anode and gridof V2 will be lengthened. In practiceit will be found that the distance be-tween Cri and the anode of V2 is ap-proximately the length of the I -wattresistor.

The standard by-pass condensers areof the normal mica type, but it is anadvant&ge if these are of the specialceram,Wtype owing to the reduction insize.

No less than five coils, however; arerequired to cover this range owing tothe use of miniature tuning condensers.It is not thought advisable at this pointto describe the construction of the coilsowing to the difficulty that experimen-ters may have in building their ownunits, but coils are available ready. -built and it is hoped that by the nextissue we shall have found some meansof describing the construction of thecoils in some way that experimenterswill be able to copy them exactly.

A maximum of 200 volts 1I.T. is re-quired with a minimum of 18o, with atotal current consumption of 4o mA.Valve heaters are fed from a 4 -volt ac-mumulator, while for mobile use theentire receiver can be run from a single4 -volt cell plus a vibrator converter orrotary converter for H.T. supply. Fullconstructional and operating data willbe given in the February issue.

The receiver is not intended for pro-gramme use owing to the fact thatquality is not of the highest order withthe super -regenerative detector. How-ever, the receiver is quite suitable forlong-distance reception of 7 -metre tele-vision sound signals for those who careo use it for that purpose.We have made arratgemmts for the

original receiver to be on show atWebb's Radio for a short period, com-mencing January 1, 1938.

JANUARY, 1938

The New Cossor All -waveConsole.

NEW standard in value has beenreached by the introduction of anall -wave super -het console re-

ceiver by A. C. Cossor, Ltd. 'This in-strument, model. 598, is a very satis-factory family set that will give a verygood account of itself,

The wavebands covered are 16 to52.2 metres, 196 to 556 metres, and 968to 2,050 metres, so that 011 the moreimportant channels that cover enter-taining stations can be tuned in.

This model is one of the first at itsprice to be fitted in a console type ofcabinet, and, as may be expected, inview of the extra baffling effect ob-tained with a large cabinet of this kindplus the use of a io-in. energised mov-ing -coil loudspeaker, quality is ex-tremely good. We also noticed thatwith very low volume, such as wouldbe used in a small private house, thatthe overal response did not fall off.

This model 598 is excellent value formoney, and the average listener, want-ing a reasonably priced set and withmaximum value for money, will beagreeably surprised with both the ap-pearance and the performance of thisset. Full details can be obtained fromthe manufacturers, Messrs. A. C. Cos-sor, Ltd., Highbury Grove, N.5.

YOUR NEW YEARRESOLUTION!

it?

RSGB

If by some chance you have been ploughing alonely furrow in your experimental work why not makea New Year's Resolution to join the Radio Society ofGreat Britain ?

The annual subscription Is moderate (London 21/-,Provinces 15/-).

The Society, founded in 1913, to -day has amembership well in excess of 3,000. Each month theT. & R. Bulletin is sent post free to members. ThisJournal contains on an average 60 pages of up-to-datetechnical and topical information-written by and forradio amateurs.

The December issue contains a full length con-structional article dealing with a 50 watts 66 Mc. crystalcontrolled transmitter; descriptions of typical 56 Mc.stations, and new ideas for 56 Mc. receivers. In additionthe usual regular features apps r including "The Monthon the Air"; Calls Heard; New QRA's ; District Notes,and a host of other up-to-the-minute technical andtopical information.

In addition, the second of a new series of monthlycontributions by " Uncle Tom " under the title "TwelveYears Back." appears in our January issue.

Resolve to -day to write for a copy of this issue(price 11- post free) and ask for full details of membership.

All communications to :

The Secretary :

RADIO SOCIETY OF GREAT BRITAIN53, VICTORIA STREET, LONDON,

S.W.I.

TELEVISIONDEVELOPMENTSAre you keeping abreast of modern televisiondevelopments ? Whether amateur or professionalyou must be in touch with the latest trend ofresearch.The Television Society enables you to meetfellow workers in the field of television, both inthis country and abroad.Founded some io years ago The TelevisionSociety provides a scientific and non-partisanplatform for discussion on all aspects of the subject.Meetings are held monthly during the session(October -June) and are reported in full in theSociety's Journal which is sent free to all members.The Society's activities are shortly being enlargedto meet the growing interest in the subject andmembers will have a unique opportunity of further-ing their knowledge by contact with well-knowntelevision engineers.

Full particulars of membership qualifications may be had from the Hon.General Secretary J. Denton, 25, Lisburne Road, Hampstead,

I,ondon, N.W.3.

THE TELEVISION SOCIETY(Founded 1927)

President : Sir AMBROSE FLEMING. M.A., D.Sc., F.R.S.

64

JANUARY, 1938 ANDSHORT-WAVE WORLD

OPPORTUNITIES SECTIONBUSINESS :: EQUIPMENT :: EMPLOYMENT :: MISCELLANEOUSThe charge for miscellaneous advertisements on this page is 12 words or less 2/-, and 2d. for every additional word. Single -column inch ratedisplayed, 12'6. All advertisements must be accompanied by remittance. Cheques and Postal Orders should be made payable to Bernard Jones

Publications Ltd., and crossed, and should reach this office not later than the 15th of the month previous to date of issue.

INDEX TO ADVERTISERS

A.C.S., Ltd. 32A.P.A. Metalworker ... .. .. 32Automatic Coil Winder & Electrical Equipment

Co., Ltd.Baird Television, Ltd. ... ... 15Belling & Lee, Ltd. ... ... ::: :::Cover iiBritish Institute of Engineering Technology ... 62British Mechanical Productions, Ltd.... ... 31Brown, S. G., Ltd. ... ... ... 59

apBulgin, A. F., & Co., Ltd. ... 1Chapman & Hall ... ... 57Clix ... ... ... ... ... 31Dubiller Condenser (1925), Ltd. ... 57Edison Swan Electric Co., Ltd. ... 2Fluxite, Ltd. ... ... ... ...Cover iiFoyles, Ltd, ... ... ... - Cover iiiG5KA Cover iiiG5NI (Birmingham), ma'. ... 62Galpin's .. . . ... ... 63General Electric CO., Ltd. ... ... 16Heston Mfg. Co. ... ... ... 59McGraw-Hill Publishing Co. ... ... 31Mullard Wireless Service Co., Ltd. - Cover in'

... 59Premier Supply Stores ... ... ... 5:3Quartz Crystal Co., Ltd. ... ... 59Radio Clearance ... ... ... 61Radio Resistor Co., Ltd. ...Cover itRadio Society of Great Britain ... ... 64362 Radio Valve Co., Ltd. ... ... - Cover iiiScophony, Ltd. . . .. -. ... 4Technological Institute of Great Britain 59Sound Sales, Ltd. ... ... ... 1Television Society, The ... ... ... 64Ward, Chas. F. -. ... ... ... 31Webby Radio, Ltd. ... ... ... 55Westinghouse Brake lt Signal Co., ltd. 82

PATENT AND TRADEMARK AGENTS

GEE & CO., patents and trade marks throughout theworld (H. T. P. Gee, Mem.R.S.G.B., A.M.I.R.E.,etc.). Estab. 1905. 51-52, Chancery I,.ane, London,W.C.2 (2 doors from Government Patent Office).'Phone Holborn 4547 and 4548. Handbook free.KINGS PATENT AGENCY LTD. (B. T. King,Patent Agent) 146a, Queen Victoria Street, London,E.C.4 ADVICE, Handbook and Consultations free.Phone City 616r.

SERVICE BOOKS"Servicing Stmerheterodynes." Rider. 5/5"Automatic Frequency Control." Rider. 5/5" Cathode Ray Tube at Work." Rider. 11/ -"Modem Radio Servicing." Ghirardi. 18/-."Radio Physics Course." Ghirardi. 20/8."An Hour a Day with Rider" series. 3/3 each.

All prices post free.

HOLIDAY & HEMMERDINGER LTD.,74-8 Hardman St., Deansgate, Manchester 3

FOYLESBOOKSELLERS TO THE WORLD

Stock of nearly three million new and second.hand volumes. Catalogues free on mentioning

your interests.119-125 Charing Cross Rd., London, W.C.2.

Telephone: Gerrard 566o (/4 lints)

11111,

THE HAMS' BOOKSHOP.RADIO AMATEUR'S HANDBOOK (1938 edition). Completelyrewritten and enlarged, with two new chapters. The only thing unalteredis the price (5/6d. post free).HINTS AND KINKS (A.R.R.L.). The 2nd edition of this book is a biggertime and money saver than its predecessor. (Don't save the money-send2/6d. at once.)RADIO AMATEUR CALLBOOK. December number with thousandsof new QRAs, 61- post free. (A two year old copy isn't good enough !)AMATEUR RADIO GLOBE OF THE WORLD. Marked with allamateur prefixes, time bases and W.A.C. divisions. 276d. post free. (Don'tlet the price stop you-it will always keep the baby quiet.)RADIO MAP OF THE WORLD (A.R.R.L.). The only difference betweenthis and the Globe, is that the Map can be nailed on the wall. 4/6d. post free.(Can also be used to cover up holes left by QSLs.)

G5KA. 41, Kinfauns Road, Goodmayes,Ilford, Essex. Dept. "T 1."

EXTRA SPECIAL362 RB 750/ 150

BI -PHASE 750 VOLT

RECTIFIER 150 M A IN STOCK !!

16/. POST FREESEE ADJOINING ADVT.

MISCELLANEOUS

AUSTRALIAN MUSEUM requires specimens ofearly television apparatus. Write particulars, Room518, Australia House, Strand, London.CONDENSERS. Fixed paper in Metal Containers.moovolt Wkg. xmfd 2/6. 2ffifd 4/9. 4mfd 7/6.8mfd to/6. Bakelite Tubulars. x000volt Wkg.rrnfd 2/6. ox5mfd o.25mf4 r/9. o.rmfd. x/3.o.ormfd 9d. The Static Condimser Co. it, EdenStreet, Hampstead Road, London,N.W.i. "Makers ofGood Condensers."RADIO AMATEURS HANDBOOK 1938, Soopages, Soo illustrations, Formulas, charts and Tables.5/5d, post free. Smith, bookseller, 6, Racquet Court,London, E.C.4.SHORT WAVE Variable Directionat Aerials. 360*rotating armchair control O -V-1, -V-2, T.R.F.,Super, state receiver and enclose lid.'istamp for fulldetails to A. W. Mann, 82, Costa Street, Ittiddlesbrough,Yorks.BEAUTIFULLY built coo watt C.W. 3C wattPhonetransmitter in black crackle finished cabihet for 7, 14and 28 Mc. with separate power pack. commercialequivalent would retail at over Oo. First 3o ornearest offer will secure. Demonstrate i..London,Box No. 16.OFFERS.-Ediswan r2H, Gth, N43, D44, KT66,Oil condensers, etc. Seen Appointment. Bot No. /8.AMERICAN A.R.R.L. Type Sectional Woo'd AerialMatt% complete, insulated guy wires, strainers,. pulley,coo ft. cotton rope, all metal galvanised. 5o ft..64/6d.Creosoting extra 2/6d. White Painting, two coats, 7/6d."Fibromet " panels and chassis," Works like wood,"but screens like metal," -4", 1?- sq. ft. to size. LowImpedance feeder, 75 ohm approx., 2/9d. 5o ft. Cashwith Order. Carr. Fd.-Radio-,CNT. Co (G.6.Q.F.),Atherton.

BINDING CASES AND INDEXES FOR 1937Binding cases and indexes for the 1937 volume of "Televisionand Short -Wave World " will be available on Jan. 14th. Thecases are full brown cloth with stiff boards lettered in gold.The price, including the index, is 2/9 post free. Indexes maybe obtained separately and the price 's 6d post free.Orders should be addressed-

BERNARD JONES PUBLICATIONS, LTD.,CHANSITOR HOUSE, 37/38 CHANCERY LANE, LONDON, W.C.2

and should be accompanied by the remittance.

SPEAK TO THE WORLDWITH AN

R.F.P.30 TRANSMITTING

PENTODECHARACTERISTICS :Filament Volts 4v.Filament Amps 2a.Anode Volts 650v.Max. Anode Input 50 wattsMax. Anode Dissipation 30 wattsAnode Load 10,000 ohmsScreen Volts 400v.Speech Input 2 wattsRadio Input 2 volts

(R.M.S.)

PRICE 45/- POST FREE

THE 362 RADIO VALVE CO., LTD.324-6 LIVERPOOL ROAD, LONDON, N.7

TELEPHONE : NORth 1853

iii

1.agYISJOAAND

SHORT-WAVE WORLDJANUARY, 1938e

Mullard introduce a

new type of Cathode

Ray Tube

TYPE A.4 1 -G4

o.

HIS NEW TYPE 4 Mullard-I- Cathode Ray Tube has been

specially designed to permit thepair of plates nearer the screen tobe used with a non-syMmetricaldeflection circuit, thus considerablyreducing the cost of the associatedapparatus.

The design of the focusingelectrodes enables an exceptionallysmall spot size to be obtainedover the entire screen area.

Price 16 ISs.COMPLETE WITH SPECIAL SOCKET

ABRIDGED SPECIFICATION

Screen Diameter - 4 ins. (approx.)H6ter - - 4 volts, 1.2 amperesMaximum Final

Anode VoltageDeflection - Double ElectrostaticFocusing - - - - Electrostatic

- 1,200 volts

FLUORESCENT COLOUR

A.41 -G4

A.41 -B4

A.41 4

GreenBlue

White

Full technical information fromTHE MULLARD WIRELESS SERVICE CO. LTD.

Cathode Ray Tube Department,225 Tottenham Court Road, London, W.1

PriT

..$1- Great Britain for the Proprietors and Publishers, BERNARD JONES PUBLICATIONS, LTD., Chansitor House, Chancery Lane, LondonoF.C.2, byCOQMBELANDS LTD., Addlestone, Surrey. Sole Agents for South Africa: CENTRAL News AGENCY, LTD. Bole Agents for Australia and New

Zealand : GORDON AND GOTCH ( AUSTRALASIA), LTD. -

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