Today, there are easier ways of handling electrons. Yet, the vacuumtube was, is, and will remain one of the 20th century's greatestinventions . . .

THERE ARE SOME who say that the electron tube has been doomed by thetransistor and its semiconductor "cousins," and that the tube, as the dinosaur, willbecome extinct. To these statements, the tube, if it could speak, might well replyin the words of the proverbial old man—I ain't dead yet. Actually, the tube is verymuch alive and kicking, with new types being introduced on an almost day-to-daybasis, and handling more different kinds of jobs than ever before. The tube"family tree" is a vigorous, strong, and healthy growing plant. Lets examine itclosely, starting with the roots and exploring the many branches and twigs

May 1963

Birth of the Tube. The electron tube had its beginnings in distinguished company. ThomasAlva Edison, one of the greatest inventors of all time, was experimenting with his newly inventedincandescent lamp bulbs one day when he discovered a curious phenomenon. When he installed asmall metal plate in his lamp bulb near the glowing filament, and connected this plate through asensitive galvanometer to the positive filament connection, he found that a small, but easilymeasured, current flow took place. Unable to explain the reason for this at the time, he

nevertheless realized that it might be potentially significant, so he obtained apatent in 1883 on what came to be called the "Edison effect." For many years,the Edison effect remained a classroom curiosity without any commercialvalue. Edison had, however, unknowingly invented the first true electrontube—the elementary two-electrode type we now call a diode.

In the meantime, scientists in other fields had started a chain of events,which eventually, would have a profound influence on the application of thebarely understood Edison effect. In 1887, Heinrich Hertz had demonstratedthat electromagnetic waves operate in accordance with the laws governinglight and heat waves, and described the basic theory upon which modernradio communication is based. Just after the turn of the century, J. A.Fleming, searching for an improved detector for electro-magnetic waves,found that the Edison effect could be used advantageously. It was Fleming,then, who invented the first practical diode—the Fleming valve. The name"valve" stuck, incidentally, and, even today, electron tubes are called "valves"in most parts of the world outside of North America. The next great stepforward came when Lee DeForest added a grid-like wire structure between

the filament and plate of the diode, patenting his new device, which he dubbed the Audion, in1906. It was DeForest who gave the electrical valve (or electron tube, as you prefer) an entirelynew capability—the ability to amplify as well as detect weak electrical signals. He invented thetriode tube, and, in so doing, laid the basic foundation for our great radio, television, andelectronics industries.

Three Basic Jobs. The electron tube's operation is relatively simple, once the idea of electronemission is accepted. Free electrons are liberated (or emitted) by the glowing hot filament. Sincethese elementary particles are negatively charged, they are attracted to the positively chargedplate, moving across the intervening vacuum to it. This current flow is unilateral—that is, fromfilament to plate and not vice versa. It is this property, which permitted the early Fleming valve toact as a detector. The filament, since it served as a source of electrons, came to be called thecathode. Later, this term was applied generally to any electron source in a tube (or other device),whether a filament or not. When DeForest added his grid-like wire between the filament and plateof the Fleming valve, he found that a small voltage applied to this structure could influence theplate current. Again, the operation is relatively simple. A negative voltage applied to the grid (asit came to be called, in deference to its original appearance) repelled the negative electrons andreduced plate current; a positive voltage applied to the grid attracted additional electrons andaccelerated them towards the plate, increasing plate current. Since the grid, an open-likestructure, intercepted few—if any—of the electrons moving towards the plate, its powerrequirements were very minute —but it could control a relatively large plate current. Thus, asmall voltage applied to the grid was able to control plate current and hence amplify signals.

The ability to amplify made another feat possible. Once a device can be used to amplify, asmall part of the amplified signal can be fed back to the unit's input (grid). The device then servesas its own source of signal, -and this feedback can generate alternating currents — so, it becomesan oscillator. With the addition of the grid, the electron tube became capable of performing the

three basic jobs it has handled from the first decade of this century to the present day: detection,amplification, and oscillation.

It all started with Edison’s observation that electric current flowed back to the battery inthis experimental light bulb.

Over 20 years later, DeForest found that he could control current flow by placing a grid-like structure between plate and filament.

The Tube's Evolution. Fleming's valve was a two-element tube, or diode, and consisted of afilamentary cathode and a plate. If, as was discovered, the filament is operated on 60-cycle a.c.line power instead of d.c., a certain amount of the line "hum" will appear in the plate circuit. Thisled to the addition of a separate cathode essentially a metal tube coated with chemical elementswhich emit electrons when heated. The tube is still a diode, however, with the indirectly heatedcathode and the plate serving as its principle elements. The filament is reduced to the simple roleof heating the cathode and, quite appropriately, is often called a heater.

When DeForest added a control grid, the tube became a three-elementdevice, or triode. In practice, the triode was found to suffer from aserious disadvantage when used as a tuned r.f. amplifier.There is a fair amount ofelectrical capacity between the grid and plate. This permitted a percentage of the signal inthe plate circuit to be coupled back to the grid, setting up the basic condition foroscillation. Thus, early r.f. amplifiers tended to be quite unstable, and a variety ofschemes were developed to neutralize the effects of grid/plate capacitance. In general,these consisted of introducing enough inverse (or negative) feedback from the plate tunedcircuit into the grid circuit to counteract the direct feedback inside the tube and preventunwanted oscillation.

The next forward step came with the introduction of a second grid to serve as ashield or screen between the (control) grid and plate. By reducing grid/plate capacity,the four-element tube, or tetrode, permitted r.f. amplifiers to be assembled withoutspecial neutralization circuits. It was soon discovered, however, that the tetrode, as thetriode before it, had a peculiar disadvantage of its own. For effective shielding, thescreen grid was given a positive charge. This accelerated the electrons tremendously intheir trip to the plate, causing the electrons to strike the plate with sufficient force to

"knock" other electrons off the plate material. In a sense, the plate became an electronemitter, secondary to the cathode. With the proper combination of plate and screenvoltages, more electrons would be emitted by the plate than were received by it, andthese, traveling back to the positive-charged screen grid, caused a curious phenomenon.'Under certain conditions, an increase in plate voltage would cause a decrease in platecurrent as if the tube acted like a negative resistance. Again, the net result was instabilityand the tendency to oscillate when the tetrode was used as an amplifier. In an effort toreduce, or "suppress," the plate's secondary emission, a third grid was added between thescreen grid and plate and connected back to the tube's cathode, creating the five-elementor pentode tube. This third grid was called, appropriately, the suppressor grid. It servedto repel the secondary electrons back to the plate without appreciably affecting thenormal cathode-to-plate electron flow. Today, the pentode is perhaps the most widelyused basic tube type.Somewhat later, it was found that a single cathode could be used formore than one function if additional electrodes were added outside the normal controlrange of the grid elements. This, in turn, led to the development of multi-purpose tubes.

Beam Power Tube . The basic screen grid, whether in a tetrodeor pentode, requires a fair amount of power for operation — power,which does not, however, contribute to the strength of the amplifiedsignal. Where very small amounts of power are handled, as in weak-signal amplifiers, the power loss is small and relatively unimportant.If a large amount of power is handled, as in tubes designed to handleseveral watts, then the power loss becomes significant, contributingto a loss in operating efficiency.

To make greater use of the stream of electrons moving from cathode to plate, this system of beamconfining was developed. Practical all modern power amplifier types use these beam-forming plates.

This led to the development of the beam power tube in which the control and screengrid wires were aligned in the same plane, so that the resulting flow of electrons wasformed in parallel sheets or "beams" between the cathode and plate. Beam-confiningelectrodes were added to shape the resulting beams and confine the electron flowbetween the grid wires, thus preventing electron movement to the support leads on whichthe grids were wound.

The resulting tube had much smaller screen currents than earlier tetrodes (andpentodes) with comparable power-handling ability, and thus was more efficient. At thesame time, it was found that the beam power tube had much greater power sensitivitythan earlier types. Today, beam power tubes are used extensively at both r.f. and audiofrequencies where more than a few watts are involved.

Gas-Filled Tubes. The majority of electron tubes are designed to operate within avacuum, so that there will be no gas molecules present to interfere with the freemovement of electrons between the cathode and plate electrodes. In fact, during themanufacturing process, a metallic substance is evaporated within the tube, forming a filmon its walls and absorbing the last traces of gas: this element is called the getter. Forsome applications, however, limited amounts of specific gases may be introduced in atube. Where a gas is present, the gas atoms can, under certain conditions, ionize; the gasatoms are partially stripped of their outer electrons and become positively charged ions.The gas ions move towards the cathode while the free electrons move towards the plate.Gas may be ionized by the application of heat and moderate voltages or by theapplication of relatively high voltages. This latter fact has led to the development ofseveral types of cold-cathode tubes – tubes, which do not require a filament or heater —the simplest examples of which are diode rectifiers such as the OZ4, voltage regulator

tubes, and neon lamps.The presence of gas within a tube has several primary effects.

First, of course, positively charged gas ions tend to reduce the tube'seffective plate-to-cathode resistance, thus reducing its internal voltagedrop. For this reason, gas-filled (generally mercury-vapor) rectifiersare extremely popular where large currents are handled. Second, an

ionized gas gives the tube an "all or nothing" characteristic. Until ionization occurs,relatively little current can flow. Once the gas is ionized, however, current reaches amaximum very quickly, and stays at that maximum value (determined by load resistancesand supply voltages) until the plate voltage is reduced to a very low value or cut offaltogether. Gas-filled triodes, or thyratrons, like gas-filled diodes, have an "all-or-nothing" characteristic, but with one difference. The difference is that the control grid inthe thyratron, being relatively close to the cathode compared to the plate, can be used to"trigger" ionization even though the plate voltage is below the value normally required toionize the gas. Thus, a small signal (or trigger) voltage applied to the grid can switch thetube from a non-conducting state very quickly. Thyratrons are used extensively asrelaxation oscillators and for control and switching applications.

A number of gas-filled cold-cathode triodes have been manufactured for specialapplications. Their operation is somewhat similar to that of the conventional thyratron,except that no filament is used. The most familiar example of this type of tube is the flashtube used in photographic electronic flash lamps.

Receiving Tubes. By far the most popular general class of tubes are low- to medium-power types primarily designed for use in radio and television receivers. This generalclass encompasses all the basic tube types — diodes, triodes, tetrodes, pentodes, beampower, and multipurpose tubes.

In the early days, there was little need to identify tubes except by their manufacturer'sname, for most units were essentially the same. Later, as more types were developed,identifying type numbers were introduced. These served to identify a particular type oftube in terms of its characteristics, permitting tubes produced by different firms buthaving the same type number to be used interchangeably. The first type numbers weresimple numerical designations, such as OIA, 15, 19, 20, 42, 45, 76, and 80. As more andmore types were developed, a different numbering system became necessary. The tubemanufacturers decided to adopt a system of numbers and letters such that the typenumber itself would give an indication of the tube's basic application. With this system,the first number would indicate the tube's nominal filament voltage, a middle letter theintended application (amplifier or rectifier, for example), and the last number the numberof active elements. Amplifier type tubes were to receive letter designations from the firstpart of the alphabet, rectifiers from the end of the alphabet. How Numbering SystemWorked. A type 6A3 was a tube with a nominal 6-volt filament (actually 6.3 volts), anamplifier type (A), with three active elements-

Multiple triode sections in one glass envelope are not as new as you might think. The 6A3 tube of the1930's (right) had two triode sections, but the were wired together. The current compactron (left has threetriodes in its envelope, and each grid

Gone but not forgotten are these three pioneers. The200A (left) was one of the first mass-producedtubes. A 954 Acorn helped introduce VHF to communication services. At right is an 1851, a super. high-gain tube developed just before World War II.

filamentary cathode, grid, and plate The 2A3 was a similar type, but with a 2-volt(actually 2.5-volt) filament. Similarly, the 6D6 was a tube with a 6-volt filament, anamplifier (D) type it with six active elements — filament, cathode, control grid, screengrid, suppressor grid, and plate. A 5Y3 was a tube with a 5-volt filament, rectifier type(Y), It with three elements—filamentary cathode and two plates; the 5Z4 was similar, butwith an extra element—an indirectly heated cathode. Unfortunately, even this seeminglyideal system was inadequate, for the introduction of new types soon outran available typenumbers. Today, type number designations still use number-letter combinations, and the

first number generally (not always) indicates nominal filament voltage, but the remainingpart of the type number does not always hold to its original meaning.

Tube Envelopes. Tube construction techniques, too, have undergone many changesas the "family tree" has grown. Originally, tubes were assembled in glass envelopesalmost identical to those used for incandescent lamp bulbs. Later, refined designs wereintroduced with connection pins, which allowed tubes to be plugged into their sockets(rather than screwed in, as with lamp bulbs). Metal envelopes became popular becausethey carried the advantages of built-in shielding and were less prone to breakage thanglass types. Special shapes were designed for high-frequency tubes to reduce electrodeleads to minimum length such as the now-famous Acorn tube. Still later, miniature 7- and9-pin glass types were developed. Today, the miniature glass tube is the most populargeneral type, although several new types of construction have been introduced in recentyears.

Pentagrid Converter. As radio receiver circuits became more and more complex,multi-purpose tube designs became increasingly popular. The use of these permitted morecompact receiver chassis layouts without, at the same time, compromising circuitsophistication. Some of these multi-purpose units were (and are) simply the elements oftwo or more tubes combined in a single envelope, but some special types were developed,the most popular of which is the pentagrid converter. The pentagrid converter has, as thename implies; five grid structures, its primary application is in superheterodyne receivers,where it is used a combination local oscillator and mixer.

In operation, the cathode, control grid, and screen grid are usedas the essential elements of a "triode" tube with appropriatecomponents to form oscillator. The incoming r.f. signal applied tothe second control grid (grid 2), which is shielded by the two-element screen grid. The cathode-to-plate electron stream iscommon to both control grids, hence the locally generated sign andincoming r.f. signal are combined the stream and electrically

"mixed" by the time the stream reaches the plate.

The Compactron. Developed primarily for TV applications and representing thepresent-day "ultimate" in multi-purpose tubes, the compactron is a squat, 12-pin tubewith a glass envelope similar to that employed with 7- and 9-pin miniatures (but broader),and may combine the functions of as many as three or four different tubes in a single unit.

As the internal structure of vacuum tubes became more complex, the necessity arose to provide pinconnections. Here are some examples of the changing styles from 1925 to 1963. Along the back row (left toright); 4-pin and 5-pin tubes, circa 1925-1935; and the octal, 1935-19-. In the Front row: the loctal (left),and the popular 9-pin miniature tube.

Ceramic Tubes. An ultra-miniature type designed for high-frequency applications,ceramic tubes are made up "sandwich-fashion" with alternate metal electrodes andceramic spacer-insulators. They are used primarily in VHF and UHF receiver designs, inradar equipment, and in similar applications, although a few types have been designed forTV receiver and FM set work.

The Nuvistor. Just as the compactron represents the present-day ultimate multi-purpose tube construction, the nuvistor is the latest version of the metal envelope tubepopular a few years ago. Nuvistors are manufactured in the basic generic types (triodes,tetrodes, etc.), and are extremely small physically… not appreciably larger, in fact, thanthe typical transistor and actually smaller than some power transistor types. They areespecially well suited to compact receiver design and are used extensively TV, FM, andVHF receivers.

Part 2 of this article will explore the transmitting tube "branch" of the tube "familytree," as well as the development of various types of phototubes and cathode-ray-tubes.

THE TUBE FAMILY Part 2

The “second generation” of vacuum tubes, offspring’s of the first simple types, found new jobs to be done

By Louis E. Garner, Jr.

In the early days of radio, essentially the same tubes were used for both transmitters and receivers. Even today, although transmitting tubes are considered a distinct class, there is a considerable overlap between higher power receiving and lower power transmitting tube type – in construction, in design and in electrical characteristics. Hams, for example, frequently use receiver power tubes, such as the 6L6, in their radio transmitters. The low-power transmitting tube does not differ appreciably in appearance, size or power-handling capacity from the tubes used as horizontal deflection amplifiers in television receivers. There is also a good correlation between transmitting and receiving tube as far as generic types are concerned. Both classes

can be divided into such groupings as diodes, triodes, tetrodes, pentodes, and beam power tubes. Both filamentary and indirectly heated cathodes are used in each class. The tube electrodes have the same designations- plate, grid, cathode, and so on—in both. And the same general characteristic terms are used in describing both.

When we turn to specifics, on the other hand, we find that there is a considerable difference between transmitting and receiving tubes. Transmitting types, in general, are constructed of sturdier materials, and, as a result, are larger, heavier, and more expensive than their receiving type counterparts. Two extreme examples may be helpful. The 6AQ5 is a typical beam power receiving tube, while the RCA 2039 is transmitting tube. The basic specifications of the 6AQ5 include: filament volt age, 6.3 volts; filament current, 0.41 amp.; peak positive-pulse plate voltage 1100.0 volts; peak plate current, 0.11 amp.; average plate current, 0.040 amp.; and plate dissipation, 10.0 watts. The same basic specifications of the 2039 are filament voltage, 7.3 volts; filament current, 1140.0 amp.; peak positive-pulse plate voltage, 40,000.0 volts; peak plate current, 92.0

amp.; average plate current, 5.7 amp.; and plate dissipation 150,000.0 watts. These comparative specifications emphasize the primary difference between transmitting and receiving tubes: their power-handling capacity.

Components of a high-power Federal Telephone & Radio transmitting tube. Electrodes are heavy and special insulation is used to withstand high voltages and heat; heavy-duty terminals take care of high currents. Tubes of this type may be forced air-cooled or water-cooled. To obtain high powers, very high voltages and currents are required. This means that the tube's electrodes must be very heavy in order to handle the large currents without melting, and widely separated to prevent arcing at the high voltages. (Arcing can destroy a tube.) Special insulation must be used where the electrodes are mounted to withstand combination of high heat and tremendous voltages. And, of course, heavy duty terminals are needed to handle the currents. Finally, all of the above construction factors must be taken into account and balanced against the tube's designed operating frequency (which may require close spacing) and desired electrical characteristics. While maximum electrical ratings, amplification factor, mutual conductance, and similar characteristics are all important, the transmitting tube's most important single characteristic is probably its rated maximum plate dissipation. Specified in watts (or kilowatts), this is directly proportional to the amount of power that the tube can handle and hence the r.f. power it can deliver. In practice, the tube's actual plate dissipation is the difference between its d.c. plate input power (plate voltage multiplied by average plate current) and its r.f. output power. For example, if a Class C r.f. power amplifier is 70% efficient and has a d.c. input of 10 kw. (5000 volts at 2 amperes, say), it will deliver 7 kilowatts r.f. (approximately) and will have a plate dissipation of 3 kw.

With plate dissipations running into the kilowatt range for some types of tubes, it is obvious that a means must be provided for removing the heat generated if the tube is to be kept from melting. While lower power transmitting tubes are invariably convection aircooled, higher power types are either forced air-cooled or water-cooled. Medium-power tubes are often provided with radiating fins, while high-power types are equipped with water jackets. The cooling device, whether a radiating fin system or a water jacket, may be either an integral part of the tube or a separate accessory. Industrial Tubes. Except for a few special types, industrial electron tubes correspond in most ways to receiving and transmitting tube counterparts. Low-power receiving types are used in industrial controls, alarm circuits, counters, protection devices, and similar equipment, while transmitting types are found in high-voltage and high-current power supplies, welders, and induction and dielectric heaters. In general, industrial receiving type tubes, while basically similar to ordinary receiving tubes, are usually of sturdier construction and designed for continuous operation under rigorous physical conditions. Industrial tubes, as a rule, must have extremely long filament life, for equipment shutdowns—even for short periods—can be extremely costly to a manufacturer. In addition, the tubes must be able to withstand extremes in temperature, shock, and vibration.

Designed for forced air-cooling, this Amperex high-power transmitting tube has a finned radiator fitted over the plate. Gas-filled tubes are used extensively throughout industry. Thyratrons and cold-cathode tubes are utilized for motor, electromagnet, and solenoid control, while mercury vapor

rectifier tubes are employed in heavy-duty d.c. power supplies for electroplating, electrolysis, and similar work.

There is one type of electron tube that is used in many industrial applications but which is not, however, found in communications equipment: the ignitron. Used for high-capacity switching and in heavy-duty d.c. power supplies for welding, motor control, and certain electrochemical processes, the ignitron is basically a special type of cold-cathode tube in which mercury vapor is produced by a controlled electric arc. In one sense, it is a type of rectifier. Some types are capable of handling voltages as high as 20,000 volts and conducting currents as great as 35,000 amperes for short periods. In its basic form, the ignitron consists of an evacuated metal envelope (which may be double-jacketed for water cooling), a pool of mercury which serves as a cathode, a heavy metal anode, and a special ignitor of rough-surfaced material which resists "wetting" by the mercury but which projects into the pool of liquid metal. In operation, the tube will not conduct until "fired" by current applied to its ignitor electrode. A moderate-current pulse here creates high-current densities at the rough points of contact with the mercury pool,

establishing a hot arc which vaporizes the mercury, filling the tube with vapor and allowing conduction to take place between the cathode and anode. Afterwards, the anode-cathode current is sufficient to keep the arc established and to maintain current flow.

Details of the ignitron are shown above-top. At the above-bottom is the schematic symbol for this industrial tube. Phototubes. When light falls on certain metals and metallic compounds, such as cesium, cesium oxide, potassium, and zinc, electrons are emitted from the material's surface. This photoemissive effect was first noticed, although not fully understood, by Heinrich Hertz in 1887. Like many early discoveries, this one eventually led to the development of the phototube: a light-sensitive electron tube with an electrical output proportional to the amount of light falling on its sensitized surface. Phototubes are used extensively in both industrial and commercial applications - burglar alarms, automatic door openers, electronic counters, doorway annunciators, safety equipment for industrial machines, sound motion picture projectors, etc.

The phototube is a special type of cold-cathode diode. The cathode is generally a semicircular metal plate coated with photo emissive metallic compounds, the plate a small rod or wire. In operation, light falling on the cathode causes electrons to be emitted. If a positive voltage is applied to the plate (or anode), these free electrons migrate to it, producing a minute output current. Like human eyes, phototubes differ in their response to light. While their current output is directly proportional to light intensity, the current may vary considerably with identical light levels in different colors. Depending on the types of photo emissive compounds used, phototubes may be made more sensitive to infrared, ultraviolet, or to the whole spectrum of visible light. Except for physical construction and type of lead connections, the chief differences between phototubes are found in their spectral responses.

Above left, basic phototube symbols. At left: the standard phototube. At right: a photomultiplier. Above right, a typical phototube, by General Electric. Sometimes, a small amount of selected gas will be introduced in a phototube. The gas ionizes and reduces the tube's internal cathode-anode resistance, permitting it to deliver a greater current output for a given cathode illumination. Gas phototubes have a higher sensitivity than high-vacuum types but are more easily damaged by excessive voltages and are somewhat less stable. Photomultipliers. Unfortunately, the current output of standard phototubes is extremely small—on the order of a microampere or less at typical illumination levels. This fact has led to the development of a class of special phototubes called photomultipliers. Used in scintillation counters, automatic light dimmers, and in similar applications, photomultipliers make use of the principle of secondary emission (which we discussed in Part 1) to increase their current output. The photomultiplier consists of a photo emissive cathode, a series of secondary anodes called dynodes, and the output anode or plate. Depending on tube type and physical design, the dynodes may be arranged in a circle around the cathode, or in parallel lines behind the cathode, which is tilted at a small angle. Cathode-Ray Tubes. By definition, a cathode-ray tube (CRT) is a device which utilizes cathode "rays," i.e., "rays" emitted by the device's cathode. Cathode rays are, of course, streams of electrons.

Although often considered a relatively modern invention, cathode-ray tubes are, historically, even older than more familiar electron tubes. Various types of cathode-ray discharge and display tubes were used extensively in physics laboratories and schoolrooms before the turn of the century, and as early as 1897 Karl Braun developed a cathode-ray display tube very similar to modern television tubes.

The "heart" of most present-day CRT's is the electron gun. The gun is made up of a filament, an indirectly heated cathode, a disc-shaped control grid, and disc- or cylindrical-shaped focusing and accelerating grids (or anodes). Its purpose is to produce a sharp stream of accelerated electrons. The number of electrons in the stream (and hence its intensity, as well as the brightness of the spot it produces when it strikes a screen) is controlled by the voltage applied to the control grid. The beam's sharpness of focus is determined by the voltage relationships between the focus and accelerating anodes.

Display Tubes. Direct descendants of the early Braun tube, display CRT's are used extensively in TV receivers and monitors, oscilloscopes, radar equipment, and in a variety of test and research instruments. As the name implies, these tubes serve to display electrical phenomena on a fluorescent screen, either as a line, pattern, or reproduced picture... In general, display tubes are made up of an electron gun assembly, a means for focusing (if not contained within the gun itself) and deflecting the electron beam, and a fluorescent screen. Manufactured in sizes ranging from tiny units with a l''-diameter screen to giant picture tubes with 30" screens, they are usually funnel-shaped. The screen itself may be round, square, or rectangular. The envelopes or "funnels" are made either of metal, or glass, or a combination of both. Most display tubes are identified by a combination numeral-letter type number. The first number indicates the nominal size of the tube's screen, the first letter (or letters) the particular tube, and the last letter and numeral the type of fluorescent material (or phosphor). Phosphors. Typically, a type 5BP1 tube has a 5" screen with a type "P1" phosphor. Similarly, a type 20DP4 has a nominal 20" screen and a "P4" phosphor. Cathode-ray tubes used as TV picture tubes generally have rectangular screens and their size designation refers to a diagonal measurement across the face of the tube. In some cases, TV picture tubes are called kinescopes. An arbitrary system is used for identifying the various phosphors used. A type PI phosphor, for example, has green fluorescence and medium persistence; you'll find this type in most oscilloscope tubes. Type P4 phosphors have white fluorescence and medium persistence, and are employed primarily in television tubes. Type P5 phosphors have a bluish-white fluorescence and very short persistence; tubes with this type of phosphor are used for high-speed photography of electrical phenomena having a short time duration. The P11 phosphor is similar to the P5 type, but has a slightly longer persistence. Types P7 and P14 are both two-layer phosphors. The P7 type has a long persistence, first emitting a bluish light, then a greenish-yellow. The P14 type has medium persistence, first emitting a bluish, then an orange light which persists for over a minute. These two types of phosphor are useful in instruments employed to observe low-speed recurrent and non-recurrent phenomena. The last type of phosphor, P15, has a very short persistence in the near ultraviolet region, emitting a visible blue-green light afterwards; its principal application is in flying-spot scanner tubes. Electrostatic and Electromagnetic. Electrostatic CRT's are those, which employ electrostatic fields to move the electron beam obtained from the gun assembly. Electron beams, may also be deflected by magnetic as well as electrostatic fields, however. Most TV picture tubes are electromagnetic types.

In many cases, the beam may be focused as well as deflected by magnetic fields, with a permanent magnet or electromagnetic coil placed around the tube's neck near the gun assembly. In some tubes, the electron gun is aimed at an angle, rather than straight towards the center of the screen, so that gas ions (in the cathode beam), which may be produced, are sent to one side and do not strike the screen (where they could cause a damaging "burn"). Where this technique is used to "trap" ions, a separate ion trap magnet restores the lighter electron beam to its straight-line path before deflection. Some CRT's combine the basic operating features of both electrostatic and electromagnetic types. Electrostatic focusing may be employed, for example, by using a suitable electron gun, with electromagnetic means used for deflecting the beam. Cathode-ray tubes designed for color television receivers are basically similar to the tubes described above, except that several electron guns are employed and a special screen is used which fluoresces in the three primary colors: blue, green, and red. The screen itself is made up in a repetitive triangular pattern of small phosphor dots and protected by a mask, aligned so that each of the electron guns excites only its particular phosphor (blue, green, or red). Flying-Spot Scanner. The flying-spot scanner is a special type of display tube, similar to more conventional CRT's except for its phosphor. In general, it is used in conjunction with picture transparencies (such as motion picture film or slides) and a phototube to produce a sequential electrical signal (or video signal) which can be televised or used to reproduce the original picture. In operation, a raster, or rectangular light pattern of fixed intensity, is formed on the flying-spot scanner's fluorescent screen as the spot of light produced by the electron beam "flies" across the screen. This moving spot of light is transmitted through the transparent film to the phototube, where it develops a varying electrical signal, dependent on the film emulsion density at each spot and hence on picture content. The video signal obtained from the phototube is similar to that produced by a TV camera and is used in the same way. (To be continued)

THE TUBE FAMILY TREE PART 3 By LOUIS E. GARNER, Jr. August 1963

Tubes amplify weak signals, but they also do many other vital electronic jobs, as this final Part shows

WHAT ABOUT THE FUTURE of the vacuum tube? Will designers continue to develop tubes based on new principles, and improve tubes employing already-known ones? The answer to this question is probably "yes," and a good look at the types discussed in this final portion of the tube "family tree" should convince anyone that the end is certainly not in sight. The family history of the cathode-ray tube alone ably illustrates how present-day tubes are built on past developments and discoveries. The first ancestor of the CRT was in actual operation in 1897, nine years before De Forest's triode put amplification into the hands of electronic researchers. Even so, practical television had to await development of more sophisticated CRT's, and particularly that much more unprecedented invention, the camera tube.

Camera Tubes. Used in television cameras, these tubes produce a video signal corresponding to the light image of a picture or scene which is to be televised, recorded on tape, or transmitted over a wired installation. In general, camera tubes consist of an electron gun assembly, a means for deflecting the electron beam in a regular pattern, and a photosensitive target assembly of some type which can convert light patterns into electrical charges or signals when struck by the electron beam. The electron guns and deflection techniques used with camera tubes are basically like those used with display-type CRT's. The iconoscope was, for a number of years, the

most practical type of camera tube. The heart of this device is a sensitized target on which the scene to be televised is focused by a lens system. The target is of sandwich-like construction and consists of a conductive coating or signal plate, a layer of insulation, usually mica, and a mosaic pattern of tiny photoemissive globules. Each globule acts somewhat like a. miniature photocell. When light strikes a globule, it emits electrons to a greater or lesser extent depending on the intensity of the light. These electrons are picked up by a collector ring in front of the photo-mosaic, leaving a greater or lesser positive charge on each globule. The globules and the insulated signal plate behind them thus act as a capacitor. As the beam from the iconoscope's electron gun strikes each globule, it restores the electrons lost by photoemission and, in effect, discharges the capacitor, developing an output signal at the signal plate. The signal corresponds to the image of the scene being televised.

The Image Orthicon. Today, the most sensitive and widely used type of camera tube is the image orthicon. It is made up of three principal parts: an image section; a scanning section; and an electron multiplier.

The image section of the tube contains a semitransparent photocathode on the inside of the glass faceplate, an accelerating grid (No. 6), and a target which consists of a thin glass disc with a fine mesh collector screen positioned very close to it on the photocathode (front) side. Focusing is accomplished by means of a magnetic field produced by an external coil and by the proper selection of photocathode and accelerating grid voltages. In operation, the scene to be televised is focused on the photocathode, which emits

electrons proportional to the intensity of the light striking each of its areas. The electron streams are focused on the target and cause secondary electrons to be emitted by the glass disc. These secondary electrons are collected by the adjacent wire mesh, leaving the photocathode side of the disc with a pattern of positive charges corresponding exactly to the image being televised. Since the glass is quite thin, a similar charge pattern is set up on its opposite side. The glass is scanned by a low-velocity electron beam produced by an electron gun and deflected by external electro-magnetic coils. This beam is made to decelerate and to approach the glass vertically at essentially zero velocity by the potentials applied to the decelerator grid (No. 5) and the field mesh. Some of the electrons are deposited on the glass to neutralize its positive charge while others are repelled to form a return beam. The return electron beam, then, is modulated by the positive charge pattern on the target and hence in accordance with the original light image. On coming back to the electron gun area, the return beam passes through a five-stage electron multiplier similar to that employed in photomultiplier tubes, developing an output video signal. The dynodes in the multiplier section may amplify the modulated beam by 500 times or more, with the result that the image orthicon is basically more sensitive than the human eye in picking up faint light images. The monoscope is a special type of camera tube. Its basic principle of operation is similar to that of other CRT's, for it incorporates essentially the same type of electron gun and deflection systems. However, it is fitted with a permanently installed fixed pattern - such as a TV test pattern - and develops only a repetitive video signal.

Special CRT's. In addition to the cathode-ray tubes we've discussed, there are a number of special types which depend on electron beams for their operation. Among these are a variety of discharge and demonstration tubes used for classroom study and laboratory experiments, but by far the most common type is the X-ray tube. The basic X-ray tube consists of two principal electrodes: an electron source (cathode), and a target anode. The anode is of dense metal and set at an angle with respect to the electron source. In operation, extremely high voltages are applied to the two electrodes, accelerating the electron stream to tremendous velocities. On striking the target anode, the electron beam excites the metal atoms, causing them to emit ultra-short electromagnetic radiation—X rays. Since the target is set at an angle, the X rays are radiated out through the side of the tube's glass envelope, where they can be photographed and used to trace in outline the interior make-up of solid matter. UHF Tubes. Conventional receiving and transmitting electron tubes cannot be used effectively at ultra-high and super-high radio frequencies, that is, from five hundred to tens of thousands of megacycles. At these frequencies, short lead lengths begin to have considerable inductive reactance and act like coils or even r.f. chokes, minute inter-electrode capacities become short circuits, and even the time required for an electron to move from a cathode to a plate may represent several cycles of the frequency to be handled. When even higher frequencies are considered, familiar tuned circuits cannot be used and are replaced by resonant cavities—essentially hollow, metal-enclosed spaces, which behave like, tuned circuits.

The lighthouse design raises the upper frequency limit for the conventional negative-grid vacuum tube amplifiers. High transconductance, close element spacing, and very low lead inductance are the design factors responsible for the good performance of the lighthouse tube at frequencies up to 3000 mc. Tube manufacturers have designed a number of special tubes for use at extremely high frequencies. In general, these tubes have close electrode spacing to reduce electron transit time and, often, disc-shaped electrodes to reduce terminal lead inductance. Interelectrode capacities are minimized by keeping electrode supports small and shaping them for maximum spacing with respect to other tube elements. Due to their construction, many UHF tubes take on strange and unusual shapes, and are often named after their physical appearance. Thus, one firm may offer long, slim "pencil" tubes, while others produce stepped tower-like "light-house" tubes, and so on. Quite frequently, the tubes are manufactured with resonant cavities as an integral part of their structure. One pencil-type triode oscillator tube, the RCA 7533, is made with two built-in resonant cavities, one between grid and cathode and another between grid and plate. The tube looks very much like a small can. Designed for use as an oscillator in the 1660-1700 me. band, the 7533 has a plate dissipation rating of 3.6 watts and can deliver approximately 500 milliwatts. Another interesting UHF tube is the RCA 7457, a beam power type which can be used at frequencies up to 2000 me. With a maximum plate dissipation rating of 115 watts, it can handle input powers as high as 180 watts up to 1215 me. Used as a class C amplifier with 900 volts on its plate, it can deliver approximately 40 watts at 1215 me. It is designed for forced-air cooling and has a built-in finned radiator. In general, the 7457 is used with external cavities, coaxial-cylinder, or parallel line circuits. The GE GL-6299 is a co-

planar triode suitable for use as an amplifier at frequencies as high as 3000 mc. Designed for use in receivers, it has an extremely low noise rating. As a rule, it is used with external cavities or coaxial circuits. Of ceramic construction, the GL-6299 generally resembles the "light-house" tube of a few years ago. The UHF tubes we've just examined, as well as many similar types, operate on the same principles as more conventional electron tubes, except for their frequency of operation and the types of tuned circuits with which they are used. In addition, however, there-is a group of high-frequency tubes which operate on entirely different principles: magnetrons, klystrons, traveling-wave tubes, and related types. We'll examine these next. The Magnetron. Although used extensively since World War II as high-power oscillators in radar transmitters and other types of ultra-high frequency equipment, the magnetron is basically a diode. In its common form, it consists of a coaxial cathode and a circular anode (or plate) which may, or may not, be split into two or more segments. This assembly is placed between the poles of a powerful permanent or electromagnet and aligned so that the magnetic field is coaxial with the cathode and plate.

The Magnetron operation depends upon the interaction between the electron stream and a strong, constant magnetic field.

For efficient magnetron operation, a definite relationship between plate voltage and magnetic field strength in the interaction space must always be maintained.

The multi-cavity magnetron has advantages that are important in practical radar applications.

In operation, a high positive voltage is applied to the magnetron's plate. If it were not for the magnetic field, the electrons emitted by the cathode would travel in a straight, radial line directly to the plate. The magnetic field, however, forces the electrons to travel in a spiral or circular path; and if the field is made strong enough, most of the electrons swing in complete circles, returning to the cathode. These high-speed electrons, whizzing by the

plate structure, induce high-frequency currents. To obtain oscillation, then, a proper balance between anode voltage and magnetic field strength is needed, for the electron resonance must approximate that of the resonant cavity formed by the plate structure.

A split-anode magnetron can be made to oscillate at frequencies much below the natural electron resonant frequency by connecting the segments to a tuned circuit, such as a tuned line. In higher frequency types, the tuned circuit may be little more than a heavy bar of metal connecting the segments together to form a simple closed loop. Split-anode magnetrons need not be limited to two segments; four, six, eight, or more segments may be used. A different type of magnetron employs a solid anode in which small resonant cavities have been formed. The high-speed electrons moving past the cavity openings shock the cavities into oscillation. The action is somewhat analogous to what happens when a person blows sharply across the open end of a small closed tube to produce a whistle. Commercially available magnetrons operate at frequencies from a few hundred to as high as 30,000 mc. and can deliver peak output powers ranging up to 2000 kw. (2 megawatts!) when used as pulse generators, or hundreds of watts when used as c.w. sources. The Klystron. In one sense a special type of cathode-ray tube, for it utilizes an electron gun and a stream of electrons for its operation, the klystron can be used as an ultra-high frequency oscillator or amplifier. When first invented, the device was originally dubbed a rhumbatron, for the electrons were said to be made to "dance the rhumba" within the tube, since they were velocity-modulated. The components of the basic klystron tube include an electron gun assembly, a pair of closely spaced grids called a "buncher," another pair of grids called a "catcher," and an anode or plate called, in this case, a "collector," since it receives the electron stream sent down the tube by the gun assembly. There is a narrow "drift space" between the buncher and catcher grid assemblies.

<<< Klyston plate voltage must be accurate and have good regulation for efficiency. Bunching of groups of electrons as they move through tube is the basic principle of the klystron. >>>

In operation, the electron beam is aimed down the tube by the electron gun, and an r.f. voltage is applied to the buncher grid. As the electrons approach the buncher and pass through it, they are alternately slowed and speeded up, that is, velocity-modulated. To visualize how this occurs, consider that the first buncher grid is momentarily negative and the second positive. Those electrons which are approaching the first grid are repelled and slowed down. Those which are between the first and second grids are repelled by the first and attracted to the second and hence speeded up. Those which have passed the second grid are attracted "backwards" and hence slowed down. On the next r.f. half-cycle, when the first grid is positive and the second negative, the action is reversed. Thus, the net result is that the electron stream is separated into tiny bunches corresponding to the applied r.f. frequency. As the velocity-modulated stream moves along the drift space, the faster moving electrons in each bunch (or bundle, if you prefer) overtake the slower moving ones and the bunch, in one sense, becomes "stronger," for a greater number of electrons are compacted together. When these bunches pass the catcher grid assembly, they give up most of their energy, shock-exciting the tuned circuit into oscillation. Afterwards, the spent electrons are accumulated by the positive-charged collector. In practice, klystrons are operated at such high frequencies that resonant cavities, rather than conventional tuned circuits, are used to tune the buncher and catcher grids. The electron stream is generally focused by a strong permanent magnet or electromagnet placed on the outside of the tube. A tunable –klystron can be assembled by using a bellows-like arrangement for the cavities, permitting the cavity size to be reduced (to increase frequency) or expanded (to reduce frequency). Since the output signal is much greater than the input signal applied to the buncher, due to the electron concentration which takes place in the drift space, the klystron may be used as an amplifier. It can also be used as an oscillator by coupling the catcher cavity back to the buncher. While the two-cavity klystron is basic, it is not the only type produced. A single-cavity type, called a reflex klystron, uses the same cavity as both a buncher and catcher; here, a negative voltage is applied to the collector, repelling the electron stream back on itself so that it passes the double-grid assembly both "coming" and "going." More recently, a three-cavity electrostatically focused klystron has been developed. Commercially available klystrons operate at frequencies from a few hundred to over 120,000 me. (120 gigacycles), delivering output powers from less than a milliwatt (for receiver applications) to many watts ( for transmitters). Traveling-Wave Tubes. Utilizing some of the basic operating principles of both magnetrons and klystrons, traveling wave tubes (or, simply TWT's) may be used both as amplifiers and oscillators. Like the magnetron, these tubes depend on the interaction between moving electrons and a magnetic field, and, like the klystron, they employ the principle of velocity-modulation.

The traveling-wave magnetron combines functional principles of traveling-wave tube and magnetron.

Traveling-wave tube provides broadband amplification in the 3000 to

50,000 mc. Frequency range. Wave motion in the backward-wave oscillator is in the opposite direction to wave motion in other traveling-wave tubes, but the principle is the same.

The traveling-wave magnetron is one type. Consider the multi-cavity magnetron in the drawing (to the left). Suppose the circular anode were split at one point and "straightened out." The result would be a multi-cavity anode similar to the tube shown on this page (top). To this we add a plane electrode to serve as a cathode plate, an electron gun, and a collector, plus a focusing magnetic field (not shown) to keep the electron stream projected by the gun from actually touching either the anode or cathode. The anode and plane

cathode form a wave guide. If an r.f. signal is introduced at one end, it will travel to the other end. Now, if the velocity of the electron stream is adjusted to match the phase velocity (speed at which a constant phase progresses) of the electromagnetic wave moving down the tuned wave guide, the electron stream will be velocity-modulated and will transfer some of its energy to the traveling wave. The result, then, is that the output wave collected at the far end is stronger than the input signal, thus fulfilling the basic condition for amplification. A different type of TWT consists of an electron gun, a wire helix, and a collector. A tube connects the input and output wave guides at each end of the helix. In operation, a stream of electrons is sent down the axis of the helix and the input signal is fed in. The helix, acting as a coiled transmission line, transmits the input signal to its far end at an axial velocity determined by the ratio of the pitch to the circumference of the helix. If the

electron stream velocity matches the traveling wave's axial velocity, there is an interaction between the wave and electrons, transferring energy from the electron stream to the wave and thus amplifying it. Since the currents induced in the helix are displacement currents, the electron stream need not actually touch the helix and hence a strong magnetic field is generally used to focus the electron stream and to keep it from diverging over its relatively long path. The backward-wave oscillator tube (or BWO) operates on general principles similar to those employed in conventional TWT's except that the traveling wave moves in a direction opposite to that of the electron stream (hence the name). In one sense, the tube serves to supply its own "input" signal. Sometimes, a strong transverse magnetic field is used to bend the electron stream in a circular path and thus to reduce the over-all size of the tube. Modified versions of magnetrons, klystrons, TWT's and BWO's are made by a number of manufacturers under special trade names, such as "Amplitron" and "Stabilotron." Special-Purpose Tubes. In addition to the basic electron tube types we've discussed, the tube "family tree" has one branch which is literally "loaded" with twigs. These are the special-purpose tubes—those types designed for one or more specific functions and, therefore, of limited general application. A prime example is the electronic flash tube used in photographic equipment, essentially a gas-filled triode with a trigger electrode. A mere description of all the various special-purpose tubes would fill a book, so we'll just examine a few representative types. The voltage-regulator (or VR) tube is a diode filled with an inert gas having a specific ionizing potential, such as neon, argon or krypton. In operation, this tube acts like an open circuit until sufficient voltage is applied to ionize its gas. At this point, it “fires” and maintains a constant voltage drop, drawing a greater or lesser current (within its rated limits) as the applied voltage varies. The purpose is to hold the output or resultant voltage constant.

The gas-filled regulator has a constant drop between anode and cathode.

Used in radiation detectors, the Geiger counter tube is also a gas-filled, diode. Generally, the tube is a thin metal shell with a coaxial wire or rod-like electrode. In use, a high d.c. voltage is applied to the two electrodes. If an alpha or beta particle or a gamma ray enters the tube, the gas is ionized momentarily, permitting conduction to take place and delivering a pulse of current. Each time another radioactive particle enters, the tube

delivers another pulse. These pulses can be amplified and used to drive a loudspeaker or headphones or, if preferred, fed to an electronic counting circuit. The number of pulses in a given period of time (that is, the pulse rate) is proportional to the number of radioactive particles or rays which enter the tube, and hence to the intensity of the radioactivity measured.

The gas-filled radiation detector tube depends on ionization of gas by high-velocity atomic particles. There are a variety of indicator tubes, with the simple neon bulb being a prime example. Another type is the Nixie tube. A

cold-cathode gas-filled type, this tube is equipped with a number of cathodes, each shaped to represent a numeral from 0 to 9. When an ionizing voltage is applied between one of the cathodes and the common plate, that cathode glows, rendering the numeral visible. Nixies are used as read-out devices in computers and counters.

Nixie indicators are gas-filled tubes having cathodes that glow when passing current.

The famous tuning-eye tube is another type of indicator tube. In its basic form, it consists of a cathode, control electrode, and fluorescent screen to which a positive voltage is applied. The electrons leaving the cathode are attracted to the fluorescent target, causing it to glow. When a negative voltage is applied to the control electrode, it repels these electrons, leaving a "shadow" on the target, with the shadow area proportional to the amplitude of the applied d.c. voltage. Sometimes, a tuning-eye type indicator tube and triode are combined in one envelope. Used in computers, counters and similar equipment, the beam-switching tube is made up of a cathode, beam forming and holding spades, shield grids, switching grids, output (target) electrodes, and rod-type permanent magnets. In operation, the electrons emitted by the cathode are formed into a narrow beam by a combination of magnetic and electrostatic fields. This beam is held in a fixed position by the potentials applied to the various electrodes, but can be switched from one target electrode to another, in rotation, by applying suitable voltages to the switching grids.

In the beam-switching type of indicator tube, deflector electrodes direct the electron beam to the desired target anode. Such tubes may have 20 or more anodes.

Other special-purpose tubes include ionization and thermocouple-type vacuum-gauge tubes; tubes in which one of the control elements is mechanically linked to an external pressure button so that the tube can be used as a mechano-electronic transducer; types with built-in fixed resistors used as ballast tubes; special T-R (transmit-receive) tubes to prevent the application of transmitter power to a receiver when both units share the same antenna system; and many, many other kinds. The Future. Electron tube manufacturers are constantly seeking ways to improve their tubes and to develop new types to meet the needs of equipment designers and manufacturers. Great efforts are being expended in the development of UHF and microwave tubes. Several firms are working on tubes requiring low operating voltages, which will be competitive with the transistor. One firm has developed a tube without a filament—it's designed to be used in an environment so hot that the cathode requires no additional heating. And an envelope-less tube has been developed for use in the vacuum of outer space. How will the tube "family tree" branch out in the future ? Even an educated guess is likely to be wrong. Only two things are certain: There will be many new types of tubes introduced in the next several years, and the "tree" will keep right on growing!