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Designing simple low-voltage vacumm-tube regens

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Designing Simple Low-Voltage Vacumm-Tube Regenerative Receivers Ramon Vargas Patron [email protected] [email protected] INICTEL-UNI The design and construction of RF and AF electronic circuits using vintage discrete technologies has been a very rewarding part-time entertainment for decades, and will certainly continue to be so in the future. You can find everywhere experimenters acquainted with solid-state technology replicating with great interest old technology approaches for amplifiers, oscillators and radio receivers, and even mixing them in a particular design. We are referring to the use of Galena (PbS), Pyrates (FeS), germanium and silicon diodes, vacumm tubes, germanium and silicon bipolar transistors (BJTs), junction field effect transistors (JFETs), 555 IC timers, and even logic ICs as signal amplifiers when biased for linear operation. We can find there are myriads of possible applications. This article will show how to build an AM MW BCB regenerative receiver using mixed technologies, specifically, vacumm tube, JFET and BJT. We would like a vacumm-tube RF stage followed by a JFET buffer stage and a BJT pre-amplifier, just for headphone listening. The direct approach for a regenerative receiver design employing vacumm-tube technology usually starts with a review of the technical specifications of the devices selected, the required power supply, parts and components. Amplifying tubes have generally been designed to work with more than 40Volts DC on plate (anode), imposing a threat on unaware folks. Portable electronic equipment made use of “B” batteries for the plate voltages. Common values for these were 45 Volts, 67.5 Volts and 90 Volts. “A” batteries were used for filament heating, being 1.5 Volts, 2 Volts and 7.5 Volts the most used types. A careful study of the plate-current vs plate-voltage characteritics (Ip vs Vp) of some amplifying tubes led curious experimenters to test the operation of these devices with low plate voltages. These tests were successful for many of those tubes designed for portable operation on batteries. The author tested a couple of these, i.e., a 1A5GT, a filament-type power-amplifier output pentode for use in low-drain battery-operated equipment (see Annex I), and a 2SH27L, a Russian universal pentode featuring also directly-heated cathode operation (Annex II). The suppressor grid of the first tube is internally tied to the filament (cathode), while the second tube has that grid wired for external connection. In this article we shall dedicate efforts towards the construction of a regen with the 1A5GT pentode. In a next article we will describe a Transitron negative-resistance type AM MW BCB receiver using a 2SH27L pentode. Let see Fig.1. It shows the schematic diagram of an experimental regen for medium wave frequencies built on a solderless breadboard using the 1A5GT. It uses a 1.5-Volt alkaline battery for filament heating and a 14-Volt DC supply for the “high tension”of amplifier stages. Regeneration is adjusted varying the screen grid bias voltage, and is quite smooth on its action. The “high tension” supply can be reduced to 12 Volts DC and still get
Transcript

Designing Simple Low-Voltage Vacumm-Tube Regenerative Receivers

Ramon Vargas Patron

[email protected]

[email protected]

INICTEL-UNI

The design and construction of RF and AF electronic circuits using vintage discrete

technologies has been a very rewarding part-time entertainment for decades, and will

certainly continue to be so in the future. You can find everywhere experimenters

acquainted with solid-state technology replicating with great interest old technology

approaches for amplifiers, oscillators and radio receivers, and even mixing them in a

particular design. We are referring to the use of Galena (PbS), Pyrates (FeS), germanium

and silicon diodes, vacumm tubes, germanium and silicon bipolar transistors (BJTs),

junction field effect transistors (JFETs), 555 IC timers, and even logic ICs as signal

amplifiers when biased for linear operation. We can find there are myriads of possible

applications.

This article will show how to build an AM MW BCB regenerative receiver using mixed

technologies, specifically, vacumm tube, JFET and BJT. We would like a vacumm-tube

RF stage followed by a JFET buffer stage and a BJT pre-amplifier, just for headphone

listening.

The direct approach for a regenerative receiver design employing vacumm-tube

technology usually starts with a review of the technical specifications of the devices

selected, the required power supply, parts and components. Amplifying tubes have

generally been designed to work with more than 40Volts DC on plate (anode), imposing

a threat on unaware folks. Portable electronic equipment made use of “B” batteries for

the plate voltages. Common values for these were 45 Volts, 67.5 Volts and 90 Volts. “A”

batteries were used for filament heating, being 1.5 Volts, 2 Volts and 7.5 Volts the most

used types.

A careful study of the plate-current vs plate-voltage characteritics (Ip vs Vp) of some

amplifying tubes led curious experimenters to test the operation of these devices with low

plate voltages. These tests were successful for many of those tubes designed for portable

operation on batteries. The author tested a couple of these, i.e., a 1A5GT, a filament-type

power-amplifier output pentode for use in low-drain battery-operated equipment (see

Annex I), and a 2SH27L, a Russian universal pentode featuring also directly-heated

cathode operation (Annex II). The suppressor grid of the first tube is internally tied to the

filament (cathode), while the second tube has that grid wired for external connection. In

this article we shall dedicate efforts towards the construction of a regen with the 1A5GT

pentode. In a next article we will describe a Transitron negative-resistance type AM MW

BCB receiver using a 2SH27L pentode.

Let see Fig.1. It shows the schematic diagram of an experimental regen for medium wave

frequencies built on a solderless breadboard using the 1A5GT. It uses a 1.5-Volt alkaline

battery for filament heating and a 14-Volt DC supply for the “high tension”of amplifier

stages. Regeneration is adjusted varying the screen grid bias voltage, and is quite smooth

on its action. The “high tension” supply can be reduced to 12 Volts DC and still get

satisfactory operation of the circuit. Exact limits for an acceptable operation will depend

on the tube and the state of the 1.5-Volt battery. The autor has been testing the circuit

with a partially exhausted battery measuring 1.38 Volts with very good results.

The stage following the 1A5GT is a buffer designed around an MPF102 N-channel JFET

for driving the rather low input impedance of the BJT output stage. The JFET stage has a

voltage gain of 1.46. The bipolar stage with a 2N3906 has a voltage gain of 310. Thus,

the overall AF voltage gain is 1.46 x 310 = 452.6, enough for comfortable hearing, as

tested. Current drain from the solid-state audio stages is about 6.8mA. The tube’s plate

(anode) current is less than 100uA DC at pleasant hearing levels.

Due to operating the receiver from two different types of supplies (one is an isolated

battery and the other is a mains-operated DC source), some 60Hz hum is noticed,

however, wrapping the vacumm tube with a coil made up with AWG#22…24 PVC-

insulated stranded copper wire and connecting one end of the coil to ground, the hum is

reduced to very tolerable levels. An aluminum ground plane was used to mount the

solderless breadboard, the air-variable tuning capacitor and the ferrite antenna assembly.

Listening is through a low-cost ceramic piezoelectric earphone or magnetic 2k-ohms

headphones. The screen-voltage control potentiometer acts doubly as a volumen control

also for listening. Sensitivity is very good as well as selectivity. A vernier reduction drive

is recommended for the tuning capacitor because tuning is very sharp due to regeneration.

Figures 2.A, 2.B, 2.C below show the receiver’s experimental layout, a front view and a

top view, respectively.

Fig. 1 Experimental Hybrid Regenerative AM MW BCB Receiver

Fig. 2.A Layout of the receiver

Fig. 2.B Front view of the receiver

Fig. 2.C Top view of the receiver

ANNEX I

Fig. ANX 1.A Power Amplifier Pentode 1A5GT – Description and Rating

Fig. ANX 1.B Average Plate Characteristics – Operation Characteristics of the 1A5GT

ANNEX II

Fig. ANX 2 Russian Universal Pentode 2SH27L

2SH27L Universal Pentode with directly heated cathode

Electrical data Limit Data

Heating voltage: 2.2 V Heating Voltage: 2.0 V min - 2.4 V max

Heating current: 57 mA Anode Voltage: 200 V max

Grid 1 voltage: 0 V Voltage Grid 2 [screen grid]: 120 V max

Grid 2 voltage [Screen grid voltage]: 45V Anode Power Dissipation: 1.0 W max

Voltage at Grid 3: 0V Screen Grid Load: 0.3 W

Anode Voltage: 120V Maximum Cathode Current: 5.0 mA

Grid 2 current [Screen grid current]: 0.35mA

Anode Current: 1.9mA Tube’s Base circuit

Slope: 1.25mA /V 1 - cathode [heater]

Capacitance Input / Output: 0.015pF (max) 2 - inner shield

Input Capacity: 5.3 pF 3 - anode

Output Capacity: 4.9 pF 4 - grid 3

Capacitance Anode/Cathode: 0.01pf (max) 5 - grid 2

End of Life Slope: 0.85mA/V 7 - grid 1

8 - cathode [heater]

Ramon Vargas Patron

[email protected]

[email protected]

Lima – Peru /South America / October 6th 2021


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