Circuit :  Ramon Vargas Patron - Lima Peru
Email :ramonvargas15353@gmail.com

Introduction

Below is a new hybrid design receiver featuring a thermionic valve (tube), FET detector and bipolar transistor amplifier, designed by Ramon Vargas. The full design notes, circuit and pictures of the finished project are available on this page, or if you prefer use the link to download and view the article in PDF format:
Article in PDF format:

Designing Simple Low-Voltage vacuum-Tube Regenerative Receivers
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, vacuum 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, vacuum tube, JFET and BJT. We would like a vacuum-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 vacuum-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 40 Volts 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 characteristics (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 author has been testing the circuit with a partially exhausted battery measuring 1.38 Volts with very good results.

Fig. 1 Experimental Hybrid Regenerative AM MW BCB Receiver

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 vacuum 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 aluminium 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.

1A5GT Datasheet
1A5GT Datasheet