The voltage is amplified by V1 and a larger version of it appears at the anode (pin 8) of V1. The high frequency voltage across L1 is fed to the grid of V1 via capacitor C2. A tuned circuit works by swapping energy stored as a magnetic field in the coil and the energy stored as charge in C1. L2 and C1 form a tuned circuit, which separates out the station tuned. Signals arrive at the antenna and cause small voltages to be developed across coil L1. The present circuit operates the valve at starvation levels with about 2 volts at the anode of the triode, nevertheless, the valve operates in regenerative mode, and with a small aerial will break into oscillation if the reaction control is advanced too far. In this application, it would have run with high voltages of about 170 volts at several milliamps. The PCF 801 was designed to be used in television tuners. (There is an ECF801 with 6-volt filament – but you might have to pay more for it). Although the PCF801 should have an 8.5 volt filament supply, it seems to operate quite well from 6 volts. I found the best solution to be a 4 Amp-hour lead gel battery. The low tension (filament) supply is a nuisance. Use the cheapest batteries you can find for the 36 volt supply – current drawn by the radio is less than 1 milliamp. Although the set will work off a single 9 volt high tension supply, it seems to work best from 36 volts (four 9 volt 6F22 batteries). Since this set is intended for young persons, I didn't want to have any high voltages, such as one normally associates with valve equipment. The circuit is based on a design by EN Bradley for a pocket one-valve receiver, which used a 3A5 (DCC90) valve.
The triode section is used as a regenerative detector, and the pentode section (wired as a triode) is used as a stage of audio amplification. Since there are really two valves in one glass envelope, the set is really a two-valve receiver. The only reason for this choice was that my local electronics supplier had a glut of these valves. This design uses a PCF801 triode-pentode valve. I built it to fill the gap between crystal sets and superhet radios in my collection. Layout of parts is not critical and although I have not tried it with other valves, I suspect that any triode-pentode or double triode may be used. It should be possible to build it over a weekend and it doesn't require any special tools other than a soldering iron, multimeter and electric drill.
This is a simple one-valve radio for AM reception from approximately 500kHz to 1.8MHz.
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