One Valve Battery Superhet

Valve era parts can sometimes be hard to find, so I decided to see how relatively modern transistor circuit parts would work with valves. One of my first experiments was to make a superheterodyne, using a 1R5 pentagrid with transistor coils.

The first working revision, with 1920's headphones.

This radio uses a cheap ferrite coil, 60pF/160pF plastic variable capacitor, and 455kHz oscillator and IF output coils.

Initial circuit

The circuit was taken from the mixer stage of an example in the back of an RCA tube manual. (RCA Portable Superheterodyne Receiver, RCA-16, 16-1, 1950 edition.) The full circuit is a 4 tube radio, mixer, IF amp, detector & 1st audio, and speaker output. I'm just using the mixer/oscillator stage to tune a radio frequency signal and convert it to an intermediate frequency, then detecting that with D1. I made no attempt to optimise the audio out stage, it could be improved by treating it as a crystal set, and matching the headphones accordingly. I had other plans...

Building notes:

Had to reverse the polarity of the main oscillator coil! The obvous direction for the secondary didn't work. At that I was lucky that it burst into life, a 1/4 turn of the adjustment slug in either direction silences it.

Used the final IF can, since I plan to connect the secondary straight to a detector.

Tuning using Test Oscillator & Frequency counter was rather easy. Attached 455kHz signal to grid 3 of 1R5, adjusted IF slug for for peak volume in headphones. Used 600kHz & 1600kHz to peak oscillator can, osc. trim and antenna trim, feeding input through antenna winding of coil.

Tuning range: 600kHz - 1650kHz. (Should be able to manage 530-1650, but it doesn't.)

The oscillator is tuning 1.04MHz - 2.04MHz according to my frequency meter, however, my meter is pulling the circuit noticably down. The real frequencies are somewhat higher. The range is about right though.

Current Draw: 1.5V 50mA, 45V 1.5mA

Tried using it with 22.5V, 0.6mA, much quieter, not really usable. At 67.5V, 2.5mA, noticably louder. However, the capacitors are rated at 50V, and the IF transformers are probably not rated any higher. So I stick to 45V.

Checked the signals over the IF transformers, at 45V, with 10Meg probe on the oscilloscope, the output transfomer has ~2V AC across it's primary. The oscillator transformer has ~1V AC across it's primary, and ~6V AC across it's secondary.

Using it:

Needs good aerial and earth, doesn't have the sensitivity to pick up anything with just the ferrite. Gets a few stations, but the ones it does get are very sharp. My local, overly-strong station 3MP doesn't smear across a hundred-plus kHz like it does with the crystal sets and regens.

Tried feeding in an artificial AGC voltage to the L1-C1 junction while listening to 3MP to see what would happen. Peak volume was around -1V. Probably less than that was shorting signal to ground through the pot. Volume faded out at about -2.5V. Adding the pot & meter noticably dropped the volume though, probably pulling the valve and reducing some of the signal to it.

IF transformer experiments

All of the above was done with the black output transformer. That probably has an imperdance ratio of around 20K:5K. I don't have any data on the particular IF transformers I'm using, but that seems typical for the small can type.

I tried putting two white IF coils back to back. They are probably something like 30K:500 ohm. That should be giving me significantly higher impedance for the 1R5 to work into, at the expense of higher output impedance. However, it also gives two tuned circuits to filter though.

Seems to perform better overall, although the tuning range dropped for some reason. Maximum is now about 1450kHz.

Final circuit

Conclusions

In general. It works! I should try winding my own oscillator coil, to see if I can get back the tuning range that it should have. The back to back IF transformers works to keep impedances higher for valve circuits.