I've looked for a while, but I've never found a design like what I imagine: a single-tube (valve) regen CW transceiver (no cheating with a tube/valve that has two or more triodes/tetrodes/pentodes in a single envelope).

The way I imagine this working is that you'd receive like any other regen receiver used for CW ‐ oscillation of the regen detector provides a BFO to make the CW signal audible ‐ but when the key closes, the coupling between tickler and tuning coil is increased to a level that oscillates independent of received signal. Regen receivers are well known for causing "interference" on the tuned frequency if the regeneration is increased too far (oscillation couples back to the antenna, if there's no RF amplifier stage to prevent this).

A single tube/valve regen usually produces an audio output signal audible only with a very sensitive high impedance earphone, but that signal can be fed to any common audio amplifier to provide speaker output.

I'm interested in this in part because it's the simplest possible transceiver -- a dozen or so components (not counting a modern digital frequency display, necessary to ensure it's in-band and to tune a specific frequency), capable of running on battery power, potentially small enough to carry in a light backpack (including A and B battery). I know it won't be the best sort of circuit, which I presume is why I haven't found such a design (most regen designs put effort into avoiding emission).

@tomnexus found a listing for such a one-tube transceiver, with AM from a carbon microphone, but per the article linked, "It’s doubtful whether this simple circuit would meet the current FCC spectral purity requirements for use on the ham bands. After all, even while receiving, the regenerative receiver is radiating." Now, I know it's possible to operate a regen at a level of regeneration that doesn't radiate enough to cause interference, and with a dual tickler still get enough oscillation to transmit at low power, but there's still the question of chirp, drift, and harmonics to be concerned with.

Harmonics can be addressed by a low pass filter, either directly in the oscillator (on the transmit tickler loop?) or as part of the antenna coupling, but chirp and drift are harder to address.

Is this even possible, in terms of meeting FCC standards for spectral purity, etc., or in terms of staying on the same frequency for transmit that it's tuned to receive?

  • $\begingroup$ That would be quite retro-cool to have a backpackable QRP tube regen rig! Not the lightest, most practical trail rig, but cool nevertheless. $\endgroup$
    – rclocher3
    Nov 11 '21 at 17:35
  • $\begingroup$ Well, a one or two cell lithium A battery and 4 or 5 9V for the B, whole thing would probably weigh less than a 40 m half wave wire antenna. $\endgroup$
    – Zeiss Ikon
    Nov 11 '21 at 17:38
  • $\begingroup$ How much might the radio weigh? If you build such a thing, I hope you make a YouTube video! $\endgroup$
    – rclocher3
    Nov 11 '21 at 20:06
  • $\begingroup$ One tube, a coil with receive and transmit ticklers, a few passive components, a 3D printed chassis (because I can) half a dozen rather small batteries -- whole radio less antenna should be under five pounds, possibly as little as two if built with lightness in mind. I'm not a YouTube guy, however, much less a tube circuit designer, so there'd be a lot of cut-and-try before I have a working set. And I lack an O-scope, so I can't tell if I'm violating FCC regs every time I key down (they get upset about stuff like chirp and drift). $\endgroup$
    – Zeiss Ikon
    Nov 11 '21 at 20:10
  • $\begingroup$ And I thought my Altoids-tin solid-state QRP kit radio was a strange combination of old and new technology. Your idea goes one step further! $\endgroup$
    – rclocher3
    Nov 11 '21 at 20:27

Is this even possible, in terms of meeting FCC standards for spectral purity, etc., or in terms of staying on the same frequency for transmit that it's tuned to receive?

The biggest problem is almost certainly going to be frequency stability, especially as the tube heats up while transmitting. The interelectrode capacitances will change; not to mention that the heat from the tube will change the values of the nearby resonant circuit components, causing significant frequency drift. Therefore, it's very doubtful that you would be receiving on the same frequency until it slowly cools down after switching back to receive (not so important for AM).

It's been done on VHF in 1944. But the pertinent FCC regulations have almost certainly changed since then.

And what is more, there has been no AM activity on VHF in many decades.

For HF, the regulations historically been more strict about this on HF than above 30 MHz, especially if the transmitter drifts too much. But the higher the band, the more relaxed the rules. Therefore, there very well may be an amateur band in the UHF or microwave region where it might be perfectly legal, provided that you can reduce the second harmonic and any other out-of-band emissions. (Having said that, that tube is not rated for those regions.)

Check out the FCC rules. Who knows? Maybe it's okay in some bands. Then you could get a friend to build one too so that you have someone to QSO with. :)

  • $\begingroup$ My mobile rig can do AM on 10 m, says the manual (compatible with Korean War walkie-talkie units, they say). Not much activity, but there's still a designated sub-band. $\endgroup$
    – Zeiss Ikon
    Nov 12 '21 at 17:03
  • $\begingroup$ Just looked at the reg sections linked in the article -- 100 mW input from the power source? Takes about that just to heat the filament! Even if you exclude filament heat, 100 mW in probably won't get the circuit into oscillation even without an antenna. That's hearing aid battery output levels. $\endgroup$
    – Zeiss Ikon
    Nov 13 '21 at 17:09

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