I'm interested in building a VFO based single tube CW transmitter.

These devices are known to have issues with frequency drift and chirp; frequency drift can apparently be mitigated by ensuring constant temperature for certain components (coils and capacitors), but chirp is an effect of changes in current within the active element(s) -- in this case, the vacuum tube.

My plan is to use a pentode, since I have a couple miniatures on hand and they offer more gain and more options than triodes. In order to get more than milliwatts to the antenna, however, I need to run about 90V on the plate (better than 400+ on some larger tubes, which might be a second iteration for this project). The question, then, is whether there's a way to key the output of a single pentode while keeping it running continuously as an oscillator, while avoiding running high voltage to the key.

Obviously it would be easy to key a low voltage to one of the grids to start and stop the oscillator, or to do the same for a second tube acting as an RF amplifier -- but is there a way to key a low voltage and keep the single tube oscillating stably?

One way that comes to mind is to key a relay in the output side -- the relay coil runs on 6V or 12V, but depending on the relay part, can switch several hundred volts. With a double throw relay, this would also let me automatically switch the antenna from TX to RX (normally necessary by one means or another anyway to protect the RX from TX power) and be able to hear between code pulses. The output from the TX, in this case, could be routed to a dummy load when the key is up.

Yet this doesn't seem to have ever caught on in the general amateur radio world -- despite setups I've seen that automatically switch antenna on key down for commercial transmitters. Burned contacts in the relay? Other reasons?

  • $\begingroup$ I think leakage across a relay will still be too much, and there will still be clicks. How about having the key just shift the oscillator frequency by 5 kHz? Maybe the residual chirp would be more manageable? Especially if you switch something soft like a varactor or grid? and not just a capacitor in the tank circuit. $\endgroup$
    – tomnexus
    Sep 22, 2023 at 5:33
  • $\begingroup$ Not enough for an answer but maybe a pointer for further research. Since you are basically building a "retro" rig it may be a good idea to go back to sources of the time. E.g. the ARRLHandbook of 1941 has a chapter on transmitter construction, including a "simple tetrode oscillator transmitter" and also a chapter on "Keying the Transmitter" containing among other subjects also a section on oscillator keying. $\endgroup$ Sep 22, 2023 at 18:19
  • $\begingroup$ Thanks, @henryflower. I'll check the 1959 book I already have as well as "Radio Elements" which I also have. What I recall from previous reading of those, however, is that chirp and drift weren't as big a deal in the 1940s, or even the 1990s. Spectral purity requirements from FCC would get a perfectly normal novice rig (even a crystal controlled, aka "rock-bound" one) from the tube era confiscated and fined today... $\endgroup$
    – Zeiss Ikon
    Sep 22, 2023 at 18:34
  • $\begingroup$ I seem to recall back in the day the "good" transmitters used a "screen block" method of keying, apply a voltage to the screen of the appropriate stage to stop transmitting while allowing the VFO to run. That may not apply in a single tube rig though. $\endgroup$
    – Duston
    Oct 7, 2023 at 19:54
  • $\begingroup$ @Duston That might be possible with a pentode -- the second grid might act as a plate to run the oscillator and the third be keyed to block power from the actual plate. It would require a pentode with the suppressor grid not tied to the cathode/filament, though; I'd have to look up what tubes I have, since some are internally tied and some aren't. $\endgroup$
    – Zeiss Ikon
    Oct 8, 2023 at 11:28


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