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One of my goals as a new ham is to homebrew my own transmitter (using vacuum tubes, aka valves). Initially I'll stick with CW for the transmitter, though over time I might build an AM modulator or even attempt an SSB modulator/filter. For the CW iteration, I plan to use a vacuum tube Hartley oscillator, with the tap on the oscillator coil positioned to minimize drift on key-down.

I've read of some CW transmitters with high voltage at the key, suggesting they were interrupting the B+ to the output tubes (for power levels above QRP, this may be several hundred volts; for higher power, it may run to several thousand). Obviously, if I do this, I'd use a relay to keep the high voltage inside the case.

For other designs, however, I've seen reference to keying the oscillator -- either interrupting the output of the oscillator before it reaches the first output tube's grid, or interrupting the plate or grid on the oscillator's own tube to completely stop the RF (potentially also leading to relatively high voltage at the key).

As I understand it, both can work well enough to have survived until transistors replaced vacuum tubes for most applications, and obviously the quality of the keyed output depends on multiple other factors (stability of the oscillator, rise and fall time of the chosen keying control, etc.).

From the standpoint of signal quality, however, is there a good reason to prefer one over the other?

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  • $\begingroup$ How will you control frequency - crystal or VFO? $\endgroup$ – Brian K1LI Mar 22 at 18:57
  • $\begingroup$ My intention is to use a Hartley oscillator -- the coil tap can be positioned to virtually eliminate chirp, but let me cover a whole CW sub-band. $\endgroup$ – Zeiss Ikon Mar 22 at 18:58
  • $\begingroup$ I'm not overly familiar with tube-based designs: Is the Hartley oscillator's output already what you put onto the antenna, or is there another PA stage after that? $\endgroup$ – Marcus Müller Mar 22 at 19:01
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    $\begingroup$ @ZeissIkon we accept that not only because of clicks, because of physics not allowing a finite source of power to have non-zero spectral power density over an infinite bandwidth ;) You can't have minimal bandwidth and fast on/off at the same time – they are mathematically the opposite! Like, literally 1/switch time = bandwidth. $\endgroup$ – Marcus Müller Mar 24 at 6:48
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    $\begingroup$ @ZeissIkon sure, a drifting oscillator is bad for SNR, but you don't gain SNR by decreasing the switch speed beyond what the filter bandwidth in the receiver lets through. $\endgroup$ – Marcus Müller Mar 24 at 11:27
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Highest frequency stability is generally achieved when the oscillator is kept running while some following stage (or stages) is (are) keyed. However, the very high Q of a crystal-controlled oscillator permits direct keying. While the cathode, grid or plate of any stage can be keyed depending on design goals and available parts, an oscillator should be keyed outside of the primary frequency-determining circuitry. Cathode keying does not place high AC or DC voltage on the key terminals.

"Chirp" can also result when the load on the oscillator output varies from key-up to key-down. This can be avoided by providing a "buffer" amplifier stage between the oscillator and the amplifier. For example, K5DH's 807 CW Transmitter uses a 6AG7 pentode either as a cathode-keyed crystal-controlled oscillator or as a cathode-keyed buffer amplifier according to the position of SW1, which brings the V1 cathode to AC ground through a .01uF bypass capacitor. Note that the cathodes of both the oscillator/buffer and the amplifier stages are keyed to prevent oscillator feed-through to the antenna during key-up.

Radios like the Heathkit DX-60 and the Drake 2-NT employed a similar architecture but used grid block keying, which is thoroughly explained in the cited manuals, placing a high negative DC voltage on the key terminals.

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