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If I'm building a vacuum tube HF transmitter (say, 40 m band), obviously the simplest mode to build is CW; it's common to key the oscillator input to the final (in order to keep the oscillator running stably, minimizing chirp and drift).

If I want to add AM to this simple transmitter, one method of modulating the output might be to use a saturable reactor, aka "magnetic amplifier," to boost the output of a microphone to be added to the B+ plate voltage (coupled through a suitable capacitance, to keep high voltage DC off the microphone); the fluctuation of the plate voltage would control the gain of the PA, modulating the output.

I don't see plate modulation used much in radio circuits I've found, so there must be some disadvantage, but I don't see offhand what that would be; I think it's just simpler in a "normal" circuit to feed the output of one stage to the grid of the next.

Is there a big problem with using a magnetic amplifier to modulate the plate voltage this way?

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  • $\begingroup$ An internet search for "AM plate modulator" turns up lots of references. There's an interesting idea titled The Simplest Modulator that claims to plug into the key jack of a cathode-keyed CW rig! $\endgroup$ – Brian K1LI Jun 5 at 19:07
  • $\begingroup$ Are you asking whether this method is more efficient –or has other advantages– than standard high-level audio plate modulation using a transformer with separate windings? That is, one winding in series with the anode line, and the other connected to an audio amplifier? $\endgroup$ – Mike Waters Jun 5 at 23:05
  • $\begingroup$ @MikeWaters If I knew exactly what questions to ask, I'd probably already know the answers. $\endgroup$ – Zeiss Ikon Jun 6 at 11:08

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