Wolf DL4YHF wrote:

I think the PA supply voltage is variable, but where does the "envelope" signal come from ?

Another way to control the output power is to vary the phase
of the driving signal to one of the sides of a full bridge amp'.
Some kind of variable delay / phase shifter would do the trick.

If the two "totem poles" (a word used in the ancient world of TTL
logic - meaning two transistors in series between +V and GND)
in the bridge are driven in phase, there will be no voltage difference
between the "midpoints" - zero output power. If they are driven in
antiphase, 180 deg's, like in the Decca (Jeep!?) full output will result.
The output power can be set to any convenient value by simply
adjusting the phase shift between 0 and 180 deg's.

No efficiency penalty and no need for a PWM modulator!

Any ideas for a digtally (or otherwise) controlled 0-180 phase
shifter?

73
Johan SM6LKM