Another idea would be to use a second low power modulation transformer with a
separate amplifier feed off the same mike amp and with it's own level
adjustment to only modulate the screen. You'd need two level adjustments in
addition to the main mike gain control to set the proper levels for the high
power plate modulator and the low power screen modulator. The screen
modulator would only need to be a few watts, maybe single class A 6L6 or 6V6.
I realize this is getting more complicated than just the choke in the screen
circuit, but that's the beauty of home brew, trying things out just for the
heck of it.

I sometimes tend to be perfectionist, and I would then not like to start a
project pursuing a compromise solution from the very beginning. So, I would like
to have the screen modulated too, not just the plate.

With regard to the separate audio amplifier for the screen, it should work but I
am not sure whether some phase shift could occur between the two amplified audio
signals (apart from the 180-degree phase ambiguity that can be easily resolved
crossing the wires of one of the two modulation transformers).

I have come to the conclusion that using a filter choke on the screen probably
is the simplest way to go, and the safest for the final tube too.

73

Tony I0JX

Antonio Vernucci wrote:
I sometimes tend to be perfectionist, and I would then not like to start a
project pursuing a compromise solution from the very beginning. So, I would like
to have the screen modulated too, not just the plate.

I think that if you do this, you have higher audio distortion even though you
have a slightly higher modulation level.

With regard to the separate audio amplifier for the screen, it should work but I
am not sure whether some phase shift could occur between the two amplified audio
signals (apart from the 180-degree phase ambiguity that can be easily resolved
crossing the wires of one of the two modulation transformers).

It depends mostly on how well matched the modulation transformers are. Most
of the group delay will be the result of the transformers.

Better to use a transformer with two secondaries than two individual amps.

I have come to the conclusion that using a filter choke on the screen probably
is the simplest way to go, and the safest for the final tube too.

Is the best possible fidelity important to you? Is having a tiny little bit
more modulation important to you? I think in the manuals to the Bauer 707
broadcast transmitters there is a paragraph or two about the compromise.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."