Antonio Vernucci wrote:
There are a number of rigs that measure cathode current rather than
plate current. Depending upon the tube, screen current is only a
small percentage of plate current, so even a 10% change in screen
current may result in only in a 1 or 2% in cathode current.
I have one other bizarre thought - which I admit has not been given much
thought - suppose you fed the screen with a constant current source,
so that screen current would not change at resonance?

That was the very initial question of my post. My concern was that
tuning for the minimum cathode current may not be as sharp as tuning for
the minimum plate current, because when plate current decreases the
screen current increases (though only about 1/10 in magnitude). I am not
feeding the screen through a resistor from the HV because I do not wish
to add a clamp tube to avoid screen voltage rising too much in absence
of drive. I am feeding it from a very stable low-voltage supply, so
screen current will vary.

Also, off this subject and back to screen modulation techniques, the
old Radio Handbook mentions that many beam power tubes will self
modulate, with no additional screen audio supplied. Apparently, it
can vary from tube to tube. Despite that stateement, I know of many am
transmitters using beam plate final tubes that do have audio applied
to the screen, so take the statement with a grain of salt.

The self-modulation method is just the one I am using. I have a 20H
choke on the screen path, and that will (I hope so!) self-modulate the
screen. I plan to finish work in a month from now. At that time I will
be able to report to which extent screen self-modulation works.

73

Tony I0JX
Rome, Italy

Depending on the choke inductance value and the final tube you may or
may not get full 100% modulation. But it will be close enough for
government work and will sound just fine.