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