Hi,

I know the screen is drawing excess current causing the screen
voltage to fall off. The control grid current looks normal and
behaves normally with tuning controls. I don't get it?? How can
I have excess screen current and yet have the low plate current?
The tank circuit seems normal, the tuning controls appear to work
properly, in fact every once in a while the unit will work perfect
for a minute or two making 250 watts output, then it goes back to
acting weird again.

I would say that the screen current is high because the plate
voltage is swinging down too far (i.e. there is little or no
loading on the tank circuit or it has become mis-tuned). One
possibility, if it has one, is an open plate blocking capacitor.

BTW, the screen current on that series of tubes is often used
as a tuning aid; some metering circuits even having a negative
screen current position for when the grid gets too hot and emits
electrons.


Cheers - Joe, G3LLV