Doug Smith W9WI wrote:
semi-dumb question: where are you measuring the bias voltage? (directly
at the tube, or on the other side of the grid resistor?)

I'm wondering if a bad coupling cap is causing the bias at the tube to
drift towards 0v or even positive.

Well, I WANT the bias to be circa -20V or -25V. There's a negative
supply and some pots which form a stiff voltage divider and supplies
grid bias through a 100K resistor.

On the "bad" tube grid current is so high that it does indeed drift up
to 0V in a minute or so. Yeah, I know, there's not supposed to be grid
current until the grid goes positive. I suspect this is the root of the
problem.

The coupling capacitors are indeed good and moving the tube around it
follows the tube.

Tim.