Bias issue

[boltd]

Thanks much, Paul.  The degradation rate is now at ~0.3V per day on both sides, so a periodic (weekly or so) fine tuning is ok with me.  I am curious about the cause though; I do fairly random but infrequent checks, I classify as degradation since all I ever see is lower voltages. 

Guy

[Paul Joppa]

This is probably the cathode being "formed" - once the process is complete, the drift will stop.

Cathode forming takes place as one of the last steps in manufacturing the tube, but it is often either incomplete (if the tube is new) or has degraded (if the tube has sat in the box for a few decades).

[boltd]

Thanks Paul. I was going to otherwise spend money a higher quality Vishay trim pot.

Guy