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Original Message
RE: 300B - Is this thermal runaway ?
Posted by Paul Joppa on October 28, 2021 at 13:10:41:
A few ideas re: "300B grid to ground goes up to almost 275 VDC and the Cathode voltage similarly climbs to about 120 VDC before I switch off."
If this is accurate, it seems likely the grid resistor is not connecting the grid to ground. Could be a cold solder joint or a bad socket connection/corroded grid pin on the 300B. Both might make contact when cold, but open up when warm - possibly triggered by movement when adjusting the volume control.
The other alternative is a bad coupling cap between the two stages. What cap are you using, in particular what is the voltage rating?
The low current is a separate issue. Possibly an exhausted tube, especially if it has been running excessive current reccently.