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In Reply to: RE: 300B - Is this thermal runaway ? posted by Burney on October 26, 2021 at 06:46:35
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.
Follow Ups:
Thanks Mr Joppa for the pointers.
I should have added in my OP that I have rechecked all solder joints, caps etc several times. Coupling caps tried included Mundorf Al oil 450v, Soviet K75 0.1uf 500v and some Arcotronics FF.
The 300B tubes work fine in an ebay PCB module that uses 6SN7's in cascade. In that board Vk is about 70vdc with an 1k Rk.
For 300B filaments I am using an ebay module that uses 1 qty of LM317. This ramps up the 5vdc slowly. Could this module be causing this grief? Current starvation? Filament voltage is stead at 5vdc as set.
Would it be helpful if I remove this regulator module and go AC or a simple bridge+caps arrangement for the filaments?
Thanks once again.
As you have tried 3 different caps, that seems unlikely to be the problem. But the grid is being raised 275v above ground, and there must be a reason. Several candidate reasons (oscillation, open grid-ground resistor or connections, leaky coupling cap) have been covered in this thread, and rejected for reasons - it may be necessary to question those reasons more deeply. Or come up with another cause, such as a short from B+ to grid
I wouldn't worry about the low current draw until that issue is resolved.
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