Rimshot
fireberd
Is that 12AX7 running low plate voltage ("starved plate") operation?
Hi fireberd, I am sorry but I don't know the answer to your question. Where can I find out?
I'll offer a creative way to do this safely for a non techie person:
Look at the largest capacitors (the blue canisters with a white stripe) and read their voltage rating.
The 12ax7 is nominally run at 200vDC. A cap on the "B+" voltage supply will be rated just above what this amps run the tube at. It could be a 250v cap on a 200vDC supply or it could be just about anything lower now that using tubes, in solid state amps, as a bullet list feature has become so routine.