SET Asylum

Single Ended Triodes (SETs), the ultimate tube lovers dream.

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RE: 300B - Is this thermal runaway ?

Posted by 33dot33rpm on October 30, 2021 at 15:52:24:
Can you supply more info regarding the problem?

When the sound cuts off (SCO), is it for both channels?
Have you tried to run the amp since?
When SCO what is the voltage at the 100uF cap?
Are you using one LM317 regulated DC supply for both 300b?
It is strange that the 300b grid goes to 275VDC when SCO, are you sure that is the correct reading (grid to ground) and you are probing the correct pin (pin 3)?

Something else is not adding up with your build. If you are sticking strictly Joe's circuit (except for 300b rk), with B+ at 413Vdc and 300b rk = 1K typically the cathode voltage should be between 65 to 70 volts while you are seeing 55 at idle. Can you test the Edcor OPT primary impedance?