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Switchable SE NFB Problem
So I tried to design a switchable NFB loop for a SE EL34 amp. Everything was going swimmingly with first power up until I flipped this feedback switch, switching from grounded to output transformer.

I saw a spark near the cathode bypass cap/resistor and heater connections, heard a pop and saw some slight smoke. Turned everything off immediately and couldn't find any obviously burned or damaged components. So I turned the switch back to the grounded position and powered up again. 

Voltages look good everywhere, the 356 ohm resistor wasnt shorted, but now the EL34 is red plating. I cant get a new tube for a while. So in the mean time what else can I test and rule out? 

A question and a suggestion (please forgive my ignorance):

why did you apply NFB to the Output stage? was there a specific design requirement? I read in Merlin's book that most designs apply NFB to the driver stage before the output.

If your output tube is redplating , I would start with checking your bias supply.

I'd be interested in whatever you find.


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