Recently went back inside my 6LU8, as I had had an issue previously where one tube had a much higher triode plate voltage, 170, vs the 155 design. It moved with the tube.
So, I bought a new tube, and swapped it out today. While I was in there, I found that the plate resistor on one side was poorly connected, so reflowed that joint. Poked around a bit to see if all voltages were proper etc.
When I reconnected the amp to the source and speakers, it sounded better than ever! REally enjoyed about two-three hours of listening. Then the speakers were making loud popping noise. I shut it down, checked all connections, later powered it up, and the popping started right away, shut it down.
Later, just now, I put it back on the bench and carefully brought up the mains to about 100vac, and the tubes started conducting, speakers were quiet unless I touched the input, wherein it made a hum. So far so good. Nothing seems out of the ordinary. I cranked up the power to 115vac, and the popping started again, this time I see that one cap is sizzling, and then popped! I shut down immediately.
Now this is the cathode bipass capacitor on the right channel. It is also the channel with the new tube.
What would make the cap fail? it is a 220uf, 50v cap. It was installed with the correct polarity. I can replace the capacitor, but what should I look for in order to determine what made it fail? How can I do this safelt without damaging anything else?
I will place this question in AK tube forum as well, hopefully someone in either of these places can help