After a thorough inspection against the schematic I noticed that resistors R311 & R312 are missing. So in the past these got bad and somebody decided to just leave them out of the circuit... I soldered new resistors into place (100ohm/2watt) and when I start up the amp, still in standby mode, everything seems to work just fine. The heater wires of the power tubes start to glow. But when I switch the amp off standby mode, I hear a loud pop and the amp starts to buzz. Immediately followed by all power tubes red plating. I tried to measure the voltages on pin 3 and 5 but I get zero results.
I noticed someone has been inside the amp before. They already changed a filter cap and screen grid resistor in the past. So now I'm thinking what could be causing this.. It's a bias issue so what should I look for next... Screen grid resistors? Grid leak resistors? Grid stopper resistors? Diode? Bad filter caps? The resistors seem to look just fine. I'll give everything another inspection try to see if I missed something... But I would love to hear the opinions from you guys, maybe I'm missing something?
