Greetings,
Working on some 8 watt mono bloc 300B amplifiers with a solid state power supply. The amps work flawlessly for a few days. On the third or fourth day, one of the amps will burn-up the 300B's cathode resistor (10 watt units) within a minute upon turning the amp on. The resistor does have a bypass cap of 100uF which will also go up in smoke after the resistor goes bad (obviously). Replace the resistor (2 pieces 5 watt WW in parallel for testing) and the amp is good till the next time.
I would suspect the 10 watt resistor rating should handle things according to the math as I am showing a dissapation of about 6.5 watts. I am wondering about the tube itself but if it makes through the power-up, it's fine. What is going on upon the initial fire up that would cause this.
Should I install a separate switch to turn on the filaments first and let them come up to temp before applying the B+? If so, what would be the best way to configure this? Although, I would rather figure out the problem first rather than install a band-aid.
I have gone over the circuit many times and even installed all the values into SE Tube Cad and everything looks good there also. I am now using 25 watt aluminum finned Dale resistors for testing which I know will handle the rating.
Any ideas would be greatly appreciated. This one has me baffled.
Thanks in advance for your throughts.
