Hey all,
I have a MA50H that I've had for about 6 months which just developed a problem today. I turn the head on to warm up, no problem, no hum. When I take it out of standby the head develops a really loud hum after 15 or 20 seconds (after the power tubes warm up).
Question 1 - Sounds like I have a power tube issue to me. What do you think?
Question 2 - Can I take out each tube individually then run the amp long enough to determine if the hum is eliminated thus identifying the bad tube? I know this is a common practice in other amps, just wanted to make sure it's ok to do with the MA50H.
Question 3 - I notice there are two trim pots located in the area of the PCB were biasing is adjusted. Is there a trim pot for each pair (1 trim pot for the 50 watt model and 2 trim pots for the 100 watt model)? If one trim pot, then which one do I use to adjust the pair of EL34 tubes? I'm sure I can figure it out but I wouldn't mind taking the mystery out of which one to use.
Note: I'm familiar and have experience with adjusting the Bias on an amp and know the precautions to take. Somewhere in my notes I have the current to shoot for, I think it's in the neighborhood of 35mA or 37mA. Can someone confirm this?
Thanks in advance to any and all advice.