Mark III Blue resurrected, but now Full Power not working

[bennyjett]

Hello! New guy here.

I searched this forum for similar issues to no avail. So now I'm asking for anyone's help before I call Mesa.

Long story short: I flipped the switch on, then a few minutes later, took it off standby. Nothing but an unusual hum on both 1/2 pwr and Full pwr, even when plugged in to the effects loop. The outer right power tube glowed a bright orange at 1/2 pwr. At Full pwr, the inner right tube glowed as well. I then replaced all 4 tubes but still had the same problem.

I opened up the chassis and discovered the screen grid resistor was split in half on the outer LEFT socket. Mesa sent me a new resistor and I replaced the broken one. Now at 1/2 pwr my amp is as awesome as ever (with a lot less noise), but at Full pwr she sounds like I'm playing through a small transistor radio! Interestingly, both right tubes STILL glow with both the old tubes and the new tubes at Full and 1/2 pwr. What to do?

Any input would be greatly appreciated!

Ben

 

[Restless Rocks]

Sounds like you have a loss of bias voltage to the output stage, either from a shorted tube or damage to the circuit from a previous shorted tube. Take it to an authorized Mesa Repair Tech.