I am having a problem with one of my MKIV's. It's intermittent which makes it tough to troubleshoot. When I power up the amp, one of the power tubes has a pulsing blue glow around the interior metal parts. The power transformer develops a very loud hum. I have a switch which allows for the amp to be run in ultra-linear to triode mode. Only one of the power tubes is reacting this way, and when I switch from triode to UL the problem switches to the other power tube. So as you look at the amp from the crossover side; in triode the right tube has the problem, in UL it is the left tube. I am not sure how to troubleshoot the problem. I am using Roy Mottram's high-gain 12AT7 boards which are excellent. I have checked the power tubes which are 6P3S-E, and they are fine.
I would appreciate any thoughts, or advice you might have on this issue