I have a pair of Minnesota Audio Labs MAL 6550 mono block amplifiers,
very nice sounding amps copied after the Dynaco Mark III's. They have
cathode biasing, a set screw to get the tubes to read the same amount
of millivolts and an adjustment knob, which adjusts the bias in both
tubes. I was told by a local tech that they were very nice sounding
amps, good build quality, etc. The problem is I adjust bias to
..049-.050mv and they are good for maybe 10 minutes and occasionally
one tube goes to zero and the other jumps to .089 or so. I turn off
the amp and wait a few minutes and turn it back on and it is fine,
both tubes are back to .049. Sometimes it can be fine for several
hours. I have 3 sets of tubes(Valve art, Svetlana, and EI KT90's) and
it doesn't seem to matter, the problem is with all 3 sets and with
both amps.

The amps are about 7-9 years old. Any ideas as to where to start?

Thanks,

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