AudioBanter.com - View Single Post - Bias problem in KT88-6550 mono-block amps

(tomrp) said:

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?

Replace the coupling capacitors between the driver and power tubes,
and the cathode decoupling electrolytics, if present..
Check the grid leaking resistors for drifted values.
They may be of a too high value, usually 100k is max. for autobias.

--
Sander de Waal
" SOA of a KT88? Sufficient. "