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Bias problem in KT88-6550 mono-block amps
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, please respond to |
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