Hi Tomrp,
I had basically the same thing happen in my HK Cit II (but with fixed
bias). I thought the grid 1 pin was loosing contact, so I cleaned the
sockets (why I didn't change them when I rebuilt the amp is another story).
I slapped another set of tubes in the amp and haven't had a problem since.
The old tubes getters were gray in color and were a RCA and GE mix, the new
set is Chinese KT88's. I haven't tried the Svetlana 6550C's yet. I still
want to test it out a bit longer before I risk the "Good Ones". BTW, I
reduced the main fuse from I believe 8amps to 4or5amps without any ill
effects. Save them tubes!

- Thank God We Don't Get The Government We Pay For. keithw...


"tomrp" wrote in message
om...
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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