"tomrp" wrote

[succinct description of problem, see below]

your test suggests the bias circuit is at fault.

does your circuit connect the two cathodes together with a pot,
whose wiper goes to ground via another pot configured as a variable
resistor? Each cathode would then have its own bypass capacitor to
ground.

apart from the operation of the valves, there are two ways in which
current can rise in one valve. Either the resistance it sees to
ground reduces, or the voltage across the bottom pot must fall. For
the current to rise in one valve, the opposite of one of these
conditions must be true.

if one capacitor were leaky, then the first condition would be true,
and more current would flow in that valve. But less current would
flow down that leg of the pot, and so less due to that valve would
flow through the bottom pot. The voltage across the bottom pot would
fall, and the other valve would also pass more current.

if the bottom pot were leaky, both valves would pass more current.
If open circuit, then neither would pass current.

if one side of the top pot is leaking to the wiper, the valve on
that side will pass more current, and this extra current will pass
through the bottom pot, increasing the voltage across it, so the
other valve would pass less current.

if one side of the top pot is rising in resistance (eg going open
circuit) then the opposite of the above would be the case, giving
the same scenario, but with less total current.

I would guess the last of these possibilities. Could be a dry solder
joint between the top pot and the cathode of the valve that switches
off. Or a problem in the pot itself such as a crack in the track.

could be totally wrong, as ever.

cheers, Ian

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?