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Ian:

Depends on the configuration and how the supply is laid out. It would
be an unusual situation that the PS would account for the imbalance by
itself. But I suspect the imbalance has nothing to do with the common
PS caps but other effects (possibly of age) including (perhaps) weak
or unmatched tubes, an imprecise balance control or similar... or your
suggestion of bad caps after the PS. But in general when one
electrolytic goes, they all should be shotgunned, you are correct in
that suggestion.

However, and here is the serious isssue - the one that West & Bret
missed entirely: If the OP is insufficiently familiar with the
workings of tube equipment such that the diagnosis would not come
"automagically" based on prior experience - and there is certainly no
shame in that - then it is most likely that the OP should take the amp
to someone who is familiar with the species. And most certainly NOT go
prodding and poking around inside testing diodes or messing about with
a scope to check waveforms.

None of this is rocket science by any means, but first-time-through
one should have an Elmer/Mentor on-site and not over the net. Were
that my amp to diagnose, I would start with the filters, check the
tubes, shotgun the rest of the electrolytics -each step checking the
results - and then start worrying about what a scope might show.
Chances are it would be unnecessary by that point anyway. My albeit
limited experince with instrument amps vs. audio amps is that they are
remarkably simple beasts, but generally more rugged than audio stuff.
So, they typically respond well to a simple approach.

Peter Wieck
Wyncote, PA- Hide quoted text -

Thanks for the suggestions. Let me provide a little more info on
the unit and about myself. I bought this preamp used from a local
stereo dealer (now out of business) in 1991. I had to replace some
caps on the circuit board that were part of the power supply circuit
because the relay kept failing (staying "on" all the time). This was
back when I first bought it. I have since replaced the power resistors
in the separate power supply myself. I have also replaced the selector
switch with a sealed Elna (sp?) model due to failure of the original
part from oxidation. This unit has a separate gain control for each
channel as well as a common volume control. I replaced these with
sealed Alps Blue Velvet pots for increased reliability. The unit has a
separate power supply with an umbilical cord to the unit. Tube
complements are a 6DJ8 in the line stage and 2 12ax7s and a 12at7 in
the phono stage. The 6dj8 was recently swapped out and the noise did
not change. I am not a trained electronics professional but have done
some circuit board repair work (replacing caps,resistors) on comuter
CRTs and printers as well as audio gear. I own and can use a
multitester but have no clue about using an oscilloscope or other
electronics test gear. I have not done much troubleshooting in the
last several years but can probably still do simple stuff like check
parts. BTW I know about the voltages in the power supply a friend told
me that it's stepped up several times above house current and I got
"bit" once by it, LOL, shocked me from my thumb to my elbow once. I
should also mention that Scott Frankland himself ( the "F" in the
company MFA) worked on the unit two years ago and he upgraded the
rectifier and checked solder connections. It was expensive for him to
work on it and the unit was out of my hands for a month so I would
prefer not to send it out again. Do you guys still feel the filter
caps are the likely culprits?