[Peter Wieck]

On Nov 4, 10:40 pm, "Ian Iveson"
wrote:
Peter Wieck wrote in





I have a 1987 MFA Magus A2 preamp that a few weeks ago
started making
a buzzing/humming sound from both channels on any source
setting. It
is strongest in the right channel but audible from both.
I have
swapped out the line stage tube (6DJ8) to no effect. I
have verified
that it is the Magus because if the amps are on (MFA M75
monoblocks)
and the preamp off there is no noise from the speakers. I
have also
tried removing the phono stage tubes to eliminate them as
the problem
but again to no effect. The noise seems to start 20 or 30
seconds
after the preamp relay clicks on and after the amps warm
up - in other
words, it doesn't start as soon as the amps warm up but
maybe 20
seconds later. Anyone have any ideas regarding what the
problem could
be and what I should do to fix it? Local technicians have
little or no
knowledge about tube gear so taking it to a shop isn't an
option. It
seems to be slowly getting louder over time so I need to
fix it. The
noise level does increase as the volume or gains are
turned up if that
helps. Any ideas?

Filter caps.

Why would it be stronger in one channel, Peter? I was
thinking either filter or cathode resistor bypass cap but
figured one would be on both channels, and the other on just
one.

Would it be wise for the OP to renew all the electrolytic
caps he can find? Same values and voltage please, OP, high
temperature, low ESR.


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