[[email protected]]

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?