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In article ,
wrote:

hi, i have a pair of old mac 50-w2 mono blocks and one got sick tonight.

the first symptom was a rushing noise a few seconds after the amp was
turned on, much like being between channels with a old tube AM radio.

i powered it off and suspecting a tube swapped tubes with the working amp.

problem satyed with the same amp - so not likely a tube.

i then swapped outputs to the speakers - problem stayed with the amp -
so not the speaker.

when i put it all back together and tried again, one of the output
tubes flashed blue and the fuse blew.

when i've had similar problems in the past, it's been a bad tube,
but it doesn't look like that this time.

thoughts?

I have these myself,and it is quite probably the interstage transformer
has failed. They were notorious for failing. The transformer driving
the output tubes.

This is a problem, as both this transformer and the output transformer
are solidly potted in the bottom of the case. If you remove the top
cover you will find that there are very few components under there.
There are only one or two coupling capacitors in the entire amp, the
whole output stage and driver stage being coupled with the interstage
transformer.

The power supplys are of course in a separate box/chassis coupled to the
amplifier with a multi-wire cable.

Have you tried exchanging power supplys between amps?

The interstage transformer can be rebuilt, but you have to get at it, and
that is quite difficult without destroying the paint job on the chassis
box due to the heating up needed to unpot it.

Gene McCluney