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Re: I too am a novice..so take this..

Eric -

As a first step, unplug all inputs and turn the input gain all the way down (essentially shorting out the inputs). If the hum goes away then Jim may be correct about a ground loop. If the hum persists, something is going on with the amps.

Simultaneous failure in both amps is certainly less likely, but I've seen lots of Mc30's that had poor condition filter caps.

"Refurbished" can mean anything from "I cleaned off the dust" to "every capacitor was replaced, every resistor measured and replaced if out of value, all tubes checked, sockets retensioned, bias circuit rebuilt, all voltages measured and documented, amplifier tested at full output for distortion".

The filter caps on the Mc30 are difficult to source, and may not have been rebuilt or replaced. If the hum is from the amps, make certain that this was done - or at the very least they were measured for leakage at full voltage and ESR. Also the bias circuit used a selenium rectifier which will need replacement and then a dropping resistor added to keep the bias in range.

Good luck, the Mc30's can sound very nice and are very collectable.






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