Masco MU-17 hum
Hi, Skip!
I have a Masco MU-17 that looks beautiful, but has an unacceptable 60 Hz hum. After replacing caps with original value caps, the hum persisted. Increasing values of caps didn't make a notceable difference.
It doesn't get louder with volume knob changes.
It is NOT present if the the PI tube (12AX7) is REMOVED, but replacing the PI tube did not help either (except that it might have been a little less loud when I tried a 12AT7 in there).
Heater wires are twisted. Resistors are in spec. Changing all of the tubes (6L6s and all) made no difference.
So, I thought I'd ask you if you have seen this problem in particular in the MU17. That is, is there a problem that these particular amps have that you have a cure for?
If not, I guess it's time to resolder every connection and see what happens?
Thanks
(Still loving the Kalamazoo and the MA-17 that you Simmonized for me!)
Hello Marcos- That model uses two chassis; one sits on the bottom of the cabinet and one (with all the controls) is at the top. This layout requires miles of shielded cable to route your signal back and forth between the tubes and the pots and is a major hum producer. I think I'd start by carefully checking those shielded cables and making sure that both chassis are grounded.
Another possibility would be heater hum, so check the heater wiring to be certain it is correct.
Of course it is still possible that your problem could be solved by some careful servicing, but those amps can be very difficult to make quiet, even for someone experienced with them. That can sound great, but noise is one reason I prefer the Masco heads over the combo amps like the MU-17!