Masco MU-5 -Be nice to your rectifier tube

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Masco MU-5 -Be nice to your rectifier tube

Postby JJman on Sun Nov 29, 2015 10:24 pm

Working on a friend’s Masco Mu-5. He accidentally left it on for a few days and it died. When I opened it I spotted the 47u cap floating on the 1st node of the cap can. The scheme calls for 16u for that cap. Also the can had been changed from 10-10-10 to 20-20-20. So there was 20u+47u=67u on the rectifier tube. The scheme shows 26u total and the spec is 32u max for a 6x5.

My tube tester is from 1947 or so and does not list a 6x5 so I checked its continuity between all the pins and it is shorted between both anodes, the cathode, and one end of the filament. Ouch.

I then pulled the fuse and it was 5amps (not 0.5.) A huge mistake by whoever put it in. I moved on to the power tranny and it is shorted from the primary to the HV secondary and the shield wire. Very smelly! A proper fuse could have prevented the power tranny from blowing, maybe.

All else looks good so far. Parts are on order. Be nice to your rectifier.

-If it says "Vintage" on it, it isn't.
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