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Old 13th July 2008, 07:25 PM   #1
pjanda1 is offline pjanda1  United States
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Default Hum trouble on old PA amp

I'm trying to convert an old Rauland PA amp into a bass/guitar/musical instrument amp. After a fired it up (in a thread a few weeks ago), it had a decent hum to it. As you all suggested, I replaced all of the electrolytics (I replaced the big 30/10/10/10 twist lock with the JJ 40/20/20/20). I pulled the mic input transformers and added some insulated 1/4" mono jacks (tip to grid of the first 12AX7's via .047 cap, ring to ground points used on original inputs). I also added a 3 prong cord. When I got it all running again, I've got as much hum as I had before. I think the problem is in the vicinity of the big ground pictured below. Whenever I get anything metal near it, the hum changes. The point is highly microphonic. I've tried connecting it to other ground points (test lead), but it doesn't change a thing. I've reheated the joint a couple of times. I don't have a schematic for the amp (still working on that), but I've changed nothing (other than described above). I'm a tube newbie. Do these symptoms point to an obvious solution?

pj
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Old 13th July 2008, 07:27 PM   #2
pjanda1 is offline pjanda1  United States
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For reference here is a pic of the whole thing. I also swapped what I think are the bias supply diodes (old ones shown) with new IN4005's.
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