Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jbefumo on January 17, 2018, 09:01:21 am
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Sorry for the subject -- couldn't resist ;^)
This issue has me utterly flummoxed ....
Amp: Originally a 70W UL Super Reverb that was competently converted (not by me) to AB763 specs, including all Mercury transformers, tube rectifier, etc. Now housed in a 2X12 Twin Reverb cabinet (the UL Super chassis was same width), with a pair of 4-ohm speakers presenting the proper 2-ohm impedance.
I further modded the amp with individual bias controls for each tube.
Since owning it I've gone back an forth between 6L6s and KT66s (after confirming with Mercury that the PT would handle the extra filament draw), rebiasing appropriately.
Anyway, I had some KT66s around, one of which I was pretty sure was bad, and since my tube tester no longer works, popped a pair (one known good, and one suspect) into the amp, and sure enough, blew a fuse. Tossed the bad one, put a good pair into a different amp, and all was well, however, replaced the 6L6s, and the amp had no power, and a wicked hum with all the controls at zero.
checked/reset the bias, problem persisted.
Did some research, opened it up, and checked all the primary suspects: PS caps, screen resistors (more in a moment), etc. -- some connections on the tube sockets are loose, so ordered a new pair, as well as a new pair of screen resistors (3W X 1K).
Parenthetically, one of the 470R resistors actually measured 570 or so (color code is 470), which seemed way beyond tolerance.
At no point did I keep the amp powered up for more than a minute or so at a time.
At one point, decided to pull the power tubes to check something or other, and I noticed that one was too hot to handle (though not visibly red-plating), and the other was warm/normal.
After a bit more research, I swapped them (right to left), and lo and behold -- Amp sounds .... perfect. No hum. Tons of power.
That's as far as I've gotten while I await the arrival of the new tube sockets and screen resistors.
Still trying to wrap my (chemo-brained) mind around what's going on here.
Any ideas?
I haven't yet powered up to check if one or the other of the tubes is noticeably hot because, even if one is, I'm not quite sure how I'm going to interpret the results, so no point in performing potentially destructive experiments.
(When it's working properly this is the best sounding AB763 I've ever owned, so I don't want to do anything to endanger it!)
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After a bit more research, I swapped them (right to left), and lo and behold -- Amp sounds .... perfect. No hum. Tons of power.
You may have inadvertently corrected the problem when you swapped the tubes around. Swap the back. Still sound perfect?
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Well I'll be dipped! Good call, sounds fine now.
So I'm guessing it's the socket?
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Maybe the socket. Maybe the tube pins. But definitely was a poor connection. You may not need to change the sockets. I'd just use some electro cleaner like DeOxit and clean the socket pins. Spray the cleaner into the socket then insert the tube, then pull it almost out, reseat, pull it almost out, reseat, etc, etc. This will give a good wiping to the tube pins as well as the socket. May be all you need to do.
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Yeah, no sense rushing in with soldering gun blazing -- if it happens again I'll know where to look and have parts on hand.
Thanks much!
Joe
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Well, have her all sorted out now -- new tube sockets, new tubes, new speakers, and .. WOW!
http://www.rocktrollamps.com/home/amps-effects/commercial-gear/fender-super-pro-reverb/
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Way to hang in there and get it done. Nice work.
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THANKS!
Been a long time since I had a right that really made me want to drop everything else and PLAY!
Way to hang in there and get it done. Nice work.