Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: p2pAmps on December 01, 2016, 09:00:24 am
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Guys, I will try and make this brief.
I had a custom-shop Low Powered Tweed Twin come in the shop yesterday. The complaint was barely any sound and some smells. This is a dual rectifier (5U4) amp but showed with only one rectifier in place.
After replacing all the tubes with UBGs (used but good) to test, I brought it up slow and the 1st thing I noticed was it was pulling just south of 2A on my variac @ 90vAC. As the rectifiers started coming up I heard some light popping and my amp meter was bouncing around, I immediately shut it down. Next I killed all the lights in the shop and brought it back up slow to see if I could detect any arcing on the sockets. That went ok, no arcs.
Next move, I pulled both 5U4s and turned it back on @ 90/100 vAC. This time I did not get current draw. I was getting around 350vDC on the filter cap. I put the rectifiers back in and fired it again @ 90v, once the rectifiers started to warm the voltage on the filter cap started to fall instead of rise. Hummm
I am very suspicious that MM power trannies is bad and that may very well be where the popping and smells were coming from.
Am I making any since whatsoever …
Now remember on any replies to speak simple English as I am not at the level as some you you experts LOL :icon_biggrin:
Scaggs
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Next move, I pulled both 5U4s and turned it back on @ 90/100 vAC. This time I did not get current draw. I was getting around 350vDC on the filter cap.
Huh??? How could you possibly get any voltage on the first filter cap if both 5U4s were pulled? Totally discharge that cap and try that again.
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Agreed Steve, the caps were discharged. This is a very strange issue
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Humor me. Please try that again.
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LOL, ok Steve... I know it sounds crazy. I checked it twice too. Hopefully Mrs Scaggs will let me escape to the garage tonight.
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You said it's custom shop. Maybe the builder put diodes || with the rect. tubes? Although I'm sure you'd have seen those.
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John has an excellent thought. Are you inside the chassis and can you check for diodes? With no 5U4s in there it can not work (by "work" I mean, there ain't no rectifier in there so no DC so no amp).....w/o diodes.
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Yes it was a bad MM power trannie... Yes Steve, I did lose my mind temporarily on the FC voltage thing :)
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PT's don't usually fail without some help...
I'd consider fusing the secondary for future protection.
http://www.valvewizard.co.uk/fuses.html (http://www.valvewizard.co.uk/fuses.html)
Keep up the good work Mike!
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I'd consider fusing the secondary for future protection.
http://www.valvewizard.co.uk/fuses.html (http://www.valvewizard.co.uk/fuses.html)
Yeah, this is one of those Fender Custom Shop amps and the secondaries are fused from the factory... It's all fixed and gone now, next......
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It's all fixed and gone now
know what killed the tranny?, did you wind up re-tube-ing ?
EDIT: Fixed moderator-modified message - HBP
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It's all fixed and gone now
know what killed the tranny?, did you wind up re-tube-ing ?
Never found the actual source of the (Mercury) tranny issue. Tubes all checked out and after changing the PT all the voltages became normal and as expected. I bench tested for 48 hours and it was stable. Not a big Mercury fan
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This is a dual rectifier (5U4) amp but showed with only one rectifier in place.
If the missing one shorted out, would that have done the dirty?
EDIT: Fixed moderator-modified message; you're killing me p2p! - HBP
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This is a dual rectifier (5U4) amp but showed with only one rectifier in place.
If the missing one shorted out, would that have done the dirty?
Possible I suppose. I did add a second rectifier back to the amp so it would be stock.