Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on April 07, 2020, 05:22:03 pm
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I've been googling this issue the last two hours, a 6v6 Plexi l just changed the OT how the PT has a low pitched hum. I thought at first it might be due to the OT change, l didn't notice it with the original AO-43 OT but was probably there..? The new PT is an 18 watt, 8k. I have three 6V6s built all exactly the same and this latest one is the only one that has a humming PT, all are the original AO-43 PTs. The hum comes as soon as I turn on the power with amp in standby mode (no signal to OT), just a/c and filaments. So, l can't see the OT I replaced having any effect? I pulled each tube, hum doesn't leave. It's not real loud, not 60 cycle hum, more like a power transformer on a telephone pole that's humming, like buzz. At first l thought it had something to do with the new OT but hum with no DC signal or current to amp. Comes with power on immediately, l pulled speaker cab for the heck of it, still there. I don't think it was there before the OT change but it might have been and l didn't notice it. I noticed it when l turned on the standby changing guitars and the room was quite. This is driving me crazy, all voltages are correct, amp sounds great, PT doesn't get hot or even warm, even when playing it hard. Please don't suggest l just forget it, none of the other AO-43 conversions do this and all transformers are located in same place, all exact copies. One thing l'm thinking, since l didn't notice it before OT change maybe something to do with the new OT not properly grounded? Ground is threw the spk jack only, maybe l should run a wire to the star ground but again buzzes in standby mode, no current to OT... ?
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Hum - the tube nut's most annoying problem.
I had a build where there was a lot of hum. I had the OT grounded at the speaker jack. I moved it to star ground near the PT, and the hum went away. But the weird thing is that whenever I've tried it with other builds, it is no longer the magic bullet I was hoping it would be. Sometimes, it's a matter of getting lucky.
Today, I had a lot of buzz/hum in a build. I found my vintage 12ax7 preamp tube was microphonic and was picking up noise from the OT cable running nearby. Even though the cable was underneath the chassis, and the tube was obviously above the chassis, the tube still responded to the cable's field. Weird.
All trans have some hum, and there does seem to be a balancing act between the PT and the OT. I have found that if I replace one in a build, the other may cause problems and also have to be replaced. But I'm usually trying to make salvaged trannies fit together, and I'm not an expert, unfortunately. Wish I knew more about it myself. Good luck.
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I like hum.
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Since l don't know if the hum or buzz was there before l replaced the OT, l'll try running a ground from jack to star ground. But after searching and reading l think l made another dumb ass mistake. When l bought the complete AO-43 chassis someone spray painted everything gold, I mean everything, sockets, OT, PT, everything. I stripped all the paint off, yes everything including the PT. Not knowing the iron is potted, l put stripper on the PT. I was careful not to over do it, just lightly brushed the paint stripper on the PT and washed it in paint thinner. To get the thinner out of the laminates l used compressed air. I remember seeing the thinner being blown out of the laminate layers, of course taking the potting with it. I'm lucky at this point that l only have a slight buzz. Now that l read about PT's construction l can see l destroyed the iron. I saw a video on taking the bells off, pulling the laminates apart and using an acid brush getting some varnish back in the grooves best you can, clamping overnight and praying for the best. In the video it worked. I can hear you guys laughing, and l deserve it. Live and learn, :w2:
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I'm not laughing, and I think most guys wont laugh either.
We all make mistakes, live and learn. It's not an irreplaceable PT.
Carry on. :icon_biggrin:
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https://www.google.com/search?sxsrf=ALeKk031DSbW7jfw2I4anzT4tNx4lI9EVA%3A1586367593373&source=hp&ei=aQyOXrrTEqu5ggeesoCAAg&q=how+to+fix+a+buzzing+transformer&oq=how+to+fix+a+buzzing+tran&gs_lcp=ChFtb2JpbGUtZ3dzLXdpei1ocBABGAAyAggAMgYIABAWEB4yBQghEKABMgUIIRCrAjoHCCMQ6gIQJzoECCMQJzoFCAAQgwE6BQguEIMBUPceWMVhYMt2aAFwAHgAgAGZBIgBnBWSAQkxNC4xMC41LTGYAQCgAQGwAQ8&sclient=mobile-gws-wiz-hp#kpvalbx=_eAyOXtG0NcWvggfunq-gBg45 (https://www.google.com/search?sxsrf=ALeKk031DSbW7jfw2I4anzT4tNx4lI9EVA%3A1586367593373&source=hp&ei=aQyOXrrTEqu5ggeesoCAAg&q=how+to+fix+a+buzzing+transformer&oq=how+to+fix+a+buzzing+tran&gs_lcp=ChFtb2JpbGUtZ3dzLXdpei1ocBABGAAyAggAMgYIABAWEB4yBQghEKABMgUIIRCrAjoHCCMQ6gIQJzoECCMQJzoFCAAQgwE6BQguEIMBUPceWMVhYMt2aAFwAHgAgAGZBIgBnBWSAQkxNC4xMC41LTGYAQCgAQGwAQ8&sclient=mobile-gws-wiz-hp#kpvalbx=_eAyOXtG0NcWvggfunq-gBg45) Don't know if this link will open, but don't try this on an AO43 Hammond PT. On those old PTs the bells are glued on filled with muck, not mean to be taken apart, l did and stop after the bells wouldn't come off without destroying the PT, l just brushed lacquer all inside a where the cover started to come off, around all the laminates and clamped. Tomorrow l'll try it...? Maybe l got lucky. I hope the lacquer dries that hot inside, the clamps are tight, needs air to dry..? Don't hear from me in a while, fried myself.... :icon_biggrin:
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A nice shellac bath will be a try option for your PT
you will hardly have the possibility of a vacuum bath, so put it in the shellac (diluted in alcohol) bath for at least an hour then wait 5-6 hours till shellac has dried, before to give it a try on the circuit
Franco
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Thanks Franco,
Didn't want to disconnect wires, as I couldn't get bell covers off. Brushed on industrial conversion lacquer best I could into laminates. Lacquer I used has a rating of 5, the hardest rating, clamped, we'll see tomorrow. The buzz wasn't that loud, I should have left it alone, don't fix it if it ain't broke, ha.
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If you obtain something that way ... OK, but if you don't, really a bath is a better choice
Franco
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Luck'd out, PT dead quite. Just took screws out, noticed no loose laminates but l had stripped all the shellac off the outside of PT getting spray paint off. Just thinned the lacquer l used and brushed outside. It worked, lesson learned. Gotta say these Plexi's look good, still have six AO-43's left. Maybe a Plexi combo and maybe something else, have to check Sluckey's site out, his work is impeccable. Cheers, stay safe.
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Ciao dude
Translation problem
PT dead quite
do you mean it is definitely unusable or do you mean that now it is fine and works well without humming ?
Thanks
Franco
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Works great, dead quite in English means, no noise, no hum, no buzz, problem fixed. Dead quite is kind of like English "slang", not literally takes as written, using dead as an adjective describing quite, as in dead, no movement, still, quite.
al
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I think you want the word "quiet".
E on the end is "extreme".
E in the middle is not-loud.
No E means you gave up.
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:laugh:
I have the same finger/brain farts, English is such a foreign language :icon_biggrin:
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I think you want the word "quiet".
E on the end is "extreme".
E in the middle is not-loud.
No E means you gave up.
Ha, that's "quite" a correction, probably early Alzheimer's disease.
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OK, Understand
May Thanks PRR
now all is really clear :smiley:
Franco
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> May Thanks
That is next month.
(I wish our keyboards did not drop or twist our letters....)
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wish our keyboards did not drop or twist our letters
:laugh:
kinda glad they can't type what we're thinking while we type :icon_biggrin: