Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on March 22, 2012, 09:25:36 pm
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One trick to chase a hum / buzz in an amp consists in amplifing (with any amp...guitar, bass, hi-fi ) the signal in every stage of the faulty amplifier with a probe made with a capacitor.
Everyone knows that. SHAME ON ME: I have never experienced this method ( cause I did not have to ). I'd like to know: Are you EXPERIENCED ? I mean , did you have some kind of success using this method ?
I have a buzz /hum issue with an Ampeg J12, and in a couple of days, if I can't make the devil out of it, I'll have an issue with it's owner....
Regards
Colas
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Colas,
Is it an old one? I have found lots of ground loops in old Ampegs.
One trick I use, (although it doesn't necesarily stand up to scientific scrutiny), is I hook an alligator clip to every connection that goes to ground one at a time and ground the other end of the clip somewhere completely away from the other ground connections.
If I find one that effects the hum (good or bad), I disconnect that component from ground and reground it elsewhere.
Although, this is my own made-up method (as far as I know) and the science doesn't hold water, it has worked for me many times.
Dave
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I like Dave's quick and dirty method however be methodical about it. I'd need to see a pic of the layout/chassis gut shot. Could be an easy heater raising job, check of the OT wiring, or something similar too? I suspect you've already checked your caps?
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yeah it is an old one. when I first opened it, I saw the OT has been replaced but with the heater center tap not connected: I thought then that was the cause of the hum. But it wasn't, surprisingly.
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Some tips I've been given in the past... does the hum change when you change the volume? If not then you could check heaters or power rail filtering.
Does it change when volume changes, then it's possible the issue is before the volume. And you can check for lead dress, grounding, etc.
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Do you still hear a hum after pulling the PI (phase inverter) tube?
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yes, Pi and first tube removed, still noisy, even one 7591 left in any of the two sockets , noisiy..........damn!
Colas
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Well it's got to be coming from the power transformer in possable conjunction with those power tubes. I have seen certain power tubes interact with the power transformer. Why? The power tubes just just will not work with that transformer. It's a eddy current frequency coupling thing. Pretty rare but when it happens it will be last thing you will expect and it will drive you nuts trying to figure the problem. Try some different power tubes. This is a PITA to fix sometimes but not impossable
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and pain in the brain too !!!
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the owner of that Ampeg J12 ( I call it now ''Humpeg'' ) told me the issue first began with a white noise, at fire up . It stopped when banging on the amp. Then, it started humming permanently even when banging on it. Does it ring some bells to you ?
Colas
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Man there no easy answer on those ampegs. Cold solder joints etc: Some of those wires in the amp look fat and thick but the gauge of the wire inside the plastic shieth is smaller than what you would expect. There brittle about now and may just be soldered by a thread. And the solid core wires in these amps are real brittle and snap right off easy. The only advice i can give is check all ground connections and especially look for any ground loops that could be causing the hum
Daves idea of checking for possable ground loops is pretty good. Maybe not always acurrate but some times will show itself quickly
Just clip a wire on the chassis and probe all the grounds under working condition.
Did you try another set of power tubes? Does this amp have a heater filament balance pot? Thats some thing to look at/
Come to think of it i had a problem with one of these 4 months ago where the bias resistor was bad and the tubes would redplate
Maybe related,maybe not
I forgot to ask: Is this the reissue amp?
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Hi, no it is not a reissue ( re-issue !!!! :laugh:) it is an original one. I just came up from my atelier in the basement and I started to experiment with a capacitor probe plugged into a guitar amp. I can say that the hum is coming from the B+ rail, from the multiple cap can........I had to stop my tests when I heard LOUD: '' jACQUES, VIENS SOUPER''. And when she says that, better listen to her. I'll continue my tests tomorrow, and will write a full detailed report on what I did ( if i can find that f......Am, in this Humpeg amplifier.
See you later buddy, you're a bro.
Colas
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I know you already located the source of your problem, but a listening box has helped me figure out where noise was coming from a few times:
http://www.el34world.com/Forum/index.php?topic=3950.0 (http://www.el34world.com/Forum/index.php?topic=3950.0)
Pulling different tubes (in a scientific manner, of course) and injecting a signal at different points in the circuit together with the listening box is great for diagnostics.
Cheers,
Chip
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cool, that's the thing to chase hum
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TROUBLE FOUND: 2 caps of the multiple cap can with 0 capacitance. Thanks for your help, fellas.
Colas
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TROUBLE FOUND: 2 caps of the multiple cap can with 0 capacitance. Thanks for your help, fellas.
Colas
Do you mean that the multican cap had no continuity in either polairity (direction) ?
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I mean that two of the four caps, from + to gnd show no capacitance, 0 uF , I can't say more , sorry ! Oh yes I can say more: the first cap busted was the reservoir cap, right after the 5Y3.
Colas Copppola
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Makes ya feel good to have found the problem dosent it?
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It is always a great satisfaction , indeed ! Just like putting a 9mm bullet in the bullseye at 300 feet.
I am developping new methods to chase invisible hum warriors. Since I have no great talent for troubleshooting and that I have many amps to fix, I gotta do it right and fast, if I wanna eat hey hey.....
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oh no, the hum is back !
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Hit it with a fly swatter or use a vacum cleaner and suck it out :laugh:
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hey, i don't know you,( I only saw you a couple of times here) but you're very funny, where did you learn that ? This forum is serious, buddy. It is all about amps tweaking, not flies busting. It is strange though, sometimes in my head I hear mosquitoes fly ( amp head, I mean ). Some say it's beacause it is infected with parasits. When I shoot it with RAID, the fuse blows :think1: Should I get slow blow ones ?
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Should I get slow blow ones ?
Yes. But kiss her and say you love her first. :wink:
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It only hums because it doesn't know the words.
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Sluckey, concerning my woman, it is not a fuse I've got, it is a breaker that she resets everytime she wants.