Another hum debugging question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by MisterAgreeable, Jan 26, 2008.


  1. MisterAgreeable

    MisterAgreeable Member

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    Hi all,

    I know this is a common question, and I've read through the archives, so here goes:

    I have a vibro champ clone that has a constant 60 cycle hum. It's a "soft" sine wave type hum, not buzzy. The amp works otherwise.

    The volume knob makes no difference. Nor does jumpering the control grid to ground, so I don't believe the hum is coming from the preamp stage.

    It is quiet until the tubes have warmed up, so I don't think it's magnetically induced.

    I've replaced all three filter caps. I've replaced all preamp tubes, the power tube (a 6V6) and the rectifier tube.

    The filament leads from the PT are not center tapped, but it came with a positive reference voltage. I tried moving that reference to ground but it didn't make a difference. Leaving the filament circuit ungrounded makes a hiss in the preamp stage, so I don't think that's the cause.

    Does anyone know of anything else I should try?
     
  2. MisterAgreeable

    MisterAgreeable Member

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    Aug 11, 2007
    Well, I do have a brief update on this.

    I've gotten the circuit pretty much stripped down to nothing. A power supply and a power tube. Nearly everything else is bypassed, and the tube's signal is just jumpered to ground. Still hums.

    I tried upgrading the filter caps instead of merely replacing them. And each time I do, it seems to help a bit. It started with 3 22uf capacitors in the filter array, and finally I added three extras, bringing the whole thing to 62, 42 and 42uf.

    Which strikes me as absurd for such a low-wattage amplifier. I guess I can sit and add capacitors for days and maybe eventually it will quiet down enough to be usable, but this seems like an awfully heavy handed approach. Also, I would imagine eventually it will start to really tax the rectifier tube.

    Any suggestions?
     
  3. phsyconoodler

    phsyconoodler Member

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    There are lots of areas that can cause hum in single ended amps.The heaters are one source.Then grounds can be an issue.You must make sure all grounds are in the right locations and they are secure.If it is stock,make sure the brass plate and pots are secure and resolder all grounds and clean behind the pots where they contact the plate.If it has been repaired over the years and someone has moved stuff around,that can be a problem.
    Does it have a centre tap for the heaters or is it the old series wired?If it's wired the old way,try using twisted pairs and use a virtual centre tap with two 100 ohm resistors off the pilot light.
    Then make sure the power tube socket is clean and all pins are making tight contact.Move wires around inside with a chopstick and listen for any changes.
    Sometimes a rectifier tube can cause it to hum.Try another one.Remove tubes til the hum stops and concentrate on that area of the amp.Move filter cap grounds away from preamp grounds.Make sure the preamp cathodes are grounded neat the input jack.If you used individual caps intead of the cap can,make sure they are grounded accordingly.Don't try and reinvent the wheel and ground where you think it's convenient.

    Sometimes the power transformers are on their last leg and nothing will work for you,but rather than assume that,check all the other things first.
    Divide the amp into sections to help you diagnose the problem.Power section and preamp and rectifier.

    Edit:I just saw that it's a clone.look at the original layout and follow it to a T.
     
  4. Big Tim

    Big Tim Member

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    I'm having the same problem, did it work out for you misteragreeable?
     
  5. mark norwine

    mark norwine Member

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    First & foremost...is it 60Hz or 120Hz? Have you measured with a meter?

    Chances are, it's 120Hz and is coming from the B+ rail. The fact that added capacitance helps leads me down that road.

    You need to first understand that in a push-pull amp, a lot of 120Hz hum is cancelled by the opposite polarity of the output tubes.

    In a single-ended amp, no such cancellation exists: as a result, almost all SE amps exhibit some hum.

    It's easy to get fixated on the hum, but I'd focus on the signal-to-hum ratio: Once you start playing, is the hum even noticeable?

    OTOH, if you've determined that it is, indeed 60Hz, AND you've indeed stripped this chassis down to just a PS & output stage, then it's likely coming in through the filaments. I'd try to "raise the filaments up" by connecting your filament CT not to ground, but to the cathode of the power tube. This adds a DC bias to the filaments, reducing (or eliminating) filament-induced hum.

    mn
     
  6. VikingAmps

    VikingAmps Member

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    Also the original speaker in those help a lot since theyre about 100 db down at 120 HZ. If you have it hooked up to a 2X12 for instance you'll notice it a lot more.
     
  7. mark norwine

    mark norwine Member

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    at quiescence.....sure.

    But the signal-to-hum ration shouldn't change. Stated another way, the hum will get louder, but so will the music....
     

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