2 amps- same hum problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jherman, Feb 9, 2008.


  1. jherman

    jherman Member

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    AZ
    Good morning eveyone.

    I have been trimming down my Fender collection to just a few living room keepers and have noticed the same problem with a few amps.

    The first one is a 1966 Vibro Champ. It had this problem when I got it and a routine cap job, grounded cord , new speaker etc. didn't help anything.
    As soon as the power switch is turned on I'm getting a pretty noticable level of hum. Power switch off and hum is immediatally gone. Swith back on, hum is right back.

    Second amp is a 54 wide panel pro. New electrolytics before I bought it but still has the 2 prong cord. This amp starts a humming as soon as the power switch is engaged (same as the VC), even in standby on the other switch.

    I'm pretty happy with my tele into a Bad Bob boost thu the Vibro Champ into an old EV SRO 12" for playing around the house. The Pro will probaly be sold soon. I'd like to cure both amps of this hum though. One more thought they do this with volumes off and nothing plugged in so that's not an issue.

    Thanks for any advice
     
  2. 78tsubaki

    78tsubaki Member

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    Location:
    Washington (The State)
    First off are you testing the amps in the same power supply plug?
    Are you getting the noise regardless of what room in the house and what plug you try?
    Does the room you play in have fluorecent light bulbs?
    You may have an issue with that one recepticle or noise from another source.
    If you have a loose or disconnected neutral wire in that plug it could be dangerous.

    Otherwise if you are certain that your tubes are tight and right, did the same tech do the cap jobs?
    You might have a lead wrap issue but I would check out the power quality at the building supply first.
     
  3. jherman

    jherman Member

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    I just moved. It was an issue at my old place too. I don't have any flor lights in the house. Don't have any other electrical devices nearby.

    The Vibrochamp cap job- grounded cord I did myself. Done several without any problems. The Pro I bought with it done already.
     
  4. 78tsubaki

    78tsubaki Member

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    Then I am stumped.

    Some one else might chime in here.
     
  5. phsyconoodler

    phsyconoodler Member

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    Both those amps have the old style heater wiring.If you swap the cords to a three prong it sometimes makes the heaters noisier than they were before.Try switching to a twisted heater arangement with the champ and then with the older amp try moving heater grounds around.A picture is worth a thousand words.
    The 54 likely has a ground switch?It may have a bad cap to ground that is causing the hum.Commonly referred to as the death cap.I can't say if your's has it but that may be the area you need to look at.
     
  6. jherman

    jherman Member

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    Thanks phsyconoodler. Last night I was flipping through one of the Gerald Weber books and read a chapter about gettind rid of filament hum in a single ended amp. It dealt with making the heater the twisted pair you mentioned but I wasn't sure if the blackface Fender was a canididate because the book only refered to tweed champs.

    The Pro doesn't have a ground switch if I remember correctly(5D5?). Does an amp only have the death cap if you have a ground switch?
     
  7. brad347

    brad347 Member

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    check that there are no dimmer switches around. If there are, turn them off.

    Put your ear beside the power transformer and see if the power transformer is physically making noise (rattling at the laminations). If so it may need to be re-potted. For the record, this noise can be amplified through the amp as well as buzzing acoustically.

    Before re-potting it, just grab it and hold it still and see if that stops the noise. If so, tightening the screws to the chassis may help.
     
  8. Buffaloamps

    Buffaloamps Member

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    Aug 20, 2006
    Location:
    Texas
    A bad coupling cap will cause hum too. When they deteriorate over time, they lose their ability to block high voltage DC. And when HVDC is making its way to the grid of the output tube, "BVVVVVVVVVVV"!

    And with your amps being this old chances are that perhaps they are bad.

    Perhaps its worth a look into.

    Good luck,

    Rob
     

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