Nasty Amp Hum...Please Help

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by schmidlin, Apr 10, 2015.

  1. schmidlin

    schmidlin Member

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    This one of my builds I finished a while ago, and lent it to a prospective buyer. He LOVED it but wanted to tweak out a small tube rattle: easy fix. He brings it back and now it has this hum.

    Can anyone identify this? There is one ONE gain stage before the volume pot. Swapped tubes, wires, subbed in caps etc. The guy had his WALLET OUT for pete's sake and I am at my wit's end. Please any and all help would be appreciated.

    Video:
    https://youtu.be/iQqf7qEFRLA
     
    Last edited: Apr 10, 2015
  2. Xlch721

    Xlch721 Member

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    Cold Solder joint most likely or a bad ground . I have also had this happen only to find out it was a bad cable. Have you tried using a chopstick and gently moving the different wires around to see if the hum goes away. Another thing to check is make sure the wires on the output trans are not wired up backwards. I had an 18 Watt build that I ran into that issue on.
    Thats all I can come up with off the top of my head at the moment since you said you have already swapped out the tubes.
    Good Luck let us know what you find out it may help someone else later.
     
  3. zenas

    zenas Member

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    Have you tried a shield on the open side of the chassis? Piece of tin, or tin foil if your lucky it's just picking up some buzz maybe from the light over your bench or something.
     
  4. zenas

    zenas Member

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    Oh some new shorting jacks don't short. And double check the grounds.
     
  5. pdf64

    pdf64 Member

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    Try re-establishing all grounding points, eg slacken off the nuts from pots, jacks, bolts, then twist the star washers a little and re-tighten, so that they bite into fresh metal.
     
  6. schmidlin

    schmidlin Member

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    I honestly can't remember being so frustrated. It still makes the noise with a guitar plugged in, so it's not the shorting jack. And since I used the JJ 20/20/20/20 cap can, this is a true star ground at the PT lug. Can't be a design flaw as it used to work fine. I've replaced the first blocking cap and it is not leaking. I am just at a loss here...not 60hz, not 120hz, more like 200 on up.
     
  7. EFK

    EFK Supporting Member

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    I've heard quite a number of oscillations that sound like that, primarily around the output section. I'd second the notion above of going to town with a chopstick with special attention being paid to the wires to the output grids, just for a quick start.
     
  8. Xlch721

    Xlch721 Member

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    Try taking some gut shots maybe someone can see something that may be causing the issue. Take multiple shots and zoom in close to the board and sockets ect. Its alot easier to troubleshot when you can see things rather than just guessing from the noise in your clip.
     
  9. swiveltung

    swiveltung Member

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    Are you saying it didn't hum when you built it? I 've had problems with where the grounds are on some builds. Be sure to ground the first stage and input jacks etc near the jacks.
    I also recently had hum problem in a build that I put a shielded cable from the jack to V1. (one end not grounded.) I replaced it while debugging with a non shielded wire and it is totally quiet... go figure.
    I've had some non grounding switchcraft jacks in the last year also....
     
  10. V2

    V2 Member

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    start pulling tubes one at a time. Does the noise go away when you pull out a pre- or PI tube?

    Try re-flowing solder on the pot and its connections. Move the pot's ground. If that fails, replace the pot.
     
  11. schmidlin

    schmidlin Member

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    Well I got it rather resolved. I built this thing a while back and remember firing it up and saying "well it works" and setting it aside. Maybe I just have a crappy memory.

    I rearranged some lead dress, but what I think really cleaned it up was going with the 'dual star' grounding scheme. All the grounds BEFORE the PI were re-directed to a lug mounted right next to the input jack. This also included an extra PS cap to get that ground out of the can. Not sure why I didn't it that way in the first place. Still has a little noise, but a lot better. Thanks for your help, gang.
     

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