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Nasty hum in build

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by samwheat, Mar 9, 2006.

  1. samwheat

    samwheat Member

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    Aug 14, 2005
    Location:
    Scranton, PA
    I have a nasty 60 cycle hum in my SE ax84.com type high octane (jcm800 type amp) ..... it has selectable 6v6gt, el34 and 6550 output tubes

    Pics at: http://photobucket.com/albums/f252/samwheat/High%20Octane/

    The heater wires aren't twisted ..... same as my plexi clone here: http://photobucket.com/albums/f252/samwheat/Plexi%2050%20Clone/ ..... the clone has no hum

    the board layout is via hoffman

    the grounding "star" is at a single point with a 12 ga copper wire bolted to the chassis

    the output jack is grounded at the hole at the jack
     
  2. donnyjaguar

    donnyjaguar Member

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    Have you ruled out a bad tube? Current leaking from the filament to the cathode will cause hum. Had this problem last month with my build.
     
  3. brad347

    brad347 Member

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    once i had a hum that I couldn't track down. Did a cap job and everything, did some better shielding, it all helped but didn't make the hum go away. I even tried different power tubes. Eventually I sold the amp for pretty cheap "as-is" because I just didn't feel like dealing with it.

    The guy I sold it to e-mailed me within an hour saying that the hum went away when he replaced the rectifier tube. :jo
     
  4. VacuumVoodoo

    VacuumVoodoo Member

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    How is the input jack grounded ?
     
  5. jpervin

    jpervin Supporting Member

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    My Bassman combo had a bad hum up until recently. I'd flip the power switch and let it warm up, and when I hit the standby switch, it would hum...and then squeal...very loudly. In checking the tubes, one of the 6L6s had what looked like a yellowish burn near the pins. After buying new tubes and plugging them in, no more hum. I then tried using one of the new 6L6s with the old 6L6 that didn't have the burn mark and it worked fine.
     
  6. samwheat

    samwheat Member

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    I tried a different tube in V2 and no avail

    Tried a different EL34 .... same thing

    I'll try replacing V1

    I had a hum in another build and it just went away

    thanks for the replies
     
  7. jbltwin1

    jbltwin1 Member

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    alton, il
    Wild ass guess-did your PT have a center tap? If not, did you use a couple of 100 ohm resistors to ground for the artificial center like fender did? Just guessing but that would do it. Mike.
     
  8. Shea

    Shea Member

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    Make sure the input jack is grounded well - with that plexiglas panel, you might consider using an isolated jack and running a ground like to that buss you have on the terminal board.

    Looks like there are B+ lines that run under the terminal board over to the filters. Make sure all those lines are in place and connected where they're supposed to be - I could easily see myself making a mistake there, which wouldn't be easy to spot because it'd be under the board.

    Shea
     
  9. John Ward

    John Ward Member

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    It will help to use shielded wire from the input jack to the first grid. I recommend getting the grid resistor off the board and onto pin 2 or 7 of the first tube. Same goes for the wiring between the first/second and second/ third gain stages, it will help to run shielded wire between the coupling caps and the grid of the next stage, put all grid resistors on the socket pins. But shielded wire to the first stage is most important.
    jbltwin1 had a good point, you should reference your heaters, but I can see you referenced them to ground. That was also my first guess.
    John
     
  10. samwheat

    samwheat Member

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    I separated the preamp ground from the power amp/100 ohm filament reference resistors ground

    All hum gone
     
  11. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    This might be a dumb question but have you posed your question on the AX84 discussion board? There are loads of knowledgeable folks here but we might not have so much experience debugging homebrewed amps.
     
  12. samwheat

    samwheat Member

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    I did but didn't get much of a response
     
  13. RyanPitch

    RyanPitch Supporting Member

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    When you run shielded wire in an amp, do you connect the shield to anything? Like, John said above, I'm running shielded wire from my input jack straight to a grid resistor, then to the grid of the first 12ax7 preamp tube (pin 7).

    Do I have to connect the shield to ground on each side of the run of wire? I know, dumb question, but I'm working on my first build. Thanks
     
  14. GearHeadFred

    GearHeadFred Member

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    Not a dumb question at all!

    Ground ONLY ONE END of the shield.. It doesn't matter which one, but you don't want to ground both because you may induce a ground loop.

    I found this page very helpful in working the hum out of my build (Fender BF Super Reverb): http://el34world.com/charts/grounds.htm
     

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