120 Hz Hum in 135 watt Twin, P.I. pulled

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

  1. gtrnstuff

    gtrnstuff Member

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    Hi all. My nephew was given a well-kept 135w Twin. While going through it, we noticed a 120 Hz hum, even with the PI pulled.
    Since the ser. # dates it to 1979, I figured filter caps should be replaced anyway, so I just put in the new ones I got from Doug Hoffman.
    Plate voltage is about 505. I measure 38 to 42 ma average on the original 6L6s (using a Weber bias adaptor). As you know, the bias voltage on this thing is not easily adjustable, so I haven't tried my newer 6L6s that might not handle that.

    I've poked around with a Sharpie, listening as I lift wires, tap on things, etc.

    No change. Do we have a bad tranny?
    Thanks in advance.
     
  2. Blue Strat

    Blue Strat Member

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    Not a bad tranny.

    Have you replaced the bias supply capacitor and is it polarized correctly (+ side to ground)? If not, do this first.

    Next, check the 470 screen resistors with your DMM. If any of these have drifted by more than 15% this can cause hum.
     
  3. gtrnstuff

    gtrnstuff Member

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    Thanks Mike.
    The good news is...thanks for the confidence in the tranny.
    But...
    The bias cap(s) are apparently in the can with the filters, and I did replace them, being careful to orient everything like the old ones.
    And I did check the 470s, they range from about 480 to 500.

    FWIW my only faint copy of the schematic for this is in an old Aspen P Tube Amp book. Kind of hard to see. I'm no EE, but I usually can read a tube amp schematic enough to find the parts. The bias circuit is like no other Fender I've seen. Has a balance pot that affects relative bias voltage on the two halves of the output section, but how it gets there sure looks different.
     
  4. SatelliteAmps

    SatelliteAmps Member

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    The bias cap (there is only one) should be near the diodes in the corner near the pilot light, not under the can.
     
  5. Blue Strat

    Blue Strat Member

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    I'm 99% certain that www.schematicheaven.com has this schematic on line.

    One thing I forgot to mention is a possible "hummy" power tube. Try the amp with only 2 6L6s at a time, one in either of the right 2 sockets and one in either of the left two sockets. Try every combination and see if any two get rid of the hum.

    There should also be a hum balance pot seperate from the bias balance pot. I'm pretty sure this affects the preamp more, but it only takes 3 seconds to try it.
     
  6. mark norwine

    mark norwine Member

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    Another thing to consider.....

    If the problem was with the bias circuit, the hum would be 60Hz, not 120Hz.

    120Hz is on the B+.......that's the only place in the amp where 120Hz exists.
     
  7. Blue Strat

    Blue Strat Member

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    I never take the reported frequency of hum to be gospel given the innacuracies in most of the posts here. Also, since it takes 3 seconds to do some of these things I always recommend doing them.
     
  8. VikingAmps

    VikingAmps Member

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    I'd also check the hum balance pot.
     
  9. mark norwine

    mark norwine Member

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    I just figured, since he was specific, he took a measurement of the frequency.....(Since most people call all hum "60 cycle hum")

    What do I know? (less & less with each passing year!)

    MK...while I got you: did you get an order from a guy to dropship tubes to me?
     
  10. Blue Strat

    Blue Strat Member

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    Never assume anything no matter how reliable the source seems:)

    Yep, got that order and shipped yesterday. Should arrive tomorrow.

    How'd he get you to work on his amp, extortion? :roll
     
  11. mark norwine

    mark norwine Member

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    Something like that!

    No.....we're just really, really great friends & I'm doing him a favor. Back when I was making amps, he was the guy who did the artwork for my faceplates. Did the original art concepts, created the files, pulled the color seps, made the films......all I had to do was hand off to the silkscreener.

    In addition to being a great guitarist, he's a pretty swell graphic artist.

    Besides, he's been after me to restore this particular amp since about 1988....
     
  12. Blue Strat

    Blue Strat Member

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    Damn! 20 years to finally wear you down!!!:BEER
     
  13. mark norwine

    mark norwine Member

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    typo.....1998.

    So it only took 10 years!!!!:BEER
     
  14. gtrnstuff

    gtrnstuff Member

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    Yeah, that's where they usually are, on any other Fender BF or SF I've seen. Are you looking at a 135w Twin layout?
    [​IMG]

    I recognize the diode near the rectifier bridge, and the blue wire from it leads to the cap can underneath. Notice the two small caps with their + connected to the black wire. The yellow from the other cap goes to the tube balance pot on the back panel, if I traced right.

    Edit: This was shot before I replaced the caps.
    [​IMG]
     
    Last edited: Jun 27, 2008
  15. gtrnstuff

    gtrnstuff Member

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    Thanks again. I already did try various combinations of the old tubes like you said, no change. I can hear a slight change in the 60Hz with the hum balance pot that's wired into the heater supply of V1 and V2. No change in the 120 Hz though. I'll try some other tubes before I give up.
     
  16. mark norwine

    mark norwine Member

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    Back up.

    Are these pictures shown the actual amp in question?

    If so.....these caps (all of them) are OLD. Your 120Hz hum (if, indeed, is 120Hz) is most likely because your entire B+ supply is built with old, original, [most likely] dried-out caps!

    I'd do a complete cap job before anything else....

    EDIT....Sorry, I just re-read the original post. Are you saying that you've already done a cap job and you now have hum, post cap job? Is that it?

    Never rule out a bad "new" cap. Happens all the time.....
     
  17. gtrnstuff

    gtrnstuff Member

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    Yeah, that was taken before the cap job. Hum the same before and after. No worse, no better, so I'm checking whatever else I can before I change out the new caps.
     
  18. mark norwine

    mark norwine Member

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  19. gtrnstuff

    gtrnstuff Member

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    TUBES! Sorry for yelling. A pair of GT 6L6 GE #5 were nice and quiet, and read a more healthy 33ma without mucking with the non-adjustable bias.
    Now to convince my nephew to spring for a new quad of those or similar.
     
  20. Blue Strat

    Blue Strat Member

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    Bingo!
     

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