speaker hum from pro reverb

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by spinfresh, Apr 14, 2016.

  1. spinfresh

    spinfresh Member

    Messages:
    25
    Joined:
    Oct 13, 2014
    Location:
    Portland Oregon
    I have a customers 1981 Pro Reverb and it has a hum from the speakers here is what I have done and now I am thinking it could be the output transformer any ideas before I go down that road.

    here is what i have done so far. // some of this was done before I got the amp so I checked all the work.

    • removed all tubes except outputs (6l6's) including the phase splitter (still hums)
    • Swapped out the 6l6 tubes for a different pair
    • new power supply caps (I also checked the filtering with a scope) this seems OK
    • Checked the bias filter caps - they appear OK on a scope
    • someone before me replaced the tube sockets
    • I converted the tube balance / matching to a standard bias configuration
    • replaced the hum balance pot (it was burned and measured at over 1m) and I put in an artificial center tap on the heaters (2 100ohm resisters to ground)
    • all resistors measure within spec (both hot and cold)
    • all grounds appear correct and good

    any one out there have any other ideas of what it could be?

    thanks in advance
     
  2. swiveltung

    swiveltung Member

    Messages:
    14,550
    Joined:
    Feb 5, 2011
    Location:
    PNW
    So it hums with all the pretubes out of it?
     
  3. spinfresh

    spinfresh Member

    Messages:
    25
    Joined:
    Oct 13, 2014
    Location:
    Portland Oregon
    all of preamp tubes removed including the phase spliter, volume control not affecting the hum either when pre tubes are installed.

    also I used signal tracer with pre tubes installed, and power tubes removed and no noise from pre-amp.
     
  4. pdf64

    pdf64 Member

    Messages:
    5,485
    Joined:
    Aug 9, 2008
    Location:
    Staffordshire, UK.
    What are the plate or cathode currents of each 6L6? If there's too much discrepancy there will be hum.
     
  5. spinfresh

    spinfresh Member

    Messages:
    25
    Joined:
    Oct 13, 2014
    Location:
    Portland Oregon
    the are matched 430v 36ma on tube one 428v 34.5ma on tube two. -39.6v bias
     
  6. Geetarpicker

    Geetarpicker Member

    Messages:
    2,676
    Joined:
    Aug 31, 2004
    Location:
    Nashville, TN
    Ah replaced tube sockets...

    Check the polarity of the filament leads on the output tubes. The outputs should be wired in phase with each other (as in, pin for pin the two filament wires match on the output) then when it operates in push pull there is hum cancelling going on. I recently found a '66 Bassman where the output sockets were wired wrong from the factory. IF the tubes were considerably miss matched it didn't hum that much, but with a matched pair it was bad. I noticed the wiring error (it had been that way for 50 years!) and corrected it. Now with a matched pair the hum level is just about gone. Also since you swapped from the bias balance to older style bias this is even more critical, with the balance setup you can sort of fudge the noise away. Most amps have the preamp tubes wired with each phase flipped, but it usually isn't as critical there. Worth checking!

    I also typically up the bias cap to 100uf, as the stock value usually induces a little hum.
     
  7. pdf64

    pdf64 Member

    Messages:
    5,485
    Joined:
    Aug 9, 2008
    Location:
    Staffordshire, UK.
    Have you got a scope? It would be helpful know whether the hum was 60 or 120Hz.
     
  8. spinfresh

    spinfresh Member

    Messages:
    25
    Joined:
    Oct 13, 2014
    Location:
    Portland Oregon
    I am going to check this now -- since someone replaced the tube sockets maybe they did it backwards.
     
  9. spinfresh

    spinfresh Member

    Messages:
    25
    Joined:
    Oct 13, 2014
    Location:
    Portland Oregon
    I have a scope I will check shortly - it sounds like 60hz but not sure till I measure it
     
  10. xtian

    xtian Member

    Messages:
    1,790
    Joined:
    Dec 15, 2010
    Location:
    NorCal
    One more test, before you bother rewiring any heater leads. Pull the power tubes also--pull ALL the tubes out. Now, with power and standby on, how's the hum level?

    If there's still hum, you've got EM radiation from the PT inducing hum in the coils of the OT. Typically, this is avoided by proper positioning of the two transformers during installation. But Fender's amps aren't built one-by-one, they're made on an assembly line, so they won't have taken this step.

    Let me know what you find.
     
  11. spinfresh

    spinfresh Member

    Messages:
    25
    Joined:
    Oct 13, 2014
    Location:
    Portland Oregon
    heater leads are correct they go from and to the same pin on each of the output tubes
     
  12. spinfresh

    spinfresh Member

    Messages:
    25
    Joined:
    Oct 13, 2014
    Location:
    Portland Oregon
    all tubes out = no hum - I am hoping it is not a partially shorted output transformer, I need to scope it to see if it is 120 or 60hz later tonight
     
  13. TimmyP

    TimmyP Member

    Messages:
    2,473
    Joined:
    Nov 1, 2007
    Location:
    Indianapolis
    Bad first leg filter cap?

    You don't want a hum balance control and an artificial center tap - the hum balance control is the artificial center tap - adjustable style.
     
  14. spinfresh

    spinfresh Member

    Messages:
    25
    Joined:
    Oct 13, 2014
    Location:
    Portland Oregon
    i have swapped out different new ones and still the hum, that was one of the things I did first, - do you know if a bad output transformer could hum and still pass signal and work?
     
  15. xtian

    xtian Member

    Messages:
    1,790
    Joined:
    Dec 15, 2010
    Location:
    NorCal
    Don't think so. OTs don't hum. They just translate AC from primary to secondary, so, garbage in, garbage out.
    I think you've ruled out mechanical hum, and EM radiated hum from PT.
    That leaves poor filtering or ground loops.
     
    spinfresh likes this.
  16. spinfresh

    spinfresh Member

    Messages:
    25
    Joined:
    Oct 13, 2014
    Location:
    Portland Oregon
    I think it is ground loop hunting tonight...
     
  17. swiveltung

    swiveltung Member

    Messages:
    14,550
    Joined:
    Feb 5, 2011
    Location:
    PNW
    Not likely, but seeing's how you just got into this after someone else: is it possible the OT Blue and Brown are swapped at the power tubes? Usually this would squeal loudly, but sometimes not. Is the OT stock? If not it could very likely be this.
     
  18. spinfresh

    spinfresh Member

    Messages:
    25
    Joined:
    Oct 13, 2014
    Location:
    Portland Oregon
  19. Structo

    Structo Member

    Messages:
    9,573
    Joined:
    Apr 10, 2007
    Location:
    Oregon
    Double check soldering on the new sockets and grounds.

    120Hz hum many times is the power supply.

    Can also be the tubes as well.
     
  20. spinfresh

    spinfresh Member

    Messages:
    25
    Joined:
    Oct 13, 2014
    Location:
    Portland Oregon
    tried 2 different sets of tubes, so I know that is not it, but the powersupply is still suspect.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice