1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

'78 Princeton(non-reverb) Humming

Discussion in 'Glowing Bottle Tube Amp Forum' started by cyborg_stew, Feb 20, 2020.

  1. cyborg_stew

    cyborg_stew TDPRI Member

    Age:
    32
    Posts:
    37
    Joined:
    Feb 20, 2020
    Location:
    USA
    My Princeton has recently developed a 60hz hum when powering up. The hum "surges" shortly after the amp is turned on, then it quiets down to being barely audible. It isn't affected by the volume or tone controls; however, the hum is pulsing and can be altered by turning the intensity knob. Knowing the little that I do about the tremolo circuit in these amps, I'm wondering if there is a problem in the bias circuit or possibly a coupling capacitor. The tremolo seems a bit weak as well. Any help would be greatly appreciated. I'm new to the tube amp world, and trying to learn as much as I can about them.

    Also, I've measured the bias a few times over the last 2 years and noticed a trend. The resistance of the OT has been dropping evenly on both halves, and the plate voltage of the 6v6's is slowly on the rise. Not sure if this has anything to do with the recent hum or not.
     
  2. schmee

    schmee Doctor of Teleocity

    Posts:
    13,413
    Joined:
    Jun 2, 2003
    Location:
    northwest
    My PR hums when warming up. I thought maybe it was the 5V4 rectifier tube. But it quiets down after warm up.
    I have been amazed at how the bias changes over several minutes. My standard for biasing is now ~10 minutes to let things stabilize before final biasing.
    The bias is taken off the trem intensity on those amps, so that may be normal.
    How old is the filter cap can? That could be your issue.
     
  3. cyborg_stew

    cyborg_stew TDPRI Member

    Age:
    32
    Posts:
    37
    Joined:
    Feb 20, 2020
    Location:
    USA
    I was suspecting the 5u4gb in mine as well. Sounds like it is buzzing/rattling.
    Not sure how old the cap can is, but the previous owner said electrolytics have been replaced. The ones on the board and the bias cap are new. And as I understand it, hum caused by the filter caps is usually 120hz as opposed to 60hz. Correct me if I'm wrong
     
  4. cyborg_stew

    cyborg_stew TDPRI Member

    Age:
    32
    Posts:
    37
    Joined:
    Feb 20, 2020
    Location:
    USA
    DSC09125.JPG DSC05574.JPG DSC09123.JPG
     
  5. BobbyZ

    BobbyZ Doctor of Teleocity

    Posts:
    13,277
    Joined:
    Jan 12, 2011
    Location:
    Snellman MN
    I think I'm seeing the original cap can with a newer cap added in?
    If so I'd replace that cap can.
     
    slider313 likes this.
  6. elpico

    elpico Tele-Afflicted

    Posts:
    1,177
    Joined:
    Sep 14, 2011
    Location:
    Vancouver BC
    The resistance of the OT windings changes with temperature. Less current = less heating = lower resistance
     
  7. schmee

    schmee Doctor of Teleocity

    Posts:
    13,413
    Joined:
    Jun 2, 2003
    Location:
    northwest
    The numbers on your cap can should tell us the date...
     
    BobbyZ likes this.
  8. koolaide

    koolaide Tele-Afflicted

    Posts:
    1,010
    Joined:
    Oct 11, 2009
    Location:
    south carolina
    Any specific tune...sorry couldn't resist.
    If your are concerned. check your filter caps, and look for any broken connections.

    good luck
     
  9. slider313

    slider313 Tele-Afflicted Silver Supporter

    Posts:
    1,398
    Joined:
    Jan 6, 2011
    Location:
    NC
    It looks like someone already added a cap to one section of the can. Time for a new can.

    If it's my amp, I go with a 40/20/20/20 @525v.
     
    BobbyZ likes this.
  10. cyborg_stew

    cyborg_stew TDPRI Member

    Age:
    32
    Posts:
    37
    Joined:
    Feb 20, 2020
    Location:
    USA
    Thanks to all for the replies.
    I wasn't aware that temperature has an effect on the resistance, but that makes perfect sense. Explains the difference in readings that I got.

    I will check the date on the cap can. I suspect it is original since it has "Mallory" stamped on it. If so, then it surely needs to go.
     
    BobbyZ likes this.
  11. cyborg_stew

    cyborg_stew TDPRI Member

    Age:
    32
    Posts:
    37
    Joined:
    Feb 20, 2020
    Location:
    USA
    I'm guessing this means the 12th month of '77?

    DSC09224.JPG
     
  12. cyborg_stew

    cyborg_stew TDPRI Member

    Age:
    32
    Posts:
    37
    Joined:
    Feb 20, 2020
    Location:
    USA
    I monitored the AC and DC voltage of the caps while feeding a guitar signal into the amp and turning the volume up. The ACV only rose from 1.4V to 2.5V or so, and the DCV hardly drops at all. According to a video I watched on youtube (the reputable Terry from D-Lab), this is normal. If the caps were bad, there would be more AC voltage, and it would vary more than a couple volts. And the DC would drop more as well, as in the video.

    Even if this can is operating normally, I know it should be replaced since its over 40yrs old. I'm just wondering if this information rules it out as a cause of the hum.
     
  13. cyborg_stew

    cyborg_stew TDPRI Member

    Age:
    32
    Posts:
    37
    Joined:
    Feb 20, 2020
    Location:
    USA
    One interesting thing I found in my probing is this: on pin 7 of V2 there is 29VDC. This doesn't seem right, as this is the grid, and the schematic doesn't have any voltage listed. I'm thinking the .022 coupling cap must be leaking from the plate of V1.

    Another thing is I can't get a steady reading on pin 1 of V2. Is that normal since it's part of the tremolo circuit? When I disengage the trem via the footswitch jack, the reading is a steady 261VDC.

    Also, should there be 83VDC on pin 8 of V2?
     
  14. cyborg_stew

    cyborg_stew TDPRI Member

    Age:
    32
    Posts:
    37
    Joined:
    Feb 20, 2020
    Location:
    USA
    Why would you choose 40 for the 1st, instead of 20? And is 525V necessary, or would 475V be enough? My B+ is right around 440V.
     
  15. slider313

    slider313 Tele-Afflicted Silver Supporter

    Posts:
    1,398
    Joined:
    Jan 6, 2011
    Location:
    NC
    40uf on the first node (rectifier) will keep the amp quieter at idle and tighten up the bass a bit. If your B+ is 440v at idle, then at start up; before the tubes start to draw current, it may be over 500v.

    Also, the cost of the 525v can isn't much more than the 475v.

    The 12th week of 1977.
     
    BobbyZ likes this.
  16. cyborg_stew

    cyborg_stew TDPRI Member

    Age:
    32
    Posts:
    37
    Joined:
    Feb 20, 2020
    Location:
    USA
    Thanks slider313. That makes sense. Better safe than sorry.

    And I meant to say "12th week". Brain fart. lol
     
  17. cyborg_stew

    cyborg_stew TDPRI Member

    Age:
    32
    Posts:
    37
    Joined:
    Feb 20, 2020
    Location:
    USA
    I'm going to replace the original Mallory cap can on my '78, and I have a question before I order a new one.

    There's a 22uF soldered from one of the leads of the can to ground. Is this wired in parallel in order to change the 1st filter cap to 42uF? Or is this added capacitor there to replace the original 20uF? I love how the amp sounds, so I want to keep the values the same as they are now.


    DSC09125.JPG

    DSC09236.JPG
     
  18. slider313

    slider313 Tele-Afflicted Silver Supporter

    Posts:
    1,398
    Joined:
    Jan 6, 2011
    Location:
    NC
    Yes, the 22uf is in parallel with the 20uf in the can making it 42uf. If it were there to replace the 20uf it wouldn't be connected to the can.
     
  19. cyborg_stew

    cyborg_stew TDPRI Member

    Age:
    32
    Posts:
    37
    Joined:
    Feb 20, 2020
    Location:
    USA
    Thanks again slider313. I suspected that was the case. Just wanted to ask someone who is more knowledgeable than I am.
     
  20. cyborg_stew

    cyborg_stew TDPRI Member

    Age:
    32
    Posts:
    37
    Joined:
    Feb 20, 2020
    Location:
    USA
    Well, I installed a CE cap can in place of the original, and the hum is completely gone now. However, I was checking B+ voltages at the can, and 2 out of the 4 capacitors are reading significantly low. (They're numbers 3 and 4 in the circuit. The first 2 are fine.) The 4th filter cap supplies the plate voltages to V1. It should be getting 190VDC according to the schematic, and I only read 50VDC.

    The voltage is normal (420VDC) after the 1k resistor. Then, after it passes thru the 18k it drops down to 241VDC. Much too low for this point in the circuit. I checked the resistor value, and it checks out at 17.8k.

    What could be causing this? I can't imagine that I did anything wrong installing the new can, and I have no guesses as to what's going on with the voltages. Any help would be greatly appreciated.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.