1968 Princeton Reverb Problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Angle Loss, May 18, 2015.

  1. Angle Loss

    Angle Loss Supporting Member

    Messages:
    1,497
    Joined:
    Oct 18, 2004
    Location:
    Orange County, CA
    Hi all,
    I need a little help diagnosing a problem with my vintage PR. It is a '68 that has been recapped (by me) and adjustable bias added (edit: about 5 years ago-just saying it has been maintained). While I was playing, it started making a ring-modulator type sound (I'm assuming some sort of oscillation?), and there is a loud hum now.

    I changed tubes--no difference. I left the rectifier and power tubes in and powered in up and it is still humming loudly. Pulled the chassis and nothing looked wrong visually to me.

    Where do I need to start checking first? Thanks!
     
    Last edited: May 18, 2015
  2. swiveltung

    swiveltung Member

    Messages:
    14,566
    Joined:
    Feb 5, 2011
    Location:
    PNW
    Well....... if it was fine before than you need to retrace what you did.
     
  3. Angle Loss

    Angle Loss Supporting Member

    Messages:
    1,497
    Joined:
    Oct 18, 2004
    Location:
    Orange County, CA
    The recapping was done about 5 years ago and other than replacing an addition cap in the power section a few years ago, it has worked fine and has been my daily player. This problem started out of the blue.

    I edited the original post as I did not make that clear.
     
  4. swiveltung

    swiveltung Member

    Messages:
    14,566
    Joined:
    Feb 5, 2011
    Location:
    PNW
    many of those type of problems come from the reverb section. Try diff reverb tubes, clean the RCA connectors and tighten them on the back of the amp. I had that problem once, took for ever to figure those connectors were just not connecting well. Ringing went away after that.
    I have had trouble with some bad input jacks before too.. the grounding tang was dirty or bent.
     
    Angle Loss likes this.
  5. Blue Strat

    Blue Strat Member

    Messages:
    30,033
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Check the bias on the power tubes. Should be set to about 20mA. If that's ok, check all cap wiring, grounds, etc. Report back.
     
    Angle Loss likes this.
  6. pdf64

    pdf64 Member

    Messages:
    5,150
    Joined:
    Aug 9, 2008
    Location:
    Staffordshire, UK.
    To me 'a ring-modulator type sound' suggests that the cap section for the power tube plate or screen node has failed (ie excessive ESR).
    Is the solder joint between can cap and chassis good? A monster iron is needed for that job, so a DIY repair may result in that being less than ideal.
     
    Angle Loss likes this.
  7. zenas

    zenas Member

    Messages:
    5,994
    Joined:
    Jul 1, 2011
    A section in the cap can may have failed. Take a single 20uf cap and sub it in a section at a time.
    Just don't shock yourself !
     
    Angle Loss likes this.
  8. Blue Strat

    Blue Strat Member

    Messages:
    30,033
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    And if you do this, you have to disconnect the old cap first.
     
    Angle Loss likes this.
  9. Angle Loss

    Angle Loss Supporting Member

    Messages:
    1,497
    Joined:
    Oct 18, 2004
    Location:
    Orange County, CA
    Thanks for the help. I'm hoping to take a look at it again later today and report back.
     
  10. Angle Loss

    Angle Loss Supporting Member

    Messages:
    1,497
    Joined:
    Oct 18, 2004
    Location:
    Orange County, CA
    Success! I checked bias (which was way off from where I had set it). After checking some soldier joints, I got a 20uf cap from another project and subbed it for the square cap-can capacitor and behold the problem was gone :) I soldiered it in within the chassis and reset the bias and it is sounding good (other than some reverb hum-but I haven't used the reverb in a while). That should do it until I get around to replacing the cap can.

    Thank you all so much for your help! I really appreciate my fellow Gear Pagers for your help. My humble thanks for your wisdom and experience.
     
  11. zenas

    zenas Member

    Messages:
    5,994
    Joined:
    Jul 1, 2011
    Glad to hear it worked out ! When you replace it just get a big soldering iron. I found a big old 200 watter in an antique shop for 5 bucks. Almost makes those cans fun to replace.
     

Share This Page