Princeton NR Odd Problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by slider313, Mar 10, 2015.

  1. slider313

    slider313 Silver Supporting Member

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    I'm working on a non reverb Princeton with very low output. Someone had done some work to it before I got it. Everything checks out; the voltages are a bit above spec, all resistors are within spec, good tubes, re-flowed the grounds, cleaned the sockets, etc., etc. Here's the problem; there was a 250k pot in the volume position, so I installed a 1 meg. No change. When I read the 1 meg pot installed in the amp, it reads 215k. If I remove it from the control panel, it reads 1 meg. The left tab is grounded and it's wired correctly. I cleaned the buss and re-installed it with no change. Any ideas?

    [​IMG]
    [​IMG]
     
  2. Washburnmemphis

    Washburnmemphis Member

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    Did the problem exist before you changed the volume pot?
     
  3. tresspassor

    tresspassor Member

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    The tonestack pots are in parallel with the volume pot. Should measure ~300K total at full volume.
     
  4. slider313

    slider313 Silver Supporting Member

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    Yes, the volume was low but, whoever worked on the amp had installed a 250k volume pot. I replaced it with the correct 1 meg. The volume is still low and the amp sounds thin until you get it up to 7 on the dial. I tried other speakers, thinking the old C10NS may be weak, with no luck.
     
  5. slider313

    slider313 Silver Supporting Member

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    I believe I'm getting 215k at full volume. I'll look into the output transformer next.
     
  6. telenut62

    telenut62 Member

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    Wouldn't be the footswitch?
     
  7. zenas

    zenas Member

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    If every thing else checks out tubes are good, voltage checks good and all the rest of it and the cap can doesn't say Mallory or Astron. You may have a bad output transformer.
    Not real common but it happens like almost never but it happens.
     
  8. mark norwine

    mark norwine Member

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    I had a princeton in my shop years ago.....complaint was "It's never been loud", and indeed it wasn't! On my scope it was a not-very-nice sine-ish wave, and made about 6 watts.

    I was told that the owner was the 2nd owner, and that the original owner also indicated that it had never been right.

    Look what I found, stock from the factory:

    [​IMG]

    Things are not always what they seem. You're chasing a volume pot, but I assure you your issue is elsewhere.

    Keep looking & good luck.
     
  9. Avatar Tech

    Avatar Tech Member

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    Did they build a half wave rectifier at the factory?
     
  10. pdf64

    pdf64 Member

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    Maybe you used a 10 or 15% audio taper type, instead of the correct 30%?

    How do you intend to do that? Other than the primary being badly shorted, most any fault that you could measure with a meter would prevent the amp from working at all.

    What is the static dissipation of the power tubes?
     
  11. mark norwine

    mark norwine Member

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    Not intentionally, but yes.....

    ....and it had significant consequences on the output (as you'd imagine)
     
  12. zenas

    zenas Member

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    Mark I'm thinking the voltage was off on that amp?
     
  13. mark norwine

    mark norwine Member

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    on a DMM.....not really. Of course things fell apart rapidly as current was drawn, but at quiescent, a meter didn't indicate much.

    on a scope, it was plain as day.

    To be clear, I didn't initially spot this mis-wired socket......I saw that the rail was half-waved on my scope, which made me look at the socket.

    Like I said before: things are not always what they seem.

    Whatever the problem is, it's not the pot.
     
    Last edited: Mar 11, 2015
  14. Sam Sherry

    Sam Sherry Supporting Member

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    That's a Princeton NoVerb in a nutshell. Volume at 3, apartment level. Volume at 5, conversational. Volume at 7-8, clean and honkin' -- ready for action.

    If somebody thinks otherwise by all means speak up . . .
     
  15. mark norwine

    mark norwine Member

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    Actually, I was having the same thought.....

    Nowhere else in the Fender product line is there such a HUGE difference between rev & non-rev versions of the same amp.

    A non-rev Princeton doesn't have the git-up-and-go that the rev has. If you're not familiar with a non-rev, it's entirely possible that you could think there's something wrong although there isn't.

    Is that a possibility?
     
  16. slider313

    slider313 Silver Supporting Member

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    I'm not sure of the % of the taper. The product listing does not offer that information. It's a CTS 1 meg audio taper.

    I'm going to sub in another transformer.

    The tubes are running 19.2ma and 20.7ma with 440v on the plates and 435v on the screens.
     
    Last edited: Mar 11, 2015
  17. slider313

    slider313 Silver Supporting Member

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    Yes, that is a possibility. I seem to remember these amps having low output yet still having some fullness at lower levels.

    I've had a couple of old Ampeg Reverberockets that lacked "punch", for lack of a better term, and came back to life with a replaced output transformer.
     

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