No output from Princeton Reverb

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Oct 12, 2019 at 11:47 AM.

  1. itsGiusto

    itsGiusto Tele-Meister

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    I've been doing some mods on my vintage silverface Princeton Reverb: switchable NFB, switching between tube and diode rectifier, and also black facing it and adding a 50k bias pot in place of the 22k bias resistor.

    After all of the mods, there seems to be some problem, because there's no sound coming out of the speaker. I'm very uncertain what the problem could be. I've used my debugging tracer to probe the circuit and listen to the audio signal at various points. The amp is working fine right up until the power tubes - the input signal in the grid of each power tube sounds fine. I've tested my speaker and it's able to work fine with a different amp. This means the problem is either with the power tubes or the output transformer, neither of which I really messed with. I did rewire the output jack, though, to remove the extension speaker jack (so I could put the NFB switch in it's place)

    Swapping the power tubes did not fix the issue, and neither did rebiasing them. Here are the voltages I'm measuring for each power tube (volume set at 0, Reverb jacks not plugged in, tremolo intensity set at 0, switched to tube rectifier):

    Pins tube right (DC unless noted otherwise):
    1: .8m
    2: 2.82 ac
    3: 364.3
    4: 350.7
    5: -30.25
    6: 15mv
    7: 2.792 ac
    8: 0

    Pins tube left:
    1: 10mv
    2: 2.802ac
    3: 364.2
    4: 350.2
    5: -30.2
    6: 25mv
    7: 2.811ac
    8: 0


    Edit:
    Nevermind, I found the issue: I left the output transformer's secondary ungrounded.
    This is why you shouldn't take a month-long break on a project, you completely forget what you were doing...
     
    Last edited: Oct 12, 2019 at 3:47 PM
    DSharp, OlRedNeckHippy and jhundt like this.
  2. jhundt

    jhundt Doctor of Teleocity

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    that's good - you had a problem; you analyzed it; you took measurements that might have been useful; you found the problem yourself, and fixed it!

    And it was a simple problem, as most of them are. Sometimes we are so busy trying to track down a problem that we overlook the simplest things.

    But you found it and fixed it, and for that you deserve a big gold star!
     
  3. itsGiusto

    itsGiusto Tele-Meister

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    Incidentally, I'm wondering: everyone always says "don't ever run a tube amp with the power tubes in with no speaker load attached, it'll blow up."
    But I just ran my amp with no load (unintentionally), while trying to debug this for an hour or so, and there's seemingly no problem now. And I've done this many times before, thinking I've plugged a speaker in, but I didn't, or I used the wrong jack, or the cable got accidentally detached from the speaker terminal, etc. Have I just been extraordinarily lucky? Or is that "your amp will blow up" advice just overblown?
     
  4. OlRedNeckHippy

    OlRedNeckHippy Friend of Leo's

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    Geeze, little things like that can make you crazy.
    My Princeton had no sound, at a gig, years ago. Fortunately, I bring a backup amp ...
    Turned out to be corrosion, greenish blue stuff, on the wire connections from the amp to the reverb tank.
    Cleaned them up and it was back in business.
    TDPRI diagnosticians saved me a trip to the amp doctor that day!
     
  5. King Fan

    King Fan Friend of Leo's Ad Free Member

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    This came up in another thread recently -- in fact it comes up all the time because it happens all the time. :)

    The big danger is if you don't have a shorting jack on the speaker output; you can readily kill your OT. If the shorting jack is wired right and working right, the OT will (as you've seen) often survive. Our all-seeing friend @Wally mentioned there that OTs tend to tolerate a short much better than an open circuit.
     
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  6. muchxs

    muchxs Doctor of Teleocity

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    It's still easy to defeat the shorting jack. I can't begin to tell you how many amps I see with half- assed connections at the speakers, twisted wire shoved into the terminal with a spade lug jammed over it, stuff like that.

    If the speaker gets disconnected your OT is still seeing an open circuit.

    Solder everything.
     
  7. itsGiusto

    itsGiusto Tele-Meister

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    I've definitely heard that about shorting vs non shorting jacks. But in my case this time, since I had a speaker plugged in, but the ground of the OT was unattached, it was actually non-shorting, and still came out fine.
     
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