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Vintage Silverface Princeton Reverb has alternate values for v2 cathode

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Oct 29, 2019.

  1. itsGiusto

    itsGiusto Tele-Holic

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    I figured my Princeton Reverb from 1972 probably needed new electrolytic caps or would need it soon (it had the original cap can and original white electrolytic cathode bypass caps). While doing this work, I found something weird.

    V2 (the reverb driver tube) doesn't have the "standard" cathode resistor and cap values. On all schematics I've looked at, I see a 2.2k resistor and a 25uf cap connected the cathode of both triodes. Mine instead has a ~470k resistor and no bypass cap.
    Edit: I meant to say 470 ohm, not 470k

    Could this be some sort of aftermarket change, or is it just an original quirk? If I changed the values to the standard values, what would be the effect? I imagine the reverb would end up being even louder and harder to dial in (the current reverb goes from too little to too much between 2 and 3 on the knob).
     
    Last edited: Oct 29, 2019
  2. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    1968-~1978 = Silverface, "1270" circuit.

    Here's a decent scan of that schematic: http://ampwares.com/schematics/PRINCETON_REVERB_AB1270.pdf

    You can possibly address that ^ with a differently tapered pot.

    And IMO I'd put if back to stock in that V2 area (caps, resistor values).
     
    Last edited: Oct 29, 2019
  3. itsGiusto

    itsGiusto Tele-Holic

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    Thanks, that's the best quality scan I've seen of that, for sure. Usually it's borderline unreadable.
    IDK, I think it might be how it came stock. I see no strong indication that it was messed with. At the very least, it's been that way for the full 5 years I've owned the amp. As far as I've always been told, they've always been screwing around with putting in random other components at random times at Fender.

    Yeah, I probably should. It's not completely terrible, there's small range from 2.3 to 2.6 that is usable. I usually just set it and forget it.
    Looks like the pot is 100k L. Would audio-taper be better, or would I want reverse audio? I think audio is right...
     
    Last edited: Oct 29, 2019
  4. muscmp

    muscmp Tele-Afflicted

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    you could try a 12au7 for the reverb driver. that will increase your range to give you better adjustment.

    play music!
     
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  5. itsGiusto

    itsGiusto Tele-Holic

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    Tried it previously. It caused massive howling feedback
     
  6. muscmp

    muscmp Tele-Afflicted

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    worked wonderfully in my buddy's 68 twin reverb. shouldn't have caused feedback.

    play music!
     
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  7. D'tar

    D'tar Friend of Leo's

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    470k is very odd cathode resistor value. Does this have the pull for boost volume pot?

    I'd like to see inside that! That sounds more like a grid stop value and very well could be a mistake Lupe' would not approve of!
     
  8. itsGiusto

    itsGiusto Tele-Holic

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    No, no pull-boost.
     
  9. D'tar

    D'tar Friend of Leo's

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    I could see possibly a 470 ohm, shared cathode resistor there and no bypass cap. 470,000 ohm not likely. I wouldn't hesitate to make the change to stock values. A 2.2k will cool down the bias on that tube and may help your reverb easier to dial in. the tube will last longer as well.
     
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  10. itsGiusto

    itsGiusto Tele-Holic

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    Whoops, sorry, I meant to say 470 ohm, not 470k!
     
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