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Vintage Princeton Reverb volume fluctuations

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Jun 23, 2020.

  1. itsGiusto

    itsGiusto Tele-Holic

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    My Princeton Reverb is having volume fluctuations. It'll be normal, then maybe get quieter for the next few minutes, then get loud again in a small burst, then quiet again, etc.

    Does this mean the vintage RCA per tubes are on their way out? Or something else? I hope the work I did on it recently to change some old caps and cap can and install a bias pot didn't inadvertently cause some sort of issue. Or maybe since I biased the tubes hotter than they'd been for the last 50 years, that caused the tubes to start to fail.
     
    Last edited: Jun 23, 2020
  2. tubegeek

    tubegeek Friend of Leo's

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    You are somewhere close to 100% certain to have made at least one cold solder joint. Start in the bias circuit and reheat each one until the solder flows and fully wets all the components that meet at each joint. Not smooth? Not soldered.
     
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  3. charlie chitlin

    charlie chitlin Doctor of Teleocity Silver Supporter

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    Rule 1: Check the last thing you f@#&ed with.
     
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  4. wabashslim

    wabashslim Tele-Afflicted

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    Not necessarily failing, but just wearing out. Old cathodes, like old people, can't shoot the juice like they used to. Go easy on 'em, give 'em a comfortable soft bias they can chew easily, and when it's replacement time, wrap 'em up in tissue and put them in the box before getting out those young insolent JJ's and pushing them into screaming grit. Have some respect.
     
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  5. Paul G.

    Paul G. Friend of Leo's

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    I'm also betting on a cold solder joint, probably somewhere near the newly installed bias pot.
     
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  6. tubegeek

    tubegeek Friend of Leo's

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    @Paul G. and I are both pretty sure.

    So it must be something else.
     
  7. Dacious

    Dacious Poster Extraordinaire

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    Play the amp while looking in the back at the tubes. Watch what happens when the volume drops. You're looking for arcing or difference in appearance in the tubes.

    I would pick a solder join, or possibly a worn out pot. But if they're really old tubes coukd be worn out.
     
  8. itsGiusto

    itsGiusto Tele-Holic

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    Well, I went ahead and tried replacing the tubes for some EHX 6V6GTs I had lying around. I swapped and biased them, and I think the amp is sounding pretty good now, though maybe less bright than it used to be, and maybe louder than it was. Kinda maybe sounds a little like a Twin at loud volumes (but way quieter, of course).

    So I'm guessing that it was just that the old RCA tubes were worn out, and didn't like being biased hotter? I THINK I haven't heard any volume fluctuations since I swapped out the tubes on Sunday or Monday night... Of course now that I'm posting this here, I'm sure the amp will want to prove me wrong and will start fluctuating volume tonight.
     
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  9. Peegoo

    Peegoo Poster Extraordinaire

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    It may also be the preamp tube sockets need re-tensioning.

    Anytine you re-flow a solder joint, give it some help with a flux pen or a little brush-on flux.
     
  10. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Cool, I hope that was it. You got lucky, old tubes like that can take out a transformer. Especially the fragile PR transformers. Don't ask how I know! Those old PR's run pretty high voltage like the DR's. Probably 450 plate volts?
     
  11. itsGiusto

    itsGiusto Tele-Holic

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    Actually, it's interesting, I was expecting to have to bias it so the voltage was very high like that. But to get these EHX tubes to 70% dissipation, I had to bias it to where the voltage was about 392v on the plates.

    I'm moderately new to biasing, but it seems that to get higher dissipation, you have to make the plate voltage lower, or at least in this case. It's weird though, cause I thought I remembered it being the other way around... Did I get something wrong?
     
    Last edited: Jul 2, 2020
  12. schmee

    schmee Doctor of Teleocity Silver Supporter

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    IME it's usually not a huge voltage difference from biasing, but some. Do you know what the voltage was with the RCA's? 392v isnt too bad. My BFDR's run 460+ v and I only use JJ's now after losing an output transformer to NOS RCA's about 15 years ago.. I tried EHX's which sounded good, but found gigging they only lasted about 3 months at my voltage! Two sets. So went to JJ's and haven't looked back.
     
  13. itsGiusto

    itsGiusto Tele-Holic

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    Unfortunately I didn't keep great records... I can't remember if it was ~410v or ~430v... One record I think refers to this amp indicates 418v, maybe?

    But is the directionality I mentioned correct or wrong? Is it
    lower plate voltage -> higher current and higher %dissipation​
    or should it be the other way around?
    higher plate voltage -> higher current and higher %dissipation?​

    Or does it differ by case?
     
    Last edited: Jul 2, 2020
  14. tubegeek

    tubegeek Friend of Leo's

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    It kind of depends - if you're starting with a "clean sheet of paper" designing an amp, if you go to a higher B+ you will get more plate dissipation at the same current.

    However, if we're talking about what happens with an existing amp when you adjust the bias:

    Compared to wherever you start out:

    If you bias the amp hotter (bias voltage closer to 0) you will draw more current from the power supply. This will tend to drop the B+ plate supply due to losses in the power transformer and also voltage drop in the rectifier tube if your amp uses one.

    So simultaneously you are doing a move that will increase plate dissipation (more current) and reduce it (lower B+.)

    Exactly where you end up %-wise needs to be calculated for your actual setting.
     
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  15. King Fan

    King Fan Poster Extraordinaire

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    Without knowing the bias numbers, I *really* hope I'm wrong, but your theory that you may have killed your vintage RCAs sounds sadly plausible.
     
  16. itsGiusto

    itsGiusto Tele-Holic

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    Yeah, I guess that's too bad. For some reason I don't really mind, they're just tubes, they're meant to be changed at some point. I wouldn't have spent lots of money on vintage RCAs, they're just the ones that happened to come in the amp when I bought it in 2015.

    But for the sake of completeness, are they definitely dead? They were fluctuating in volume but still working. Maybe if I dropped the bias down to what it was originally, they'd work again?
     
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  17. King Fan

    King Fan Poster Extraordinaire

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    Just tubes, except soon there won't be any vintage tubes, and they'll never make 'em like that again. I'm not trying to guilt you. I just like old things and I happen to think vintage 6V6s sound better.

    Yes, I would be curious if they still work at lower bias (and I'd like to know what bias that was). My secret fear would be they were cruising along happily at age 50+ and then they got cooked. *But* that's the gloomy worrier view. Too bad, as you say.
     
  18. itsGiusto

    itsGiusto Tele-Holic

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    I'll have to try it out sometime. Fortunately, I still have the old bias resistor that I replaced with a pot. It was a 22k carbon comp. I could measure what the value actually is sometime.
     
  19. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    OP:

    What exactly is plugged into the amp when you hear the volume fluctuations?

    What is the model/make of your rectifier tube and how new is it?

    (asking bc I just ran into a similar situation with guitar and bass feeding the amp, and changing out a 15 yo rectifier tube seems to have resolved it, for now. Could be a red herring but the rect swap made a difference everybody could instantly hear)
     
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