Princeton Reverb & Bias

Discussion in 'Amp Central Station' started by stefanhotrod, Aug 15, 2019.

  1. stefanhotrod

    stefanhotrod Tele-Meister

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    Hi and good evening...

    I‘d like to know how hot you run the bias in your Princeton Reverb?

    Got mine back from the tech, he swapped the original 110V transformer for a TAD 240V and I‘ve asked him to set the Bias cold (19 ma) which is a bit too cold :rolleyes: for my liking. It sounds great like before, but there was a bit more „soul“ with a tad more bias.

    Thanks for your suggestions!
     
  2. dan40

    dan40 Tele-Afflicted Silver Supporter

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    We would really need the plate voltage to know if 19 ma is running it cold or hot. If plate voltage is higher than normal with the new PT, 19 ma may be a bit too hot. If the new PT is delivering a lower plate voltage, you may be biased a bit cool. Are you comfortable taking a plate voltage measurement with the amp powered up?
     
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  3. schmee

    schmee Poster Extraordinaire

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    Be sure to keep your vintage transformer!
     
  4. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Stefan, please forgive me if you know all this. If you calculate your bias in percent dissipation, everyone can answer your question. As noted, percent dissipation depends on plate voltage *and* current (and power tube max wattage).

    Do you have a way to measure bias? Have you tried a good online bias calculator like Rob Robinette's?

    Anyway, once all that is taken on board, the 'right answer' is probably a number between 55 and 70% dissipation. In my personal PR, I like 70%.
     
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  5. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    I’ve monkeyed around quite a bit with this, with calculators+ears, and have 3 PRs. Each one I eventually walked into between 22 and 23.5 mA.

    I like the way they all sound, each one is just a little different component-wise. My only “complaint” is I’d still like a much slower trem on all of them, but it isn’t that big of a deal and I’d rather have a phatter power tube sound (which I have), than a slowww trem at intensity 2.5 which hardly anybody else can hear in a group setting.

    So IMO you could set the bias a bit higher and it’ll probably sound better (sub 24, it isn’t going to gobble down tubes - true in my case at least).
     
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  6. DavidP

    DavidP Friend of Leo's

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    If you want to slow down the trem, change the 1M resistor to 470K; and one .01uF cap to .02uF -- worked great for me!
     
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  7. King Fan

    King Fan Friend of Leo's Ad Free Member

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    You’re right, and usually in fact you can just change that one .01 cap (the bottom cap in the array is already .01). Too hard to take out the .01? Just reflow the eyelets and stick another .01 alongside.

    To be clear, 'too high' bias affects trem intensity, not speed. FWIW my PR has plenty of wah even at 70%, but my 6G2 is a bit trippier at 60%.


    Sent from my iPad using Tapatalk
     
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  8. stefanhotrod

    stefanhotrod Tele-Meister

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    Thanks for all the input guys. I‘m a complete Neanderthal when it comes to high voltage, but I‘ve got a nice tech ;)

    I guess I‘ll have to setup the Bias to the sweet spot in numbers AND ears together with him :)

    The original tremolo cap is already swapped for a 500k one. A gorgeous tremolo, but I‘m totally blown away from this 40years old Accutronics spring reverb. Fantastic.

    Yes, the transformer, the original speaker and the completely useless pushpull pot will be stored.

    0C538939-6228-4C17-802D-1A6EFF2AF7B6.jpeg
    This guy is a tone machine.
    I surrender to that vintage twang.
     
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