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What do I do with a bias adjustment on a Princeton Reverb?

Discussion in 'Amp Central Station' started by dnauheim, Oct 15, 2020.

  1. dnauheim

    dnauheim Tele-Meister

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    I just got a '69 Princeton Reverb. It has a bias adjustment installed. Never had one of these--what the heck do I do with it? Will it change the tone? Do I risk burning up my tubes if I set it wrong?
     
  2. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Yes you can burn up the tubes if it's set wrong. Setting bias can make an amp sterile or sweet or in between. You need something to measure the bias. It can be a Volt Ohm Meter and a $14 item that goes in the tube socket, or you can buy a little device that makes it easier for more money, or there are other means to do it.
    Probably best to google how to do it.

    Nice find on the amp! I'm 70 miles north of Seattle.
     
  3. wmsimpson

    wmsimpson Tele-Holic

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    Congrats on the new '69 PR! Super nice amp! I picked up a '71 PR a couple years back and installed a bias adjustment pot in mine. If you ever need to have it checked or adjusted, you'd be welcome to drop it off here at my house, and then pick it up in an hour or so. I have all the needed equipment... and I'm just out in North Bend.
     
  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    If it's a 69 it would be good to know the service history, but for normal amp use you never need to touch the bias adjustment.
     
  5. EsquireBoy

    EsquireBoy Tele-Afflicted

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    Since the PR has a bias tremolo, you can use the latter as a reference when adjusting the bias by ear: the hotter the bias, the less depth the tremolo has.

    Set the tremolo intensity halfway up (NB: the knob won't necessarily be at noon, depending on the taper of the pot). Then play with the bias adjustment knob by very small increments and hear the difference in the amp's tone: as long as you still have good intensity on the trem, the bias should be within safety for your output tubes. When the tremolo starts to really fade (while still set halfway to the max intensity), you may be entering the danger zone.

    This method surely does not replace proper measurements but it's a good safety net when you just want to experiment a bit with the bias knob to hear what it does to the sound of your amp.
     
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  6. dnauheim

    dnauheim Tele-Meister

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    Thank you Esquire, I'll try that!
     
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  7. dnauheim

    dnauheim Tele-Meister

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    That's very kind of you! I am not in Seattle anymore though, but I can't figure out how to update my profile!
     
  8. EsquireBoy

    EsquireBoy Tele-Afflicted

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    You're very welcome. That's how I set the bias on a friend's PRRI without anything to take actual measures, and it's running nice and smooth.
    Just remember to take note of the initial position of the bias knob in case you want to come back to it.
     
  9. wmsimpson

    wmsimpson Tele-Holic

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    Go to the top of the page and set the cursor to hover over your user name. A drop down menu should appear, click on personal details. Change your location.
     
  10. dnauheim

    dnauheim Tele-Meister

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    Thank you! If only adjusting bias was that simple!
     
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  11. Dacious

    Dacious Poster Extraordinaire

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    Be careful. If you have a 69, it may have the older Champ-based power transformer, which are marginal - does it have a GZ34? Some people crank the bias to try to get more power, but that will just heat up the PT and you'll see your B+ voltage drop. As Esquireboy noted, the trem drops out as bias goes up.
     
  12. dan40

    dan40 Tele-Afflicted

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    And be careful if the bias pot is located inside of the chassis. There will be 400vdc inside the chassis that can get you if you touch the wrong spot. Use an insulated screwdriver and do not touch anything else unless you are experienced working on live circuits.
     
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  13. dnauheim

    dnauheim Tele-Meister

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    It has a 5U4GB. Not sure the significance of that, though.
     
  14. Paul G.

    Paul G. Friend of Leo's

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    If you don't know what you're doing, best to leave it alone or to a tech.
     
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  15. Dacious

    Dacious Poster Extraordinaire

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    Ok that's good. 68/69 was the crossover year, Fender went to the 5U4 and a bigger PT which had 3A vs 2A and 90ma vs 70ma B+. They're a bit tougher.
     
  16. dnauheim

    dnauheim Tele-Meister

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    I just checked and the tube chart actually calls for a GZ34 so . . . what then? It is alslo lists AA764 as the circuit. However, there is not guarantee the chassis is the original that went with that cabinet, that went with that tube chart . . .
     
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