Do you really need to bias your tubes?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by fishlog, Aug 21, 2008.

  1. fishlog

    fishlog Member

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    How important is it really...
     
  2. jawjatek

    jawjatek Member

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    It only matters if you can actually play the guitar, and the guitar is in tune. :dude
     
  3. Steve Dallas

    Steve Dallas Supporting Member

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    Absolutely. It matters a great deal.
     
  4. TweeDLX

    TweeDLX Member

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    Hmmm...if you don't, (and the tubes are over-biased) at the very least you may wind up spending more money than you want on new tubes. At the worst, red-plating tubes may fail and take other more expensive parts with them. Or if they're under-biased...your amp probably won't perform at its best. Why spend good money on vintage amps only to run them at less than peak performance? Just my opinion. :)

    Mike
     
  5. slider313

    slider313 Silver Supporting Member

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    It's like adjusting the carb in an older car. If it's running too lean the cars power will suffer, if its too rich you'll burn too much fuel and foul the plug. If it's at its optimum point everything runs the way it should.
     
  6. rockon1

    rockon1 Member

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    In all my years swapping out tubes(many times without rebiasing ) Ive never seen one redplate because of it. They would have to be running way hotter than not biasing would make them run.

    Still optimum performance requires rebiasing. They could be running too hot and wear way too fast or be to cold and sound poor.

    A good vendor should be able to provide you with a set that at least wont run too hot though. Bob
     
  7. groovy daddy

    groovy daddy Member

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    Some tubes vary quite a bit you might want to check it.
     
  8. TweeDLX

    TweeDLX Member

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    A friend of mine once put a set of used tubes into his Boogie (un-biased) with disasterous results. Smoke poured out and it fried on the spot. That's my basis for the line about red-plating. Cost him a few hundred bucks in tech time to get it fixed.

    Mike
     
  9. rockon1

    rockon1 Member

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    Id be willing to bet he had a shorted/bad tube but I can understand your cause for concern in the matter. I'd probably feel the same way. Bob
     
  10. VikingAmps

    VikingAmps Member

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    If you don't just keep plenty of tubes, fuses, resistors, capacitors and transfomers on hand and you should be fine.
     
    Last edited by a moderator: Aug 21, 2008
  11. fishlog

    fishlog Member

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    Well that answers that!

    Thanks everyone!
     
  12. rockon1

    rockon1 Member

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    I agree it should be checked. That said I can say that for over 20 years I just swapped out tubes without rebiasing.I wasnt until recently I started rebising my amps thru knowledge gained here. I never had a "meltdown" or problem caused by it. I probably had amps that didnt sound thier best or wore out tubes quicker though. I still have a few amps I dont worry about it in. Just my experience with many power tube changes. Yes,I may be extremely lucky or that many of the amps I routinely retubed had cold fixed non adjustable bias points. I dont condone it but thats just my experience. Bob
     
  13. mooreamps

    mooreamps Senior Member

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    Depends.... :]


    For power tubes, ya they need to be biased. However ; I saw one schematic where, on the first gain stage on the preamp, was a dead short on the cathode, and a 10 meg input grid leak resistor on the grid. I've not tried this myself, but supposedly, the triode will conduct hard enough to generate about a 1/2 volt of internal bias for itself.


    -g
     
  14. doctorx

    doctorx Member

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  15. hasserl

    hasserl Member

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    This is called Grid Leak Bias and it's not that rare. But I think you missed the point of the question, that being whether it is necessary to check and adjust the bias of tubes (I assume he means when you change them)? Not whether tubes need to have some kind of bias circuit.
     
  16. GearHeadFred

    GearHeadFred Member

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    It's a numbers game.... mainly because most amp manufacturers factory setting is a cold bias, almost any tubes you install will not cause a problem.. This is "by design". BUT, any amp that has been bias adjusted out of the "safe zone" for optimum tone should definitely re-bias when changing power tubes.

    IMHO, I feel that bias has somehow attained a mysterious black voodoo aura.. I chuckle when I read about guys hearing a .1 mA change in bias adjustment.. but to each his own.
     

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