What could happen if tubes are not properly biased?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by lgehrig4, Mar 16, 2008.

  1. lgehrig4

    lgehrig4 Silver Supporting Member

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    Can it hurt the amp or just the life of the tubes? I just received a Louis Electric KR12 and I need to change the tubes. I have a new matched pair of EL34's, but I ordered a bias meter and have to wait until it arrives until I can bias them.

    Would it hurt the amp to play these lightly for a few days until I can bias them? They sound real good, and I don't mind if tube life drops a bit, but if the amp could be harmed in any way I won't do it. I only play at home so we're talking maybe 1/2 a day ay maybe 1/2 power.

    Thanks!
    Jeff
     
  2. phsyconoodler

    phsyconoodler Member

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    If they are biased too cold it will sound bad and if they are too hoy they may redplate.If they are not redplating while you are playing then you may be ok til your meter gets here.Turn the lights off and fire up the amp in a dark room.Play while looking at the back where the tubes are.If they look normal,no plates glowing red,then you are fine.
    What amp is that you have?

    I looked at their website.Pretty cool amps.It sounds VERY EL34 to me!
     
  3. Blue Strat

    Blue Strat Member

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    Note that redplating doesn't occur until you hit about 200% of maximum dissipation and you're not supposed to exceed 60%.

    Using replating as an indication of anything other than imminent disaster is a very bad idea though it may be safe in this case for a few hours of use.
     
  4. phsyconoodler

    phsyconoodler Member

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    What tubes are you taking out and what tubes are going in?That would help determine whether it's safe or not.
     
  5. PRNDL

    PRNDL Member

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    It is possible to bias them without a meter.
    There are several methods, but the best is to put in 1 ohm resistors at the cathode and measure the voltage drop.
    In cathode biased amps, measure the cathode voltage and divide it by the resistor value. This, however, usually shows the total in both tubes.
     
  6. lgehrig4

    lgehrig4 Silver Supporting Member

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    But is is bad for the amp or just the tubes? I am taking out Svetlandas and can either put in new Mullards or NOS Teslas.

    It can be biased without the probe, but I would have to desoldier a resistor and that I'm not doing.

    Again, the only thing I'm concerned about is causing damage to the amp.
     
  7. rockon1

    rockon1 Member

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    Ive changed power tubes without biasing for 25 years without "problems". I guess that means that its generally not much of a risk to the amp as some would lead you to believe. Ive probably shortened some power tubes lives and not heard the amps at thier best from underbiasing though. I may have been extremely lucky also.

    Lately I think its better to know the tubes are running optimally and have learned to bias my amps. Ironically the only time Ive had a problem changing tubes is from slipping with a meter probe and damaging the amp while trying to bias it! lol! However that was my negligence!


    Still its been a good learning experience. I also use a probe now as it reduces the amount of poking around inside the amp I need to do! Bob
     
  8. Blue Strat

    Blue Strat Member

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    It's unlikely to cause amp problems but not impossible. If a power tube shorts it can take out your screen grid resistor which isn't a big deal but requires work. When that resistor goes, it can leave carbon deposits on the socket which often can't be removed and will cause the amp to blow fuses until the socket is changed. Not a huge deal either, but do you really want to go there?
     

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