bf bassman, Bias at 100v or 120v?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by alivegy, Jul 1, 2008.

  1. alivegy

    alivegy Member

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    Hey guys, I've got a 1965 Bassman that I'm going to drop some new SED power tubes into. I also just got an Ultimate Attenuator with the 100v tap option, the bassman will always be plugged into the attenuator. Should I use that outlet with my bassman and bias the tubes with the 100v, or stick with the 120v wall power? Since the amp will be running full bore into the attenuator all of the time, I thought it might be a good idea to save it some stress and bias to the lower voltage. Thoughts, opinions, ramblings?
     
  2. Blue Strat

    Blue Strat Member

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    The Ultimate Attenuator has different voltage taps? You actually plug your power cord into it? If so, it's a lot more than just an attenuator.

    Anyway, bias it under whatever conditions you use it.
     
  3. VikingAmps

    VikingAmps Member

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    I guess it's for brown and serve:

    100 volt AC tap
    3 prong female AC tap for plugging in your amp to drop the voltage to around 100v. Useful to slightly drop plate voltage on amp. Tonal effects are minimal. Feel may have slightly more give. Useful for older British amps that may run current production tubes a little hot. Easier on vintage transformers. Also useful for extended coffee fueled jams with extreme attenuation for easier (cooler) amp operation. Not to be used as a courtesy outlet.
     
  4. alivegy

    alivegy Member

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    Right I had heard of most people using it with vintage marshalls and hadn't heard of people using the tap with bg fenders. I might use it just because it will be attenuated all of the time and to go easier on my vintage transformer.
     
  5. Blue Strat

    Blue Strat Member

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    Using a lower AC input voltage is no easier on your transformer. The only reason to use it is to lower the voltages in the amp and "brown out" the tone a bit.
     
  6. phsyconoodler

    phsyconoodler Member

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    When you drop the voltage down like that before it gets into the amp,it causes the heater voltages to drop also.This may cause the tubes to malfunction earlier.Ask Eddie Van Halen how often he had to change tubes when he used a variac.
    Where did you get the idea that it would be easier on the transformer?
    Keep a spare set of tubes on hand if you are gigging with the amp.
     
  7. alivegy

    alivegy Member

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    Good point about the heater voltages, hadn't thought about that. I think I'll stick with good old 120v for now. Thanks for the responses guys.
     
  8. teleamp

    teleamp Senior Member

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    +1, on the filament voltage.
     
  9. gearitis

    gearitis Gold Supporting Member

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    I don't think that using the 100V outlet is a very good idea. It seems that having a 110V-115V outlet for vintage amps would be better.
     
  10. Lyle Caldwell

    Lyle Caldwell Member

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    109VAC is about as low as you can safely go without hurting the heater supply on older amps. Depends on the amp of course.

    Generally speaking you want to be within +/-.35VAC of 6.3VAC on the heaters.
     

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