Swapping Power Tubes W/out Bias

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by trower, Feb 17, 2006.


  1. trower

    trower Silver Supporting Member

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    What Are The Dangers Of Swapping Out Power Tubes In An Amp On The Fly'?.... Is It O.k To Do A Ruff Comparison On Quads Of El34 Type Tubes For A Few Minutes In Say.. A Marshall And Then Lug It Over To A Tech To Bias?
     
  2. brad347

    brad347 Member

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    I do this sometimes... if using quality tubes they are often fairly close, not always. Listen for crossover distortion (a sign that the tubes are biased too 'cold' and won't give you an accurate assessment of what they sound like) and especially watch for signs that the tubes are getting too hot, if the plates start glowing, especially a deep red, then you're in trouble and you should kill power to the tubes immediately. Try to keep it at sane volumes as well.

    Putting in new tubes without bias there is always a danger of putting premature wear on the tubes.
     
  3. donnyjaguar

    donnyjaguar Member

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    I'd be interested to know if anyone has every destroyed their amplifier by doing this. I've never managed to melt down a transformer myself - although I have killed tubes by pumping too much idle current through them.
     
  4. brad347

    brad347 Member

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    I think you could conceivably kill tubes but not transformers... except MAYBE if you had the wrong fuse inserted... others correct me if i'm wrong.
     
  5. devbro

    devbro Member

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    What does "crossover distortion" sound like?
     
  6. brad347

    brad347 Member

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    crossover distortion occurs in a class a/b amp when one tube stops "pushing" before the other starts "pulling" if you know what I mean... it basically sounds like a terrible, unmusical distortion that happens at lower volumes than distortion should... you will recognize it by how terrible it sounds. That's basically the simplest way I can put it.

    To add to what john phillips said, some amps and tube types are more likely to be biased at the upper limit than others. For example, lots of amps run 6V6s pretty "hot."
     
  7. Blue Strat

    Blue Strat Member

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    The bigger risk is in not doing a fair comparison between the tubes you're comparing. Listening to one set biased way cold and the other dead on (or hot) is not even a valid comparison, hence a waste of time and potentially dangerous to the tubes.

    Here's an idea, get a bias probe. You should have one anyway. No more trips to the tech for things you can do yourself.
     
  8. brad347

    brad347 Member

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    i need to get one of those bias probes!
     
  9. trower

    trower Silver Supporting Member

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    YEA, A most valid point, Would you believe I just got an e-mail from a fellow gearpager offering to loan me his Bias probe! I'v said it before and I'll say it again..THIS PLACE IS THE BEST! best people! best advice!!
     

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