RE-BIASING for the "non-tech"

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by trower, Feb 11, 2006.

  1. trower

    trower Silver Supporting Member

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    O.K, I'm not an Amp tech but I have a good mechanical backround. How difficult is it to bais amps. I'm fully aware of the electrical dangers a loaded transformer holds for the "screwdriver probing nimrod" but I'm also smart enough to know when the waters too deep for me..any advise?
     
  2. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    Not to be a smartass but it's the capacitors that hold the juice, not the transformer.

    That said, with a tool like a Weber Bias Rite, it's pretty easy if you can avoid electrocuting yourself assuming your amp has trim pots.
     
  3. trower

    trower Silver Supporting Member

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    You see! I'm in trouble already!.. loaded meaning "plugged in" anyway, i'v seen these probes on ebay but do they include a clear step by step baising approach? Reading a bias setting and making the necessary adjustments are clearly two different things.
     
  4. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    If you can work a screwdriver and do multiplication, you can use a bias rite.
    1. Start at minimum bias and increase until things sound "right".
    2. Measure the plate voltage
    3. Calculate max "reasonable" bias current (based on 70% of max plate dissipation)
    4. Check that the bias current doesn't exceed value calculated in step 3. If it does, reduce to that value.
     
  5. trower

    trower Silver Supporting Member

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    Thanks...Think i'll pick one up and give it a shot...mind if I pick your brain down the road if I need to?, sounds like you've done this before................................multiplication...does that involve numbers?
     
  6. aleclee

    aleclee TGP Tech Wrangler Staff Member

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  7. bluesdoc

    bluesdoc Gold Supporting Member Gold Supporting Member

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    You do need to know/determine that your amp has bias trimmer/pot(s). If not, it's a whole other thing. If it does, AND it's accessible from the outside, it's really, really easy if you have a bias meter for the output tube type you have (ie, octal vs 9 pin, eg, 6L6 vs EL84).

    If it does have a bias pot and it's inside, then it's still easy, but you are near dangerous voltages from the power caps.

    If you have a meter and know the recommended bias setting for the tube/amp you're using, you won't need to do any math. Just warm 'er up and set the bias.

    jon
     
  8. Dave C

    Dave C Gold Supporting Member

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    I have the Groove Tube Bias Probe which comes with a manual. I've often felt that the manual alone was worth the price of the manual and probe. The manual has settings for many different amps and tube types . I work on a lot of amps and found the info very helpful. I've seen probes cheaper that probably work just as well but for me .......I would want that manual.
    Dave C
     

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