Self biasing amps and non self biasing

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ibenhad, Oct 10, 2008.

  1. ibenhad

    ibenhad Member

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    So I have a Fargen Bastage that self Biases to any octal tube. I have a kit amp that I changed from 6L6's to what were originally in it 6V6's and now I can't get the bias right. Why is it the Fargen self biases and other amos don't. What could I do to change my amp to self bias? Is this even possible. Is it that mine has 2 tubes and the Bastage only 1? Thanks for any help you can give...
     
  2. VaughnC

    VaughnC Supporting Member

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    Self biasing amps are cathode biased (voltage drop across a cathode resistor self regulates the cathode current). Non-self biased amps are fixed biased from a power supply voltage, and the bias voltage is usually adjustable in order to get the output tube's plate current within spec. With fixed bias amps, when you replace output tubes, sometimes you need to change a resistor to get the bias pot within range of the new tubes...which sounds like your issue.

    Any amp can be set up to be self biased or fixed bias, but they tend to sound/feel different...so you might not like the change.
     
  3. WesKuhnley

    WesKuhnley Member

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    Not entirely true. If you try to set any old fender that uses power-tube-bias-vary trem as a cathode biased amp, you're going to have issues with the tremolo. In my experience, amps designed to output under 45 watts or so are relatively easy to convert to cathode bias, amps over this mark tend not to benefit from the mod at all.
     
  4. skipm45

    skipm45 Member

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    Crate's Blue voodoo combos from the mid 90s had an interesting "self biasing" circuit that allowed the use og 6L6s or El34s. The design is basically fixed bias with an additional bias circuit connected to the cathodes.


    Skip
    www.skipzcircuits.com
     
  5. andrekp

    andrekp Member

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    Something that can do it all, doesn't necessarily do it all equally well. I am not familiar with your amp, but generally speaking, different tubes have different requirements that are not 100% compatable. You can swap octal tubes in an cathode biased amp (provided it was designed for that feature) without having to rebias. HOWEVER, what you may be running into is that a plate voltage which is great for one tube, is so-so for another type. Or maybe the plate voltage is only good, but not great for any of them (compromise to allow compatability).

    So that might be what you are experiencing. Meaning that your amp might be "capable" of using three different tubes, but really works best with one, or maybe there's a mod you can make (extra resistance on a switch or something) that will allow you to compensate for the preferences of another tube.

    Just a shot in the dark, since I don't know your amp.

    Andre
     
  6. skipm45

    skipm45 Member

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    I agree. Just beacuse there is an "auto-bias" circuit that allows the use of various octal based tube types doesn't necessarily mean that it's optimised for all of those types. The designer may have tried to hit the "middle ground" in the circuit design. Withouit using a bias probe of some type in conjunction with a tube swap you'll never know.
    my .02


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  7. VikingAmps

    VikingAmps Member

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