Power Scaling: Fixed Bias or Cathode Bias ?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by yZe, Aug 25, 2006.

  1. yZe

    yZe Senior Member

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    How can i tell when my amp has either a Fixed Bias or Cathode Bias ?

    Seems like I have to know this before i decide which kit to install:

    http://www.londonpower.com/psk-faq.htm

    I plan on doing this to a both a red Bear MKE 60 and MKE 120

    both are P2P based on a JCM 800 circuit

    What does fixed bias and cathode mean to the laymen ?
     
  2. TheAmpNerd

    TheAmpNerd Member

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    OTM Does that mean we actually might have to read and (hopefully) understand those little squiggly things which represent an amps circuitry?
     
  3. yZe

    yZe Senior Member

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  4. trdlasvegas

    trdlasvegas Member

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    Their Fixed Bias.

    -Tony
     
  5. yZe

    yZe Senior Member

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    Thanks, Tony. If you don't mind me asking - How did (or do) you know?
     
  6. TheAmpNerd

    TheAmpNerd Member

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    The classic JCM800 circuit is fixed bias (with a pot in there).

    I assume those were your pics posted. If you look at the
    bias circuit you can see (as best as I can tell) a silver pot.

    It would be my recommendation that you have a tech
    install the PSB for you. No offense, but if you have a
    difficult time reading a schematic and therefore not
    knowing which type of bias arrangement your amp
    is, it would be well worth the money not to fry
    yourself, the amp or components.

    The time you save will be well worth the investment of having
    a power scaled amp and your pleasure of playing through it.

    Then when you get a little champ amp, buy a smaller
    scale box and tweak that first and learn from it.
     
  7. yZe

    yZe Senior Member

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    Actually, I am looking for a tech. i posted that i am looking for a tech in Tampa to install it in the Amp forum.

    I am just trying to educate myself as to the how and why's so i know that some tech isn't taking me for a ride.

    I was just curious for educational purposes only

    So, in light of the fact that I am in no way attempting or ever have intended to install this myself; would it be presumptuous of me to ask how I can tell where and what the bias circuit is in the first place from the schematic?
     
  8. trdlasvegas

    trdlasvegas Member

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    Because the schematics you listed show the all the output tubes cathodes connected directly to ground, no cathode resistor. Also there is a extra winding on the transformer producing a bias voltage. (It's called -V on those schematics)

    -Tony
     

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