What is "cathode biasing"?

Discussion in 'Amps and Cabs' started by phoenix 7, Sep 28, 2006.

  1. phoenix 7

    phoenix 7 Silver Supporting Member

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    Someone mentioned this on another thread as a way of lowering the wattage on a Bassman (mine's a Victoria Bassman) by about 10 watts. I'm interested in having this mod done, if it works well. So, what is cathode biasing, and would it work well to get my power tubes cooking at lower volumes? Thanks!
     
  2. jamison162

    jamison162 Member

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    What's the difference in the two as far as volume level, tone, etc?
     
  3. Blue Strat

    Blue Strat Member

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    It'll lower the wattage and make the amp compress and break up at slightly lower volume. It won't cut the volume in half.
     
  4. drbob1

    drbob1 Silver Supporting Member

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    It's more of a difference in feel and harmonic content than a difference in volume. Like triode mode or 1/2 power modes, since the iron stays the same the volume is almost the same...
     
  5. billyguitar

    billyguitar Member

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    That was me. I said it would reduce the wattage by maybe 10 watts, depending on which rectifier tube you use. A 5Y3 gives the most sag. At low and moderate volumes the amp sounds the same but at some point it smooths out more than gets louder, that's the sag. The amp slams less and should be easier on the speakers. One of my 57s came with 59 speakers, unreconed. I probably only gigged this amp about 10 times before I started using other amps but I couldn't hear the original coned speakers straining. I also made a junction box so I could attenuate the amp with an attenuator if i wanted to. It never needed much attenuation.
    To me the amp sings better and reducing the punch is easier to control the volume.
     
  6. phoenix 7

    phoenix 7 Silver Supporting Member

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    Thanks for the helpful info -- it's much appreciated. This may work well for my amp.
     
  7. daddyo

    daddyo Guest

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    The most appealing thing about cathode biasing is that it is self biasing - no adjustments with a voltage meter.
     
  8. Jimmydeez

    Jimmydeez Supporting Member

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    How hard it is to change an amp to cathode bias?
     
  9. Blue Strat

    Blue Strat Member

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    Not always. Not every pair of the correct type of tubes will bias correctly in every self biasing amp.

    In EL84 amps, EL84s are typically run at least 20% over their specified limits. Ask the designers of these amps why. One resistor change would correct this.

    Then you have the, usually, single ended (one power tube) amps that can use "any" octal power tube. It's not possible to select a cathode resistor which allows all the different power tube types to bias optimally or correctly.
     
  10. Blue Strat

    Blue Strat Member

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    LOL! Technical quote of the year!!!:crazyguy
     
  11. billyguitar

    billyguitar Member

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    Jimmydeez: One of Gerald Weber's books tells you how to do it, that's how I did it. It's more than I can write here, though. It certainly wasn't hard to do or I couldn't have done it. I believe the amp must have a tube rectifier or it can't be done.
     
  12. Blue Strat

    Blue Strat Member

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    No, a tube rectifier is not required.

    Check some of the Fender schematics at www.schematicheaven.com
     
  13. billyguitar

    billyguitar Member

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    I don't have many amps without a tube rectifier so I never thought about that, thanks. With the tube rectifier you can adjust how much sag you get, from 5Y3 soft to 5AR4 hard.
    I need to call you and get some tubes for my Stang Ray. One day soon........
     
  14. Fuchsaudio

    Fuchsaudio Member

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    And condoms too, oh ummm, nevermind.
     
  15. Blue Strat

    Blue Strat Member

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    "Glow in the dark" or "ribbed for her pleasure"? :dude
     
  16. Jimmydeez

    Jimmydeez Supporting Member

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    Hey Andy is it possible to mod the ODS20 to make it cathode Biased?
     
  17. Blue Strat

    Blue Strat Member

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    Any amp can be converted to cathode bias.
     
  18. Jimmydeez

    Jimmydeez Supporting Member

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    I guess I was wondering how easy/hard it would be.

    Scottl had the proto 20 watter and it had a cathode bias switch. I loved the way that amp sounded in cathode mode.
     
  19. Fuchsaudio

    Fuchsaudio Member

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    Sure, a switch could be added to go from fixed to cathode bias.
    Drop me an e-mail and we can go over the details.
     
  20. Jimmydeez

    Jimmydeez Supporting Member

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    Thanks.... I just emailed you.
     

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