Bias Circuit Question

Discussion in 'The Squawk Box' started by Cjsinla, Feb 11, 2019.

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  1. Cjsinla

    Cjsinla Senior Member

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    I was looking at the way that Mojotone implemented a way to adjust the bias on their Vibrolux kit. It seems to me that a similar mod could be done to my tweed Super build as well.

    Vibrolux
    AA47A141-9B80-49AE-B28C-787BF8CEB2F6.jpeg

    I mean that I should be able to replace that 56k resistor with a pot and smaller resistor, right?

    Super
    F3C232CD-0048-42C7-83F4-EA1BC870EF99.jpeg
     
  2. ErictheRed

    ErictheRed Senior Member

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    Yes, except that pots can physically move with a little vibration and whatnot over time. So you'll want to keep checking it, I guess, before you fire it up.
     
  3. Cjsinla

    Cjsinla Senior Member

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    I have an extra bias adjustment pot that came with my Marshall clone that I could use, that should hold tight against the vibration I would think. I also have another regular sized pot, the type that I used in the Vibrolux. It’s not as loose as a regular pot I’m just not sure if I have enough room to mount it. The small Marshall type pot might fit on the board.

    C90230E9-6E71-4B9B-8D32-F9D45B85101E.jpeg
     
  4. sonar

    sonar Senior Member

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    You might be able to mount a mini pot right on the board. Common mod with Princeton Reverb’s, but more real estate on the PR bias board.
     
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  5. Cjsinla

    Cjsinla Senior Member

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    I do have one. As soon as the new set of tubes comes in the mail I’ll take the amp apart and take a look.
     
  6. sonar

    sonar Senior Member

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    Cool!

    Post pics of your solution if it isn't much of a bother. It makes the forum a bit more interesting.
     
  7. Cjsinla

    Cjsinla Senior Member

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    No problem.
     
  8. Cjsinla

    Cjsinla Senior Member

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    I started looking inside the amp in preparation to adding a couple of 1 ohm resistors to help with biasing. The Weber grounding scheme has only pin 8 grounded. I assume that I can connect pins 8 and 1 to ground via the 1 ohm resistor.

    8AAB76D2-6388-4306-8507-66900DDE3064.jpeg
    I also have an extra bias adjustment pot left over from my 2204 build. It looks as if it is supposed to be PCB mounted. I plan to use it despite that. Do you think that I need to ground the unused terminal?

    6F72A575-9E47-475C-9B68-F103ECB9AC8D.jpeg

    @V2 @Soul Tramp
     
  9. Cjsinla

    Cjsinla Senior Member

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    Am I right to assume that the Super amp is fixed bias even though it has no bias adjustment pot? If so, this amp reads at 30.4 and 32.8 mA’s on the new power tubes I just put in. I was looking at the Robinette bias calculator discussion about fixed bias vs cathode bias. If this amp is fixed bias it is right where it’s supposed to be. It’s running too cold for it to be cathode biased.
     

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