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Thread: Please advice me how to do 5150 combo bias mod

  1. #1
    Junior Member
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    Please advice me how to do 5150 combo bias mod

    Hi

    I want to mod my 5150 combo. I find out the schematic is different with 5150 amp head.

    Please see the schematic, is this 47k a bias resistor?
    If yes, I should choose 22k or 47k trimmer? And what value resistor ? 10k?

    And I want to add 1ohm resistor to the power tube pin, to check the ma. I did it for vintage Marshall. How can I do it?

    Thank you

    Is img_8451.jpgimg_8451.jpg

  2. #2
    Supporting Member Jazz P Bass's Avatar
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    And keep in mind that the 5150 was deliberately designed with a lowish bias in mind.

    If you goose up the current it messes with the 'Resonance' control.
    Dave H, g1 and The Dude like this.

  3. #3
    Supporting Member The Dude's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    And keep in mind that the 5150 was deliberately designed with a lowish bias in mind.

    If you goose up the current it messes with the 'Resonance' control.
    Agreed. IMO, there is nothing to be gained by this mod.
    g1 likes this.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  4. #4
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    I'm sure you are right. I've deleted my suggested mod.

  5. #5
    g1
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    Quote Originally Posted by The Dude View Post
    Agreed. IMO, there is nothing to be gained by this mod.
    How about reduced tube life?
    (I'm sure the tube dealers would disagree with you)
    The Dude likes this.
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  6. #6
    Senior Member Enzo's Avatar
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    Yeah, if you are not wearing out a set of tubes every few months, you must not be rocking.
    Education is what you're left with after you have forgotten what you have learned.

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