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Peavey Butcher Bias Mod Help!

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  • Peavey Butcher Bias Mod Help!

    Hello. Very new to electronics modding and have little experience unless there is a very clear set of instructions in front of me. I successfully done a couple mods to pedals and other tube amps but this one is confusing me.

    I have seen multiple different people add the bias mod to a peavey butcher and explain it in different ways, and go off on a tangent with numbers I have no idea about can somebody explain what needs to be done in the most simple, dumbed down way for a dummy? it looks like to me that I'm just replacing R52 with a 10k pot but then another person did something else different and someone said that was wrong lol. if someone would dumb it down for me I'd appreciate it very much haha
    Schematic: https://irationaudio.com/2015/09/25/peavey-butcher/

  • #2
    Whatever method you use, keep in mind that pots can fail, so ALWAYS consider what would happen to your bias if the wiper breaks. In other words, disconnect the wiper adn what goes on?

    I myself don't like your R52 becomes 10k pot idea. If your pot dies, it all reverts to the hottest setting. My preferred way is to raise R52 to maybe 3k or thereabouts. At 470 ohms there is no room for voltage division. I then make R53 the variable. That way if the wiper dies the whole thing reverts to the coldest setting.

    Also be aware of the range of settings. The amp maxes out at -57v, and maybe the lowest I might expect to ever need might be -45v. But certainly we never want to be able to turn it to zero, right? R53 is 47k, a reasonable value. Replace it with a 50k pot seems reasonable. But we could turn that to zero, so what we do is instead of just a 50k pot, we add a series resistor to make up the rest of the 50k at low settings. I'd say maybe 15k. Now you can't turn it to zero.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Originally posted by Enzo View Post
      Whatever method you use, keep in mind that pots can fail, so ALWAYS consider what would happen to your bias if the wiper breaks. In other words, disconnect the wiper adn what goes on?

      I myself don't like your R52 becomes 10k pot idea. If your pot dies, it all reverts to the hottest setting. My preferred way is to raise R52 to maybe 3k or thereabouts. At 470 ohms there is no room for voltage division. I then make R53 the variable. That way if the wiper dies the whole thing reverts to the coldest setting.

      Also be aware of the range of settings. The amp maxes out at -57v, and maybe the lowest I might expect to ever need might be -45v. But certainly we never want to be able to turn it to zero, right? R53 is 47k, a reasonable value. Replace it with a 50k pot seems reasonable. But we could turn that to zero, so what we do is instead of just a 50k pot, we add a series resistor to make up the rest of the 50k at low settings. I'd say maybe 15k. Now you can't turn it to zero.
      just wanna get this right. 50k pot, 15k resister into R53? and the resister, I'm guessing I'm getting 1/4w? on the schematic. it says all are 1/4w unless otherwise noted. thank you so much for this. it paints a much clearer picture for me

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      • #4
        I am wiring a 50k pot and a 15k resistor in series. THEN that little circuit replaces R53.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          not trying to be a noob but i got around to doing this, and there doesnt seem to be any difference. anyone able to tell me where i dun goofed? lol.

          https://drive.google.com/file/d/1zTr...ew?usp=sharing

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          • #6
            What is the photo of? I see a circled cap and some wires passing by. I don't see an adjustment pot or added resistors. I see an emepty fuse holder
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              green and brown wires are all the mod. the original resister is right under the cap but I wanted to get it all in 1 photo. on the bottom is the pot with resister. brown going in, pot, resister, green wire

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              • #8
                Did you remove the original R53?

                (edit: deleted comment about C28 being wrong value, there is a typo on the board layout that shows 100 instead of 1000uF, schematic is correct)
                Last edited by g1; 04-22-2021, 08:32 PM.
                "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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