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Peavey VTM adjustable bias - each tube

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  • Peavey VTM adjustable bias - each tube

    Attached is the schematic. I saw other people replace R9.

    how hard would it be to add an adjustable pot for each tube?


    Attached Files

  • #2
    I would raise R9 to around 3k, then play with making R8 a pot and resistor series combination for adjustment.

    The feed to the two sides of power tubes doesn't split until you are on the preamp board, and there is little room to play with. I am sure I could engineer a quad bias adjustment into this if I had to, but really, why? Use matched sets of tubes and they will be close enough. Just my opinion. The output transformer won't likely have the same resistance on the two sides, which affects the idle current readings. Plus the two sides of the phase splitter circuit are not really balanced either, so I don't know what all that adjustment actually accomplishes.

    In short 4-way bias adjust could be done, but would require some serious circuit work.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Originally posted by beedoola View Post
      how hard would it be to add an adjustable pot for each tube?
      If you are sure in your knowledge, and want to play with VTM120, separate R24 and R25 from J5 (pin 4) and make OTM (output tube matching) on a small PCB.
      This will enable bias balance of output tubes (V1, V2 / V3, V4) as at Twin SF135.
      Personally, I don't think you'll get anything spectacular, only you'll must to adjust OTM more often, depending how the output tubes is old.

      http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Twin-Reverb-SF-135-Schematic.pdf
      output tube matching
      www.peel.dk/Dynacord/pdf/Gigant%20(unknown%20revision).pdf
      adjustable pot for each tube
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