Bias mod issue

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Markorock37, Jan 26, 2012.

  1. dualhumstrat

    dualhumstrat Member

    Messages:
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    Jan 26, 2012
    Okay, I played with some numbers, and this is what I came up with.
    The same setup as mine, but with a 25k pot might actually work out okay for you.

    That's 3K for R9, and a 10K in series with your 25K pot for R8.

    I get an approximate range of about -43 to -52 volts with that setup, which is
    more range than I expected. Your actual numbers will probably vary somewhat
    but it looks promising.

    Pot at zero resistance:
    40*sqrt(2)*(10)/(10+3)=43.514246

    pot at 25K resistance:
    40*sqrt(2)*(10+25)/(10+25+3)=52.102584
     
  2. Markorock37

    Markorock37 Member

    Messages:
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    Jan 12, 2009
    Location:
    Flint, Mi.
    Yeah that looks really good. I bought a handful of resistors yesterday I was out of so I'll make that change today and give it a whirl. I ordered a new presence pot for the preamp board so until that comes in I won't be able to fire it up. Should be here in a few days but that gives me time to yank the power board and change those resistors out. A lot better number than the 70 or so I was frying my tubes with!

    [​IMG]
     
  3. dualhumstrat

    dualhumstrat Member

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    Jan 26, 2012
    Last edited: Jan 28, 2012
  4. Markorock37

    Markorock37 Member

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    Flint, Mi.
    Wow, that is some extensive modifications dualhumstrat! Nice!
     
  5. Markorock37

    Markorock37 Member

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    Flint, Mi.
    Well, I didn't have a 3K but had a 4.7K so opted for that. With the 10K to ground I was still up around 60ma. I changed out the 10K to 22,33, and ended at a 47K which got me to 55ma with the bias pot all the way down. Not much change with the ground resistor. I'll pick up a few other values to replace the 4.7 to get a more useable range. Also going to get some new tubes, the original Sylvanias are still in it for the power section, with Ruby's in the pre. I'm pretty much settled on some Tung Sol 6L6GC STR's due to the 516V plate voltage. Any ideas for preamp tubes?
     
  6. dualhumstrat

    dualhumstrat Member

    Messages:
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    Jan 26, 2012
    I'm thinking that you're not going to be able to squeeze much more out of it
    at this point. With a 47K bottom resistor, a 25K pot and a 4.7K top resistor
    you've got 94% of the maximum possible bias voltage already. Going to 3K
    will get you to 96%. My setup gives me 95%. The stock circuit using 470R and 47K gives you 99% of it. There just isn't much more to be had.

    It could be that your tubes are just too hot to run with the bias voltage available.
    Did they run this hot before the bias mod? It's always good to have a baseline
    to compare to.

    The tubes that came with my VTM were Peavey branded Sylvanias.
    One ran ok but the other was at about 60ma with the stock bias circuit.
    No amount of resistor tweaking would help with that tube, because the
    stock bias circuit is already maxed out. The only answer for me was to
    run cooler tubes or totally rework the bias circuit for more negative voltage.
    I might still do that some day just so I can run my old Svetlanas in it.

    As for preamp tubes, I usually just go for the cheap chinese ones. I think
    they sound fine, and they're pretty tough. Stay away from Tung Sols and
    Sovtek LPSs in the cathode follower position. They are great sounding tubes
    but won't last long in that position. Sometimes only minutes, from what I've
    heard. I kind of like a Tung Sol in V1, with Pentas (rebranded chinese) for the
    rest.

    God luck.
     
  7. Markorock37

    Markorock37 Member

    Messages:
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    Jan 12, 2009
    Location:
    Flint, Mi.
    I don't remember it running hot before but I also didn't take a baseline measurement before I started. I used to own this amp in the early 90's than sold it because the high gain jack never worked so I would always run a distortion pedal into the clean channel. Fast forward I just bought it back from the guy I sold it too for $100 so I figured on modding it quite a bit plus I fixed that bad chAnnel which was nothing more than a fried resistor on the preamp board. It still has the same tubes in it that I sold it with. If I knew how to work on these 10 years ago I would've never sold it bUt I'm glad its back!
     
  8. Markorock37

    Markorock37 Member

    Messages:
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    Location:
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    I'm ordering some tubes today then I'll give it another go on the bias. It is quite useable now so I may just keep it how it is.
     

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