Bias mod issue

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

  1. Markorock37

    Markorock37 Member

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    Recently I modded a Peavey VTM 60 head for adjustable bias. Its running too hot even with the pot all the way down. I get one of the tubes just faintly glowing red. I have a 15K resistor feeding a 25K pot then returning to the power board with a 33K resistor. Plate voltage is 475V running 6L6 power tubes. I was following someone else's mod when I did this and it originally called for a 10K pot. What value resistors should I be running to make this work? I rock with a solder gun but not so much on theory.
     
  2. Blue Strat

    Blue Strat Member

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    Assuming it's a standard circuit (listing resistor values doesn't confirm that) increasing the resistor between the pot and ground (33K?, I don't know) is what you need to do.

    Hopefully you're taking a measurement to set the bias as opposed to looking at the plates which is worthless.
     
  3. Markorock37

    Markorock37 Member

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    Right. I have a bias probe and an oscilloscope. Guess I will play with a few resistors when I get outta work.
     
  4. Blue Strat

    Blue Strat Member

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    Forget the scope. The bias probe is all you need. Whichever resistor goes to ground is the one to increase.

    BTW, what bias current are you shooting for? The "burning red" comment concerns me.
     
    Last edited: Jan 26, 2012
  5. dualhumstrat

    dualhumstrat Member

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    I modded my VTM 60 for adjustable bias as well.
    I find you need to squeeze every volt out the bias tap, and I still have some
    tubes that run too hot to use at all. The bias tap is a bit low on voltage compared to
    other PTs.

    As I recall, I used something around 2 or 3k for the top resistor, a 47k trim pot
    and a 10k resistor to ground. Most of my tubes will bias ok with this arrangement.

    Good luck
     
  6. donnyjaguar

    donnyjaguar Member

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    There's always a way to get w-a-y more bias voltage than you'll ever need.
     
  7. RedRock

    RedRock Member

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  8. cap47

    cap47 Member

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    If your bias resistor is 33K You should have at least that much resistance in your circuit since the amp design is at 33K. Solder a 15k resistor in series to a 25 K or 50K bias pot. Set the knob to read 33K across the pot and resistor. Solder it in and you are at factory spec before you even start to fine tune the bias.
     
  9. dualhumstrat

    dualhumstrat Member

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    Yes, you can always take the bias voltage off the HT, but why go to that much trouble when you can just optimize the existing circuit for maximum range, and then
    just select your tubes a little more carefully. Most good tube suppliers can supply tubes that will bias up in whatever range you want.

    I am someone who makes all kind of needless changes just for the fun of it, but I
    still see no need to change the bias circuit any more than I have already.

    I am running EL34s in my VTM, and I can just order 34/4.7 matched pairs and know
    that they will work every time.
     
  10. Markorock37

    Markorock37 Member

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  11. dualhumstrat

    dualhumstrat Member

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    Yes, I thought that might be the mod you were referring to.
    Those resistor and pot values would probably work well in a lot of amps, but
    I don't think they work well in this particular amp because of the relatively low voltage
    available in the stock circuit. 40V vs 45 to 55V in most other PTs I've seen, at least.

    Give my setup a whirl, and see how that works for you. I find that about 80% of the
    tubes I tried worked fine. If you need more bias voltage, then by all means follow the
    other posters' suggestion and take the bias voltage off the high voltage winding.
    There are lots of examples of this available. Check out a marshall 1987 schematic
    for another example.

    Good luck
     
  12. Markorock37

    Markorock37 Member

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    __________________
    That means red plate. I am only shooting for just a little hotter bias, just want to see what the amp is capable, for all I know it might not be worthwhile but I'm having fun doing it and getting my feet wet.
    I'll bias using both methods.
     
  13. Markorock37

    Markorock37 Member

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    Thanks dualhumstrat, I think I can get what I need off the stock location, getting plenty of power just need to cut it down a bit. BTW I am using 6L6's instead of EL34's, would the resistor values be the same as 6L6's need more ma?
     
  14. Markorock37

    Markorock37 Member

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    Can I just change the 33K to 15K (giving me a total w/ pot of 58K)? I'd hate to change out the $10 25K pot I drilled in.
     
  15. dualhumstrat

    dualhumstrat Member

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    It's really more about ratios than specific values - you have to look at the circuit as a whole.

    You could either make the 33k resistor larger, or make the 15k smaller, or a
    combination of both.

    Either way, though, I think you'll find the range of adjustment rather narrow
    with the 25K pot, but you should be able to find a spot that works for you.
     
  16. dualhumstrat

    dualhumstrat Member

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    Oh, and I can run 6L6GCs fine, too, with the same caveat - I can't run ALL of them, but I can find sets that work in the range I have available.
     
  17. dualhumstrat

    dualhumstrat Member

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    Having thought about it a bit, I would change the 15K to about 5K and go from there.
    The problem is that the top resistor needs to be relatively small so that you
    don't lose too much off the max bias voltage, but as it gets smaller the range
    of adjustment also gets smaller. It's a balancing act.

    I figure you would have only about a 3 volt range to work with if you keep the
    25k pot, but maybe that's enough for your needs.
     
  18. Markorock37

    Markorock37 Member

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    So no change to the lower resistor? Seems I would have to increase that one too to bring the voltage down. I appreciate all the input so far.
     
  19. Blue Strat

    Blue Strat Member

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    It's a voltage divider circuit. Changing the ratio of upper to lower resistors is the goal here so either change might work depending on what the voltage at the transformer tap is.
     
  20. Markorock37

    Markorock37 Member

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    Cool. I'll play around with a few values and let you know what I get.
     

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