12AX7 cathode resistor in series with pot

Discussion in 'The Workbench' started by StingRay85, Mar 17, 2020.

  1. StingRay85

    StingRay85 Active Member

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    Short question. If you put a 10k pot in series with the cathode resistor going to ground, how do you properly wire this? In the middle goes the signal, then decide which part of the pot goes to ground, and leave the other side untouched? Closing the pot means 0 ohms in series with the cathode resistor, opening it up means 10k in series (I will use a 1k probably). So that's the reserved way a volume pot is grounded.

    Am I correct on this one?

    The goal is to control the cold clipper on a 2204 preamp, and to see if there is a sweet spot around 5k that can be found.

    Is a regular pot ok for this? I assume no high wattage or voltage is needed on the preamp cathode resistors.
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    When you turn the pot there is going to be crackling.
    That's because the cathode has DC voltage.
    The pot can be rated for DC voltage but it's normally only about 2 volts or less at the cathode of a gain stage.
    Normally the cathode resistor is changed with a switch or with a relay, not a pot.

    cathode pot.png
     
    Last edited: Mar 17, 2020
  3. South Park

    South Park Well-Known Member

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    Any time you choke off power it is the amp rating you look at . Because you are baking off power you don’t use. Check the wattage of the pot
     
  4. RickyLee

    RickyLee Well-Known Member VIP Member

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    I do this method quite a bit for fine tuning my projects.

    But, lately I have been wondering if I should not be leaving the pots in permanent on some amps I have. This is due to some odd sounds and things I have been encountering and some things have improved when I just pulled the pot and put a fixed resistor. I have been very curious if these pots in various places have been affecting voicing, noise ect. Hmmm. I use quality 1/2W Bourns pots as well.
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes I understand.
    He wants to tweak it just right.
    I want to do the same thing with the slope resistor in the tone stack.

    The wiper of the pot can lose contact when the internal contact surface oxidizes (like from age).
    This could cause intermittent or erratic result of gain stage.
    Which is probably why fixed resistor is used more often.
    But there is pot like wire wound or pot designed for power application in DC circuit.
    The DC between the contact surfaces eats away the surface of the metal slowly or otherwise.
     
  6. mickeydg5

    mickeydg5 Well-Known Member

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    How about this? No DCV worries.
    The 820R or some low value can be added in series for a minimum low resistance.


    upload_2020-3-17_22-9-11.png
     
    Last edited: Mar 18, 2020
  7. RickyLee

    RickyLee Well-Known Member VIP Member

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    I usually use a resistor value that is the lowest cathode value I would want to try - like a 470R or 680R. Then put that in series with the pot. I try to put the pot on ground side as let the resistor drop the first voltage. But then in theory, what is correct there? lol Current travels from negative/ground to positive, so actually, I should be doing the opposite?
     
  8. thetragichero

    thetragichero Well-Known Member

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    just finished a build with a 6k8 resistor in series with ~40k pot in a jcm800-based preamp. love it!
     
  9. RickyLee

    RickyLee Well-Known Member VIP Member

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    Is that in place of the 2nd stage cathode 10K?
     
  10. StingRay85

    StingRay85 Active Member

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    Interesting approach. Are you sure it will work? Will this have the same effect?
     
  11. mickeydg5

    mickeydg5 Well-Known Member

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    Nope, I am a dumbass.
    Its in my signature if you can see that.
    It states:
    "WARNING!
    I have not a clue and no one should take advice from me especially regarding repairs and/or modifications. :D"
     
    Last edited: Mar 18, 2020
  12. thetragichero

    thetragichero Well-Known Member

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    yup. didn't do a lot of empirical testing for the values: wanted to be able to get lower than the Marshall 10k and higher than the soldano 39k. values seen to work nicely for my tastes
     
  13. myersbw

    myersbw Well-Known Member

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    Actually, no, it won't quite be "the same". His approach allows the ac gain to vary, but maintains the same dc bias setup on the tube with respect to the load line. You'll actually be altering the frequency spectrum within the same bias. Doing the strictly pot-in-series will change the operational curve lines as portrayed on the tube's data sheets. Much different result per given swing of the pot.

    AMS has good points of DC interaction on pots with metal material on the wiper disc.

    Ricky - shouldn't matter which resistive value is "first" in the cathode unless you're tapping off in between the pot & resistor. Overall, the whole bias circuit determines current and then each resistive value presents a respective dc voltage drop...matters not unless you're tapping for something in between them.

    I prefer mickeydg's method in regard to safety, no noise, etc. Otherwise, I find the inline pot (no cap) ok if just tweaking to determine the permanent resistor to use. Any tube can short and you don't want any high voltage on a pot...for however short a time before a fuse blows.
     

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