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DC and Potentiometers

Discussion in 'Shock Brother's DIY Amps' started by andrewRneumann, Aug 24, 2020.

  1. andrewRneumann

    andrewRneumann Tele-Holic

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    When it comes to scratchiness on potentiometers due to DC, does it matter if DC flows through the resistance element only, and not the wiper?

    Say I used a pot for a 100k load resistor with the wiper attached to the coupling capacitor. DC would not flow through the wiper, so would it not tend to be scratchy?
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    IDK. You could try it and report back.:)

    Be careful standard pots are rated for 1/4 watt or less.
     
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  3. D'tar

    D'tar Friend of Leo's

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    Eve though the "load" is not on the wiper, the DC is still present and potentially emit noise while adjusting the pot. Not something you would likely be moving during active playing. No worries. Scratchy pots usually indicate an issue(usually small voltages also), but yours would be by design(much larger voltage) therefore you know what to expect.
     
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  4. andrewRneumann

    andrewRneumann Tele-Holic

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    Thanks I had forgotten they were so low.
     
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Parallel or series with a resistor would make things more comfortable. For instance a 500k pot with a paralleled 130k resistor would get in the neighborhood so the wattage rating would not be an issue.

    As @D'tar pointed out, a resistor/pot connected to high voltage is not something you would want to touch when in service.
     
  6. D'tar

    D'tar Friend of Leo's

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    I'd opt for plastic shaft pot there;)
     
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    And stand on a rubber mat with one hand in the back pocket. Just like adjusting a VVR.:eek:
     
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  8. D'tar

    D'tar Friend of Leo's

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    Good for test bed but maybe not for end users.
     
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  9. Nickfl

    Nickfl Friend of Leo's

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    Alpha liner taper are 0.5w.
     
  10. XTRXTR

    XTRXTR TDPRI Member

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    On the load resistor? Hmm for experimental use to find a gain or how it affects your amp...Why not? Most amps call for .5w resistors at the load resistor. Whats the worst that can happen it burns to an open then the tube just isn't getting any B+ its not gonna blow up your amp or ruin your power transformer. If it shorts then the tube might blow but the rest of the amp is gonna be fine. The cathode follower has no load resistor so that is unlikely too. Since there is no physical electrodes touching inside the tube you will NOT short to ground. And further more even if somehow that happened your HT fuse will blow to protect the PT. Worse case is a bad pot or blown tube $1.5 to $14 dollars of experience to see what your amp might sound like, I have a bag of pots, fuses, and tubes sitting around. I'd try it. If you only have metal shaft pots use some rubber gloves to turn it to keep yourself safe.

    I would love to hear from highly experienced tech how this is a catastrophic fail waiting to happen?
     
  11. XTRXTR

    XTRXTR TDPRI Member

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    If I understand the 12AX7 right. By reducing the Load resistance you are going to rotate the Load Line clockwise which makes the grid have a much larger affect on the output. You will have less of a change in Anode voltage with a larger change caused by the grid - you will increase Anode current. Don't get me wrong. Anode voltage will go higher as well as Anode current not linearly because of the grid so a tiny change in grid could get a huge change in dissipation but not necessarily more gain. Likely less gain. V/R=I when R goes down what happens to V and I

    I'm basing this on Valvewizard section 1.8 concerning the load line.
    http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

    Particularly the part near the end of that section last paragraph something suspicious. As the load line rotates right the grid lines are spaced out further away from each other meaning much larger change for small change in grid voltage.

    Also read section 1.15

    Heck that whole file is full of some good stuff.

    I think you'd be okay trying out your idea just be careful of HT on the wiper and make small rotational changes and measure and play your guitar to hear it.
     
    Last edited: Aug 29, 2020
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