JVM Negative Feedback Pot Question

Discussion in 'The Workbench' started by L902, Jan 22, 2021.

  1. L902

    L902 Member

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    Hey amp gurus!

    I have recently done the negative feedback mod by splicing the blue NFB wire and putting a 500k pot between it to use as a variable resistor. I used an audio pot I had laying around but plan on getting a linear pot so the taper is smoother.

    One thing I noticed is when I have the pot all the way to the left it is stock and the farther to the right I have the more resistance and it gets beefier. But the last say 10% of turning the pot wide open I notice a large volume drop. Is that normal?
     
  2. South Park

    South Park Well-Known Member

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    No it is not normal it sounds like you have to much resistance to ground on that pot . Are you hooked up to the 4ohm tap
     
  3. Lowlife

    Lowlife Well-Known Member

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    Agreed, mine doesn't drop the volume at all
     
  4. L902

    L902 Member

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    I am hooked to the 16ohm speaker output. I wired the pot like this:

    Terminal 1 on the pot is bent and soldered to under the pot since it came from a guitar at one point and was like that when I took it out
    Terminal 2 on the pot is wired to one side of the blue wire from NFB
    Terminal 3 on the pot is wired to the other side of the blue wire from NFB

    Maybe I wired it wrong?
     
  5. South Park

    South Park Well-Known Member

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    This circuit is not easy to get . You are taking power from the 4 ohm tap and rolling off frequency’s and feeding it back into the tubes to cancel out frequency’s . The tone cap and the pot control how much this works .
     
  6. L902

    L902 Member

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    Here is how I wired the pot, is this correct? One lug connects to ground on the pot. The other 2 lugs connect to each end of the blue wire I cut. It's not possible to wire it backwards correct (sorry I am not good with the theory behind electronic components)

    If it's wired correct could it be the pot itself causing the issue? Can it have too much resistance, maybe 500k is too much?
     

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  7. spacerocker

    spacerocker Well-Known Member

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    Disconnect/cut off the lug soldered to the body of the pot. You should not have that connection. The pot should be wired so that you are only using ONE of the end lugs and the centre lug. By having one of the end lugs soldered to the pot body, you are shorting out the negative FB signal via the chasis of the amp!

    As you say - the pot (presumably) came from a tone circuit on a guitar. For use as a negative feedback pot, you DON'T need the end lug to be grounded!
     
  8. Redguitar

    Redguitar Member

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    I haven't seen/heard how any frequencies are being cancelled out.
     
    Last edited: Jan 23, 2021
  9. L902

    L902 Member

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    Thanks for the help. I tried what you said but unfortunately it's still doing the same thing. I'm going to buy a new pot over the weekend and see if it helps. Does it matter what 2 out of the 3 lugs I hook the wires up to?
     
  10. spacerocker

    spacerocker Well-Known Member

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    If you number the lugs 1 to 3 clockwise (with the shaft facing you) - use lugs 1 and 2. That way, as you turn the shaft clockwise, the resistance goes UP and the amount of negative feedback reduces. The amp should also get LOUDER for a given volume setting as you reduce the amount of negative feedback!

    I used a 1Meg LOG (Audio) taper pot because this gave finer adjustment in the 0-200K region where the change is most noticeable.

    I thought I was on to something with my theory :lol::lol::lol:! Never mind......

    Let us know how you get on with a new pot? I would be curious as to what sort of pot defect could cause the behaviour you describe....:shrug:
     
  11. L902

    L902 Member

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    Just wanted to report back that switching the wiring from lugs 2+3 to 1+2 fixed the issue! The volume now goes up instead of down :)

    Thanks everyone!
     
    MatFnEhUK likes this.
  12. spacerocker

    spacerocker Well-Known Member

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    That's brilliant news! Thanks for letting us know!
     

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