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Dual gang pot terminals wired together?

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  • Dual gang pot terminals wired together?

    I'm trying to write out a schematic for my amp, but I'm having trouble understanding what's going on at one point. The gain knob is a dual-gang A500k pot. I've attached a picture of the pot from the top of the board and it's traces on the bottom. The problem is, it looks to me like terminals 1 and 2 on the pot (and terminals 4 and 5) are connected by traces on the board. This doesn't make sense to me. I would have expected 1 and 2 to be connected only by the variable resistance provided by the pot, which seems to be shorted by the trace in this case. Can someone help me figure out where I've gone wrong?

    Click image for larger version

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  • #2
    Who says you did anything wrong? Other than to assume.

    It appears you are tracing out a commercial product, rather than recording something you built, yes? So chances are good a schematic already exists. SO what is the make and model of the product?

    Wiring the wiper to one end makes the control a variable resistor. That can work with another resistor to form a voltage divider and thus control signal levels or control voltage levels. Looks to me like pin 3 is grounded, so we are not looking at an op amp feedback resistor. But in op amp based amplifiers, we often see the feedback resistor of an op amp made variable to use as a gain adjust. That is often wired as a variable resistor.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Excuse the mess-something like this ? Click image for larger version

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      (Resistors bottom left may be wrong from top photo)

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      • #4
        Thanks for your replies. Yes, it's a commercial product, but it's a recent model amp, and I haven't found a schematic for it so far online. It's a Randall RVC (guitar-center model sharing most in common with the Randall RD5).

        Pin 3 is ground. I'm attaching a portion of the schematic I'm drawing up for the amp. If you follow the trace in my original pic from the top pin 1 down and to the right, it connects to the output side of R10, which is also connected in parallel (the horizontal trace in the pic) to R11 which feeds into the second stage (pin 7 of V1a - a 12AX7).

        I got a little stuck at this point, because I didn't understand the way the dual-gang pot was being used in this case. It sounds like it may be worth my adding both sides of the pot's circuit to the schem in order to really understand what's going on here.

        Click image for larger version

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