I have recently acquired a TC Electronics Tube Pilot guitar effect pedal.

While exchanging the vacuum tube in it, I noticed that there was a 22k ohm variable resistor on the board. Can anybody tell me what the function of this resistor is? As far as I can tell, I only hear a volume difference when I adjust the potentiometer.


Edit: I've added an image to the question.

The resistor is labeled 'VR1'.


  • 1
    \$\begingroup\$ generally, it allows the maker to use cheaper parts and calibrate the output levels after assembly, canceling-out loose tolerances. Certain high-precision devices also require such "trimming" to achieve spec. Without seeing the schematic, we can't say what any given trimmer does, or what side-effects adjusting it might have... \$\endgroup\$ – dandavis Mar 18 at 18:12
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    \$\begingroup\$ Odds are its, as @dandavis said, a trimmer to adjust stuff with wide tolerances -- likely to adjust the bias of the tube itself. Without a schematic or a photo, though, we can't know for certain. \$\endgroup\$ – esilk Mar 18 at 18:17
  • \$\begingroup\$ it would adjust the tube current for optimum gain and temp rise tradeoffs. Too much accelerates wear. running them at half bias current/rated current for lower power dissipation is best for lifetime. But this also reduces gain. \$\endgroup\$ – Sunnyskyguy EE75 Mar 18 at 19:12
  • \$\begingroup\$ A picture of the board including the pot would help -- but it's probably as @SunnyskyguyEE75 said. \$\endgroup\$ – TimWescott Mar 18 at 19:24
  • \$\begingroup\$ As others are saying, it is almost certainly to set the bias on the 12AX7 tube (but without a schematic or even picture of the board we cannot be sure). Adjusting it would affect the volume, but also the distortion and lifetime of the tube. \$\endgroup\$ – evildemonic Mar 18 at 21:08

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