The Punisher is a dynamic power supply sag simulator. The Punisher is not an audio effect but a companion effect that is to be used to control another effect to provide new sounds. You plug your audio signal and power supply into Punisher in series and connect the output audio signal and power supply into the effect to be controlled.
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If you want to vary the voltage and not the current of the power supply you can sub in this circuit for the FET that is in series with the power supply:
Your Tone God,