How this tone control works

This is the tone stage made famous by the Tube Screamer. The signal first passes through R1 and C2, a simple low-pass filter that rolls off treble. From there it hits the op-amp, which is wired as a gain stage whose boost only applies to high frequencies, thanks to C3 in the wiper leg: at low frequencies C3 blocks, the stage sits at unity gain, and nothing changes. At high frequencies C3 conducts, and the stage boosts treble by up to 1 + R3/(R2 + R_p).

The tone pot blends between those two behaviors. With the wiper toward the low-pass side (0%), you hear the darkened R1/C2 signal. Sweep it toward the feedback side (100%) and the treble boost takes over, lifting the highs that the low-pass took away. R2 sets a floor on the boost so it can never run away, and R3 sets how much boost is available in the first place. Bigger C3 moves the boost down into the mids; smaller C3 confines it to the very top.

The fixed parts do the housekeeping: C1 and C4 are coupling caps that block DC at the input and output, R6 feeds the op-amp its bias voltage (vref), and R4/R5 set the output impedance and give the output cap a load. The dashed curve shows what you would actually hear if you built it with the nearest standard E12 parts.