Volume Control
Miller C of the following stage
How it works
A bright cap is a small capacitor connected from the top of the volume pot to its wiper. Treble frequencies bypass the upper part of the track, so at low and medium volume settings the high end is attenuated less than the rest of the spectrum: the amp sounds brighter. At full volume the wiper is at the top, the cap is shorted out, and it has no effect.
Because an audio-taper pot has most of its resistance in the upper part of the track at low settings, the effect is strongest at low volume, a classic Fender trait. The input capacitance of the following stage (Miller effect) works the other way, shunting treble when the wiper sits mid-track (where the source impedance seen by the grid is highest).
The response is computed by complex nodal analysis including source impedance, both sections of the pot, the bright cap, and the load capacitance.