From MetroAmp Wiki
"Bright cap" is the name of the capacitor mounted on a potentiometer that allows some high frequencies to pass at lower settings. This is to prevent the tone from being dull when the volume is turned down.
Values range between 47-5600pf. But the function is best realized with a value in the 100-500pf range. Large values, such as 5600pf tend to be a band-pass capacitor as opposed to tailoring the frequency balance with respect to volume setting. This is illustrated in many Marshall amps. The volume knob work more like an on/off switch and the amp is overdriven even set to only 3 or 4.
It's notable that a bright cap is often the only component that designated early Marshalls as a Bass or Lead amp.
Tech
The cap is installed between terminal one (the ungrounded terminal) and the wiper (the middle terminal) of the volume pot.
Here are bright caps installed on the VOLUME I pot of a few amps. Note that the first image shows a 4700pf cap (47 and two zeros).
