Vol/Tone in a 5C5?

[Bruce / Mission Amps]

A common failure mode for a pot is for the center terminal to become an open circuit. If the bias adjust pot is connected like a voltage divider, this means the bias will be lost and the output tubes will red plate. If arranged correctly, the bias pot is connected like a rheostat in such a way that if the center terminal becomes open, the bias increases thus saving the tubes.

Accept that it can load down the bias supply when the pot is turned all the way to the least negative voltage.
To be honest, I've never experienced a bias supply "pot" failure or ran into an open one in repairs but theoretically LD is right.
Some guys never build their amp using the adjustable voltage divider with out a shunt resistance around the pot's wiper from -vdc source so negative voltage is always supplied to the power tubes if the wiper goes open.

[RC99]