Filter capacitor

A filter capacitor is a large capacitor in a power supply that smooths the pulsing DC coming out of a rectifier into a steadier voltage. In tube amplifiers these capacitors are charged to hundreds of volts and can hold that charge long after the amp is unplugged. They wear out over time, especially when kept hot.

Example: Before working inside an old tube amp, a repair tech bleeds the filter caps through a resistor and checks them with a meter to avoid a dangerous shock.