Microphonic valve

A condition in which a valve's internal structure vibrates and converts mechanical shock into a signal, so the amp rings or howls at certain frequencies.

The valve behaves like a microphone: tapping the chassis or turning the volume produces an audible ping through the speaker, and at high gain the vibration feeds back into a howl. It usually comes from a worn internal support or a valve that was always prone to it, and the first valve in the chain is the one most likely to be the culprit. Swapping the valve is the standard test and the standard cure, though a damper ring can help.

Example: an amplifier that rings audibly when the panel is tapped needs the offending preamp valve replaced.

Microphonic valve

See also