Valve

A vacuum tube, the glass device used in traditional guitar amplifiers and some pedals, which amplifies by controlling the flow of electrons between electrodes in a vacuum.

Valves are valued for how they behave when pushed: rather than clipping sharply, they compress and round the waveform, adding harmonics gradually, so an overdriven amplifier keeps its dynamics and responds to the player's touch. They also need care, since they age, run hot, can become microphonic and need periodic replacement, and matched output valves must be biased. Their scarcity and cost are why so much modern equipment models their behaviour digitally.

Example: a small valve amplifier turned up to the point of breakup responds to the volume knob in a way that a hard-clipping pedal cannot.

Valve

See also