Valve sound

Valve sound refers to the characteristic sound of music amplified by a valve amplifier[?], generally believed to be generated by the addition of second harmonic distortion from the triode characteristic of its valves. The sound is generally considered "warmer" than the sound produced by transistor amplifiers.

Transistor amplifiers or DSP algorithms can approximate a valve sound by adding square law distortion, approximating the effect of the triode characteristic of the original valve amplifiers. Adding a slight DC bias before the distortion results in even harmonics that make the sound somewhat more pleasant.

"Valve amplifier" is an English expression for what Americans call a "tube amplifier".