A vacuum tube that performs the rectifier function in the power supply of an amplifier, converting AC into DC.
Using a tube rectifier versus a solid-state rectifier does not directly change the tone of the amp, but it will affect the “feel” of the amp for the player and may affect the response of the amp to audio signals. For example, a tube rectifier will typically “sag” more than a solid-state rectifier, providing a softer feel for the player, affecting the sustain of notes, and possibly affecting the “presence” of the sound.