Sag

The momentary drop in supply voltage that a valve amplifier's power supply experiences under a loud transient, which softens the attack and adds compression.

When the player hits a hard chord, the output stage demands more current than the power supply can deliver instantly, so the voltages dip and the note blooms rather than snapping. That behaviour is what players mean by a valve amplifier feeling elastic, and it depends on the rectifier type, transformer size and filter capacitance rather than on the preamp. Solid-state designs usually hold their voltage and therefore feel more immediate, which is either their virtue or their shortcoming depending on the music.

Example: A valve amplifier that seems to lean back and then swell on a hard low chord is demonstrating sag.

Sag

See also