This term refers to the selectable solid-state/tube rectifier found on certain Mesa/Boogie amplifiers. Most modern tube amps use solid-state rectifiers to convert AC current to DC. These rectifiers typically provide more volume and headroom than their vintage counterparts, the tube-based rectifiers found in vintage amps. But some players prefer the “sag” that occurs when the tube rectifier’s output decreases slightly as it deals with incoming power surges — its compression-like effect lends a looser (some say more organic) feel to the amp’s response. Mesa addressed the issue by implementing a rectifier that can switch between tube and solid state operation. This feature allows players to choose between “tight” and “loose” amp response characteristics.