Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A valve (UK) audio amplifier or vacuum tube (US) audio amplifier is a valve amplifier used for sound recording, reinforcement, or reproduction. Until the invention of solid state devices such as the transistor, all electronic amplification was produced by valve (tube) amplifiers. While solid-state devices prevail in most audio amplifiers today, valve audio amplifiers are still used where their audible characteristics are considered pleasing, for example, music performance or music reproduction. Valve amplifiers for guitars (and to a lesser degree vocals and other applications) have different purposes from those of hi-fi amplifiers. Distortion may be considered a desirable characteristic, and the valves are electrically driven much harder to maximize output power.