Valve audio amplifier

Until the invention of solid state devices such as the transistor, all electronic amplification was produced by valve (tube) amplifiers.

The distortion and overdrive characteristics of valves are quite different from transistors (not least the amount of voltage headroom available in a typical circuit) and this results in a distinctive sound.

Amplifiers for such performance applications typically retain tone and filter circuits that have largely disappeared from modern hi-fi products.

Guitar amplifiers are often designed so they can, when desired by the guitarist, distort and create a tone rich in harmonics and overtones.

Guitar amplifiers are typically designed to withstand a lot of abuse both electrically and physically (since guitarists often travel to gigs, etc.)

Most modern valve guitar amplifiers use a class AB1 push-pull circuit with a pair of power pentodes or beam tetrodes, 6L6 or EL34 but occasionally KT88, 6550, or the lower-power EL84 in Ultra-Linear connection.

Cinema sound systems of this period were predominantly supplied by "Westrex", related to the Western Electric company, a telecoms supplier, who were also the makers of the 300B DHT tube that today is central to current production DH-SET audiophile amplification.

Today this type of circuit retains a niche following at the very extreme of audiophile hi-fi, where it is often referred as DH-SET.

In this case, distortion is largely relatively unobjectionable second harmonic, with percentage closely proportional to the output amplitude.

Widespread adoption of push pull allowed smaller (and thus cheaper) transformers,[citation needed] combined with more power (typically 10 to 20 watts) to handle peaks.

[2] These later radios were hybrid designs which used transistors only for the audio output stages because a 12 volt power amplifier valve was not practical.

Due to the very poor technical performance of early gramophones, the lack of standardised equalisations, poor components and accessories (including loudspeakers), preamplifiers historically contained extensive and very flexible equalization and tone and filter circuits designed to adjust the frequency response of the amplifier and so the sound produced by the system.

A representative valve preamp from the 1950s is the Leak 'varislope' series of preamps, which included a switchable rumble filter, a switchable scratch filter with selectable slopes and corner frequency, continuously variable treble and bass tone controls and a selection of 4 different gramophone equalisations (RIAA, ortho, RCA, 78).