Until the invention of solid state devices such as the transistor, all electronic amplification was produced by valve (tube) amplifiers.
Valve amplification peaked as the mainstream technology during the 1960s and 70's, with device and circuits being highly developed, there have been only minor refinements since then.
The last generation of power tubes, typified by KT66, EL34 and KT88, in many ways represent the pinnacle of the technology, and also of production quality. Valve amplifiers produced since that time usually use one of these tubes, which have remained in continuous production (apart from KT66) ever since.
Negative feedback (NFB) was invented by Harold Stephen Black in 1927, but initially little used since at that time gain was at a premium. This technique allows amplifiers to trade gain for reduced distortion levels (and also gave other benefits such as reduced output impedance).
The introduction of the Williamson amplifier in 1947, which was extremely advanced in many respects including very successful use of NFB, was a turning point in audio power amplifier design, operating a push pull output circuit in class AB1 to give performance far ahead of its time.
In the closing years of the valve era, valves were even used to make simple "operational amplifiers" - the building blocks of much modern linear electronics. An Op-amp typically has a differential input stage and a totem pole output, the circuit usually having a minimum of five active devices. A number of "packages" were produced that integrated such circuits (typically using two or more glass envelopes) into a single module that could be plugged into a larger circuit (such as an analog computer). Such Valve op-amps were very far from ideal and quickly became obsolete, being replaced with "integrated" (planar silicon) types.
Today the main application for valves is audio amplifiers for High-end HiFi and performance (notably Guitar) use, although these applications have different requirements regarding distortion which result in different design compromises, although the same basic design techniques are generic and widely applicable to all broadband amplification applications, not only audio.
Post WWII the majority of valve power amplifiers are of the Class AB1 Push Pull ultralinear topology, but niche products using the DH-SET and even OTL topologies still exist in small numbers.
