A valve amplifier or tube amplifier is a type of electronic amplifier that uses vacuum tubes to increase the amplitude or power of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers in the 1960s and 1970s. Valve amplifiers can be used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS, high quality stereo amplifiers, military applications (such as radar) and very high power radio and UHF television transmitters.

Until the invention of the transistor in 1947, most practical high-frequency electronic amplifiers were made using thermionic valves. The simplest valve (named diode because it had two electrodes) was invented by John Ambrose Fleming while working for the Marconi Company in London in 1904. The diode conducted electricity in one direction only and was used as a radio detector and a rectifier.

In 1906 Lee De Forest added a third electrode and invented the first electronic amplifying device, the triode, which he named the Audion. This additional control grid modulates the current that flows between cathode and anode. The relationship between current flow and plate and grid voltage is often represented as a series of "characteristic curves" on a diagram. Depending on the other components in the circuit this modulated current flow can be used to provide current or voltage gain.

The first application of valve amplification was in the regeneration of long distance telephony signals. Later, valve amplification was applied to the 'wireless' market that began in the early thirties. In due course amplifiers for music and later television were also built using valves.

The overwhelmingly dominant circuit topology during this period was the single-ended triode gain stage, operating in class A, which gave very good sound (and reasonable measured distortion performance) despite extremely simple circuitry with very few components: important at a time when components were handmade and extremely expensive. Before World War II, almost all valve amplifiers were of low gain and with linearity dependent entirely on the inherent linearity of the valve itself, typically 5% distortion at full power.

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 surpassing its contemporaries.

World War II stimulated dramatic technical progress and industrial scale production economies. Increasing affluence after the war led to a substantial and expanding consumer market. This enabled electronics manufacturers to build and market more advanced valve (tube) designs at affordable prices, with the result that the 1960s saw the increasing spread of electronic gramophone players, and ultimately the beginnings of high fidelity. Hifi was able to drive full frequency range loudspeakers (for the first time, often with multiple drivers for different frequency bands) to significant volume levels. This, combined with the spread of TV, produced a 'golden age' in valve (tube) development and also in the development of the design of valve amplifier circuits.

A range of topologies with only minor variations (notably different phase splitter arrangements and the "Ultra-Linear" transformer connection for tetrodes) rapidly became widespread. This family of designs remains the dominant high power amplifier topology to this day for music application. This period also saw continued growth in civilian radio, with valves being used for both transmitters and receivers.