Until the invention of the transistor in 1947, all practical amplifiers were made of thermionic tubes.

The simplest tube was invented by John Ambrose Fleming while working for the Marconi Company in London in 1904 and named the Diode, as it had two electrodes. The diode conducted electricity in one direction only and was used as a radio detector and a rectifier.

Although he may not have at first realized the significance of his invention it was Lee De Forest who 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 for a given voltage between the 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.

Tubes are high voltage/low current devices in comparison with transistors (and especially MOSFETs). The high working voltage makes them well suited for radio transmitters, for example, and valves remain in use today for very high power radio transmitters, where there is still no other technology available. However, for most applications requiring an appreciable output current, a matching transformer is required. The transformer is a critical component and heavily influences the performance (and cost) of the tube amp.

Many power amp tubes have good linearity but modest gain or transconductance. Signal amplifiers using tubes are capable of very high frequency response ranges - up to radio frequency. Indeed, many of the Directly Heated Single Ended Triode (DH-SET) audio amplifiers are in fact radio transmitting tubes designed to operate in the megahertz range. In practice, however, tube amplifiers designs typically "couple" stages either capacitively, limiting bandwidth at the low end, or inductively with transformers, limiting the bandwidth at high end.

source : wikipedia