Triode

ECC83, a dual triode used in 1960-era audio equipment, showing the orange glow of the hot cathode.

3CX1500A7, a modern 1.5 kW power triode used in radio trans­mitters. The cylindrical structure is a heat sink atta­ched to the plate, through which air is blown during operation.

A triode is an electronic amplifying vacuum tube (or thermionic valve in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 1906 Audion, a partial vacuum tube that added a grid electrode to the thermionic diode (Fleming valve), the triode was the first practical electronic amplifier and the ancestor of other types of vacuum tubes such as the tetrode and pentode. Its invention helped make amplified radio technology and long-distance telephony possible. ^[1] Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today, their main remaining use is in high-power RF amplifiers in radio transmitters and industrial RF heating devices. In recent years there has been a resurgence in demand for low power triodes due to renewed interest in tube-type audio systems by audiophiles who prefer^[vague] the sound of tube-based electronics.^{[citation needed]}

The name "triode" was coined by British physicist William Eccles^[2][3] some time around 1920, derived from the Greek τρίοδος, tríodos, from tri- (three) and hodós (road, way), originally meaning the place where three roads meet.