Triode

A triode is an electronic amplifying vacuum tube made of three electrodes sealed inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate, or anode. It was developed from Lee De Forest's 1906 Audion, which added a grid electrode to the earlier thermionic diode, and the triode became the first practical electronic amplifier and the ancestor of later vacuum tubes such as the tetrode and pentode. Its invention helped make amplified radio and long-distance telephony possible, and triodes remained common in radios and televisions until transistors replaced them in the 1970s, though they are still used today in high-power radio-frequency amplifiers and have seen renewed demand from audiophiles who favor tube-based audio equipment.