Triode were invented, Philipp Lenard used the principle of grid control while conducting photoelectric experiments in 1902. The first vacuum tube used in radio was the thermionic diode or Fleming valve, invented by John Ambrose Fleming in 1904 as a detector for radio receivers. It was an evacuated glass bulb containing two electrodes, a heated filament and a plate (anode). Triodes came about in 1906 when American engineer Lee De Forest and Austrian physicist Robert von Lieben independently patented tubes that added a third electrode, a grid, between the filament and plate to control current. Von Lieben's partially-evacuated three-element tube, patented
Single-ended triode SET) is used when directly heated triodes are used. There are also single-ended tetrode, beam tetrode/beam power tube/kinkless tetrode, and pentode amplifiers with the same functionality and similar circuitry; e.g. this Mullard design. A typical triode audio power amplifier will have a driver that provides voltage gain, coupled to a triode (like 2A3 and 300B) or a pentode or kinkless tetrode such as EL34 or KT88 connected as a triode, connected to the loudspeaker through an audio transformer in a common cathode arrangement. The triode is biased to Class A operation by applying a suitable negative bias voltage to its