Audion Audion The Audion was an electronic detecting or amplifying vacuum tube invented by American electrical engineer Lee de Forest in 1906. It was the first triode, consisting of an evacuated glass tube containing three electrodes: a heated filament, a grid, and a plate. It is important in the history of technology because it was the first widely used electronic device which could amplify; a small electrical signal applied to the grid could control a larger current flowing from the filament to plate. The original triode Audion had more residual gas in the tube than later versions and vacuum tubes; the
Valve audio amplifier pentodes or beam tetrodes, 6L6 or EL34 but occasionally KT88, 6550, or the lower-power EL84 in "Ultra-Linear" connection. The output stage is preceded by a voltage amplification stage (pentode or twin triode) and a phase-splitter (twin triode). Twin triodes with two identical sections in one envelope are used, usually the noval types 12AT7, 12AU7, or 12AX7 or equivalents, less usually the octal 6SN7. The earliest mass usage of valve audio amplifiers was for telephony. Valve amplifiers were critical in development of long-distance telephone circuits and submarine telephone cables. Radio applications followed soon after, where valves were used for both the