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Patent Granted A triode is an electronic amplifying vacuum tube (or valve in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Invented in 1906 by Lee De Forest by adding a grid to the Fleming valve, the triode was the first electronic amplification device and the ancestor of other types of vacuum tubes such as the tetrode and pentode.