Tetrode

Vacuum tube with four active electrodes

A tetrode is a vacuum tube having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids, and a plate. There are several varieties of tetrodes, the most common being the screen-grid tube and the beam tetrode. In screen-grid tubes and beam tetrodes, the first grid is the control grid and the second grid is the screen grid. In other tetrodes one of the grids is a control grid, while the other may have a variety of functions.

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