A
tetrode is a
vacuum tube having four active
electrodes. There are several varieties of tetrode with
screen-grid tube, and the
beam tetrode being the most common. The four electrodes in order from the centre are: a
thermionic cathode, first and second grids and a
plate. 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.