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A four-element electron tube containing an anode, cathode, control grid, and additional electrode.
1. (Electronics) an electronic valve having four electrodes, namely a cathode, control grid, screen grid, and anode
2. (Electronics) (modifier) (of a transistor) having two terminals on the base or gate to improve the performance at high frequencies
a vacuum tube containing four electrodes, usu. a plate, two grids, and a cathode.