TETRODE

electronic vacuum tube with four electrodes. In addition to the cathode, anode, and control grid, as in the triode (q.v.), an additional grid, the screen grid, is placed between the control grid and plate. The screen grid acts as an electrostatic shield to protect the control grid from the influence of the plate when its potential changes. Although the pentode replaced the tetrode in most vacuum tube functions, a specially designed tetrode, called the beam-power tube, found extensive use in power amplification. A special form of tetrode was the dynatron, a vacuum tube that was operated with screen grid voltage higher than plate voltage so that the tube exhibited negative resistance (i.e., plate current decreased when plate voltage increased), a useful characteristic in oscillator circuits.