Screen grid--the tetrode

Adding another grid to a triode, between the control grid and the plate, makes it into a TETRODE. This "screen" grid helps screen, or isolate, the control grid from the plate. This is important is reducing the so-called Miller effect, which makes the capacitance between the grid and plate look much bigger than it really is. The screen also causes an electron-accelerating effect, increasing the tube's gain dramatically. The screen grid in a power tube carries some current, which causes it to heat up. For this reason, screen grids are usually coated with graphite, to reduce secondary emission and help keep the control grid cool.
Many large radio and TV stations use giant metal-ceramic power tetrodes, which are capable of high efficiency when used as RF power amplifiers. Power tetrodes are also sometimes used in amateur radio and industrial applications. (Regular tetrodes are rarely used for audio applications because of an effect called "tetrode kink", caused by that secondary emission. Most of it is due to electrons bouncing off the plate, some from the screen.) This greatly increases distortion and can cause instability if not carefully dealt with in the design. See section F, "audio beam tetrodes", below.)
Large ceramic tetrodes are often called "radial beam tetrodes" or simply "beam tetrodes", because their electron emission forms a disc-shaped beam. The wires on their control and screen grids are aligned, a special trick which improves efficiency.

By Eric Barbour
Information from www.vacuumtubes.net