Plate (anode)

The plate, or anode, is the electrode that the output signal appears on. Because the plate has to accept the electron flow, it can get hot. Especially in power tubes. So it is specially designed to cool itself off, either by radiating heat through the glass envelope (if it's a glass tube), or by forced-air or liquid cooling (in bigger metal-ceramic tubes). Some tubes use a plate made of graphite, because it tolerates high temperatures and because it emits very few secondary electrons, which can overheat the tube's grid and cause failure. See "H--the getter" below for more about the graphite plate.

By Eric Barbour
Information from www.vacuumtubes.net