Screen grid

The screen grid is a grid introduced into a vacuum tube (thermionic valve) to greatly reduce the capacitance between two other parts of the electrode structure.

In its simplest form, a screen grid is placed between the control grid and anode or plate of a triode. This turns it into a tetrode. The screen grid is usually connected to the high voltage anode supply via a resistor and is bypassed by a suitable capacitor to ground. This A.C. grounding of the screen grid causes it to act as an electrostatic shield between the control grid and the anode, reducing the capacitance between those two electrodes to near zero.

The downside is that because the screen grid is positively charged, it collects electrons, which causes current to flow in the screen grid circuit. This wastes power and heats the screen grid, and if the screen heats up enough, it can melt and damage the tube. There are two sources of electrons collected by the screen grid -- in addition to the electrons emitted by the cathode, the screen grid can also collect secondary electrons emitted from the anode when the primary electrons slam into it. Indeed, as the secondary emission increases, the actual anode current can decrease, since a single primary electron can eject more than one secondary electron. The reduction in anode current is because the external anode current (through the connection pin) is due to the cathode-to-anode current "minus" the secondary emission current. This can give the tetrode valve a distinctive negative resistance characteristic, sometimes called "tetrode kink". This is more often than not unwanted, but can be exploited as in the dynatron oscillator.

More than one screen grid can be present. For example the pentagrid converter has two.

The secondary emission can be overcome by adding a suppressor grid or even beam plates to make respectively, a pentode or a beam tetrode.