Screen grid
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A grid introduced into a thermionic valve or tube 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 valve. 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 earth (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, current flows in the screen grid circuit, which heats the screen and becomes wasted power. If the screen heats up enough, it can melt and damage the tube. Additionally, the positive charge tends to significantly accelerate the electrons towards the anode. This acceleration is sufficient to cause secondary emission from the anode's surface. The secondary electrons are collected by the screen grid to contribute to its current flow. Indeed, as the secondary emission increases, the actual anode current can decrease, giving 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. The reduction in anode current is due to the fact that the external anode current (through the connection pin) is due to the cathode-to-anode current minus the secondary emission current.
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 repectively, a pentode or a beam tetrode.