Screen Grid

Screen grid - Wikipedia, the free encyclopedia
The screen grid is usually connected to the high voltage anode supply via a ... This A.C. grounding of the screen grid causes it to act as an electrostatic ...
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The Screen Grid is an extra element added to the basic three element ... The Screen Grid usually comprises a formed coil of wound turns of round wire, ...
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screen definition | Dictionary.com
Definition of screen at Dictionary.com with free audio pronunciation. ... screen grid. 15. Photography. ... Also called screen grid. ...
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The screen grid is a grid introduced into a vacuum tube 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 (electricity)). This A.C. grounding of the screen grid causes it to act as an electrostatic shielding 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 emission 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 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. 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 tetrode to make respectively, a pentode or a beam tetrode.