The screen grid or (obsolete) protective grid is an electrode in an electron tube . In electron tubes, the screen grid (grid 2) is the third electrode of tubes with several grids, arranged between the cathode and anode.
There are electron tubes with more than one screen grid, which are usually connected to one another.
The screen grid is an auxiliary grid or an anode with a variety of tasks:
- It shields the control grid from the electrical field of the anode . This main task also shaped the name. This effect only occurs in AC voltage applications if the screen grid is connected to ground via a capacitor of sufficient capacity . This simplifies the construction of amplifiers because the tendency for the circuit to develop undesirable tendency to oscillate decreases . The IF amplifier in tube- equipped heterodyne receivers is, for example, always built with pentodes and never with triodes.
- The Miller effect is markedly reduced, which simplifies the construction of broadband amplifiers.
- With a voltage that is as constant as possible in relation to the cathode, it ensures constant acceleration of the electrons towards the anode. This decoupling makes the anode current largely independent of the anode voltage. This increases the possible dynamic range and decreases the penetration .
In the case of tubes with several control grids (heptodes and hexodes), each control grid is followed by its own screen grid. These tubes were used for multiplicative mixing in the heterodyne receiver. There are circuits like the Miller-Transitron in which the screen grid also controls the flow of electrons.
The screen grille can be seen in the picture: It is located between the copper-colored support bars (support for the control grid ) and the bars for the brake grille , which can be seen as an "airy" wrapped grid on the outside support bars.
- F. Bergtold: Röhrenbuch for radio and amplifier technology . Weidmannsche Buchhandlung, Berlin 1936.
- Ludwig Ratheiser: The great tube manual . Franzis-Verlag, Munich 1995, ISBN 3-7723-5064-X .
- Ludwig Ratheiser: Radio tubes - properties and application . Union Deutsche Verlagsgesellschaft, Berlin 1936.