Screen grid

The screen grid or (deprecated) protection grid is an electrode in an electron tube. The screen grid (grid 2), the third electrode of tubes with a plurality of grids disposed between the cathode and anode in the electron tubes.

There are electron tubes with more than one screen grid, are usually connected to each other.

Function

The screen grid is a utility grid or an auxiliary anode with a variety of tasks:

• It screens the control grid of the electric field of the anode. This main task also coined the name. This effect occurs in AC applications on only when the screen grid is connected via a capacitor of sufficient capacitance to ground. This simplifies the design of amplifiers, because the tendency is reduced, the circuit develops undesirable tendency to oscillate. The IF amplifier in tube- equipped superheterodyne receivers, for example, always use pentodes and never built with triode.
• The Miller effect is reduced significantly, which simplifies the construction of broadband amplifiers.
• It provides with a respect to the cathode voltage as constant as possible for a constant acceleration of the electrons towards the anode. This decoupling of the anode current is largely independent of the anode voltage. Thus, the possible dynamic range and the penetration increases decreases.

In tubes with multiple control grids ( Heptoden and Hexodes ) each control grid is a separate screen grid downstream. These tubes were used to multiplicative mixing in the superheterodyne receiver. There are circuits, such as the Miller Transitron, wherein said screen grid also controls the flow of electrons.

In the picture the screen grid is visible: It is located between the copper-colored support rods (support for the control grid ) and the rods for the brake grid, which is known as " airy " wrapped grid visible on the outer support rods.