Suppressor grid

A suppressor grid is a wire screen used in a thermionic valve (i.e. vacuum tube) to suppress secondary emission. It is also called the antidynatron grid, as it reduces or prevents dynatron oscillations. It is located between the screen grid and the plate electrode (anode). The suppressor grid is used in the pentode vacuum tube, so called because it has five concentric electrodes: cathode, control grid, screen grid, suppressor grid, and plate, and also in other tubes with more grids, such as the hexode. The suppressor grid and pentode tube were invented in 1926 by Gilles Holst and Bernard D. H. Tellegen at Phillips Electronics.{{cite book

In a vacuum tube, electrons emitted by the heated cathode are attracted to the positively-charged plate and pass through the grids to the plate. When they strike the plate they knock other electrons out of the metal surface. This is called secondary emission.{{cite book

In the pentode, to prevent the secondary electrons from reaching the screen grid, a suppressor grid, a coarse screen of wires, is interposed between the screen grid and plate. It is biased at the cathode voltage, often connected to the cathode inside the glass tube. The negative potential of the suppressor with respect to the plate repels the secondary electrons back to the plate. Since it is at the same potential as the cathode, the primary electrons from the cathode have no problem passing through the suppressor grid to the plate.

In addition to preventing the distortion of plate current, the suppressor grid also increases the electrostatic shielding between the cathode and plate, causing the plate current to be almost independent of plate voltage. This increases the plate output resistance, and the amplification factor of the tube. Pentodes can have amplification factors of 1000 or more.

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