THE PENTODE

The problem of secondary emission associated with the screen grid of a tetrode has been reduced by - you guessed it, the addition of another grid.

This third grid, called a SUPPRESSOR GRID, is placed between the screen grid and the plate. The suppressor grid is normally connected either internally or externally to the cathode and bears the same charge as the cathode. This is shown in figure 1-33.   Because of its negative potential (relative to individual electrons), any electrons that are emitted by the plate, through secondary emission, are repelled back toward the plate.

Figure 1-33. - Basic pentode circuit.

You might think that a grid with a negative potential placed close to the plate would interfere with the electron stream. However, this is not the case. Because the suppressor grid is negatively charged, it will not draw grid current. Additionally, the wide spacing within the mesh of the suppressor and its location between two positive elements of the tube ensures that the suppressor grid's effect on the electron stream will be minimum. Only the electrons emitted by secondary emission from the plate are affected by the suppressor grid.

Because pentodes do not suffer from secondary emission, they have replaced the tetrode in most applications.

Q.32 The suppressor grid is added to a tetrode to reduce what undesirable characteristic of tetrode operation?
Q.33 On the diagram below, name the elements of the vacuum tube and their potentials relative to dc ground.