High-Vacuum Thermionic Four-Electrode Tubes

The presence of the screen grid shields the control grid from the plate, and the screen-grid tetrode is better than the triode for radio-frequency voltage amplification. With triodes, the amplified signal variations in the plate circuit tend to feed back into the grid circuit and cause oscillations.

In the tetrode, the plate is rendered even less effective (than in the triode) by the presence of the screen grid. For this reason, the amplification factor is about 500 for a typical screen-grid tetrode. The plate resistance also is very high, because the curves of Fig. 16 will be almost horizontal, since a large change in plate voltage will cause almost no change in plate current. A value r_p = 500,000 ohms is typical.

Although the screen-grid tetrode is for some purposes superior to the triode, the characteristics of the tetrode are impaired by the effect of secondary emission. In amplifiers the screen grid is held at a potential of about 50 to 100 volts positive with respect to the cathode. This voltage and the positive voltage on the plate accelerate the electrons constituting the plate current to such velocities that the electrons cause secondary emission (release electrons) when they strike the plate. In an amplifier the plate voltage rises and falls in accordance with the signal being amplified. If the plate potential falls to that of the screen grid, some of the secondary electrons may flow to the screen grid. This phenomenon, in general, is undesired.