Basic Audio is a free introductory textbook to the basics of audio physics and electronics. See the editorial for more information....

Maintaining Constant Screen Potential

Author: N.H. Crowhurst

An audio signal applied to the grid will cause the screen current to vary

When a capacitor is connected between screen and ground, changes in screen current fluctuate charge on capacitor (1) with very small change in voltage. So current and voltage in resistor (2) remain nearly constant.

When audio voltages are applied to the grid of a pentode tube, screen current fluctuates as well as the plate current. The screen potential, however, must be held constant if we are to achieve the best operation from the tube, because a constant screen potential enables the plate current and plate voltage fluctuations to be almost independent of each other. If there were only a resistor between the supply voltage and the screen, screen current fluctuations would also be accompanied by some screen voltage fluctuations, and the plate voltage fluctuation would be affected by the screen voltage fluctuation.

To avoid this, a fairly large capacitor whose charge does not have time to change during the audio fluctuation is connected between the screen and ground. A 0.1-microfarad or smaller capacitor is usually quite large enough for this purpose, because screen current and its fluctuations are small.

Last Update: 2010-11-03