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Screen-Biasing Methods

Author: N.H. Crowhurst

For a pentode tube, a supply is also required for the second (screen) grid. This supply usually has to provide a fixed voltage not greater than the average plate voltage. It would be possible to design a completely separate supply for the screen grid, but because this electrode only requires a small current (a fraction of a milliampere for small tubes of the voltage-amplifying type and a few milliamperes for large output tubes), a separate supply would involve unnecessary expense.

Screen-Biasing Methods

Instead, the screen supply is usually derived from the plate supply, using either a series resistor or a voltage divider. The series-resistor method has the advantage of economy on supply current, because it passes only the current necessary to feed the screen. The disadvantage is that, if the screen current should vary (which it does under certain conditions), the screen potential will also vary, because of the change in voltage drop across the feed resistor. This difficulty may be overcome by using the voltage-divider method (potentiometer feed). The two resistors a current that is larger than the average screen current and thereby keep the screen potential more steady.

Last Update: 2010-11-03