Potentiometer Feed

Author: N.H. Crowhurst

Suppose that the supply potential is 250 volts, the average screen current 1 milliampere, and the desired screen potential 100 volts. A single 150,000-ohm resistor would provide the necessary 150-volt drop at this current, If the current rose to 1,2 milliamperes, however, the drop across the resistor would increase to 180 volts, leaving only 70 volts at the screen. If the current should drop to 0.7 milliampere, the resulting drop in the resistor would be only 105 volts, leaking 145 volts at the screen. It is clear that comparatively small changes in screen current result in quite large changes in screen potential, which means that the operating conditions of the tube are not very steady.

If we arrange a voltage divider that draws 5 milliamperes in addition to the screen current, this situation will be improved. The 5-milliampere current from screen to ground will require a resistor of 20,000 ohms to drop the required 100 volts; the 6 milliamperes from the supply to the screen, a difference of 150 volts, will require a resistor of 25,000 ohms. Now an increase in screen current from 1 to 1.2 milliamperes will only change screen potential to about 97.8 volts. If the screen current should drop to 0.7 milliamperes, instead of rising to 145 volts the screen potential will only rise to about 103.5 volts. In this way the extra 5 milliamperes flowing in the voltage divider helps considerably in maintaining a steady screen potential.

Advantages of the screen potentiometer feed