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

Author: N.H. Crowhurst

We can apply these facts to the load line. The actual load resistor connected between B+ and the plate controls the steady operating point, according to the steady grid bias voltage. This operating point is found by using the load line corresponding to the coupling resistor, starting from the B+ voltage used.

Audio fluctuations cause the plate current and voltage to fluctuate in a manner that can be indicated by drawing a load line through the operating point, at a slope representing the combined resistances of the coupling resistor and grid resistor in parallel.

In the example we used before, the plate coupling resistor was 20000 ohms. If we use a grid resistor of 100000 ohms, the effective resistance of these two working in parallel for (audio only) is (20000 x 100000)/(20000 + 100000) or 16670 ohms. Drawing a line representing this resistance, through the 175-volt/3.75-milliampere operating point, can be achieved as follows: three milliamps through 16670 ohms will produce a voltage drop across it of 50 volts. Hence 6,75 milliamps on this load line will correspond with 175-50 or 125 volts.