Power Output from a Class-A Stage

Author: N.H. Crowhurst

Calculations for class A push-pull. Load line is for each tube, but both works at once. The characteristics for the other tube (not shown) would be inverted underneath.

As an example, suppose that in ordinary push-pull (known as class A, which means that both tubes are conducting current all the time), the operating point for each tube is 250 volts at 30 milliamperes and that the load value presented to each tube is 5000 ohms with pentode operation. Disregarding the curves to make the calculation simpler (if approximate), the audio fluctuation should carry the plate between 100 volts at 60 milliamperes and 400 volts at zero milliamperes. This represents a peak fluctuation from each tube of 150 volts and 30 milliamps in each direction, which is a peak power of (150 X *03) or 4.5 watts per tube or 9 watts for the two tubes in push-pull. The average power, using a sine wave to drive the output, will be just half of this figure or 4.5 watts for the two tubes.