The Effect of Changing the Load

Author: N.H. Crowhurst

Distortion and power output vary with load resistance

With choke or transformer coupling, we can try the effect of applying different load values to the same tube. If we use referred resistance values slightly higher than the plate resistance of the tube, the waveform of the output follows the input waveform quite closely and there is little distortion. Continuing the increase in load resistance, however, the voltage fluctuation fairly quickly reaches a maximum and doubling the resistance value does not result in a proportionate increase in the voltage developed. For this reason, the power obtained drops off almost in inverse proportion to the increase in load value.

Making the referred load resistance the same as the plate resistance of the tube gives about the maximum power output obtainable, but causes considerable distortion in the output, due to the curvature of tube characteristics. If a high-quality power output is required, the load resistance must be at least two or three times as great as the plate resistance of the tube. Taking the load resistance value even lower than the plate resistance of the tube results in increased distortion and reduced power output.