The Lowering of Distortion

Author: N.H. Crowhurst

Push-pull operation

Using two tubes in push-pull helps the waveform problems, so that the distortion produced by one tube cancels that produced by the other. This can be understood better if we think of each tube as having a curved load line. The input voltages to the grids are equal but 180° out of phase. The plate voltages likewise are out of phase because of the coupling between the two primary windings of the output transformer. So the changes in plate current must adjust between the tubes to allow this condition, while the two of them supply the total current fluctuation to the load at all points. The ratio between total voltage and current fluctuation of both tubes is set by the load resistance matched to the secondary of the transformer, but each tube feeds a load resistance whose value is constantly changing, as represented by the curves.

This effect can be extended further to increase the efficiency of the output stage. Normally the steady plate, current is about half the maximum plate current (which occurs when the grid voltage fluctuation goes from the operating point up to zero). The current fluctuation in the tube at maximum power level swings between almost zero current and twice the steady current. This sets a considerable limitation on the power-handling capacity of the tube because the steady component is such a large proportion of the maximum current the tubes take. Using a greater negative bias on the grids of the tubes makes the audio fluctuations carry the current from almost zero up to a maximum in one direction and cuts the tube off so that no current flows in the other direction. This makes possible a considerable increase in efficiency and available power output.