Graphical Determination of Performance of Push-Pull Audio Amplifiers

Previous methods used for determining graphically the performance of push-pull audio amplifiers are inaccurate because they neglect the coupling between the tubes through the output transformer. A relatively simple method is presented which takes this effect into account. It consists in combining the plate-current-plate-voltage characteristics of the tubes to form a family of composite characteristics. This may be used to determine the performance for any load resistance in the same manner as is done for single tubes. This method is equally applicable to class A or class B conditions. It is found that each tube operates into a variable load resistance, which, under optimum conditions, is equal to its internal resistance at every point throughout the cycle. An experimental verification under a number of operating conditions is presented, showing close agreement between computed and measured values of power output, distortion, and average plate current.