Push-Pull

Author: N.H. Crowhurst

The next step in improving the power output capacity of an amplifier stage is to use two tubes in a connection known as push-pull. This arrangement uses transformer coupling, but there are two primaries (the primary winding has two halves), through which the current flows in opposite directions. B-j- is connected to the center point of the primary, with the plate of one of the tubes connected to each end. The current, therefore, flows from each plate outward through an equal number of turns to the center point. This means that the total magnetizing effect on the core of the transformer is neutralized as far as the d-c is concerned. (The transformer core only has to carry the magnetization due to the audio fluctuation.) This simplifies the design and cost of the transformer, but the big advantage is in tube operation.

With a single tube, matching the output load to the tube plate resistance results in a poor output waveform, which takes the form of a rounding at the bottom and sharpening at the top. When the tubes are worked in push-pull, the current flows in opposite directions around the transformer core and, consequently, what is the top of the current waveform in the upper part of the winding becomes the bottom of the current waveform in the lower half of the winding. Thus both halves of the current waveform have a sharpened portion added to a rounded portion, and the effect averages out, producing a much better waveform for the load value used. To achieve this, we must provide the correct audio voltages at the grids of the tubes. We shall consider this problem presently.

Push-pull reduces distortion by neutralizing curvature