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Audio-Frequency Class A1 Push-Pull Triode Power AmplifierIn this amplifier two triodes are operated as a balanced amplifier (Fig. 25), or in push-pull. The second harmonics, which cause most of the distortion when a single triode is used, are largely canceled out in the primary of the push-pull output transformer. Thus, a lower plate load resistance may be used, and the condition for maximum power transfer (page 152) can be more closely approximated. Two triodes in push-pull will handle more than twice the power handled by a single triode under comparable conditions. Calculations for two tubes in push-pull are made by first selecting the direct operating voltage Eb and then calculating the direct grid-bias voltage Ec. For class A operation this bias can be between the value for single-tube operation and one-half that required to produce plate-
current cutoff when the plate voltage is 1.4Eb.19 The maximum power output is then calculated by drawing a vertical line (on a set of static curves such as Fig, 24) at a value 0.6Eb until it intersects the zero grid-voltage curve. This establishes the Imax value, and then19
The correct plate-to-plate load that the output transformer should produce between the two tubes is then four times the resistance represented by the load line drawn as explained. Or, this value is19
where the units are ohms, volts, and amperes.
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Last Update: 2011-05-30