Transformer-Coupled Amplifiers

Author: J.B. Hoag

For audio-frequency amplification, a transformer is used as the coupling unit between two tubes only when power is to be transferred or when coupling is to be made to a push-pull stage. Usually, triodes, such as 6C5, 6J5, and similar tubes, with a μ of 20 or less, are used.

Fig. 25 G. A transformer-coupled amplifier driving a push-pull amplifier

Figure 25 G shows a series-feed circuit whose gain per stage, in the absence of grid currents, is equal to the μ of the tube multiplied by the step-up ratio (usually 2 to 1) of the transformer.

Fig. 25 H. Transformer-coupled a.f. amplifier with parallel plate feed

The parallel-feed circuit of Fig. 25 H will have better low frequency response than that of Fig. 25 G for the same reason advanced in connection with the impedance-coupled amplifier. The gain per stage with the parallel feed is nearly equal to that of the equivalent resistance-coupled amplifier, multiplied by the secondary-to-primary turns-ratio of the transformer.