Transformer Coupling

Author: N.H. Crowhurst

The first step toward improving the efficiency of a power-output stage is to eliminate this loss in the plate coupling resistor by using choke coupling. (This avoids the d-c drop in the plate load component.) We still get the audio fluctuations across the choke due to its inductance, but the fluctuations are too rapid to allow the current in the choke to fluctuate. The current fluctuations produced by the tube are all delivered through the coupling capacitor to the load. In this way we can use a B+ supply of 130 volts and still get the same total audio power from the tube as before, using a 250-volt supply.

An improved audio output stage.

We can save a component here by eliminating the coupling capacitor. We do this by putting two windings on the choke, converting it into a transformer. The winding connected between the plate of the tube and B+ has a large number of turns, whereas the other winding has a much smaller number of turns. This means the audio current fluctuations are stepped up, while the voltage fluctuations are stepped down to suit a lower impedance load; the tube, however, operates as if it had the requisite high-value load resistance connected between the plate and B+.

Using a transformer saves the coupling capacitor; enables any impedance load to be "matched" or suited to the tube

Operation of output stage using transformer coupling

Suppose that the load resistance connected to the secondary is 16 ohms and the transformer step down ratio is 20:1. This means the voltage fluctuations will be stepped down by 20:1 and the corresponding current fluctuation stepped up by 20:1. The impedance will be transformed by a ratio of 400:1, producing an effective resistance at the primary of (400 X 16) or 6400 ohms. This is called the load resistance referred to the primary or referred resistance for short. The d-c voltage drop between B+ and the plate will be quite small - only about 10 volts.

Comparison of the output stage without (left) and with (right) a transformer

Suppose that the operating point chosen for the tube is 30 milliamperes at 130 volts. A current of 30 milliamperes through 6400 ohms produces 192 volts. The transformer-coupled circuit thus works as if the B+ supply were 130 + 192 or 322 volts, with a 6400-ohm resistor connected between plate and B+ for a load. However, the 30 millamperes are not flowing through the load resistor actually in the circuit (16 ohms), but only through the primary resistance of the transformer, which may produce a drop of about 10 volts and thus require a supply of only 140 volts at 30 milliamperes to get 130 volts at the plate, instead of 322 volts at 30 milliamperes. Furthermore, all of the power developed by the tube is matched to the load, which can be any desired impedance, if the transformer ratio is correctly chosen.