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lowest frequency to be amplified must be at least twice the plate impedance of the tube. This means that the primary cannot be reduced and still have good reproduction of the low notes unless a core is used that has a higher permeability.
The drawback in the use of permalloy is that it saturates so easily, and when once saturated, or if it receives a sharp blow, its magnetic properties are ruined. It can no longer be used. That is why no plate current is allowed to flow through the windings. This must also be kept in mind when testing. To test one of these transformers in the ordinary way would ruin it. This will be taken up at greater length in the chapter on testing.
Getting back to the more conventional types of transformers again, let us see what is required to prevent saturation. The shunt feed method can be used in connection with any transformer, and in the writer’s opinion will improve the performance whenever used, with one exception. This is when a transformer is designed for a large plate current, but is used with a tube that draws only a small plate current. The term shunt feed is applied to the type of circuits we have been discussing, where the plate is fed through a resistance and
A STAGE OF PUSH-PULL
the signal goes to the transformer through a condenser.
In the design of modern transformers the manufacturer takes steps to keep the core from becoming saturat
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ed, and this is done by making the core large enough so that the primary current does not cause saturation.
Someone suggested a method to counteract saturation by putting an additional winding on the transformer. This winding is similar to the primary, and a current having the same value as the steady plate current is passed through it in such a direction that it tends to produce a flux in the core in the opposite direction that the plate current produces. The sum of the two effects is zero. No flux is produced in the core excepting that due to the signal. As far as the elimination of the tendency of the core to saturate, the idea is O. K., but there are other things wrong with it, among which is the fact that the additional winding acts as another secondary, a partially shorted secondary, which puts a load on the primary. This is not desirable as a rule. So far no one has put out such a transformer.
In push-pull output transformers saturation effects are not present to any great extent. The primary is tapped in the exact center. This point is connected to positive B. The ends of the primary connected to the plates of the output tubes. When the tubes are evenly balanced, when they draw the same plate current, the current from one tube tends to produce a flux in one direction and the current frcm the other tube tends to produce a flux in the opposite direction. The total flux due to the steady plate current is zero. When the tubes are not exactly alike a flux is set up in the core by the difference of the two currents. If one tube takes 30 mils and the other takes 31 mils a magnetic flux is produced in the core of the same strength as would be produced by a current of 1 mil flowing through onehalf of the primary winding, which is not serious. This is one of the reasons why the tubes in any pushpull stage should be as nearly alike as possible.
One maker puts out a push-pull input transformer of the Clough type.