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AUTO TRANSFORMERS
Auto-transformers are a type of transformer in which part of the winding is used as the secondary, but only a part of it is used for the primary. This gives a step-up ratio. Auto-transformers are sometimes thought of as a variation of impedance coupling, and the auto-transformer is the result. The tap is placed about 2/3 or 3/4 the way up, thus giving a step-up of about \l/2 to 1.
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In some respects the auto-transformer is similar to the transformer and also to the impedance type of coupling. The primary has a very high impedance, which is a requisite for quality reproduction. The secondary being larger than the primary causes the increase in voltage applied to the following grid . The voltage gain is not as great as it is in the regular transformer, but this is not serious, as resistance or impedance coupling produces no gain other than that derived from the tubes. As some of the best audio transformers have a ratio of only 2 to 1, the autotransformer makes a pretty good showing.
The music produced by auto-trans
formers may be as good as that produced by other forms of coupling. The type of coupling is not the allimportant thing. At one time it was thought by many that no other form of coupling could approach the quality that resistance coupling gave. The present day transformers have changed that, and now there is little difference between the two types.
Going back to the Clough transformer again we see that it is really an auto-transformer. Forget the resistance and the condenser. Then we see a coil with three terminals, one at each end and a tap near the lower end. The ratio of the Clough transformer is greater than that of the ordinary auto-transformer.
■Western Electric uses an autotransformer between the second and third tubes in the 8-B amplifier. Looking at Figure 1 it is seen that the wiring resembles that of the Clough transformer very much. In place of the resistance a choke coil is used. This is called a retard coil. The choke serves the purpose that a resistance would, that is, the result is the same, but the manner in which it works is different.
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