Singletuned If Transformers

The single-tuned IF transformer has taken two important forms, the bi-filar coil and the double coil tvpes.

In the former case, the two wires constituting primary and secondary are wound simultaneously, forming a coil that is a single physical unit yet having two independent circuits. The start of the primary was usually the plus "B" connection and the start of the secondary was ground. The outside of the primary was the plate connection and the outside of the secondary was the grid connection. These transformers were characterized by very high gain and comparatively little selectivity. They were used on receivers that had no A.V.C. and the secondary low-potential end usually connected directly to ch assis. Such a transformer could not he used satisfactorily in a receiver employing the conventional diode type A.V.C. circuit for the reason that on damp days there is enough leakage between primary and secondary to produce a decidedly positive bias on the grids of the automatically controlled tubes.

In addition, such a structure possesses such a high capacity between windings that the ripple in the "B" Supply would be transferred to the diode load resistance which would produce a bad audio hum in the output of the receiver. A third reason why this/ type of transformer would not now he acceptable, even if there w-ere no diode load resistance to pick up hum or to be incorrectly biased, is the frequent failure of windings due to electrolytic corrosion. Where two conductors are run so intimately parallel for ao many turns, with opposite D.C. potentials applied to the two wires, ideal conditions are set up for rapid failure due to electrolytic corrosion in the presence of moisture.

With this transformer redesigned to have two physically separate coils wound side by side, the objectionable features of leakage, corrosion and hum transfer are re(dueed to a very small per cent of their original importance, and transformers acceptable in today's critical market can be produced. The largest remaining objection to the single-tuned transformer is selectivity. In a low-frequency amplifier operating at. 125 KC or 175 KC, the transformers are too sharp for good audio fidelity, and at the higher intermediate frequencies such as 456 KC, the transformers do not add sufficient adjacent-channel selectivity.

Single-tuned transformers may be divided into two classes according to the circuit tuned; some have their primaries tuned while the remainder have their secondaries tuned. As far as secondary voltage is concerned, there is not a great deal of difference regardless of which winding is tuned, but if there is a question of single-stage oscillation in the tube driving the single-tuned transformer, greater stability is had by tuning the secondary than by tuning the primary.

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