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746
Radio Broadcast
The effect of the direct current in the plate circuit does not pass beyond the transformer itself, as only fluctuating current or voltage will pass through a transformer.
The audio frequency current passes through the primary, and through the large by-pass condenser (i or 2 mfd.) back to the filament of the detector tube. This large by-pass condenser is a necessity for quality reproduction, as it prevents this audio current from flowing through the leads to the B battery and the B battery itself, and conducts it through a short path to the filament. The B battery and its leads have resistance and inductance. If the audio currents flowed through these leads, it would couple the circuit of the detector path to the plate circuits of the other tubes using the same B batteries and leads, by means of this common resistance and inductance. The majority of squeals and audio howls in an amplifier are caused by common circuits and by capacity couplings.
The audio current, in passing through the primary of the transformer, induces a voltage in the secondary by means of the magnetic flux induced in the iron core of the transformer, and this voltage is impressed between the grid and filament of the first audio tube. The plate current of the audio tube is controlled by this voltage, and variations exactly similar in form to those of the grid voltage will follow
in the plate current if the operating conditions of the tube are correct.
GOOD QUALITY OF RECEPTION MEANS GOOD TRANSFORMERS
IN CHAPTER VII of The Thermionic Va* cuum Tube, by Van der Bijl, a complete discussion of the action of the vacuum tube when used as an amplifier is given, and the reader is referred to this book for complete information, some of which is too involved or too technical for presentation in this article. Proof of most of the statements made in this article can be found by the careful reader in this book.
Before discussing operating conditions of amplifier tubes it is necessary to make one further statement about transformers. Good quality of radio reproduction — to which more attention is constantly being given — can not be obtained with cheap, poorly designed transformers. In order to obtain amplification of the lower audio frequencies, such as those of a drum or bass viol, it is necessary that the impedance of the primary winding of the audio transformers shall be, at that particular low frequency, at least two and one half times the impedance of the tube connected to the primary of the transformer. Transformers made by most of the reliable manufacturers have this necessary primary impedance.
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•«-->• Radio Frequency Paths •* — »• Audio •• •
• > Direct Current •
FIG. I
The paths which must be followed by the different currents in a good audio frequency amplifier. The author tells how to keep them on the right path