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Radio Broadcast
FIG. I
The one-tube reflex circuit, which gives a stage of R. F., detector", and a stage of A. F. amplification
operate a loud speaker over distances about equal to those it is possible to hear with the telephones on an ordinary crystal receiver. When used with a headset it is capable of very long distance reception, extremely sharp tuning, and exceptionally clear reproduction of speech and music.
The receiver is essentially a one-tube reflex outfit, but involves certain modifications that make for efficiency, sensitivity, volume, clarity, and ease of control. It is:
Efficient — because the one tube is made to do double duty and because an improved circuit with correct constants is employed.
Sensitive — because a stage of tuned radiofrequency amplification is provided before the tuned detector circuit.
Volume — because a stage of audio-frequency amplification is used to amplify the rectified impulses and because both the radio-frequency amplifying and rectifying circuits are tuned— giving maximum amplification with corresponding selectivity.
('.{ear — because a crystal is used for rectification: and because, when properly adjusted, the vacuum tube does not oscillate and the howling and squealing so noticeable in regenerative receivers is totally absent.
So far, not so bad, eh?
METHOD OF PREVENTING SELF-OSCILLATION
ORDINARILY in a reflex circuit the tendency toward self oscillation is so great that a potentiometer or similar device must be employed to impress a positive charge on the grid so that the resultant grid current will prevent self-oscillation.* In a plain radiofrequency amplifier this would be quite satisfactory, but when it is desired to use the same tube for audio-frequency amplification it is necessary to operate the grid at a negative potential or else the A. F. amplification will be nil! It is evident then that reflex systems utilizing a potentiometer stabilizer are out of the question.
We could employ reversed inductive or capacitive feed-back to balance the self-oscillations, but each of these systems has certain disadvantages; especially in a circuit having a variable resistance element such as a mineral rectifier. The adjustment would necessarily be tricky and unstable.
The method of preventing self-oscillation in the receiver to be described is not new, but, to the best of our knowledge, its application and dual functioning are original.
Briefly, if the grid and plate circuits of a
*See "Radio-Frequency Amplification" by Kenneth Harkeness, published by The Radio Guild, New York.