Radio Broadcast (Nov 1923-Apr 1924)

So much corking good Lab material is piling up that we are forced to devote more space to it this month than we originally intended it to occupy. Our successful experiments with the apparatus described, and the many letters of inquiry, appreciation, and suggestions that have come to our office since the R. B. Lab was started, in October, persuade us that we could not employ these pages to better advantage. Upon observing the one-tube reflex circuit in operation and seeing a sample of a Ballantine Variotransformer sent us for test, Mr. Zeh Bouck, Editor of the R. B. Lab. suggested the unique arrangement he describes below. We then asked the manufacturers of the transformer to build up a circuit in the manner described. A comprehensive report from that company indicates that the results obtained check with our own, and that the Variotransformer works about as well in the circuit as the radio-frequency transformer and variable condenser combination described in R. B. for November and used by Mr. Bouck. Radio Broadcast will be pleased to buy from its readers, at prices from three to five dollars, commensurate with the value of the data, kinks, devices, original ideas, etc., with photographs if possible, which the editor may consider eligible for this department. — The Editor IMPROVING THE ONE-TUBE REFLEX SET THE article by iVLr. Kenneth Harkness, in the November Radio Broadcast, on the best one-tuV reflex set that has ever been brougn to the attention of this magazine, furnished the Lab with material for additional experiments. The object of the tests was the elimination of the predominant and admitted defect of such sets, namely the tendency toward self-oscillation. This fault was overcome to a considerable extent in Mr. Harkness' set, and though remarkably stable for apparatus of this type, the set will nevertheless oscillate at certain adjustments on the radio-frequency transformer (T2, page 14, Radio Broadcast for November). These adjustments are: (1) when the single tuning condenser across the secondary of the transformer does not effect sufficient resonance between the primary and secondary (when they are not tuned sufficiently near to the same wave), and (2) when a highresistance contact is made by the cat-whisker on the crystal detector, an adjustment, incidentally, which is often the most sensitive one. The reason for oscillations at such adjustments is this: If a circuit has a tendency to oscillate, such as is a characteristic of the plate circuit of the bulb in the one-tube reflex, and another resonant circuit is coupled to it, so much energy will be absorbed by this second circuit that not enough will remain to sustain oscillations. But of course the moment that this second circuit is detuned (or imperfect resonance is established, as often happens with the single tuning condenser), or the circuit is opened (as is virtually the case when the resistance of the crystal detector contact, which is in series with this additional circuit, is raised very high), oscillations will start. However, these difficulties would be obviated if the transformer unit could be so arranged that there would always be so perfect a resonance between the primary and secondary windings, that even with a high-resistance crystal contact, sufficient energy would be absorbed to smother oscillations. The development of the Ballantine Variotransformer, which is a tunable radio-frequency amplifying trans