Radio Digest (July 1924-Apr 1925)

August 23. 1924. RADIO DICES T— Illustrated Neutralizing the Super-Regenerator Circuit Part I — The Circuit Used and Description of Parts By C. E. Butterfield AIDED by Prof. L. A. Hazeltine's excellent method of controlling ■ Radio frequency amlification the writer presents herewith a circuit whereby super-regeneration and Radio frequency have been combined bringing about resulting increased volume on distant reception. Difficulties encountered in the control of oscillation, even with the use of neutralized Radio frequency, were only surmounted after several months of laboratory experiments and were met through the use of a combination of negative or reverse and capacitive feedback in the detector circuit, in connection with a high impedance coil in the grid return circuit. No control was placed on the regeneration, as it maintains its balance over the entire Radiocast wave length range, for which the circuit was specifically designed. The set, which might be termed a "neutralized super," has considerably more volume than the straight neutrodyne, is just as selective if not more so, and will function to a certain extent on an aerial alone. "While two stages of audio frequency amplification are indicated in the accompanying diagram, local stations and some of. the more powerful distant transmitters come in with volume sufficient to actuate a loud speaker without the last step. In fact on locals, when the five tubes are operating, the volume is great enough to fill a large hall and is more than is necessary for home use. The High Pitched Whistle The ever present high pitched whistle of the super regenerative set has been eliminated, as far as audibility is concerned. It is carried to such a high pitch as to be entirely inaudible. In fact, when the constants of the circuit were changed to make the whistle audible a considerable decrease in volume resulted. An analysis of the detector circuit will show it to be a combination of methods from several popular circuits so gathered to bring the desired action. Negative or reversed feedback is used in a fifty turn honeycomb ,coil fairly closely coupled to the detector input Radio frequency transformer and placed in a fixed position. Then there is the .006 mfd. fixed condenser, popularized by our old friend, E. T. Flewelling, in his single tube super. However, this condenser has slightly a different feedback. In addition there appears the 1250 turn high impedance honeycomb coil from Major Armstrong's super regenerative circuit. It is found in the grid return circuit to aid the regulating the flow of the grid current. Some builders may discover that this coil can be left out and the circuit function just as satisfactorily, apparently. Such is the case on strong local signals, but its need is quickly demonstrated when a distant Radiocast is filtering in. As the set must handle both weak and strong ^ignals, its need can easily be seen. Selection of Grid Leak Too much emphasis cannot be laid on the selection of a proper grid leak, as its faulty design or construction will result in failure of the successful operation of the circuit. A grid leak should be chosen which will have a range of from onefourth to ten megohms, be noiseless and easy in its adjustment. If the proper instrument is built into the set, its adjustment will be a simple matter and it can remain untouched unless the detector tube is changed. The same is true as to the neutralizing condensers. Those used in the ordinary neutrodyne will not give satisfaction. They should be of a type very similar to the tuning condenser, but of course built on a much smaller scale. Their maximum capacity should not be over .00005. In fact, too much stress cannot be laid on the choosing of the remainder of the apparatus. It should be of the best. The tuning condensers, of which there are three, as in the neutrodyne, preferably should be those providing a straight line adjustment and have a maximum capacity of .0004. ' The Radio frequency transformers must have a four to one ratio with air cores. Preferably those with secondary windings of the honeycomb type should be selected, as they permit the saving of space and the rearrangement of the transformer in the detector input circuit. From the secondary of this transformer three turns are removed, as the close relation of the feedback coil will throw off the tuning adjustment slightly in relation to the other two condensers and will make it difficult to reach the lower wave lengths. Tubes Used While other hard tubes than the 201 A or 301A might be used satisfactorily, the builder is advis'd to select no other, unless he has a power tube which can be placed in the detector circuit permitting a higher plate voltage and more volume. Howivcr, this power tube is not a necessity; in tin laboratory tests only 2D1A's wer. used, 'lie gave all the volume de parts values used in circuit. LI and E.2 are 4 to 1 air core transformers with secondary honeycomb windings; 13 is a 4 to 1 air core transformer with secondary honeycomb winding, with 3 turns removed from secondary; L4 is a 50 turn honeycomb coil; L5 is a 1250 turn' honeycomb coil; CI, C2, C3 are .0004 variable condensers; C5 and C6 are .00005 midget variable condensers and SI, S2 and S3 are A battery switches. sired, using 112% volts on the detector tube and 90 volts on the Radio and audio amplifiers. A C battery is shown in the diagram. While it is not absolutely necessary, the saving in the B battery voltage and resulting clarification of amplified signals is more than compensated for. Note should be made of the fact that only one fixed condenser is contained. In the preliminary tests' more were used, but it was found that they were not needed, and in fact several of them brought about a decreased volume. To date no body capacity effects have been noted, and it is possible , to touch the wiring at a ground potential and get no decrease or increase in volume. List of Parts Required LI, L2, L3, 4 to 1 air core Radio fre quency transformers, with secondaries of the honeycomb type. L4, 50 turn unmounted honeycomb coil. L5, 1250 turn unmounted honeycomb coil. CI, C2, C3, .0004 mfd. variable condensers. C5, C6, .00005 mfd. variable condensers, midget base mounting type (neutralizing). (Continued on page 16) ifki^said it couldn't be done/ MERE it is at Clarity Beauty Volume Distance Selectivity A Tuned Radio Frequency Receiver that will bring even the most distant stations to your home with surprising clarity and volume. So selective that you can pick up any station you want — night after night — at the same dial settings, and, what's more, it is the easiest set in the world operate. Chas. Freshman Co., Inc., 106 Seventh Ave., New York