Radio Digest (July 1924-Apr 1925)

February 7, 1925 RADIO DIGES T— Illustrated 25 How to Make the Two Tube Knox Reflex Part I — Analysis of Circuit and Winding of Spider Webs By W. H. H. Knox . THE writer has always been a staunch supporter of the crystal as a detector and, as the reflex circuit presents the best opportunities for utilizing this form of detector, it was naturally selected some two years ago when searching for a hook-up with which to experiment. The reflex has been faithfully adhered to during this time and, in developing the circuit to be submitted, a great deal of pleasure has been derived; also a greater amount of knowledge regarding Radio and high frequency current than it was thought possible to absorb mentally in connection with any branch of science. Analyzing' the Circuit The analysis of this circuit is about as follows (see figure 2). Signals enter the circuit through the coil LI, which is an untuned primary in the antenna circuit. Because of inductive coupling, signals are passed into coil L2, which is tuned by condenser CI and which, with this condenser, will cover the broadcasting range. At first glance it might be presumed that coil L5 introduced straight tickler regeneration, but this is not the case as coil L5 is connected to produce reverse feedback as in the superdyne. L5 is connected to the plate circuit of the first tube through a small condenser of .000045 or .00005 mfd. capacity and through the resistance R3, which is 24 ohms. Such a resistance is on the market in the form of a wire wound fiber strip intended to be used with a 6-ohm rheostat for control of dry cell tubes with 30 ohms. Signals pass from the plate of tube Tl through coil L6 tuned by the condenser C6 and through the primary of audio frequency transformer AT2 into coil L3, which is the primary of a Radio frequency transformer. L6 and C6 constitute the tuned impedance of the plate circuit and bring the plate circuit into exact resonance with the grid circuit composed of L2 and CI. The howl which would ordinarily result is prevented by the reverse feedback of coil L5 and condenser C5. The spider web L4 is inductively coupled to L3 and is tuned by variable condenser C4 with which it covers the broadcasting range. Signals then pass into the crystal detector D and the primary of audio frequency transformer ATI. They are now at audio frequencies and are passed into the grid circuit of tube Tl through the electro-magnetic coupling between the primary and secondary of audio $& < Figure 1 transformer ATI. At audio frequencies they are amplified by tube Tl, passed through coil L6 and into the primary of AT2. This being an audio transformer the audio frequency is passed into its secondary and onto the grid of tube T2, which is a second stage of audio frequency amplifiaction. The output jack is tickler coil, experimental work was gone into along this line which nearly deafened one with the groans, howls and whistles that came out of the headphones. It was finally decided that some means of bringing regeneration up gradually must be employed. After very careful observation, and plotting the charts of three element connected in this plate circuit. "We thus have a successful application of the superdyne principle to a reflex receiver with consequent gain in both selectivity and range, but retaining the economy of tubes and tone quality of the reflex. Difficulties Overcome Something over a year ago it was decided to try introducing regeneration into the reflex circuit and, starting with the The first "low-loss" condensers CARD WELL Condeuttera were the first condensers made with metal end-plates and grounded rotors. The phrase "low loss" was originally applied only to Cardwells, because engineers wished to distinguish them from ordinary varieties. Cardwell Condensers today, as in the past, are the standard of comparison. They have proved their superiority because of such a correct design that they have never been excelled electrically or mechanically by any commercial type. The correctness of the Cardwell design assures the utmost distance, smooth tuning free from noise, and prevents changes in capacity at given settings. Its ruggedness and general serviceability explain why a Cardwell Condenser re quires no attention once installed in a receiver. The Cardwell Condenser has proved so efficient that it has compelled the manufacture of more and more efficient apparatus ot all kinds in order to keep the receiving sets uniform in their low-loss characteristics. This is a truly revolutionizing achievement not to be lost sight of. Build your receiver around Cardwell Condensers. There's a Cardwell for every requirement — 76 different types. Ask your dealer today for the complete Cardwell circular and look over the Cardwell Condenser types for the one you want. Sold by all reliable dealers. tubes, it was found that regeneration is constantly in advance of amplification up CARDWELL CONDENSERS The "Goode" Two-o-One A Le Ton d'arg ent BY MAIL ONLY 2 .39 Postpaid QUARTER AMPERE AMPI.IFIE*— BHTBCTOK RADIO TUBE GUARANTEED SATISFACTORY All "GOODE" Tubes Sold Direct to the Consumer — No Dealer Profits $2.39 ... 6.42 ONE— "Goode" Detector-Amplifier THREE — "Goode" DetectorAmplifiers (All postage prepaid) The "Goode" Two-o-One A Tube amplllea or detects. It Is a Quartet ampere. Ire Tolls, standard bale silvered tube. Send express or postal money order or New York draft to — ' The Goode Tube Corporation Incorporated Owensboro (Dept. A) Kentucky to the oscillating point. Therefore, if regeneration could be retarded, thereby preventing an oscillating condition of the tube until the peak of amplification was reached, enormous amplification would most certainly result. The writer decided, first of all, to attack the problem of inductive resonance. This really means that the inductance of the plate circuit is equal to the inductance of the grid circuit, a condition that will surely result in violent oscillation. The logical way of preventing this was, of course, to make this inductance as unequal as was practical and coils of different ratios were tried until it was finally decided that this was not the cause of the instability of the set. Attention was next directed to the problem of magnetic coupling and, much to the writer's disgust, it was suddenly noticed that while coils LI and L.2 (figure 2) and L3 and L4 were about 5 inches apart, they were placed parallel to each other. The writer has never been able to figure out how they got that way, but the fact remains that they were in this position, and after turning them both to about a 57-degree angle a decided improvement was noticed. Every three element tube is subject to a malady of capacity coupling, or the interaction of the grid and plate circuits. There are several remedies for this, all effective to a certain degree, such as the Hazeltine method of neutralization, conductive reverse feedback, and inductive reverse feedback. The first two were tried but there were certain features about their action that seemed unsatisfactory, so the inductive reverse feedback was tried and it was found that the tubes were under perfect control at all times. At this stage of the work it was felt that the experiments had gotten along pretty well, so the next step was to tackle regeneration once more. There was no whistle when tuning, but, (Continued on page 26) 9.^ MURDOCH £2ARKft THERMIODYNE PHENDC DERESNADYNE MALONE LEMON AUDIOLA GATES GLOBE HARMONY ODELL FERRY PEERLESS , MASTER RADIO MUSIO j*^yj|>IANY OTHER* C/gJliicgJ TrtojssfessoN TRANSFORMERS / $rfa)dorfcy^ What better advice on transformers I could you ask than the opinions of the J leading fine set builders? The majority J use Thordarsons— proof positive they're / best I Unconditionally guaranteed by I world's oldest and largest exclusive / transf ormer makers— transformerspeclalists since 1895. Follow the lead of J the leaders— build or replace with f Thordarsons. Recommended by best / dealers. Audiofrequency : 2-1, $5; 8H1, »4; 6-M4.50. Power Amplifying:, pair, $13. Interstage Power Amplifying Transformer, $8. Write for the , latest Thordarson hook-up bulletins. ' THORDARSON ELECTRIC MFC. CO. CHICAGO.