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Show iL Q jTpli TPlir Ivwyv' v'-.V',-- N p.' 1 rip, 4 o 6 1 ki-t..UC' J BliMiiiiA. r1"1 A-e- a "p.tT,..,.i v I j Left Circuit Diagram Which Is the Principle of the Monophase. Right-Details Right-Details of Radio Frequency Coupling Transformers. All things considered, ease of control, con-trol, simplicity of construction, selectivity selec-tivity unci range of reception the five-tuhe five-tuhe tuned radio frequency set is the most eflieient hook-up that has been developed. The one stumbling block has been the proper means of controlling con-trolling the oscillating characteristics )f the tube. Several means of doing this have been devised, most of them effective, but all had their drawbacks. The monophase, recently described in the Itadio News and tested out in that publication's laboratories, seems to be one of the best circuits yet offered. The following description of this set Is taken from the Radio News: It is well known that when several stages of radio frequency amplification amplifica-tion are employed in radio receivers there is n tendency for self-oscillation to occur in the circuits. This is due to the feeding back of radio frequency energy from the plate circuit of a tube to the grid circuit of any other tu'je, or to Its own grid circuit. The Feed-Back. There are, In general, three ways of preventing this feed-back : viz., the introduction in-troduction of resistance into the oscillatory os-cillatory circuits, neutralization of the tube capacity or magnetic coupling, introduction-into the circuits of electromotive electro-motive forces which have a polarity (or phase) opposed to that of the incoming in-coming 6ignais. The last method is that employed In the superdyne receiver and is also the method which has been employed In this receiver. There Is a difference, differ-ence, however, in the way in which Ikon ij4M-,'i Mm , t n 1 4sn i - rf4 1 , -L-.8 ,j -n y- .00035 MF to J Ik r-, s , .00035 MF. g ik 1 - y00035 MF. ? T ImsJsmhsLr-flG-2 The Monophase Receiver Employs the Above Wiring Diagram. the circuit stability is accomplished, for in the superdyne this so-called "negative feed-back" is accomplished by coupling a coil in the plate circuit of a stage to a coil in the grid circuit, whereas, in the monophase system the coupling Is between a coil in the filament fila-ment circuit and the one in the grid circuit. The fundamental circuit diagram is shown in Fig. 1. Here the common terminal (the negative filament connection) con-nection) Is broken and the feed-back coil connected -.cross the break. The polarity of tlin feed-back coil is to be made such tliat It will prevent self-osclllatlon. self-osclllatlon. Fig. '2 shows the complete wiring diagram of the set. Three such feedback feed-back coils are used, one for each stage of radio frequency amplification and one for the detector circuit. One might think Unit y using so many adjustable adjust-able parts, the operation of the set would becorie very complicated. Such Is not the case, however, for it will be found that two of the feed-back colls can be left untouched once they hm adjusted, so that the number of tufiing controls in the set is reduced to the three condenser dials and one dial for adjusting the first feed-back coil or compensator. The idea is that the second stage of K.F. amplification is adjusted on the long waves, so that, as the feed increases in-creases with the short waves, there will be less tendency for oscillations. That Is, the set is tuned in on the longest wave It Is desired to receive, gay 000 meters, and the second and third compensating coils adjusted carefully to the point at which howl ing just ceases. After this, they cat be left alone, for on all wave-length, shorter than this there will be les, tendency for self-oscillation to occur on account of the increased negative feed-back on the shorter wave-lengths The Amplification. The amplification in these stage: will always be high, however; in fact it will be very close to the amplihca lion obtainable at the critical point for exceedingly small adjustment ol the compensators is necessary to stall or stop oscillations. . After the secont and third compensators are properly adjusted they are to lie left alone and the circuits can be brought up to the critical point by manipulating the first compensator alone. There is an interesting point in con nection witli the placing of the feedback feed-back coils which deals with their position posi-tion with respect to the secondaries of the tuning units. It will be noted in this circuit that the compensator is placed at that end of the secondary coil which is connected to the filament. fila-ment. There Is a very good reason for this, it being that the controlling is done at the low potential end of the coil, and the adjustment of the compensators com-pensators will not be so critical. This Is contrary to the general practice prac-tice of placing ticklers at the grid end of the coil, but this latter is done in cases where It is desired to increase the regeneration by tightening the coupling. Again, it must be remembered remem-bered that the third winding is not similar to the reversed tickler in the superdyne, but it does create t. bucking buck-ing E.M.F., which acts against the oscillating circuit and produces a tendency ten-dency toward damping. It will be remembered, re-membered, too, that the coupling' must be loosened in this circuit to increase the regeneration. It is at once evident evi-dent that we are not trying to increase in-crease regeneration. The only regeneration regen-eration that will exist in the receiver is that which it inherently possesses due to the natural tendencies of radio-frequency radio-frequency amplifiers. Constructional. The first step in assembling this receiver re-ceiver is to procure the best of the necessary materials. A 21 by 26-inch panel and a 7 by 24-inch baseboard are fastened together in the customary manner, after the panel is drilled with the necessary holes. The construction of the radio frequency fre-quency transformers is not a difiicult one. Three bakelite or hard rubber tubes 2 inches in diameter and 6 inches long are required. There are wound 45 turns of No. 22 D.O.C. wire. A G-turn winding of the same size wire composes a primary winding i of an Inch from the first winding. The secondary winding is then continued for another 45 turns. The compensating compen-sating coil is next placed on the tubing tub-ing and consists of six turns of wire wound In the opposite direction to that of the other windings. Two small brackets are used to fasten the radio-frequency radio-frequency transformers to the baseboard. The above description pertains to two of the transformers, the third being be-ing slightly different in that the compensating com-pensating coll, instead of being wound on the tube itself, Is placed on a small rotor at one end of the tube. This rotor is a piece of bakelite tubing two inches in diameter and one inch long, at one end of which is placed the six-turn six-turn winding. It Is not necessary that the wire be wound in any specific direction, di-rection, since the coll can be readily turned through 180 degrees and thus change Its polarity. The wiring of the receiver is not very difficult, since there are no set rules, except that careful work brings good results. In case trouble is experienced ex-perienced from the beginning, the connections con-nections to the compensating colls should he reversed one at a time until the undesirable oscillations are eliminated. elimi-nated. It will be found advisable to place two by-pass condensers across the "B" battery and also to use a .00025 mfd. condenser between the grid and plaU' terminals of the last audio-frequency amplifier tube. As regards the selectivity of the set, It Is safe to say that it surpasses the majorly of those on the market today. In sensitivity, too, due to the exacting values of the apparatus used and the correct engineering principles involved, in-volved, the receiver stands as a big step In the development of tuned radio frequency sets. As will be noted, the colls are fully CV2 inches upart, reducing to a minimum mini-mum the tendency towards inter-stuge coupling with resultant oscillations. Two stages of audio-frequency amplification ampli-fication will be found to give more than sufficient volume, it being necessary neces-sary to incorporate a by-pass con- ! ' denser across the grid and plate ter- I minals of the last tube. |