Radio Digest (Apr 1925-Jan 1926)

June 6, 1925 RADIO DIGES T— Illustrated 17 Recent Advances in Tuned R.F. Amplification Part II — Circuits in the 1924-5 Receivers By Milo Gurney BEFORE passing on to showing other than schematically, modifications and adaptation of Rice, Hartley and Hazeltine, it may prove interesting to digress a moment in calling your attention to circuit figure 7, which enjoys a marked likeness to Professor Hazeltine's circuit, the only apparent difference being the employment of a variable capacitance between the coils Tl-Ll which comprise series resonant circuits. "With this circuit at resonance the voltages in coils 1,1-1,2 are in phase opposition and. by inserting Tl between the input circuit and the top of L2, compensation is accomplished. This circuit, which will be given in detail in a later article, should prove superior to either the Rice or Hazeltine method of neutralization, even though it is not a perfect compensator for wave lengths foreign to the resonance point of the coupled circuits. It, however, is easily constructed and offers much toward surprising efficiency, affording in theory a much closer counterpart of Hazeltino's regeneration limiting device than that indicated in the Rice method. Figure 7 Reverting back to the more simple plotted in tuned radio frequency amplifiers wherein intentional losses are deliberately inserted, either in the primaries or secondaries, to absorb undeSired values of regeneration one will rte that in each method used it is quite apparent that the designer of Ircuit made no intelligent attempt toward providing neutralization, but introduced a loss, usually in the form of a resistance, to accomplish or cover up a bad job, while often this method of resistor insertion has been used purely to dodge patent infringements. Among such means are the well known -use of so-called stabilizers, high resistance transformers and potentiometers. Xo originality of thought is expressed in any of these adaptations, while in general, and in a Cp Figure 8 greater or less degree, they are harmful because, while they do function toward nullifying the effect of over inter-stage regeneration, yet in doing so their adoption trends toward causing distortion and the limitation of selectivity, each result to be avoided if possible. It would, however, be unfair to exclude a few diagrams showing some of the methods used and to advise as to which of these methods is most desirable should the experimenter desire to use them in an attempt to improve upon their application, which naturally is desirable. Insertion of Series Resistance Figure 8 shows a typical input and output stage of a tuned circuit as used in tuned radio frequency amplification, in which it will be noted that a series variable non-inductive resistance is inserted in the input circuit This resistance may be of the order of 1,000 ohms, must be variable and should preferably be placed as indicated at the low potential end of such input circuit. A partial variance of this method of inserting a resistance is shown in figure !), wherein the resistance is fixed rather than vari Getting Concerts WHOLE Cabinet Model "C" $30 It soon dawns on the owner of a I irist ol Speaker that he is listening in on entire concerts. Tnat roving disposition to tune in every station on the map is <\uv, much more than is generally supposed, to a yearning for really sweet music. One reason radio music does no1 always sound sweet is that certain of t lit' tones are out of tune. Coming through a Bristol Speaker, all the tones arc evenly in tune. The result is an arresting sweetness thai "invites" you to stay through a concert to the end. BRISTOL AUDI^ONE Loud Speaker I'd $25 ai >u i an gel a Bi istol Spi al and I lure are cithers a > low a ■ $12 ;li II dealer to send one out. Write u^ tor Folder No M .'^22. telling why the Bribtol i .such a delight to the ear. THE BRISTOL COMPANY, WATERBURY, CONN. BRISTOL'S able and is placed at the high? potential end of the input circuit. This method is used in the first and second stage of a well-known commercial receiver, the first stage having as its resistor value approximately 62.") ohms, while the second is near the value of 600. these figures merely being given as approximate, in fairness to the manufacturer and with the further assurance that they would not hold as accurate or correct for the receiver you may construct. Thty are values as a guidance because the total losses essential for the securing of stabilization are quite obviously dependent upon the amount of regenerative reaction one desires to control, and varies with the maximum regenerative constants indicated as to be controlled in the completed receiver. Shunt Resistance Figure 10 shows a somewhat different method of loss insertion, it in effect being comparable with, and equivalent to, placing a variable shunt across the input circuit. This resistance should preferably also he variable and of the order of 1,500 to 2,000 ohms, though once the proper Figure 9 resistor value is determined it may be locked as constant. The variable iondenser is of the standard two-plate value as it is not critical, and minimum or maximum value is not overly important Of the two circuits, viz., 8 or 10, figure S is to be preferred, first because oi its greater ease of construction and. secondly because the grid to filament resistance is not changed as in figure 10, hence the li ss likelihood of annoying distortion. while some few experimenters have directed their efforts toward the shown insertion of resistors, both variable and fixed, in the input or grid to filament circuit, another school of thought has reversed the procedure through placing variable resistances in the output or plate circuit, scum i 1 sing their location ahead of the plate inductance or between it and i!e plate terminal, while others have Figure 10 chosen to insert it between the plate return and the phis II battery feeding the plate. Irrespective of location, each remain as insertions of resistance and, while placing them in the plate or primary output circuit is less harmful than if they were placed in the grid or input circuit, they still remain as loss devises and do not, except in a measure, indicate a new thought or course of procedure coward the securing of ideal compensat ion. They do, however, act in a measure as radio frequency chokes though evidently the thought prompting the procedure of such plate circuits resistance was to reduce the plat, battc i> component and to ai tic same time ace plish this, if possible, without increasing the fundamental frequency of the primary in such a trans i -i nil i I'.rhaps if is well a l this point to touch upon the interesting subject of coil ratios, i. e., primary to secondary in radio frequency transformers, wherein both primary and secondary arc of approxi1 1 'out iniie.l .hi page l 8) **^*:*3*.#^ Vtt V* J*!&&'*3t*&yi3t*X Combination' PRICE Complete Kit ;35 Buy Only Genuint SHAMROCK HARKNESS Licensed Parts Five-Tube Volume and Shamrock-Harkne 1 1, i ,■' a pow ' rful little et I I o c|c, the work oi five. Cuts Battel istent loud within a , adiu oi 1,000 mile and w e don'l mean ma) he ! This powerful little receive! i the i , uit oi' month "i investigation b\ Shamrock engint era to build the SHAMROCK MANUFACTURING CO. 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