Radio Digest (Apr 1925-Jan 1926)

May 30, 1925 RADIO DIGES T— Illustrated 17 Recent Advances in Tuned R.F. Amplification Part I — Hartley, Minor and Hazeltine By Milo Gurney RADIO, in the first mad rush of its popularity has, of necessity, created strange bed-fellows as is evidenced by the fast growing and not unpopular family of "Dynes" and "Plexys" with which the market has been flooded during the past year, following the introduction of the neutralizing principle of tuned transformer compensation by Professor Hazeltine. It is only regretable that too many of these offerings did not represent added advancements to the art rather than portray ingenious methods of inserting "losses" into circuits as a means for securing stabilization, with resultant inefficiency. The road to further improvement is not strewn with roses and it is, therefore, the author's purpose in this series of articles to not only attempt to clarify the experimenter's mind through pointing out the pitfalls which should be avoided, but also establish a history n f the worth while research work which has been done in this field, together w ith the present trend of recognized engineering minds. No effort of this character would be of merit that did not also include fundamentals for continued experimentation by the thousands who take pride in constructing circuits representing the latest in Radio receivers and it will, therefore, be my greater pleasure to include with this series two, and probably three, relatively new tuned radio frequency circuits, each combining compensator or neutralizing methods which are improvements and not "loss, rs ' Tuned radio frequency circuits, properly constructed and correctly wired, represent Intensely interesting experimentation, as Milo Gurney inherent in them is the assurance of much greater amplification per stage, greater volume over the total broadcast wave length band, together with a selectivity factor comparable with, if not superior to, any circuit at this stage of the art. We may confidently look forward to the market being provided with some very creditable conceptions for the coming season, not as a result only of recent research but, rather, as a culmination of nearly fifteen years of engineering in this particular type of Radio receivers. Cpj;-f^oWoT| Much of this work has centered around the application of the theory or principle of the well known wheatstone bridge for the securing of compensation or neutralization, and it is the use of this balance bridge principle which will form the basic theory not only of these new and more efficient offerings but also of the circuits which the author will present. However, before explaining the theory of the wheatstone bridge principle, or presenting circuits using it, a historical survey covering the leading contributions to the science of tuned amplification occurs as important, as, without such a survey to guide us, progressive research would be impossible. In addition, it seems pertinent that the causes of excessive reactive currents and consequent regeneration beyond control should be clearly understood, not because regeneration is harmful, but rather in order that we may intelligently take full advantage of it. Regeneration is defined as any form of reaction from the plate or output circuit Atuiio Frequency Trons/ormcr Quality Parts Matched for Perfect Teamwork Your "pet" hook-up needs first quality parts - perfectly matched — to give you real radio. Every Federal Si ho Part is designed, made, matched and guaranteed by Federal. That is why you find Federal parts in ill the better hook ■ I why you should imist on Fcdcr.il parts when purchasing. FEDERAL TBLBPHON1 MANUFACTURING CORP Buffalo, v \ ' Standard RADIOProduct9 upon the grid or input circuit of an amplifying tube wherein the alternating current power supplied to the grid is increased. Note particularly that regeneration can only occur when preceded by amplification and, as amplification of the input or grid circuit can only take place within the vacuum tube, it is obvious then that so long as we find it essential to have both input and output circuits comprising inductance and capacity, coupled through the grid and plate elements of the tube, then just that long will regeneration be present at some value and we may as well give up hope of trying to eliminate it. At the same time it is well to realize that, instead of attempting to totally eliminate regeneration, particularly in tuned radio, frequency circuits, we should to the contrary use the maximum amount compatible with quality reproduction and our ability to neutralize or compensate its interstage coupling effect. Figure 1 is a schematic typical of a circuit capable of oscillation and consequent regeneration, representing the total elements in one stage of a standard receiver and comprising in its picturization all the factors common to any type of circuit you may be using or applicable to any other. It is shown primarily to convince that regeneration is ever present while SL,P L pit 1) °# figure 2 is shown as proof that no change has been made in the fundamental of figure 1 except that a portion of the plate circuit winding has been reversed, and, in the form shown, is the schematic circuit of the Minor Superdyne. It is then apparent, that our problem in all tuned radio frequency circuits is the control of oscillation and I want each of my readers to appreciate that the major portion of howling or oscillation troubles occur in the first, or antenna input stage. That is the starting point of all your troubles, therefore give it your first attention. Further realize that all compensating devices must of necessity be only a means of limiting the regeneration occuring between the fixed plate and grid elements of the tubes of the sepa Figure 3 rate stages for each tuning adjustment in the plate circuit; in other words, the ideal compensating device would be one which would change its compensation automatically for each frequency of the plate circuit. Without question, the majority of regeneration control devices now on the market comprise the introduction of resistance or loss means into either the input or output circuit, or both, so that the energy reaction, or feed back, from the plate to grid will be absorbed. This practice in itself would not be objectionable ware it not that their insertion also usually causes distortion and certainly is opposed to the securing of selectivity, hence are not to be considered as other than undesirable makeshifts and totally unworthy of serious consideration since they do not represent any attempt toward securing compensation or feed back control. Hartley (U. S. patent 1,188,875), is credited with the first effort among Amer(Continued on page 18) Last night we heard the sweetest voice!9' FEW boast of having heard a powerful voice. Radio has grown up. It is now something to listen to, not to marvel at. We arc now in the cycle of TONE! The other day a man said he was just realizing that he had a hundred dollars worth of set and a dollar's worth of horn! His next move is to balance Up his set with a good speaker. l'he BRISTOL has all the volume you will ever want, hut its line point is TONE. It is good to listen to. The notes come in in tune. You receive a wealth of music from which the cream of tone has not been skimmed. For $20, $25 or $30 you can get a Bristol Speaker, and there are Others for less. .\>k your dealer to send one out to the house. And let us send you folder No. AY-3022; it tells why the Bristol is the horn of tone. BRISTOL ^^ Loud Speaker THE BRISTOL COMPANY, WATERBURY, CONN,