Radio Digest (Apr 1925-Jan 1926)

April 25, 1925 RADIO DIGES T— Illustrated 17 Low Loss Tuned Radio Frequency Receiver Part I — Description and Winding of Coils By George Walters IN THE January 31st issue of Radio Digest there was an article by Jacques Fournier on low loss radio frequency transformers that contained data for which the writer had been waiting for some time. Using the coils described by Mr. Fournier as a basis, and a breadboard layout on which to experiment, the receiver about to be described was developed. Losses vs. Low loss There are several known characteristics I.IST OP PARTS 3 Variable condensers .0005 mfd. $15.00 1 Audio frequency trans., 4 to 1 5.00 1 Audio frequency trans., 3 to 1 5.00 2 Rheostats, 6 or 10 ohms 2.00 1 lb. No. 20 dec. wire 1.00 1 Variable grid leak 1.85 1 By-pass condenser, .5 mfd 90 1 Fixed mica cond., .002 mfd 40 1 Fixed mica cond., .00025 mfd 35 5 Sockets, cushioned 5.00 1 Two circuit Jack 1.00 1 Open circuit Jack 70 1 Pllament switch 1.00 7 Binding posts 1.05 1 Panel 7"x24"xft" 3.00 1 Panel 9"x23"x-.y 4.05 Brackets, wire, screws, etc 3.00 1 Cabinet 7"x24"xl0" 8.S5 Total COBt $58.55 Accessories 1 Storage battery, 6-40 to 6-120. 3 B battery units, 45 volts each. 5 Vacuum tubes, type 201-A or 301-A. 1 Iioad speaker. 1 Antenna, inside or ouside, 40' to 125'. of tuned radio frequency sets with which anyone who attempts to build such a receiver should be familiar. First of all it is not hard to build such a set if the elimination of losses is disregarded, as, if sufficient losses are inherent in the transformers and condensers there will be no tendency to oscillate at lower wave lengths and the amplification will be fair throughout the entire range. To offset this elimination of oscillation, however, Figure meters the range and volume are equal to the average tuned radio frequency set; above 475 the efficiency is not as great as might be desired and is probably the large dial to the left is the output of the second stage. No means is provided for plugging in after the first stage of audio since it was found that when local Figure 2 about the same as that obtained from a good three circuit regenerative receiver. On any wave lengths the tuning is far sharper than with ordinary tube form coils. It would be possible, by the use of switches, to increase the efficiency at the | higher wave lengths so that, when one Figure 3 there are the disadvantages that the set is not selective and the range is considerably cut down. The writer has endeavored to develop a receiver in which all Figure 4 of the losses that It was possible to get rid of were eliminated, so low loss condensers and low loss coils were utilized and exceptional care taken in the layout and wiring. The results as the receiver now stands might be summed up as follows: On wave lengths between 250 and 400 the range is exceptional and is not exceeded by super-heterodyne; from 400 to 475 wished to receive stations above 425 meters, he might throw in more primary turns in the second and third R. F. transformers which would bring the regeneration almost up to oscillation and the high efficiency of the lower wave lengths would also then be found at higher wave lengths. As the set stands, the writer can bring in all but one or two of the high wave length stations so this added feature was not considered sufficiently necessary to warrant putting it in. The first layout followed the usual tuned R. F. practice of putting the three large dials at the left end of the panel and the two rheosats at the right, but, after studying the circuit and trying several layouts, an arrangement was found which permitted of symmetry on the front 'panel und also on the baseboard without loss of efficiency. Analysis of the Assembly Figure 1 shows the front view of this set and in this Illustration the rheostat ,,i the left end of the panel controls the two R. P. amplifier tubes while the rheostat at the rl^lit end of the panel controls the d< tei tor and the two audio stages. The first large dial to the left is on the condenser In shunt to thi mdary of the antenna coupler, the second dial is on the condenser in shunt to the secondary of the transformer connecting the first and second stages, while the large dial to the right Is across the secondary of the transformer connecting the second stage to the del. .tor. The filament switch will be noted directly under the center dial, the lack below the right hand dial is the detector output Jack, while the Jack below stations are too strong for the second stage, the rheostat controlling the R. F. tubes served as an excellent volume control and the brilliancy of the two R. F. tubes could be reduced without impairing the quality. Figure 2 shows the rear view of this set and the reader familiar with tuned radio frequency will see at once that the energy travels logically across from the antenna and the ground binding posts in the lower right hand corner to the first coupler, then to the tube between condensers, into the next coupler and then into the second R. F. tube which is between condensers. The coupler connecting the second R. F. tube to the detector is that at the left in figure 2 and from this coupler it goes to the detector tube which is in the lower left hand corner with the variable grid leak close beside it. Its progress is then across the back of the sub base, in the foreground of the picture, and the tube socket in the lower right hand corner is the second audio frequency stage from which the energy travels beneath the sub base to the output jack below the first condenser. This arrangement makes for easy wiring and little tendency for interaction between various stages and the two types of amplification. A variable grid leak should be used by all means as the receiver was Figure 5 found somewhat critical in this respect, the final adjustment being at about six megohms. Cushion sockets are a very desirable feature as there will be no ringing noise from the microphonic construction of these tubes when the condensers are turned or the table is jarred. The writer has presented this receiver frankly, giving its results exactly as found, with no claims that it will do "coast to coast" on any and all occasions, nor that it will bring in all stations with equal efficiency. It may be possible to develop a receiver in which the efficiency is 100 per cent on all wave lengths without a multiplicity of controls but the writer has not as yet seen or heard of such a circuit. We can now proceed to the actual construction of this receiver which many can use as a finished set and many others will use as the basis for further experiments. In this issue the writer will present the construction of the coils, and take up the panel and base board layouts next week. The Winding' Form The form for winding this transformer is shown clearly in figure 3. As will be seen, a square block of wood, bakelite, hard rubber or metal . is necessary, 4 inches on a side and from % to 1 inch thick. A circle is drawn in the center of this, 2% inches in diameter and divided into nine parts so that holes will be drlled 40 degrees apart around the (Continued on page 18) WhyThe Kane Antennae Eliminates Power Noises All power lines, transformers, and other devices carrying or using electric power on a large scale are grounded, so that if any leakage develops it runs harmlessly into the earth without damaging or destroying property or life. These leakages run through the earth back to the central generating stations, or to the sub-stations that control the distribution of electric power. When your receiving set is connected to the ground, as most sets still arc, these leakages cause noises to enter your set, thus ruining your reception. Electric currents do not travel steadily along the power lines: They surge up and down, the current traveling in waves like the water in the ocean. 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