Radio Digest (July 1924-Apr 1925)

Utgust 9, 1924 RADIO DIGES T— Illustrated 'At Combination Radio Frequency and Regeneration Part I — Simplicity and Convenience Are the Chief Points in the Design By Brainard Foote THE general trend of set building seems to be along the lines of simplification. And this is but a natural reaction to the prevalence of receivers involving a multitude of dials and knobs to "wind" every time the set is used. _ A neutrodyne, for instance, already having three tuning dials, ought not to be complicated with a rheostat for every tube inside the cabinet, for such practice means a lot of extra work when you want to listen in. With the universal use of quarter ampere tubes in the sockets other than that for the detector comes the% possibility of cutting out the filament regulators to a large extent. The C-301A and the UV201A are not critical tubes — you can swing the rheostat knob quite a way back and forth without noticing any difference in signal strength. Why bother with rheostats on the panel, then? Filament Control Jacks The panel for a four-tube set may be made to have the appearance of the set shown in the illustration. There are two tuning controls and one regeneration control. Two jacks complete the exterior. Instead of employing rheostats at all, a fixed resistance is employed, and two filament control jacks permit the use of either one or two steps of audio amplification. When you're through with this sort of a set, just pull out the phone plug and the tubes go out. Insert the plug in the first jack, and three tubes light up; in the second jack, and all of them are illuminated. Don't you think that's better in every way than eranking five "handles" instead? If you're a believer in a soft tube as the detector, you will probably retain a rheostat for such a tube, but, as a matter of fact, the difference is so slight that it's scarcely worth while to use a soft tube as detector. No doubt it is worth while in a plain detector circuit, but with Radio frequency ahead of it, you'll gain but little. Then, besides, you save three-quarters of an ampere by using the other kind of tube, and your A battery lasts a good deal longer. There are other worth-while improvements to be suggested, too. All wiring is placed on a sub-base made of panel material, as shown in the second picture. The base is supported by two jacks and two standards. The coils and sockets are mounted on top of the base and the wiring beneath. The use of a base in this manner is electrically much superior to screwing the parts to a wooden base board. It's far easier, also, because you merely use % or 1-inch 6-32 machine screws and nuts for fastening the parts in place. The wiring is thus concealed beneath the subbase, and the appearance of the set bettered in that way, too. Dust cannot settle on the connections, either. Variable Features Many manufacturers are now bringing out receiving sets using one step of Radio frequency, detector, and two of audio frequency. Some of the sets are adjusted so they are near the point of oscillation — others have regeneration controls. A regenerative detector circuit preceded by a step of tuned Radio frequency is surely a combination of high sensitivity. What is more, it does not radiate appreciably on account of the blocking action of the Radio frequency amplifier tube. The circuit used for this set is not at all complicated despite the filament control jacks. The antenna is coupled to the Radio frequency amplifier tube by the variable coupling coil inside of the sta OneOne— One Two TwoFour TwoTwo Nine— TwoThree EightOneOne One— 7x18 cabinet 7x18 panel (Mahoganite, illustrated) 7x18 panel (black) as the subpanel -Vernier variable condensers, .00025 mfds. -Audio transformers -Sockets -Pieces 4x4 hard rubber tubing -Pounds Number 16 double covered magnet wire -Binding posts -Filament control jacks, 6 and 4 contact 3-inch dials — Lengths bus bar for connections Grid condenser .00001 mfds. 2 megohm grid leak and mounting Fixed condenser .001 cast band, while a larger size will give too "crowded" an arrangement of the dial settings. The 13-plate condenser with about .00025 mfds. capacity is correct. (Construction of the coils for the superdyne will be told in the 'next issue. A complete wiring diagram will be given and an explanation of the circuit as well as the assembly. — Editor's Note.) tionary coil shown at the upper left corner of the bottom picture. The coil may be rotated and left set at the best point of the coupling. This depends entirely upon the capacity of the antenna used with the set; a large aerial requiring less coupling than a small one. The coil lying on the base in the center is the plate coil of the Radio frequency tube and couples it to the detector grid circuit. The coupling 'may be varied, so that the Radio frequency tube is supplied with a definite and fixed amount of feedback. It is set near the point of oscillation, but there is not quite enough coupling between to permit the Radio frequency tube to actually oscillate. That coupling is then left set. Now comes the unusual feature — the tickler coil. Instead of securing regeneration in the Radio frequency tube, as is done with many circuits (the Superdyne, for instance) and which practice makes the first tube a strong transmitter and interferer, the regeneration is obtained wholly in the detector. This is but evenly dividing the work of the two tubes anyway. The tickler coil is mounted at the left end of the plate coil — the detector grid circuit — and it is mounted so that it can be controlled from the panel front. The center dial shown moves the tickler through a semi-circle. Hence, not only can you get regeneration and oscillation if you want it, in the detector tube, but you can also turn the tickler "backwards" so as to oppose oscillation and reduce signal strength when using headphones on a local Radiocast. / Farts Necessary You may depart from the recommendations listed below, but these are given in order that the reader may know and recognize them in the photographs: About %-inch will have to come off the 7 by 18 sub-panel in order to make it fit inside the cabinet. Any good apparatus may be chosen as desired, provided the constants are not different. This is especially true of the variable condensers, for a smaller size would not cover the Radio Doubly-Grounded Antenna To avoid trouble from power lines and local electrical machinery one ingenious French amateur has devised an interesting scheme of aerial connections which has received much approval. The principle consists in connecting the free end of the usual horizontal aerial to a ground. This connection should be made by a wire large enough to have practically no resistance. With the ground connection itself, the average resistance should be not more than 40 ohms. The other end of the aerial is connected to the receiving set and thence to the ground in the usual fashion. At this end, also, the total aerial-to-ground resistance should be low, preferably not more than 40 ohms. The ground connections themselves must be efficient ones, made preferably by burying zinc or alumihum plates about a yard square and about four feet under ground. If possible the ground plates should be placed where the soil is continually moist, even in summer. If it is inavoidable to place the ground plates in dry earth, they may be buried in beds of coke, which should be wet down by the garden hose every so often. In this case a larger piece of metal should be used. In ordinary moist soil the dimension of one square yard mentioned above will produce very good results and under these conditions, it is not necessary to make any measure of the resistance of the ground. No matter what the ground plates may be, the resistance of the ground must not be more than 80 ohms. Otherwise the mounting will have little effect in eliminating parasitic interference. POZ at Nauen, which formerly transmitted its time signals on 1300 meters spark, is now working on CW at 1800 meters. "VERY MUCH BETTER THAN YOUR ADVERTISING CLAIMED" These words from a user's letter are just a sample of nation-wide favor. Wherever your favorite circuit calls for a variometer, Coto Compact Moulded Variometer^ with the honeycomb wound stator coils is your first choice. Also available for portable sets because of small size. If your dealer fails you write us, giving his name and address and list of Coto parts you need. C0T0-C0IL CO 87 W1LLARD AVE. PROVIDENCE, R. I. BRANCH OFFICES: Los Angeles. S29 Union League Bide; Minneapolis. Geo. V. Darling, 705 Plymouth BldR. : Atlanta. C. r. Atkinson, Atlanta Tr. Co. Bldg. Canada. Perkins Klcc. Co., Ltd., Montreal. Toronto, Winnipeg.