Radio Broadcast (May 1928-Apr 1929)

250 RADIO BROADCAST SEPTEMBER, 1928 and who, 10 kilocycles away from local kyw, were received consistently on the loud speaker without interference from the neighboring local channel. Such stations as wgy, woe, and kdka could often be heard in the daytime and frequently with only a four-foot antenna at night. The set was then taken home and tested in a wooden residence. Even better results were obtained, and instead of only a comparatively few powerful out-of-town stations being heard, little 50-watt stations all over the country came in on the loud speaker, and reception of West Coast stations became the author's early evening amusement. The receiver shown in Fig. 3 has eight tubes placed in a straight line ranging from left to right in this order: UX-201A oscillator, ux-222 first detector, three ux-222 screen-grid intermediate amplifiers, UX-201A second detector, UX-201A first audio tube, and UX-171A power tube. Behind the tubes are the transformers; at the left is the special oscillator, Li, and to the right the four intermediate transformers, Ti, T), T3, and T4, with their tuning knobs projecting from their tops. At the right end of the subbase are the two audio transformers, T5 and T6, which are built following the specifications laid down by Mr. Kendall Clough, Director of the Research Laboratories of Chicago, in the July issue of Radio Broadcast. At the left front of the chassis is the large antenna coupling coil, L2, consisting of 80 turns of No. 16 enameled wire on a form 5" long and i\ in diameter. The losses of this coil are so low that extremely sharp antenna tuning is had without the necessity of regeneration, which it is almost impossible to add to a screen-grid first detector with any benefit, as the writer's investigations have revealed. At the front center of the chassis are the two tuning condensers with their drum tuning dials, the controls of which are brought out to a small compact bronze panel which also holds a filament rheostat and on-off switch, and a potentiometer for controlling the sensitivity of the intermediate amplifier and, consequently, the volume of the receiver. The curve in Fig. 1 shows the over-all r.f. amplification of the 3-stage intermediate amplifier (4 tuned circuits). This curve indicates just why the set is as selective and sensitive as it is. The intermediate amplifying transformers are very interesting, inasmuch as the design finally evolved ran contrary to ordinary engineering theory in that the transformers themselves actually had a step-down turn ratio! This was worked out very carefully in an effort to gain the 390 400 410 420 FREQUENCY, R.C. 430 FIG. I required selectivity and is justified by the theory of the screen-grid tube used as an r.f. amplifier. The plate impedance of this tube is very high, running up into the hundreds of thousands of ohms, and it is almost an impossibility to build an r.f. transformer of the conventional type with a high enough primary impedance to obtain any great amplification. The writer therefore set out to use a tuned primary and used for this purpose a bakelite tube 2" long and 1 5" in diameter wound with 300 turns of No. 37 enamelled wire. This primary was tuned by a 75-mmfd. midget condenser, and the circuit provided showed such a high value of impedance at resonance as to obtain a far higher voltage drop across its terminals, when connected to a screen-grid tube plate circuit, than did ordinary types of i.f. transformers. If the grid of the succeeding i.f. tube had been connected directly to the plate end of this primary, with, of course, the necessary grid condenser and leak, the selectivity of the amplifier would not have been very good, and it was here that the idea of a step-down transformer worked out so very well. A small secondary upon a \\" tube, consisting of 150 turns of No. 37 enameled wire, was slipped inside the primary and connected to the grid circuit of the next r.f. amplifier tube. The result was a circuit of high amplification with sufficient selectivity so that four such transformers provided positive 10-kilo cycle selectivity. By using a very large primary and a smaller secondary, tremendous amplification can be had from the tube, a little of which is then sacrificed in the transformers to gain selectivity; this is an unusual but very practical theory, judging from the results obtained. "one-spot" tuning TTHE intermediate transformers are tunable 1 by means of small knobs on top of the copper transformer shields, which may be constructed at home of 0.014" copper and which are 2j" wide, 35" high and 2J" across. The transformers are placed upright inside these cans with the tuning condensers mounted on the top and insulated from the can by insulating washers. As each transformer is individually tuned after assembly of the whole receiver, no matching is necessary and the writer's set can be duplicated without any fear of poor results due to lack of proper matching equipment, which is totally unnecessary with this type of intermediate amplifier. The wavelength at which the transformers may be operated can be varied by the tuning condenser knob, and covers the range of 600 to 1000 meters, or 500 to 300 kilocycles. There are two distinct advantages to be had in using this frequency range for intermediate amplification in a super-heterodyne. The first is that the repeat points at which a station may be tuned-in on the oscillator dial are so far separated as to render the set practically "onespot" in operation. Anyone who has ever operated a super-heterodyne will appreciate that the primary drawback of the super-heterodyne to-day is that the effectiveness of the oscillator dial is at least halved because each station is heard at two points upon the dial, instead of at only one, as in t. r. f. sets. This is due to the fact that with a 50-kilocycle intermediate amplifier, for example, the sum and difference settings of the oscillator will bring in a given station. The method of eliminating this is to raise the intermediate frequency until one set (lower) of oscillator dial heterodyne points are thrown so far away from the other set (upper) that the lower repeat points fall beyond the used tuning range. Just how this works out can be illustrated by considering the 400-kilocycle i. f. amplifier used in the author's super to cover the 200 to 550-meter band (1500 to 550 kilocycles, approximately). The oscillator settings for both extremes must be the highest and lowest signal frequency to be received plus and minus the i.f. Thus, for a 200-meter signal ( 1 500 kilocycles) the oscillator must be tuned to 1900 or to 1 100 BPl-Ground-9 BP 2 -Ant.Long FIG. 2. THE CIRCUIT OF THE LINCOLN SUPER-HETERODYNE