Radio Broadcast (May 1928-Apr 1929)

118 RADIO BROADCAST DECEMBER, 1928 and condensers are identical. The lower the resistance of the coils, the greater the amplification, but the side-band clipping depends upon the coupling between the coils which can be adjusted mechanically. Theory states, and if you care to look it up you will find it in Pierce's Electrical Oscillations and Electrical Waves, pages 73 to 85, or Morecroft's Principles of Radio Communication, pages 119 to 136, that when two such circuits are tuned to the same frequency, and coupled electrically to a sufficient degree, the circuit no longer responds to the frequency to which the individual circuits were tuned but to two new frequencies which are displaced from the singlecircuit frequency a certain amount, depending upon the coupling. In Fig. 4 may be seen two extremes of coupling. In one case (A) the coupling is very loose, so that a single sharp peak shows up, and with this adjustment little energy is transfered from primary to secondary. In the other case (B) the coupling is too close. Two peaks arise with a sharp dip between. Somewhere between these two degrees of coupling is found the type of curve we want, broad at the top and steep at the sides. COUPLING ADJUSTMENT THE coupling, then, is the important thing, and fortunately it can be controlled and adjusted until the desired type of response curve results. So much for the theory of the Master "Hi-Q 29" receiver, It employs a band-pass filter type of r.f. amplifier using screen-grid tubes (a.c. or d.c.) which selects and amplifies but which clips side bands far less than the conventional lowloss coil transformer of the type used in nearly all present receivers. The detector of the receiver is conventional — a grid leak and condenser type. The audio amplifier is composed of two stages coupled with high-grade audio transformers. The last tube is a 171 type, and, as may be seen in the picture, sufficient room is allowed on the chassis for mounting an output device, or if desired a push-pull amplifier. This chassis is made of steel, plated with cadmium which prevents rust, and has almost the same dimensions as last year's model. The individual stage coils and condenser are housed in aluminum boxes. The tubes themselves are between the shields and shielded leads make the connections from the control grid to the apparatus within the boxes The volume control governs the voltage on the screen grids. The antenna stage is tuned separately from the other circuits, 100 % 90 80 70 60 50 40 30 20 10 3 t — 1 to / \ / / \ \ / \ \ 1 n \ y\ / / V \ y f J Vv It 20 15 10 5 0 + 5 10 15 20 25 30 KC OFF RESONANCE FIG. I and an additional tapped arrangement on the coil permits some range of selectivity control in the antenna stage. Careful shielding is necessary in this circuit where the primary windings of the radio-frequency transformers are tuned. Any feedback coupling would introduce serious difficulties. For this reason the stage shields used are tight fitting, and the wires which connect coils to the tubes are enclosed in screening which is supplied as part of the Hi-Q kit. So much for the electrical and mechanical properties of the new receiver. Complete data on how to build, adjust, and operate the set are obtainable from Radio Broadcast or from the Hammarlund-Roberts organization. The list of parts on the next page is the list specified by the manufacturer. The coils and mountings are special, and difficulty would be had in at tempting their construction. Their essential dimensions are given in the complete circuit diagram, however, so that if the constructor desires he may try his hand at it What does the receiver do on the air? Just as an automobile manufacturer sends his product out over the road to see how long it will run without falling apart, how fast it can go, or other tests which the user probably never will desire to make himself, so must the radio set manufacturer make his receiver go through a "road test." The following is a report of such a test made for the Hi-Q organization. In a small town on Long Island, about 8 miles from weaf, 30 miles from wjz and 10 miles from wabc we set up the model using Arcturus a.c. screen-grid tubes in the r.f. circuits, Arcturus type 46 and 48-tubes in the detector and first a f. circuits, and a Cunningham type 371 a tube in the power a.f. stage. The antenna was used ordinarily for a 40-meter amateur transmitting station and was about 60-feet long with the lead brought in from the middle. The two ends were about 45 feet above ground, and the set was operated in a second-story room. The set was not grounded — this was accidental, not intentional— and the selectivity and sensitivity might have been improved if proper grounding had been looked after. The night was October 13, the beginning of the winter season of heterodyne notes and ether jamming. The stations whose calls were identified definitely came in with a more than ample volume for a large Peerless dynamic speaker in a threefoot baffleboard. wls and wod in Chicago were very strong, wip in Philadelphia was very loud — ordinarily he is difficult to hear on Long Island — and the old stand-by's, kdka and wgy were roaring in. wham at Rochester was easy to get, so were wbz, wtam, and one station between weaf and wjz was separated easily from these REAR VIEW OF HI-O AND POWER UNIT FIG. 4 two near-by stations. It was probably wrc in Washington. All in all, the writer had an enjoyable evening and predicts much fun for the owner of such a receiver. other models THERE are two models of the Master Hi-Q receiver. One is designed for d.c. tubes, and thecircuitdiagram for it was published in October Radio Broadcast, page 343. The other is for a.c. tubes and the diagram is published herewith. This receiver uses Arcturus screen-grid tubes which require a 15-volt filament supply. The Thordarson power-supply equipment illustrated in Fig. 5 supplies this voltage as well as the other filament plate and grid voltages for the operation of the entire receiver. The foundation unit supplied by the Hi-Q organization includes the resistors R6, Rr, and R8 which are the center-tapped 50-ohm units for the first tube. And small fixed resistors used to filter the screen-grid circuits. The picture of the receiver which appears on the next page shows several interesting features of its mechanical construction. One of the sides of the middle shield has been removed to show how