Radio broadcast .. (1922-30)

How to Construct a Two An Amplifier — Detector Unit for Use in Con- junction with a Separate Audio-Frequency Amplifier—A Loading Coil Permits Qreater Transfer of Energy — Complete Shielding is Easy of Accomplishment and Aids Materially the Selectivity in Congested Districts By JOHN B. BRENNAN Technical Editor rHEN radio broadcasting made its bow, some six or seven years ago, the types of receivers then in vogue closely fol- lowed the orthodox designs of the day—queer to us now in comparison to the fine pieces of radio furniture which may be purchased to-day. He considered himself high and mighty who boasted of a little box housing a tuner circuit plus another box on whose front dangled the precious audiotron with its double filament. Those were the days when the neighborhood hardware store was sorely pressed to supply the demand for flashlight cells with which the high and mighty one built up his B-battery. The art has progressed. But a strange coin- cidence is the fact that, to-day, designers of radio receivers are swinging around the circle, so to speak, and the more important receiver develop- ments are crowding closely the styles of yester- year. Then it was that receivers were built up on the unit principle. That is, first one would ob- tain the tuner coil box and connect to it the crystal detector and phones. Graduating from this stage on to the next the experimenter would follow with the annexation of a vacuum tube de- tector and unit amplifier of some sort. To-day that system—to be sure, in slightly altered form—is coming into its own again, and rightly so, for it has many points in its favor. Much has been said about the ability of the tuner coils and the wiring of a receiver to pick up unwanted signals without the aid of the an- tenna. To many this has manifested itself as broadness of tuning and lack of sensitivity. To overcome this un- desirable property, the tuner unit must incorporate some means for excluding the unwanted signals or at least to make all the signals enter the receiver through the antenna. Building the tuner unit in a metal cabinet, as described in this paper, very definitely overcomes this difficulty. In selecting a circuit suitable for the tuner unit, the R. B. "Lab" circuit has been chosen, since it has all the meritorious qualities of a desirable circuit. In no way does this supersede the R. B. "Lab" circuit receiver which was fully described in the November, 1926, RADIO BROADCAST. That was a complete receiver in itself, audio channel RADIO BROADCAST Photograph THE COMPLETE RECEIVER INSTALLATION and all, and the descrip- tion in that issue re- sulted from the natural desire on the part of experimenters of the "Lab" circuit to have before them a suitable receiver design embody- ing the fine points of the circuit. Unquestionably, to shield the coils within a receiver is highly beneficial, providing it is done correctly, and because of these beneficial qual- ities, the receiver described here has been shielded practically completely. Slight variations in the R. B. "Lab" circuit, as outlined in the November, 1926, issue, have been made in the circuit used here, to suit the mechanical and electrical requirements set for this model. To obtain maximum transfer of energy from the antenna to the receiver, involves the use of some sort of antenna tuning device. The an- tenna loading coil employed in the tuner unit described here furnishes exceptionally well the means to this end. Four taps have been provided in this coil so that the circuit is resonant at approximately 1200, 857, 666, and 600 kc. (250, 350, 450 and 500 meters) successively as the switch arm is turned from left to right. The value of this load- ing coil is most apparent when it is desired to •The Facts About This Receiver— J^ame of Receiver R. B. "Lab" Two-Tube Receiver Type of Circuit R. B. "Lab" Circuit; one stage tuned neutralized radio frequency amplifi- cation plus regenerative detector. Number o/Tubes Two;r.f.,2oi-Atype;detector,special detector such as aoo-A, or ordinary aoi-A tube. Frequency Range 1500-545 kc. (200 to 550 meters). Travel Features Tapped antenna loading coil; C bat- tery detector; complete metal hous- ing. The receiver described here is essentially a tuner unit in which is incorporated an r.f. and a detector tube. To it must be con- nected some sort of audio amplifying device of quality to repro- duce with fidelity the sounds entering the microphone of the transmitter. The tuner unit has been intentionally housed in a metal cabinet which affords the advantage of shielding the cir- cuit wiring from undesirable signal pickup. Consisting of the two-tube tuner unit, power audio am- plifier, loud speaker, trickle charger A battery, and auto- matic control, is shown housed in a Corbett cabinet shift tuning from a short-wave station to one operating on a longer wavelength. Shielding makes possible the use of a type of coil such as a solenoid, in the radio frequency stage input circuit which is more efficient than one of the confined electromagnetic field type, and, further this shielding aids in preventing the wiring of the circuit itself in picking up unwanted signals. A separate audio channel for this two-tube unit may employ the same parts as the receiver described in November, and doubtless this point will find favor with those who already have that material on hand or for those constructors of the November R. B. "Lab" circuit who wish to convert their receiver into the two-tube tuner circuit with its separate amplifier. The amplifier itself presents no very great problem of construction and, when completed, may be stowed away in some remote part of the console cabinet or perhaps in the basement with the storage A battery and its attendant charger, etc. Elsewhere in this maga- zine is described a combined audio power amplifier and plate power supply device which is very well adapted for use with the tuner unit described here. THE ALUMINUM SHEET FOR SHIELDING BEFORE collecting the various parts necessary for the construction of the tuner unit, it is well for the pros- pective builder to read over the descrip- tion thoroughly and acquaint himself with the various sketches and con- struction lay-outs. In this same con- nection, the reader should not fail to review what has been published on the "Lab" circuit in the June, September, and November, 1926, issues of RADIO BROADCAST. This will serve to familiar-