Radio broadcast .. (1922-30)

A Four-Tube Further Improvements for the Four- Tube "Lab" Receiver —Putting in New Coils, Including a Loading Coil —Over- loading of the Detector Tube Is Pre- vented by Using C-Battery Detection —A Different Volume Control System RADIO BROADCAST Photograph IN ITS CABINET The R. B. four-tube "Lab" receiver equals in appearance, a factory-made job By JOHN B, BRENNAN ATER the completion of the construction of the four-tube R. B. "Lab" receiver, which was originally described in the November, 1926, RADIO BROADCAST, the Labora- tory Staff felt that for a time it could sit back and rest on its laurels with the satisfaction that comes in doing a job well. However, this rest- ing period was short-lived, due mainly to the many excellent suggestions received from the readers of this magazine. The two-tube tuner unit, practically a new departure in modern receiver design, was then presented to RADIO BROADCAST readers, in the January issue. This two-tube tuner unit incorporated many novel f™""" features of design which it was thought j were worth while to apply to the previ- ous four-tube receiver. And, too, dur- ing that time which elapsed between the November, 1926, and January, 1927, issues, new material made its appear- ance, all of which gave rise to the pertinent query "Can the R. B. 'Lab' four-tube receiver be improved?" Many of the improvements contained in the construction of the two-tube R. B. " Lab" receiver can be made in the four- tube model without much difficulty. One of the most important of these improvements is the substitution of dif- ferent coils for the two binocular ones originally employed. Then too, the use of an antenna loading coil permits individual builders to adjust the an- tenna circuit of their receiver for maxi- mum signal pick-up, making it possible to utilize the antenna at its utmost efficiency. Hand in hand with the problem of providing the very best volume control is that of supplying a detector circuit capable of handling all the signal en- ergy that can be supplied by a radio- frequency amplifier operating at its peak of efficiency. It was observed, in the operation of the first four-tube R. B. "Lab" receiver, that some distortion took place even though the volume output was reduced by means of the Technical Editor volume control, because the detector had be- come overloaded. The first problem then was to employ a detecting circuit which did not tend to overload, even on very strong signals. The solution lies in the use of the C-battery method of detection, as shown in B, Fig. i. While not as sensitive as the grid leak and condenser method, this C-battery system has virtues which outweigh the point against it of poor sensitivity. The problem of providing a suitable volume control has been solved by placing this volume adjustment in the radio frequency amplifier cir- -The Facts About the Receiver™ Name of Receiver Four-tube R. B. "Lab" Receiver. Type of Circuit One stage tuned neutralized radio fre- quency amplification, regenerative de- tector, and two stages of transformer- coupled audio frequency amplification, followed by an output device. Number and Kind Four; OOI-A'S for r.f. stage, detector, of Tubes and first audio stage; ux-iyi in last audio stage. Volume Control A 5oo,ooo-ohm variable resistance in series with B battery lead to plate of r.f. tube. Regeneration Condenser feedback. Neutralization Rice. Tube Voltages R. F. and ist a.f., 90 volts; Detector, 45 volts; last a.f., 135 volts for 112 tube or 180 volts for 171 tube. Fila- ment voltage, 5 volts. C battery vol- tage, r.f. 45 volts; detector, 45 volts; ist a.f. 4j volts; last a.f., 9 volts for 112 with 13; B volts, or 40.5 volts for 171 with 180 B volts. Essentially this four-tube "Lab" Receiver is the same as that described in the November issue. Briefly, the circuit changes consist mainly in the insertion of a loading coil in the antenna circuit, the substitution of a solenoid coil for the binocular in the radio frequency amplifier stage, and the use of C battery detection instead of the grid leak and condenser method. The mechanical changes involve the substitution of a grained aluminum panel for the former insulated one, the addition of an interstage shield, and a tap switch. cuit—proper enough because, in this position, strong local signals can be reduced to the desired volume and of course no overloading will take place in the detector circuit. In the four-tube set described in the November issue, the volume control consisted of a 0-500,000- ohm variable resistance unit shunted across the secondary of the first audio transformer. In the model described here, the variable resistance has been re-located in series with the plus-B plate lead going to the r.f. amplifier. Instead of being a 5OO,ooo-ohm potentiometer type of volume control having three terminals, this new volume control is the Electrad Royalty, Type L, having a similar resistance range, but it is of the two terminal type. The two volume-control circuits, that formerly used and the one used at present, are shown in A and B respec- tively, Fig. 2. As regards the antenna loading coil, it was found, by actual test, that as great as a 4 to I increase in signal strength was obtained when the approx- imately correct inductance was included in the antenna circuit at the particular wavelength to which the receiver was tuned. The loading coil was then pro- vided with taps so that, on the first tap enough inductance was inserted in series with the primary so as to make the circuit function satisfactorily at 250 meters, (1200 kc.), the second at 350 meters (857 kc.), the third at 450 meters (666 kc.), and the fourth at 500 meters (600 kc.). For the individual experi- menter, the placement of these taps to coincide in resonance with these figures is, of course, approximate, since every- one's antenna is not exactly the same. However, not a great difference will re- sult as the loading coil taps are arranged to work with what is considered an aver- age antenna. The complete winding specifications for the new coils are given in Fig. 3. Where a solenoid coil is used in a tuning circuit it will have an elec- tromagnetic field which, if not taken :