Radio Broadcast (Nov 1926-Apr 1927)

DECEMBER, 1926 NOTES ON THE R. B. "LAB" RECEIVER 179 binations of several of the new Eveready batteries may also be used to obtain the desired C voltage. It is possible also to obtain this 405-volt potential by bucking a regular 4§-volt C battery with a light duty fig. 4 The connection of a milliammeter in the plate lead of the output tube provides a visible indication as to whether the amplifier is satisfactorily handling the signals or not. Overloading will make itself manifest by a badly fluctuating needle on the meter 45-volt B battery. That is, the positive terminal of the 4^-volt battery connects to the positive terminal of the 45-volt battery. The negative terminal of the large battery connects to the F minus post of the last audio transformer, and the minus terminal of the smaller C battery connects to the minus A lead of the A battery. Fig. 2 illustrates these connections, while Fig. 3 shows how to obtain an approximately correct value by the use of two 22^-voIt batteries. Voltage recommendations are always approximate and, for best results, the home builder should experiment with various voltages. If a milliammeter is obtainable (0-50) it should be connected in the plus B lead to the loud speaker unit. See Fig. 4. Then, when reception is taking place, any distortion which may be present in the receiver will manifest itself in an unsteady, fluctuating reading of the milliammeter. The B and C battery voltages should be adjusted until a minimum fluctuation of the needle from a normal current reading takes place. This normal reading will approximate about 25 mils. It must be remembered, however, that a certain amount of fluctuation can take place before distortion is noticeable. In other words, the fluctuation should be as small as possible, at most not over five milliamperes in twenty-five. ANTENNAS \A7TTH a high, long antenna it is not * " to be expected that the selectivity of this receiver will be as marked as when a shorter antenna is used. The ideal antenna is one of about 75 feet in overall length. Where a longer antenna is used, a fixed condenser of about 0.0001 mfd., inserted in series with the antenna and coil, will aid in sharpening the tuning. Naturally too, a long antenna will prevent the dials of the receiver reading similarly for a given station. It is not to be expected that the receiver, when built according to the constructional specifications outlined in the November Radio Broadcast, will immediately fit in with any and all conditions. The R. B. "Lab" receiver was built to meet average conditions, and when it is used under adverse conditions, such as are imposed by the use of an extremely long antenna, some means must be resorted to in counteracting these unfortunate circumstances. The method employing the fixed condenser, as explained above, is one form of remedy. A variable condenser can be used if desired, and its connections are shown in Fig. 5. To do the job completely, though, the antenna circuit might best be tuned to resonance with the incoming signal, rendering the receiver exceptionally sharp and obtaining a maximum transfer of signal energy from the 100 mmfd. FIG. 5 A condenser (it may be variable) will help to increase the selectivity when a long antenna is used antenna to the radio-frequency amplifier circuit; even with a small antenna this additional control is worth while from the standpoint of selectivity and volume. The circuit diagram, Fig. 6, shows the additional tuning unit. It consists of an inductance and variable condenser. Ci is a 0.0005-mfd. variable condenser and the inductance Li, may consist of about 80 turns of No. 24 d. c. c. wire wound on a \\" form. This coil should be placed at right angles to the antenna secondary coil. The coupling coil, P, may be the existing primary coil in the set. Worth while information relative to the tuning of antenna systems was contained in an article by Harold Jollife on page 84 of the November Radio Broadcast. It is recommended that this article be closely studied. Mr. R. S. Danforth, of San Francisco, California, has written a very interesting and informative letter which is here re printed in part for experimenters and constructors who have built the R. B. "Lab" circuit. I have been playing with the so-called Hull or Radio Broadcast "Lab" circuit since October, 1925, with very encouraging results. The action of this set was much improved by using an aperiodic antenna primary coil separated about \" from the secondary, but I found the response and selectivity was greatly improved by using a separate loosely coupled series tuned antenna coil. This antenna coil was about 3" away from the coil ahead of the radiofrequency amplifier tube, and at a slight angle to it, but at the same time kept at right angles to the detector tube coil. I used the Rice System of neutralization and had no trouble in neutralizing the radio frequency tube. As an audio amplifier, I am using an Amertran De Luxe first stage transformer with two stages of resistance coupling. I employ an ux-200-A detector tube and 201-A amplifier tubes (excepting in the last stage, where I employ a 171) and find this audio combination superior to using all resistances or two transformers. The radio-frequency amplifier tube is a 199. As regards the efficiency of this circuit, let me state that my present location is very poor yet I can bring in stations that the ordinary set will not get at all, and these with sufficient volume to overload the 171 power tube. My R. B. "Lab" Receiver is superior in sensitivity to any set which I have ever heard. The use of the loosely coupled tuned antenna coil, while adding one additional control, shows a very marked increase in volume as well as giving real selectivity. Our readers are urged to communicate with the author, or with Mr. Keith Henney, director of Radio Broadcast Laboratory, relative to their experiences with this circuit. From time to time the data collected in experimenting with this interesting circuit will be passed along to our readers. FIG. 6 A better way of increasing the selectivity is to tune the antenna primary circuit. A loading coil and condenser are necessary