Radio Broadcast (May 1928-Apr 1929)

JULY, 1928 A SHORT-WAVE ADAPTER FOR THE R. B. LAB RECEIVER 155 THE RECEIVER FOR WHICH THE ADAPTER WAS DESIGNED The a. c. model of the R. B. Lab. receiver, for which the short-wave adapter described in this article was especially designed, is shown above. This receiver was described in the May number of Radio Broadcast. As explained in the article, the adapter may be used with any other standard a.c. receiver. battery connections 01 the receiver detector, the Amperite will no longer be needed and may be shorted out or left out of the assembly. Where only broadcasting is to be received and the set is not used in an oscillating condition it is possible to use a 227 type tube. In this case a 5prong socket is necessary and four flexible leads may be run to the receiver. Two should go from the H pins of the plug to the filament binding posts, one from the P to the B plus and another from the K to the blank binding post. The K terminal of the short-wave tube should then be grounded and the return to the receiver secured through the common ground. A 227 type amperite should be used. For intermittent operation, 8 dry cells connected in series-parallel will give very satisfactory results with the 112A tube. Where, because of the layout or for any other reason, there is poor regeneration on the very high frequencies, a 10,000ohm resistor (a Durham grid leak for example) should be connected in series with the choke. This increases the impedance of this circuit at these high frequencies without introducing sufficient resistance in the circuit to lower the plate voltage appreciably. Increasing the detector plate voltage to 67.5 will result in better oscillation too. It may be thought that omitting the choke coil in the detector plate lead and substituting a fixed resistance — such as is often done in amateur code receivers — would result in as good a receiver at a reduced cost. This is true provided broadcasting is not to be received, but the 10,000-or 25,000-ohm resistance, which is often used, offers appreciable impedance to low voice frequencies in comparsion to the impedance of the audio amplifier looked .at from the detector tube. This would naturally result in poor low-frequency reproduction. The choke offers sufficient impedance at all except the very high frequencies for good regeneration and has no effect on quality of reproduction. PARTS EMPLOYED IN THE parts list which follows, the only * special parts are the coils. [For best results, the coil and the condenser used with it should be FIG. 3 In many amateur's receivers the plate choke is connected on the plate side of the regeneration coil. Better oscillation control is obtained by placing it at the ground end of the coil, as this diagram shows carefully chosen for in the high-frequency channels, slight differences in coil turns and condenser size — differences from those suggested here, that is — greatly affect the tuning range. In Radio Broadcast, June, 1928, page 78, appeared data on some short-wave coils which can be home-wound. See also, pages 13-14 of our May, 1928, issue for details of other home-made coils. — Editor]. Any other parts from well-known manufacturers may be used, provided their respective electrical and mechanical characteristics are similar to those given in the list. Li 1 Set (3) Hammarlund Short-Wave Coils. L2 1 No. 85 Hammarlund RadioFrequency Choke Ci 1 .00014-Mfd. Hammarlund ML-i Condenser C2 1 .0001-Mfd. Hammarlund MC-23 Condenser C3 1 Neutralizing Condenser — Hammarlund or Sangamo .000050Mfd. Fixed Condenser C4 1 0.5-Mfd. Parvolt Series A Condenser Ri 1 io-Megohm Durham Grid Leak R2 1 Amperite Type iA 1 Benjamin Socket 1 Durham Grid-Leak Mount 6 Eby Binding Posts 1 National Velvet Vernier Dial 1 7 x 12 x T3g Westinghouse Micarta Panel 1 10,000 or 25,000-ohm resistor (optional) ACCESSORIES 1 CX-312A tube Six-volt filament supply: storage battery or 8 dry cells.