Radio Broadcast (May 1928-Apr 1929)

98 RADIO BROADCAST JUNE, 1928 The rheostat is used to turn on and off and to control the current through the amplifier filament. It should never be turned on further than is necessary to bring in the signals at proper volume. An experiment will show that turning it beyond this point does not increase signal strength. As a matter of fact such a procedure only decreases the life of the batteries and the tube. One 45-volt B-battery block and three dry cells will last several months with such a simple receiver. After constructing such a receiver there is the possibility of adding another stage of audio amplification for loud speaker signals, and the Laboratory will be pleased to supply information on how to do this to those who write. There is also the possibility of adding a stage of radiofrequency amplification to such a receiver, FIG. I How to place and connect the parts employed SIMPLICITY ITSELF The panel is 7" X 12" and can be fitted into any cabinet which suits the owner. The Remler dial noted in the parts list was not available when this photograph was taken thereby making it much more selective and sensitive and as a result to increase its distancegetting ability. In fact, a three-tube set using the Carborundum unit as a detector provides the listener with a high quality, reasonably selective receiver for reception. from stations within several hundred miles. The Laboratory has received a number of letters from readers who have done very creditable DX work on such an outfit. How such a receiver as is described here is used in Radio Broadcast Laboratory may be of interest. At times considerable noise is picked up in the Laboratory from the presses, which print Radio Broadcast and many other magazines, making the testing of receiving equipment impossible. Down at the shack, a distance of about 1000 feet from the Laboratory where 2 gy is located the "air" is quiet. The receiver illustrated here is installed there and permanently tuned to weaf. The output from the detector is sufficiently high that it can be placed on a wire line coming to the Laboratory where it is fed into a two-stage amplifier and thence to a loud speaker. When necessary, the first-stage amplifier on the base board with the detector is thrown into the circuit by means of an extra line and the output from it is put on a third pair of wires. This output may be used in connection with a single power stage or loud speaker operation. At all times, when near-by broadcasting stations are on the air we have good signals available in the Laboratory for testing purposes. THE LIST OF PARTS The experimenter who already has odds and ends of radio apparatus in his possession can probably assemble the receiver described here without the purchase of much additional apparatus. The parts listed below were used in the receiver described, although of course any others, electrically and mechanically similar may be selected. Ci — Remler 0.0005 mfd. type No. 639 condenser Remler standard dial L> — 52 turns No. 24 s.c.c. copper wire wound on 3" form Li — 10-20 turns No. 28 q.s.c. copper wire Ti — General Radio No. 285 audio transformer Ri — Frost 30-ohm rheostat Carborundum Company detector unit Frost UX type socket Fahenstock spring clips (8) One — ux-199 vacuum tube One — Westinghouse Micarta panel One — Wood baseboard 3 — -dry cells One — set Trimm headphones Phones or Power Amplifier Three dry ' cells To one dry cell fig. 2 Circuit diagram of the crystal set and one stage of audio amplification. The insert shows how the detector bias may be obtained from the voltage drop in the rheostat