Radio Broadcast (May 1928-Apr 1929)

"Z_ RADIO BROADCAST JET Back in April 19. 28/ CeCo Announced This Type AC-22 Screen Grid Tube Five prong tube of the separate heater type operating directly on alternating current. — now recognized as the most outstandingly successful amplifying tube of the season. CeCo pioneered — and did its pioneering without the fanfare of trumpets. But it is pleasing to know that an increasing number of radio engineers and experts look with confidence to the CeCo laboratories for each new development in the tube industry. . . a reward not measured in dollars and profits. Do not miss CeCo's entertaining radio broadcast each Monday evening at 8:30 Eastern time (7:30 Central time) over the Columbia ^Broadcasting System. CeCo Mfg. Co.jItlC, Providence, R. I. Radio Tuber No. 275 Radio Broadcast Laboratory Information Sheet Obtaining Grid Bias from B-Power Units April, 1929 T>OTH B and C potentials for a 171a tube may *J be obtained readily from a simple B-power unit without adding any resistors or condensers to the circuit; it is simply necessary to change a few connections. Sketch a on the sheet shows an ordinary B-power unit and the connections which would be made to it if it were to supply only plate potential to the 171a power tube. Sketch b shows the connections if it is to supply C potential as well. In these diagrams it should be noted that the connection of negative filament from the regular B-minus terminal on the power unit has been changed to the + 45-volt terminal. With this ar B + Power OB+135 o B + 90 o B+45 o B o To Plate »» of 171-A To FN. B+ Power B + 135 B+90 B+45 B (A) (B) rangement the + 45-volt terminal then becomes B minus, the plus 90 volt terminal becomes plus 45 and so on, each terminal supplying 45 volts less than it is marked. The regular fi-minus terminal is now 45 volts lower in potential than the new Bminus terminal, and, therefore, from the regular B-minus terminal we are able to secure a negative potential of 45 volts which we can apply to the grid of the 17lA-type tube. In this way we have, by a simple circuit change, made it possible to obtain C bias for the power tube. Forty-five volts is slightly higher than normal, but not sufficiently so to affect seriously the output from the tube. This slightly higher than normal bias will help to lengthen the life of the tube. To Plate The 'arrangement described of above can be applied only to 171-A those B-power units capable of supplying under load a maximum of about 225 volts. This much voltage is necessary because 180 volts are required on _ p. . the plate and 45 volts are used to 1 0 r II. supply C bias. With this arrangement 45 volts are obtained from C~45 the tap that normally supplied 90 volts and 90 volts are obtained from the tap that ordinarily supplied 135 volts. No. 276 Radio Broadcast Laboratory Information Sheet Simple Two-Way Telephone Set April, 1929 TN THE country friends who live some miles apart often wish to establish a telephone communication channel without the expense of installing a regular pole line. Lieutenant W. H. Wenstrom, U. S. A., suggests the following simple method of building such a communication system. As can be seen from the diagram given in Laboratory Sheet No. 277 the set is simplicity itself. It is essentially a radio receiving tube (a 199-type tube is satisfactory) provided with input and output transformers. The best ratios are somewhere around 1:6 for the input and 3:1 for the output. The microphone may be an old, discarded telephone "mike." Two sets are, of course, required for one communication channel. Due to the economy of apparatus, radiophone practice must be used in operation. A definite time for communication is arranged in advance. At this time "A" calls and "B" listens. When "A" finishes calling, both operators throw their switches, and "B" then answers "A." The procedure might be compared to two unusual people carrying on a conversation where each one politely waits until the other has finished before he himself begins. Because two stages of audio-frequency amplification are used in the talking circuit, one at the sending end and one at the receiving end, the connecting wire line may be very much poorer than a standard telephone line. Continuous fence wire, or any medium-resistance metal circuit fairly well insulated from the ground may serve as one conductor, with the ground as the other conductor. Paris Required 1 Audio transformer, 6:l-ratio 1 Audio transformer, 3:l-ratio. (connected reversed) 1 Four-pole double-throw switch 1 Microphone, telephone-type 1 Pair of headphones 1 199-type tube and socket A battery, fil. ballast and fil. switch B battery, 45 volts The four-pole double-throw switch changes the set from "send" to "receive." It saves expense by permitting the use of the same tube and transformers for both operations. No. 277 Radio Broadcast Laboratory Information Sheet Simple Two-Way Telephone Set April, 1929 • april, 1929 . . . page 414 •