Radio Broadcast (May 1928-Apr 1929)

We have already f(ew Crystal mentioned the ex Control treme accuracy to which broadcasting stations must be kept on their required frequencies if there is a general exodus of stations down to the short waves — or into the higher frequencies. Now we learn from the March, 1928, Bell Laboratories Record that crystal control apparatus has been developed for use with television equipment which is capable of holding oscillators to a constant frequency to within one part in ten million. This is equivalent to one cycle in 10,000 kc. (30 meters). At present, broadcasters seem to have difficulty in holding to 500 cycles at 1000 kc. This shows that such accuracy of adjustment and maintenance can be obtained. The objection, from the broadcasters of course, is that such equipment costs a great deal of money. We do not believe this is a valid objection. There is a great deal of money invested in radio receiving equipment at the present time — much of which is wasted because of present conditions in the ether. It costs a lot of money to wage war, and many years afterward to pay for it, but at the time nearly everyone gets highly enthusiastic about it. When controlled by these crystals, a driving motor of a. television receiver would require a week to get out of step with a transmitting motor by as much as one revolution. This eliminates the tendency of a television image to slowly float off the screen. The following prices Prices of British taken from a recent issue of "Components" the British publication Wire less World and Radio Review are indicative of what the Britisher has to pay for his radio apparatus: Amplion Cone Speaker . . . . $ 7.00 Siemens B batteries, power type . . 3.00 Weston Set Tester 75. 00 Cosmos High-mu tube, [J.-35, RP 50,000 2.00 Cosmos power tube [J.-6.5, Rp-4500 ohms 2 . 50 Brown phonograph pick-up .... 20.00 Carborundum resistance-capacity unit 1 .75 Ferranti output transformer 1— 1 . 6.50 Dubilier phonograph pick-up . . . 7.00 Marconiphone power supply unit . . 37-00 Magnavox dynamic speaker unit. . 47-00 Mullard audio transformer, high quality . 5.00 Ormond five-tube portable receiver . 122.00 Benjamin sockets .50 General Radio speaker filter . . . 6.00 In this issue are no less than twelve advertisements of moving coil loud speakers, represented in this country by the Magnavox, the R.C.A. 104, and the Jensen, there is also an extraordinary radio-phonograph outfit consisting of two tonearms, two pick-ups, two turn tables, and two motors — all for $180.00. The reason for the duplicate list of parts is not stated. It begins to look as Present Interesting though the English have been Trends in Radio considerably in advance of us in the development of moving coil — dynamic type, if you will — loud speakers. For many months the English papers have had descriptions of such instruments pointing out their superior characteristics compared with both Laboratory horn and cone speakers, and the advertising pages of these magazines have been correspondingly full of copy from manufacturers of the newer type of speaker. We predict that next year will see a flood of moving coil speakers in this country, mounting to a new craze on the part of our radio public. We have already mentioned the superior qualities of the Magnavox and Jensen speakers. We have recently seen a curve which we believe to be truthful — which shows a uniform response from below 35 cycles to above 6000 when a unit of this type is used with a rather large and awkward baffleboard. We do not believe it necessary to go down to 35 cycles for excellent quality — but it is comfortable to know your automobile can go 75 miles an hour even though you haven't the nerve to drive it at that rate. The trend toward dynamic speakers is already evidenced in the interest shown on Cortlandt Street, the cut-rate market of New York. Here are a half dozen imitations of the real thing which the gullible radio public is buying as fast as it can. It is reported that several receiver manufacturers whose names are well known are interested in the dynamic speaker and that several have already made arrangements for using it in 1929 models. There is another trend which seems to be becoming more and more pronounced. This is the search for some method of solving the a.c. re ceiver problem whether by series-filament systems or by voltage control units which maintain voltages both A and B constant in spite of varying line voltages. We have seen several of the voltage regulators which as yet are not ready for the market. One in particular delivered A and B voltages with variations of not over 1 per cent, in spite of line fluctuations of over 30 per cent. There is some interest at this time in running filaments in series, such as was described by Roland F. Beers in Radio Broadcast many, many months ago. The advent of eighth-ampere filament tubes will make this problem much simpler, for it is not difficult to obtain currents as high at 135 milliamperes from modern rectifier-filter systems. Just when these tubes will appear is not known, although a recent Sonatron catalogue lists them as being ready for sale at once. And speaking of trends — someone has suggested that people who object to the high voltages necessary for 210 and 250 type tube operation, where considerable audio power is desired or necessary, might use four 171's in parallel. The plate current drain would be about the same as for a 250 type tube, but the voltage need not be over 220 for a combined B and C voltage supply system. This is to be compared with 500 necessary to provide the newest power tube with proper operating conditions. Four 171's will deliver nearly 4 watts of power, and will present an output impedance of about 500 ohms which ought to bring out enough low notes, without distortion, to suit anyone. With the proper output transformer this amplifier output ought to work into a dynamic speaker with its usual 5 to 10-ohm impedance. The curve in Fig. 1 is that of the Amertran output transformer designed to couple push-pull 210 tubes to a dynamic speaker. The Kodel Radio CorRadio poration announces it has Gossip paid $250,000 for the inven tion which led to the development of Kuprox, a rectifying element. This sum bought approximately 35 patents. During the past six months royalties in excess of $100,000 were paid to the Leibel-Flarsheim Company of Cincinnati for devices using this material which was developed in their laboratories. The Arcturus Radio Company, makers of tubes, announced a '27 type replacement tube with a guaranteed life of 1000 hours. This has been possible by designing a heater and cathode combination in which the filament or heater burns at a very low temperature. The lag in time between turning on the current into this new tube and its functioning properly, which in some tubes is about 42 seconds, has been reduced to six from fifteen seconds by decreasing the spacing between heater and cathode. The increase in heating varies roughly as the square of the de in"' 15w 10 2000 3000 4000 6000 8000 20 30 40 50 60 80 100 200 300 400 600 800 1000 FREQUENCY, CYCLES PER SECOND FIG. I Frequency characteristic of the Amertran type 200 push-pull output transformer working out of two 3000-ohm tubes into 10 and 1 5-ohm resistances 143