Radio Broadcast (May 1928-Apr 1929)

Output Transformer Characteristics An output transformer connecting a loud speaker to a power tube serves two purposes: (a.), to keep the direct current flowing in the plate circuit of that tube from circulating through the windings of the loud speaker; (b). to correct anylarge impedance differences which may exist between the speaker and the tube. The user has a right to expect that the use of the transformer will not perform the tasks mentioned above at a loss in either power or fidelity. The curve shown in Fig. i shows the frequency characteristic of the Pacent 1:1 output transformer. The upper curve represents the voltage across 5000 ohms when 15 volts were applied to the grid circuit of a 210 tube which was properly biased and taking about 20 milliamperes of plate current. The lower curve shows the same characteristic translated in TU. Anyone desiring to know the power delivered to the output load resistance may calculate it by dividing the voltage squared by the resistance. The extreme variation obtained in this manner is from 395 milliwatts at 60 cycles to 610 milliwatts at 2000 cycles, or less than 2 TU, which is quite good. The loss in power occasioned by the use of a transformer instead of placing the loud speaker directly in the plate circuit of the tube may be calculated by dividing what one actually gets into the 5000 ohms by what would be obtained without the transformer. The power output of a tube working into its own impedance (in this case the 5000 ohms is sufficiently near that of the tube) is equal to "Strays" Laboratory Wo = (j> eg)2 4 Rp where (A is the amplification factor of the tube eg is the input volts r.m.s. Rp is the tube impedance Using this formula, the power into 5000 ohms with 1 5 volts r.m.s. on the grid of the tube should be 720 milliwatts. Actually we get only 395 at 60 cycles and 610 at 2000 cycles. These are losses due the transformer of only 2.6 and 0.7 TU respectively. Listen to this from the High Powered Radio World's Fair: "'The Press Releases last frontier of radio re sistance will bow before this final stroke of air-mastery, the automatic broadcast receiver,' said Rear Admiral Bradley Allen Fiske, leading inventor of the United States Navy for the past fifty years, as he declared himself as being much interested in the automatic broadcast receiver which was disclosed to the press during the past week by Harry N. Marvin, millionaire inventor of Rye, New York." We hope Admiral Fiske didn't say any such thing because the inventor of the torpedo plane and the naval telescope sight which, again quoting from the release, "has been adopted by all the navies of the world and its use has been the main cause of the improvement in accuracy of modern naval gunnery," should know that automatic tuning has been in use many years in the navy, commercial, and amateur stations. It is a simple matter to change the frequency to which a station, or a receiver, is tuned by pressing a button. How does the Admiral, or his press agent, think the frequency of wcc, that high-pressure coastal station of the Radiomarine Corporation at Chatham Massachusetts is changed? The operator is some miles away, and while there is an attendant at the transmitter, does he listen for a call on the 'phone, "Say buddy, how's for tuning-up wcc to 2000 meters?" We have been at Marion, Massachusetts, where the transmitter is and have seen the thing in operation. There is a click of a relay and the signals go out on 2100 meters, another click and the wavelength has been changed to 2000 meters, another click and the transmitter is turned off. No one has been near it for an hour or so. No, the problem of turning on and off your receiver, or of changing its frequency setting, or regulating the volume control, all from a distance is not new. Neither is the idea that in the navy in time of war the frequency of a transmitter could be changed rapidly and at the will of the operator. Such schemes are as old as the apparatus itself and the business was reduced to practice many years ago. If anyone wants to know how to tune his radio by pushing a button instead of whirling a ■ knob, we'll tell him, and at the same time give him data on how to turn the thing off — in case of sopranos singing, for instance — or to change the volume. As a matter of fact, the interested reader may find an article on such matters in this issue. This scheme is at least old enough to have gone through the patent office, and in these days of radio inventions, this means something! Incidentally, Commander E. F. McDonald of the Zenith Radio Corporation states that his company has acquired Mr. Marvin's invention and that he, too, believes the last frontier of radio will bow etc., etc. The a. c. receiver oA. C. seems to be causing Troubles dealers and service men no end of worry. A dealer in New Hampshire writes us typical complaint: "I. wish to state that the a.c. tube sets around this section are a decided failure. Fluctuating line voltages ruin tubes in a few weeks. I know of one individual that has had four sets of tubes this winter. I spent an hour explaining why a.c. tubes were no good to one prospective customer after which he agreed with me and thanked me. However, the next day he promptly went to my competitor and purchased an a. c. set." — We should like to point out that the trouble is by no means altogether with the a.c. tube. For example, we know an editor of a nationally known magazine who owns a Radiola 17 receiver. He lives in a section of New York where line fluctuations are very severe, so severe in fact that he purchases a new 171 power tube and a.c. tubes, too, about every two months. The trouble here is patently not with the tubes, but with the bad voltage regulation. Any tube operated at a temperature above normal will not last long. This means that a tube whose manufacturer states should be operated at 2.5 volts cannot be expected to lead a long or useful life if the voltage about half the time is 3.0 volts. Tubes operated from a.c. will last a long time if the voltage is normal or slightly below. From time to time we hear rumors of voltage adjusting gadgets which are in process of development; units that plug between the house lighting socket and the receiver. Their duty is to keep the voltage down to some fixed figure. We predict a million dollar business for the first reasonably priced and successful equipment of this type. 60 100 200 83 500 1000 2000 FREQUENCY FIG. I 5000 IQP00