Radio Broadcast (May 1928-Apr 1929)

202 RADIO BROADCAST AUGUST, 1928 0.007 ,199 ¥ J L, = Hammarlund 85 mil. Choke L2=Brownmg-Drake Antenna Coil FIG. 2 the 5 per cent, that the R. C. A. engineering and test department found to be true in Manhattan? Many experimenters have A C. Screen been hoping for a.c. screen grid tubes grid tubes. The following data is the result of some measurements in the Laboratory on several tubes of this type. The filament is the standard 2.5volt, 1.75 ampere heater type with the accessory plate and grid structure around it as in the d.c. tubes. Under normal conditions of screen and plate voltage, we obtained an impedance of about 375,000 ohms and a mutual conductance of about 500 micromhos. Realizing that the most experimenters prefer actual gain measurements to those on the tubes' constants we hooked up the circuit shown in Fig. 2 and secured a voltage gain as shown below. Frequency Voltage gain 500 40 580 41 660 48 940 80 1050 120 The coil and condenser unit was standard Browning-Drake apparatus used as the input to an r.f. amplifier tube. The inductance was 200 microhenries and at 500 kc. the combined coil and condenser had a resistance of about 5 ohms. These values of resistance and inductance indicate a theoretical voltage gain at 500 kc. of about 56. It should be noted that a resistance input was used, and that d.c. was used to heat the heater. The effect of a.c. when this tube is used in an actual circuit has not been determined, although reports indicate that it makes an exceptionally good high-frequency amplifier, the question of selectivity remaining where it was when recently discussed in these pages. Last month we quoted "Strays" from some strange business about other Laboratories a loud speaker test that took place in one of our contemporaries' laboratories. During the test the output current from the amplifier ran as high as 49 milliamperes, and from 1200 to 1280 volts appeared somewhere in the circuit without the necessity of calling out the fire department. The following statement appeared recently in another radio publication: "The Raytheon BH rectifier tube is now tipless, increasing rating to 125 MA." We feel the prize, however, for technically inaccurate statements appeared in still another radio paper a year or so ago. An author was describing a world-beating receiver consisting of one or more r.f. stages coupled to each other by means of t.ansformers. Figuring that the usual voltage gain in a neutrodyne stage, which has 60 turns on the secondary and 6 on the primary of the interstage r.f. transformers, was 10, this writer suggested using only one primary turn, when the voltage step-up would be increased from 10 to 60 per stage! The field for one's imagination in radio is apparently unlimited. Making D C. Sets Comfortable Several months ago we mentioned the fact that we saw little reason why anyone should throw away a good battery-operated set in favor of an a.c. receiver, when the former only required about eight minutes a week to place the battery on charge. Mr. Beecher Ogden, of Pleasantville, New York, forces us to admit that we have no simple device for doing this work for us, and that we still get down under the table, fuss around with wires, get hands and knees dirty, and get into a generally bad humor — at least once a week. Mr. Ogden's scheme for preserving one's friendliness toward the battery-operated receiver is illustrated in Fig. 3. It consists of a switch which throws one battery on the set and another on charge — or if one has only one battery, a simple d. p. d. t. switch will throw the battery from the receiver to the charger, which may be connected into a base-plug or any other source of a.c. Say Mr. Ogden: I am shocked and grieved at the implied admission on page 352 of Radio Broadcast for March, second paragraph, that you still have to disconnect the battery from the set to put it on charge, for I can't figure any other way that could possibly take eight minutes a week. Radio Broadcast has described several switching arrangements that certainly don't need five seconds to put the battery on the charger. However, here is a real one that has been in use for years. Take a four-pole d. t. switch and cross-connect it on the back so that one battery is on charge and another on the set at the same time. A 2-ampere Tungar will give you about 30 a.h. a day, which should be enough. The following letter from Loudspeaker a Norfolk, Va., radio enthu Tests siast speaks for itself. It describes the result of two very interesting tests on loud speakers, and calls attention to the dynamic type which has ■Or" CT^ Wiring on ""O ! Bat.No.2 --back of > Switch FIG. 3 been mentioned more than once in these columns. I read with interest your department's comments on recent developments in loud speakers under "Strays from the Laboratory" in the February issue. Your readers appreciate the necessity of protecting your advertisers, but information of that nature based on the results of comparative tests in the Laboratory are of the greatest value to your readers. I hope you will continue to give us the results of your tests. I have followed the development of loud speakers for a number of years, and have known that the Western Electric has been generally favored for monitoring work in the control rooms of broadcast stations. I have had occasion to compare this speaker with others before an audience composed of people with trained musical ears and technical knowledge. The loud speakers were arranged behind a curtain so that the audience would not be prejudiced by preconceived opinions. Seventy per cent, rated the W. E. first on music, and 90 per cent, rated it first on voice. The same experiment was tried with an audience composed of people none of whom had had a musical education or technical training. Practically everybody in this audience preferred the RCA model 100-A over all other speakers on music, and they were about equally divided between this speaker and the W. E. on voice. These experiments were performed under conditions of ideal local reception, that is, without heterodyne -squeals or static. Under conditions of distant reception with heterodyne squeals, background noises, and static present, almost everyone agreed that the RCA model 100-A was to be preferred to the Western Electric. In other words, the high-frequency cut-off filter incorporated in this (RCA) speaker which eliminates the greater part of these interfering noises is certainly desirable under present broadcast conditions even at the slight sacrifice of clearness of speech which it entails. I have been testing out a Magnavox dynamic power cone under various conditions of baffling, recently, and I am surprised that you did not mention this speaker as being one of the best of the new speakers in your article on this subject. The power handling capacity of this speaker is so far superior to that of other types that it is a little difficult to make comparisons. A 5000-cycle cut-off filter effectively eliminates heterodyne squeals and background noises. On some nights when static noises are so loud on the Western Electric that reception is unsatisfactory, the Magnavox brings the music in clear with a minimum of interfering noises. Of course, this is accomplished at a slight sacrifice in "intelligibility." We have read with condecent Articles siderable interest the followof Interest ing articles in various radio and technical publications: Amplification Behind the Talking Movies, Bell Laboratories Record, May. Loud Speakers of High Efficiency, Journal of the A. I. E. E., April. Getting Started at 30 Megacycles, QST, May Practical Audio Filters, QST, May. Amplification and High Quality, Wireless World, May 2nd. The Earth as a Magnet, Scientific American, May. Geophysical Prospecting, Scientific American, May. The Inverted Vacuum Tube, Proceedings of the I. R. E., April. The Development of the 250 type tube, Proceedings of the I. R. E. April. Broadcast Control Operation, Proceedings of the I. R. E., April.