Radio Broadcast (May 1928-Apr 1929)

268 RADIO BROADCAST SEPTEMBER, 1928 Stage 1 Stage 2 FIG. 2 plifier, or 1.0-1.5 kw. as a modulator. Even if an anode voltage of 15,000 is allowed in the instructions, it is likely to be found running at 10,000, with a plate current of less than one ampere; as a modulator it will be taking a plate current of a half or third ampere at the same lower voltage. Under those conditions it is likely to run eight or ten hours at a stretch, if the cooling suits it, without knocking out the breakers and leaving the listeners in silence, and to do this day after day. It may, in fact, do it for years; the skeletons of water-cooled tubes with 3,0004,000 hours of useful service may be seen in museums. Some, again, are buried after 40 minutes. They all look the same when they are taken out of the crates. The life of vacuum tubes may be prolonged if they are carefully handled. Of course, continuous water supply is a primary requisite and pressureor flow-actuated relays are used to shut off the filament supply as soon as the water fails. When set adjustments have been changed, it is wise to resume operation at some low plate voltage corresponding to reduced power, so that undue stresses will not be set up in the tubes. Excessive vibration or rough handling of any kind should be avoided. Dropping tubes into water jackets for an inch or so, knocking off grid seals with wrenches, and similar accidents, should be as rare as announcers who shrink from publicity. Starting resistances for the filaments should be used to limit the first surge of current. The filament connections, carrying about 50 amperes, must be tight and clean, or heating at these points may cause burning and destruction of the leads. The grid and filament leads should not be so tight that any force is exerted on the walls of the tube, nor so loose that they may lie along the glass; the first condition may result in mechanical breakage; the second in puncturing following corona. Other such precautions in handling are more or less obvious. If they are not followed a water-cooled radio station can break a millionaire faster than the Street. Even if they are followed such outfits are not for the poor. 22. PHONOGRAPH PICK-UPS ASIDE from the modulation of a broadcast station from a phonograph for prok. gram purposes, as commonly used in the smaller studios, this is about the only convenient method of running prolonged transmitter tests where music is required. Phonograph transmission by radio is, of course, nothing new. In 1917 I recollect seeing and hearing the radio telephone transmitter with which Dr. Alfred N. Goldsmith communicated from New York City with Dr. A. H. Taylor at the University of North Dakota so controlled, and probably this was not the first instance of such use of the electromagnetic pick-up. Of late years the development of the electric phonograph has made it possible to impress on a suitable radio transmitter quite decent acoustic material from wax records, a frequency range of from 100 to 4,000 cycles being covered without serious distortion. The film phonographs which, aside from their use in sound-motion picture work, are not yet commercially available, can probably do better; but this article, being of the same practical nature as the others in the series, is concerned only with reproduction from the common flat disc records. A short bibliography of the subject, for those who wish to do a little reading in this field of electroacoustics, is as follows: Maxfield and Harrison: "Methods of High Quality Recording and Reproducing of Music and Speech Based on Telephone Research." Journal of the A . I. E. E. March, 1926. Kellogg: "Electrical Reproduction from Phonograph Records." Journal of the A. I. E. E. October, 1927. Millen: "The Electrical Phonograph." Radio Broadcast. May, 1927. Millen: "Building an Electrical Phonograph." Radio Broadcast. June, 1927. Wilson: "A Phonograph Amplifier." General Radio Experimenter. April, 1928. Almost all the devices commonly used for transforming sound into electrical energy, such as the condenser and carbon telephone transmitters, may be used as phonograph reproducers, the actuating force being derived from the revolving record instead of from a wave in air. The most convenient commercial form has been found to be the magnetic type, which is substantially a small alternating current generator driven by the record. The operating principle is the same as that of the electromagnetic telephone transmitter, which does not happen to be common in American broadcasting. Many variations in construction are possible; Fig. 3 shows one in general use, so that it will be unnecessary for readers to take their own electric phonographs apart. The magnetic field is provided by a permanent magnet. An armature is free to move within a small amplitude in the air-gap. The movement of the needle as it is carried along the record groove causes the armature to wiggle about its pivot. The system is so designed that the motion of the armature is proportional to that of the needle. A coil of fine insulated wire surrounds the armature. The magnetic flux through this coil varies slightly as the armature vibrates. We might say that a small electromagnetic ripple is superimposed on the constant magnetic field supplied by the permanent magnet, like the commutator ripple of a d.c. generator. The result is, of course, that a corresponding e. m. f. is generated in the coil. This may be applied to the grid of a vacuum tube and amplified. The type of reproducer thus sketchily described works with records having laterally cut grooves, as distinguished from grooves of varying depth. If suitably supported it would, of course, give some sort of reproduction with the latter style of record also. A telephone receiver with a needle soldered to the diaphragm will reproduce sound from a phonograph record, but the reproduction will not be faithful. In other words, the principle of the device is simple, but high quality output requires much finesse in design, as is shown in Kellogg's paper. FIG. 3 Coir Set-screw --Needle ELECTROMAGNETIC PHONOGRAPH REPRODUCER The output of magnetic phonograph reproducers varies, naturally, with the make, type of record, etc., but in general it is at such a level that two stages of amplification will bring it up to loud speaker volume. Assuming this to require a level of plus 12 tu at the speaker terminals, and calling the amplification 28 tu, we find a reproducer output of minus 16 tu. Probably a level between minus 10 and minus 20 tu will include the makes and conditions usually encountered in practice. In other words, the phonograph reproducer is good for an output level considerably greater than that of a high quality carbon transmitter, but not as high as that of a commercial telephone transmitter (zero level) in which, however, quality is sacrificed to sensitivity. Fig. 2 shows schematically a suitable amplifier circuit. The output of the generator is such that it will not overload a tube of the 199 size, so that this may be used for the first stage, if desired. The second tube should be of the 171 or 210 size. In a broadcast station it is often considered better practice not to use smaller tubes than the 5-watt (oscillator rating) size, so that an amplifier of the 17-B type, using two 5-watt tubes, may be associated with the phonograph reproducer. In Fig. 2 the volume control is a 25,000-ohm potentiometer. These instruments are now made very cheaply and conveniently by the use of transverse wires imbedded in a carbon composition ring. A scratch filter is also shown. It happens that the noise made by the needle as it rides along the surface of the record has some of its more annoying components in the neighborhood of 4,500 cycles, so that a series circuit tuned to about this frequency will by-pass a good part of the surface noise — and also any frequencies above 4,000 cycles which may be present in the output of the reproducer. But usually it is better to sacrifice this section of the band in order to get rid of the background noise. Inasmuch as input to the preliminary amplifier of a radio transmitter, from the wire line connecting the transmitter with the studio, is commonly at a level of about minus 10 tu, a magnetic phonograph pick-up of the type described, with a two-stage amplifier including a gain control, may readily be substituted for the line when local test modulation is desired. Likewise at the studio the same combination may be used in place of microphone pick-up, both the gain of the studio amplifier and the amplifier associated with the pick-up being kept low. With a medium gain setting the phonograph amplifier will deliver energy at a suitable level to the line, if that is desired. Of course the 25,000-ohm volume control across the input of the phonograph amplifier is not the only type which may be used. A standard high resistance (about 400,000 ohm) interstage gain control is just as suitable. But if a line (500: 500 ohm) amplifier with such a gain control is connected to the phonograph pick-up care should be taken to omit the input transformer, so that the pick-up goes directly to the grid of the first tube. Five hundred ohms is too low for connection across a device with the impedance of mos_t magnetic pick-ups. --Permanent Magnet -Pivot Armature