Radio Broadcast (May 1928-Apr 1929)

216 RADIO BROADCAST AUGUST, 1928 FIG. 2 The principle of parallel feed, familiar in oulput devices, applied to the conventional transformercoupled amplifier. microfarads. The direct plate current flows through the choke, L, which has a low impedance to direct current, while the condenser offers an effectual bar to the flow of direct current through the primary of the transformer. Alternating current is prevented from flowing through the choke, L, in appreciable amount because of its high impedance, while the condenser and transformer primary offer a path of low impedance as compared with that offered by the choke. In this way the two componentsof current existing in the plate circuit of the tube, i.e., the space current (constant and undirectional), and the audio frequency signal current are directed into different circuits. The separation of the direct and alternating componentsof the plate current of a vacuum tube is desirable for a number of reasons. Direct current flowing through the primary of a transformer sets up a field in the core which may cause magnetic saturation in the core. Saturation is a condition under which changes in magnetizing current do not produce corresponding changes in flux. Since the operation of the transformer is dependent on changes in flux, the instrument is naturally affected. The better the transformer, the more likely is this to happen. If the transformer's core is of silicon steel, saturation is not likely to occur with tubes of the 201A or 199 type, but if a 1 12 or 227 tube is used, saturation may occur. Cores of nickel steel such as are coming to be used to an increasing extent are much more subject to this difficulty than are silicon cores. Many experimental receivers employ semipower tubes, such as the 112, in the first stage. In such cases, the following transformer should always be parallel fed. FIG. 4 This balancing arrangement for adjusting cone and horn speakers connected in series is useful where one unit favors the high frequencies and the other low frequencies. Currents of more than a few milliamperes will seriously affect the behavior of the nickel steel transformer, and the instrument may easily be permanently injured by the application of too large a direct current magnetizing force. The effect of saturation of the iron is to reduce the input impedance of the instrument, resulting in a loss of amplification which is particularly marked at low frequencies. When using cores of some of the nickel alloys, the gain due to the special core material may be completely sacrificed as a result of too much direct current in the primary. The elimination of oscillation and motor-boating in the amplifier is another advantage gained by the use of parallel feed. It has been noted that the signal current does not flow through the direct current circuit, i.e., no signal or audio-frequency current flows through the plate supply unit. Since no signal current from any stage flows through the plate supply, no audio-frequency voltages are set up, and no coupling between the stages results from the common impedance in the plate supply device. The result is an increase in the stability of the amplifier, and elimination of regeneration and "motor-boating." C. T. Burke, Cambridge, Mass. Some More on Balanced Speakers OUR Readers Suggest" has devoted considerable space to the interesting possibilities of operating speakers of opposing characteristics, such as a cone and a horn, in series in order to obtain adequate reproduction of both high and low notes. Different resistor devices have been suggested to determine the correct balance. I find that the simple arrangement, employing a Duplex Clarostat, as shown, in Fig. 4, altogether satisfactory and less complicated than the systems heretofore suggested. I am using an R.C.A. horn with a Western Electric 24-inch cone. When the right hand screw is tightened, the cone is cut out of the circuit and the high notes emphasized. When the left hand adjusting screw is tightened (and the right hand screw loosened if necessary) reversed conditions prevail, and the low notes are accentuated. Once the correct balance is obtained, the resistors should never be touched, and the screw driver adjustment is well adapted to this semi-permanence. R. Mackey, Schoharie, N.Y. A Milliampere Meter Protector and Multiplier A FEW days ago the representative of a wellknown manufacturer gave me a startling figure on the number of milliammeters repaired daily at his factory. After one of mine had been laid up for repairs, I adopted the protective device shown in Fig. 5. The milliammeter is shunted with a 30-ohm General Radio filament rheostat. When the ammeter is inserted in the circuit, the resistance is adjusted so that the resistance is practically cut out. In this position, a very slight deflection of the meter is noticeable, which serves to show if the polarity connection is correct. The resistance of the rheostat may then be gradually increased and the meter reading noted. If the current in the circuit is above the meter capacity, it will be noted before any damage is done. Providing all is well, the rheostat may be turned to the "off" position, which gives normal full deflection on the meter. If the meter has a scale of fifty or more milliamperes, the rheostat may be used as a multiplier with sufficient accuracy for all ordinary purposes. While on the subject of protection, 1 might FIG. 3 An emergency storage battery connector that is quite independent of the elusive binding-post cap. add that I find the 199 Amperites a very effective fuse in the B and C battery circuits, and 1 and 3-ampere fuses useful in the A circuit. Edward T. Werden, Mt. Vernon, N. Y. STAFF COMMENT THE rheostat can be used as a multiplier regardless of the range of the meter as far as the requirements of the average experimenter are concerned. The rheostat should be set so that the indication of the meter is some easily multiplied fraction of the actual current. For instance, with the rheostat open, the current through the meter should be noted (any desired current can be secured by connecting the meter in series with a high range Clarostat and a dry cell). The rheostat is then cut in until the current through the meter drops to one half or one third or one tenth of the original reading. This fraction will then hold for all readings. For example, if the rheostat is set for a one quarter deflection, three milliamperes on the meter indicate an actual current of twelve milliamperes. Tinning Wires WHEN using an untinned wire for the internal connections of a receiver difficulty is often experienced in tinning the ends before making a joint. The process of tinning these wires may be carried out much more effectively if a groove is filed in one of the surfaces of the soldering iron. This groove will readily fill up with solder, and then by sliding the wire to be tinned into the groove, it will be tinned on all sides simultaneously. J. B. Bayley, Jersey City, N. J. Emergency Battery Connections IN THE absence of the usual battery clip a good emergency connection may be made with an ordinary one inch finishing nail, or even a flat head if it is of small diameter. First drive the nail into the lead end post about a quarter to three eighths of an inch. This may seem difficult, but is really very easily accomplished. Then remove the nail and insert the wire from the receiver or charger and drive the nail in again. Fig. 3 shows how it is done. This will make a good solid contact equal to the carrying capacity of the wire used. J. B. Bayley, Jersey City, N.J. AWAW FIG. 5 An inexpensive milliammeter protector and multiplier that will save its fifty cent cost many times over.