Radio Broadcast (May 1928-Apr 1929)

SEPTEMBER, 1928 NEW APPARATUS 285 THE MAGNAVOX SPEAKER, TYPE D-80 receiver is turned on. When the set is turned off, the power control turns off the B-power unit and turns on the trickle charger. The connection of this control to a radio receiver is shown in Fig. 2. Manufacturer: Yaxley Manufacturing Company. Price: $6.00. Application: As mentioned above, the power control is designed especially for use with radio receivers using six 199 type tubes or less. By its use the charging of the battery is automatically taken care of while the radio receiver is not in use and there is no possibility that the B-power unit will be left turned on while the set is not in use. Dynamic Speakers for A.C. and D.C. Operation X56 Device: Magnavox Dynamic Loud Speakers. The following types are available: Type R-4. The field winding of this model requires 0.65 ampere at 6 volts. This energy may be supplied from a storage battery or any other source of 6 volts d.c. Type D-8. This model requires 6 to 12 volts d.c. for field excitation. The field current at 6 volts is 1.1 amperes and at 12 volts 2.2 amperes. Type D-80. The field circuit of this model is supplied through a transformer and rectifier from the 1 10-volt a.c. light socket. The hum due to the use of rectified a.c. is prevented by means of a neutralizing coil as explained below. All of the above models are identical in practically every way with the exception of changes associated with the particular method of field excitation which is used. In the case of the type 80 unit utilizing rectified a.c. for field excitation, an additional coil is wound on the moving element and the current through this extra coil neutralizes the effect of the pulsating current in the field winding. The resultant effect upon the cone is zero and therefore no hum is produced. All of the models include an output transformer designed to adapt the impedance of the moving coil to that of the power tube. If it is desired to use one of these loud speakers in special circuits such as push-pull or a parallel arrangement of power tubes, special transformers are required. These transformers can be obtained from any of the well known manufacturers of audio frequency apparatus. It is essential that a baffle be used with these loud speakers. Two waves are generated by the Cross-section of Cabinet'' FIG. 4 movement of a free edge cone, into which class fall speakers of this type. When the cone moves forward it compresses the air in front of it and at the same time decompresses or rarifies the air at the back of the cone and unless a partition is erected to keep these two effects separate they will tend to neutralize each other and lower the efficiency of the speaker. The baffle is especially necessary to obtain satisfactory reproduction of the low frequencies, for at high frequencies the cone itself acts as an effective baffle. The baffle may take the form of a cabinet or of a large flat surface. In either case the baffle itself should be so constructed that it will not readily vibrate. Otherwise it will produce exaggerated response at those frequencies at which the baffle resonates. In the Laboratory a large flat piece of wood measuring about four feet by four feet, and an inch thick, is used with excellent results. If a cabinet is used for the baffle the distance from the center of the front of the cone around the edge of the cabinet to the rear of the cone should be at least two feet. This distance is indicated in Fig. 4. The baffle should be left open at the back and in order to prevent undesirable resonance in the cabinet itself, it may be necessary that a few small holes be placed in the side and bottom. Manufacturer: The Magnavox Company. Prices: Type R-4, $35.00; type 8, $35.00; type 80, $50.00 without cabinet. Application: Present day dynamic speakers represent one of the most important developments in radio. Theoretically the dynamic speaker is capable of giving practically uniform response over the entire range of audio frequencies, and such uniform response is realized to a considerable extent in practice under good operating conditions. A dynamic speaker is now used in the Laboratory as a standard of comparison for it is capable of giving much better quality than other types of loud speakers available at present. Many will prefer to purchase only the unit and install it in a baffle consisting of a large flat board about one inch thick and preferably three or more feet square. For Push-Pull Amplification X57 Device: Push-Pull Audio Transformer, Type H. These transformers are for use with either 171 or 210 type tubes in push-pull amplifiers. The turns ratio of the input transformer is 1.75 to 1 ; the turns ratio of the output transformer is 1 to 1 . Manufacturer: Dongan Electric Manufacturing Company. Price: $4.50 for the input transformer; $4.50 for the output transformer. Application: The circuit diagram of a single stage push-pull amplifier using these transformers is given in Fig. 5. The tubes may be either 1 7 1 's or 2 1 o's. The resistance, R, supplies C bias to the tubes; for 210's its value should be 750 ohms, for 171's its value should be 1000 ohms. Input Output DONGAN PUSHPULL TRANSFORMER Fixed Resistances X58 FIG. 5 1 watt (100 to 5000 ohms), 2 watts (100 to 7500 ohms). Super-Davohms have a rating of 1 watt and are available in sizes ranging from 10,000 ohms to 5 megohms. Manufactured by the Daven Radio Corporation. Prices, vary with rating of resistor. Application: May be used in any device where there is need for a good resistance. The Davohm resistances are excellent for use in all types of power units. Super-Davohms are useful as laboratory standards, multipliers for voltmeters, plate resistances in a. f. amplifiers, grid suppressors in r. f amplifiers, etc. A.C. Power Amplifier for Many Loud Speakers X59 Device: Distributing Power Amplifier for apartment house use. The amplifier illustrated consists of eight 210 push-pull stages each operating ten loud speakers, with approximately 500 milliwatts per outlet. The eight amplifiers with their input panel are powered by three rectifier panels employing 28 1 type tubes, each of which delivers approximately 132 mils, at 450 volts, each rectifier operating three amplifiers. The complete installation is light-socket operated, and comes in a varying number of racks to take care of varying numbers of outlets. These racks need merely be interconnected by the plugs provided, the output of the radio receiver connected to the amplifier, and the power plug inserted in an a. c. light socket, and the installation is ready for operation. Manufactured by Silver-Marshall, Inc. Price: $300.00, approximately, for an amplifier suited to the operation of from four to ten loud speakers. Application: This amplifier rack represents a development in equipment suited to public address or apartment house installation and the accompanying photograph shows a typical distributing amplifier — a good subject for a conversation with your landlord. Device: Davohm, and Super-Davohm WireWound Fixed Resistances. Davohm resistances are made in the following sizes; 7.5 watts (500 to 15,000 ohms), 1 5 watts (500 to 15 000 ohms), SILVER-MARSHALL DISTRIBUTING AMPLIFIER