Radio Broadcast (May 1928-Apr 1929)

86 RADIO BROADCAST JUNE, 1928 constructing power amplifiers using the 250 type tube especially, although the equipment is of course suited to the construction of any power unit from which it is desired to obtain comparatively large amounts of current, say 60 milliamperes or more. Dynamic Speaker Models X36 Device: Jensen Dynamic Speaker: A movingcoil type loud speaker. It may be connected directly to the output of a receiver without any need of an output device, for the transformer contained within the loud speaker insulates the windings of the moving coil from the plate current of the power tube. The field winding of the speaker must be supplied with energy from an A battery (Model D-44), or from a 90-volt d. c. source (Model D-45) or from the 1 10-volt a. c. mains (Model D-44. a c-) Manufactured by the Jensen Radio Mfg. Co. Price: Model D-44, $65.00; Model D-45, $67.50; Model D-44-AC. $75.00. Application: A sample of this loud speaker has been in use in the Laboratory for some time and has proved to be an excellent instrument. An elementary diagram of the loud speaker appears ! Intense magnetic field* Field connections to |— «• storage A Battery D.C.6Volts-0.4Amp. Circular paper cone I .-''""{''f-tT^X ' orA.C.6Volts,rectified specially treated '— f i i \ ■ and filtered (J^j^^TJ Step cJown output T"nsiformer built u. 8!$ depth overall w DIAGRAM OF JENSEN CONE herewith. The cross-sectioned part represents a magnet, energized by the current flowing through the field winding. As a result there is produced across the gap in the magnet an intense magnetic field. The paper cone is arranged as indicated, projecting into the gap, and around this portion of the cone is wound a small coil of wire. This coil is connected to the secondary of the stepdown transformer, the primary of which connects to the output of the radio receiver. The signal currents flowing through the coil react with the magnetic lines of force flowing across the gap and produce a torque which makes the co'il move left and right. The cone, attached solidly to the moving coil, must also move with the coil and its movements produce the sounds. This loud speaker gives best results when operated in conjunction with a large baffle but will also work very well in the cabinet supplied <B> To Rectifier Filter System <F> CIRCUITS FOR CENTRALAB VOLUME CONTROLS by the Jensen Company. The cone unit can also be purchased without the cabinet for installation in radio or phonograph consoles. Resistors for A. C. Sets X37 Device: Volume Control Resistances, for a. c. receivers. The method of volume control commonly used in d. c. -operated receivers, i. e., a filament rheostat in the filament circuit of the r. f. tubes, cannot be applied to a. c. circuits because lowering the filament potential of 226 type tubes will tend to produce a hum in the loud speaker and because with 227 type tubes the emission from the heater will lag behind changes in the current through the heater filament. It is therefore necessary to use some other type of volume control. The devices listed below will be found very satisfactory. RX-100 Radiohm, a special tapered resistance for use across the secondary of one of the r. f. transformers, preferably the detector stage. See Fig. 1 -A. Price; $2.00. RX-025 Radiohm, a special tapered resistance for use as a volume control across the primary of an r. f. transformer or across the primary of a tuned antenna coil. See Fig. i-B. Price: $2.00. P-112 Potentiometer. A 6000-ohm potentiometer with a special tapered resistance at the end to be used in the antenna circuit or across the primary of a tuned r. f. stage. See Fig. i-C. Price: $2.00. M-500 Modulator. A potentiometer with a special tapered resistance for the grid circuit of one of the audio stages. Used principally as an auxiliary control in a. c. circuits. See Fig. i-D. Price: 2.00. HP-050 Heavy Duty Potentiometer. A wirewound, non-inductive potentiometer used as a plate circuit control. See Fig. i-E. Price: $2.00. PR-050 Power Rheostat. A specially constructed rheostat to carry heavy currents and an excellent control for use in the primary of a power transformer. See Fig. I-F. Price: $1.25. All of the above units are manufactured by the Central Radio Laboratories. Application; The application of these units to a. c. circuits has been covered in the data given above. A useful pamphlet is obtainable from the manufacturers through Radio Broadcast describing these units in detail and giving further information regarding the circuits in which they are to be used. TRUVOLT RESISTORS AND BELDEN ANTENNA KIT JENSEN DYNAMIC CONE Resistors for Power Supply Use X38 Device: Truvolt Resistors. Wire-wound resistors, using Nichrome wire wound on an asbestos covered enamel core. Available as fixed and variable resistors as follows: Type T, variable resistors, with maximum resistances of 200 to 50,000 ohms. Type B, fixed resistors, rated at 25 watts, and available in sizes from 200 to 50,000 ohms. Type C, fixed resistors, rated at 50 watts, and available in sizes from 200 to 100,000 ohms. Type D, fixed resistors, rated at 75 watts, and available in sizes from 200 to 100,000 ohms. Type V, center-tapped fixed resistors, available in sizes from 10 to 200 ohms, and designed especially for use as center-tapped resistors across the filaments of a. c. tubes. Grid Resistors, available in sizes from 10 to 2000 ohms, and designed especially for use as grid suppressors, in radio-frequency amplifiers. Special Tapped Fixed Resistors, designed for use with the popular types of power packs such as those made by Silver-Marshall, Amertran, Samson, Thordarson, etc. Manufactured by Electrad, Inc. Prices: vary according to the style and rating of resistors. Application: The Truvolt line of resistors is very complete and units are available to meet almost every need in the construction of radio receivers and power packs. Complete data can be obtained from the manufacturer through Radio Broadcast.