Radio Broadcast (May 1928-Apr 1929)

New Apparatus and Their Applications How to Build a B-Power Unit with Recently Announced Parts A RE you operating your receiver in the most A-\ economical manner possible? If a power ^ tube, such as the 171 a, is used in the last stage it is advisable to use a socket-power unit to supply the plate power. A B-power unit designed to supply the various plate voltages required for the operation of any standard receiver may be constructed easily. The apparatus required are: a power transformer designed to supply plate and filament voltages for the operation of the rectifier tube and filament voltage for the operation of the power tube if this latter is to be operated from alternating current, filter choke coils and filter condensers to change the pulsating current from the rectifier to a steady d.c. required by the plate circuits of the various tubes, and a bank of resistances and by-pass condensers which will enable you to obtain the intermediate values of voltages required for the operation of the amplifier and detector tubes in the receiver. The illustration on this page shows how such a unit may be constructed easily and inexpensively from a group of standard parts which recently have been placed on the market. This power unit supplies 180 volts for the plate of a i7iA-type tube and also delivers the negative 40-volt C-bias potential which this tube requires. Intermediate potentials of 135, 90, 67 and 45 volts are also available. These latter voltages are variable to some extent — the voltage from the 135-volt tap may be adjusted to any desired value from about 120 volts to 180 volts, the go-volt tap is adjustable from 80 to 120, the 67-volt tap may be varied from 60 to 80 volts and the 45-volt tap is adjustable from o to 60. The voltage from the various taps is adjusted by rotating the arms on the Frost 2000ohm potentiometers; this arrangement making it possible to use this power unit with any receiver, for it is possible to adjust easily the different output potentials to give most efficient operation of the radio receiver. The power transformer at the left is a Dongan type 5509 containing three secondary windings — two 5-voIts windings and one high-voltage winding, supplying 300 volts either side of the center-tap connection. The Dongan choke coil, type 7542, contains two filter chokes in a single case. The filter condensers are three 2-mfd. 400-volt Frost type 1305. Each of the Frost potentiometers connected across the output have a value of 2000 ohms. The long resistance located at the lower left-hand corner has a value of 2000 ohms and it supplies the 40-volt C bias required for the i7iA-type power tube. To assemble this power unit the apparatus should first be mounted on the baseboard as shown in the picture. The leads from the power transformer should then be connected to the rectifier tube socket. As indicated in the picture, the two red leads from the high-voltage winding on the transformer are connected to the grid and plate terminals of the socket; the two filament posts of the socket are connected to the two black leads from one of the filament windings of the transformer. The colors of these and the other wires leading from the power transformer and the filter choke coils are indicated in the picture and if it is followed carefully no difficulty should be experienced in constructing the unit. The parts required for the construction of this unit are listed below: T — One Dongan power transformer, type 5509; Important Announcement THIS month an important change is made in the method of treating new apparatus in these pages. All of the various pieces of apparatus available for description were suited for use in B power-supply devices. Therefore, in order to provide a concrete example of an application for each of these units, it was decided to incorporate them in a B-power unit. Not only does this unit illustrate a use for the various pieces of apparatus under discussion, hut it also provides an ideal design for the set-builder to follow. The power unit is of upto-the-minute design, it will provide B potentials to any standard receiver and A, B and C potentials to a ijiA-type power tube, it is easy to build, and the cost of the parts is $38.40. In future issues, if the occasion presents itself, this method of presentation will be applied to other types of apparatus. ■ — The Editor Li — One Dongan filter choke coil, type 7542; Q, C2, C3 — Three Frost i-mfd. 200-volt by-pass condensers, Type 1 104; Ri, R2. R3. R4 — Four Frost 2000-ohm potentiometers; Rs — One Frost 2000-ohm fixed resistor; 1 Frost Socket. The various Frost resistors used in this unit are part of the "Universal Resistance Kit" which this company manufactures. New Power Transformers and Filter Choke Coil X77 Device: Power Transformers and Filter Chokes. Various types, are available to meet the requirements of all different types of power units. Manufacturer: Dongan Electric Manufacturing Company. Application: The B-power unit described on this page illustrates a typical application of these filter chokes and transformers in the construction of a power unit. Complete circuit diagram and constructional information on various types of power units may be obtained by writing the manufacturer. New Power Unit Parts X78 Device: Filter Condensers and Universal Resistance Kit, Type No. 300. These parts are for use in construction of power units. data on universal resistance kit This kit consists of three 2000-ohm fixed resistors each of the A series, wound on flexible bakelite strips, one inch wide and five and onehalf inches long; four 2000-ohm heavy-duty wire-wound potentiometers, and one 1500-ohm A series fixed resistor. This kit may be used in all present types of power amplifiers, including those using 2 10 or 250-type tubes in a push-pull circuit The total heat dissipation of the kit is 72 watts. Price: §9.00. data on filter and by-pass condensers Individual condensers in capacities ranging from 0.1 mfd. to 2 mfd. are available. These are designed to work on potentials up to 2000 volts d.c. A block condenser is also made containing four sections, the first section being a 1000-volt 2-mfd. condenser, the second section a 600-volt 2-mfd. condenser, the third section a 400-volt 4-mfd. condenser, and the fourth section a 400-volt i-mfd. condenser. Prices: Block condenser No. 690: J? 1 8.00. Prices of individual condensers vary with size and voltage rating. Manufacturer: Herbert H. Frost, Inc. PICTURE DIAGRAM OF POWER SUPPLY This picture shows the exact arrangement of apparatus and wiring in the B power-supply unit. The wires terminating in arrows on the right connect with the wiring harness of the receiver. The approximate potentials available at the various points follow: wire from Ri, 135 volts; wire from R2, go volts; wire from R3, 67 volts; wire from Ri, 45 volts, and the lower wire is the B minus connection 127