Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST 113 VUll 125VoltQ Q QlOSVolt ODyn. Spkr. Fig. 2 — Schematic diagram of power-supply circuit single balancing condenser, C4, is supplied with the amplifier unit for connection across the oscillator tuning condenser so that the two tuning dials can be made to agree closely over the broadcast band. At the center of the panel is located a switch with which is combined a 6-ohm rheostat, R2, controlling the screen-grid tubes. A protective resistor, R3, is connected in series with this rheostat so that excessive voltage can not be applied to the screen-grid tubes. Filament control of the remaining tubes of the set is automatic. Two more controls, which are of a semi-fixed nature, are located on the panel. One of these, the volume control, is a 500,000-ohm variable resistor, Rj, which controls the voltage applied to the screen-grids of the 222-type tubes and to the plate of the first detector. The other, the sensitivity control, is a 2000ohm variable resistor, Ri, which is shunted across the feed-back winding of the first detector and which controls the amount of regeneration in the first detector circuit. The receiver is tuned as is any other receiver of the super-heterodyne type. If a power tube of the 112a or 171a type is to be used and the first audio stage and second audio stage (shown in dotted lines) are both to be included in the receiver proper, the Render first-stage transformer, No. 900, and Render secondstage transformer, No. 901, may be employed. (The characteristics of these transformers were given in November, 1928, Radio Broadcast, page 29.) Render transformer No. 901 can be used in the first-stage position if desired. If only the first audio stage is to be built into the receiver proper and the Render power amplifier is to be used, the firststage transformer should be the Render No. 920. The secondstage transformer will then be the Render No. 921 which will be built into the power amplifier. In either case it is suggested that the first audio tube be of the type 112a and that it be operated at a plate voltage of 135 and a 9-volt negative grid bias. With power tubes type 112a or 171a the output transformer should be respectively either the Render No. 922 or the Render No. 923. If the loud speaker is to be of the dynamic type the transformer should be the No. 923. POWER AMPLIFIER The Render power amplifier and plate supply, Fig. 2, is ideal for use with the Render "29." It employs full-wave rectification, making use of two 281-type half -wave rectifier tubes, and incorporates a single 250-type power tube. It has ample capacity to supply plate and grid-bias voltages for any type of receiver and it will, in addition, supply field current for a dynamic speaker of the 90-110volt type. The speaker field is connected in series with the voltage divider and is supplied with about 55 rnilliamperes. When the speaker field is in use a 2000-ohm resistor in the voltage divider is shorted out. Provision is made for a 874-type voltage regulator or glow tube so that the voltage delivered to the set remains constant. The Render No. 950 power transformer and choke is designed to supply from 425 to 440 volts to the plate of the power tube under full load. The primary of the transformer is tapped for line voltages of 105, 115 and 125 volts. The filter circuit consists of two 80-henry chokes and either three 2-mfd. condensers or two 2-mfd. and one 4-mfd. condensers. The filter condensers are to be installed by the builder in a space provided in the No. 950 case. Part of the voltage divider of the Render power amplifier and plate supply consists of three 2000-ohm strip resistors to which sliding contact connections can be made and three 2000-ohm resistors of the potentiometer type. Grid bias for the first audio tube is taken from a 400-ohm potentiometer which is in series with the return path of the current from the receiver and the voltage-regulator tube. Grid bias for the power tube is supplied by a 2000-ohm variable resistor which is in series with the return path of the plate current for the power tube alone. All component parts of the power amplifier are mounted on a pressed-steel chassis, 10j" wide by 20" long, which is supplied with the No. 952 power amplifier foimdation kit. All resistors of the voltage divider must, of course, be thoroughly insulated from the steel chassis and insulating washers for this purpose are supplied with the foundation kit. Practically any voltage between the maximum for the power tube and zero can be obtained for the receiver and all of these voltages are continuously variable. A pressed-steei cover, finished in brown crystalline enamel, is available for the power amplifier if desired. It greatly enhances the appearance of the unit and insures freedom from the collection of dust. The No. 923 output transformer is supplied with three output terminals marked "Mag. Spkr.," "Speaker," and "Dyn. Spkr." The output winding between terminals "Speaker" and "Mag. Spkr." is of high impedance and is suitable for use when a high-impedance loud speaker such as one of the magnetic type is employed. The output winding between terminals "Speaker" and " Dyn. Spkr." is of low impedance and a moving-cod loud speaker may be connected directly across these transformer terminals. It is possible to connect the regular input terminals of a moving-coU loud speaker across the "Speaker" and "Mag. Spkr." terminals of the No. 923 transformer and to thus obtain fair results, but the elimination of the transformer in the speaker and the use of the "Speaker" and "Dyn. Spkr." terminals of the transformer as above described is recommended. • february, 1929 page 268 ©