Radio Broadcast (Nov 1926-Apr 1927)

AN INTERIOR VIEW OF THE GAROD EA RECEIVER Note the neat cabled wiring at the rear of the sub-panel A* C. as a Filament-Supply Source A Description of a Popular Commercial Receiver — the Qarod Model EA — Which Is Operated Entirely from the A. C. Mains By B. F. MIESSNER THE operation of radio receivers directly from the a. c. house mains has always been a problem which has greatly interested radio engineers. After extensive research, and the accumulation of a great amount of data, the problem of design has been solved in a few instances. Whether a. c. operation will entirely supplant other means of powering the receiver is a question which will have to be left for the future. Of the tubes in commercial use, the 112 type has been found to lend itself most readily to a. c. operation. This is due, in the most part, to the heavy oxide coated filament which does not fluctuate so much in temperature with the alternations of the current. By using the correct grid, plate and filament voltages as previously shown, the operation of such tubes is entirely satisfactory. As yet, no completely satisfactory means have been devised to use a detector supplied directly by a. c. To take care of this, it is customary that one of the 199 type tubes be used, its filament being energized by the total plate current of the other tubes in the receiver. In the February issue of Radio Broadcast was discussed, with the help of curves, the different types of tubes and their characteristics under actual a. c. operation. The preceding article also dealt thoroughly with the different causes of hum and the methods of eliminating it. The present article will take up the practicable application of a. c. operation to a commercial receiver. The Model EA Garod receiver has been designed and produced to meet a growing need for a receiver in which all batteries are eliminated, and with a much finer and fuller tone quality than has hitherto been available. The receiver is of the neutrodyne type, employing five tubes in the receiver and one rectifying tube in the a. c. power supply unit. It is made under Hcenses from Hazeltine and from the writer. Chief Engineer Garod Corporation The power conversion system does two things; first, it develops an alternating current suitable for excitation of the filaments of the amplifier tubes, and second, it develops a direct current for supplying the plate current of all the tubes and the filament current of the detector tube. The power-supply unit is mounted in the table portion of the receiver, and is entirely enclosed in an enameled sheet-iron box. The top and sides are removable as a whole without disturbing the enclosed apparatus which is mounted on the base-board. The opening or lifting of the lid opens an automatic safety switch which cuts the line circuit so that the user can not make accidental contact with high voltage terminals. The unit is designed to operate on a. c. lines of 60 cycles, 110 to 125 volts. It will not operate on 25 or 40 cycles, or on direct current, but seems to operate satisfactorily on a 50-cycle line. To provide for voltages which differ slightly from the usual 1 10volt standard, the transformer primary is tapped at different points which are designed to take care of any voltage variation in the mains and still provide the same value of voltage for the vacuum tubes. There are three secondary windings on the power transformer, one of 7.5 volts and 3 amperes for exciting the filaments of the amplifier tubes in the receiver; another of 7.5 volts and 1.5 amperes for exciting the rectifier tube filament; and a third of 525 volts and 60 milliamperes for providing the a. c. for rectification of the B supply power. To prevent the passage of electrostatic disturbances from the lighting circuit to the receiver circuits, a shield winding is provided, consisting of a layer of wire insulated from, and between, the primary and secondary windings of the transformer. One end of this winding is connected to the transformer clamping frame and grounded through the oower box and receiving set ground. The line current from the house lighting socket enters the power box through the attachment cord provided, one wire of which has connected in it the operating switch mounted on the front of the table. This side of the line is connected through one of the fuses on the output terminal board to one side of the transformer primary. The other side of the line enters the power box and connects directly to the other fuse, and from this through the automatic switch, to the other side of the transformer primary. These fuses are provided to protect the power converting equipment from overloading, which might result from excessively high line voltage, or from connection to direct-current or 25-cycle lines, which would produce excessive current in the transformer, and which might burn it out. House lighting fuses ordinarily are of 15-ampere capacity, and would provide no protection whatsoever for the power equipment. The power consumed by this receiver is approximately 100 watts, the primary current being about tV of an ampere at 1 10 volts. The fuses are rated at one ampere and will open the circuit should the primary current of the transformer exceed one ampere. No apparatus other than the A winding on the transformer is necessary for providing the filament current. The system, therefore, is exceptionally simple in this regard, as no conversion system from alternating current to direct current, such as is used in most power driven receivers, is necessary. The power equipment provided is used chiefly for B power of high voltage and high current capacity, which is so desirable in providing distortionless reproduction at the required volume. The B supply portion of the power equipment consists of the rectifier tube, the high-voltage