Radio Broadcast (Nov 1926-Apr 1927)

496 RADIO BROADCAST MARCH, 1927 secondary winding of the transformer, the choke coil, several condensers, and resistors. The rectifier tube and transformer winding together provide a half-wave rectifier for the a. c. supply, and the filter system converts this rectified a. c. into practically pure direct current. Directly across the rectifier output is a 4-mfd. condenser which is flash tested at 3000 volts before assembly at the factory. Its purpose is to provide a reservoir for d. c. energy and a bypass for a. c. energy. In series with the positive side of the filter line is the choke coil marked A in the accompanying diagram. It is a 50-henry choke having two air gaps of 0.0 1 inches formed by ten-mil paper between the laminations forming the two halves of the choke coil's magnetic circuit. This choke coil has a resistance of approximately 600 ohms and consists of about 7000 turns of copper wire. Across the filter line, on the output side of the choke, is another 4-mfd. filter condenser which functions as a d. c. reservoir and as an a. c. bypass to further smooth out the ripple of the rectified current The voltage across the first 4-mfd. condenser should measure approximately 400 volts (r. m. s.). The actual peak value of the voltage across this condenser is approximately 525 volts. The second 4-mfd. condenser has approximately 364 volts (r. m. s.) across it, and its rated operating voltage is 600 (the same as the first one). The plate supply of the second audio or power tube is taken from the filter output on the output side of this filter choke coil. A reduced B voltage is obtained for the radio and first audio-amplifier tubes of the receiver by connecting in series with their plate supply a 5000-ohm resistor. This resistor is at the front of the power box at the right-hand side. To ventilate it properly, holes have been provided beneath it and above it in the power box lid so that air may circulate and prevent overheating. These same holes also provide for ventilation of the rectifier tube. Directly across the filter line, between the B and the output side of the 5000-ohm resistor, is a 100,000ohm fixed resistor which is located in the rear right-hand corner of the output terminal board. It is soldered in the connecting clip provided for it. This resistor prevents excessive voltages in the filter circuit when the set load is taken off by the removal of all the tubes. To the left of the 100,000-ohm resistor is a 500,000-ohm one of similar type mounted in the same manner, which provides a plate current of reduced voltage for the detector tube. The voltage of the input side of the 500,000-ohm resistor, or directly across the 100,000-ohm resistor, is the voltage which feeds the two radio and the first audio-amplifier tube plate circuits. It should measure about 150 to 175 volts, depending upon the adjustment of grid voltage for these tubes, as will be explained later. Across the output side of the 500,000-ohm fesistor supplying the detector plate circuit is a i-mfd. condenser whose function it is to still further eliminate the slight alternating current ripple superimposed upon the direct current output of the filter system. Inasmuch as the audio frequencies of the plate circuit of the detector tube are twice amplified, that is, by the first and second audio tubes, the plate circuit supply for the detector tube must be perfectly smooth and without any variation. THE OUTPUT DEVICE INCLUDED in the power box assembly, be1 sides the power conversion equipment proper, is the output choke (B in the accompanying diagram) together with a i-mfd. condenser, the two combining to form an efficient output device for the protection of the loud speaker and to better quality. The plate circuit of the second audio tube is fed from a point on the output side of the filter choke, A, through the output choke B. This latter choke is, of course, connected in series with the lead to the plate circuit of the second audio tube. The output choke for the loud speaker coupling is mounted on the rear side of the transformer. Air gaps in the two choke coils are provided for the purpose of giving maximum in COMMERCIAL A.C. OPERATED RECEIVER It is the Garod EA receiver, the circuit diagram of which is shown on the following page ductance to the chokes at the particular value of direct current flowing through the coils. If the air gaps provided by the paper spacers were omitted entirely, magnetic saturation of the iron core would result, and this would produce a considerably lower inductance in the coil than if saturation did not occur. It has been found that, for a given amount of direct current in the coil, a definite size of air gap will provide the maximum amount of inductance. Choke A, which must carry about 60 milliamperes of direct current, requires a considerably larger air gap than the choke B, which carries only the plate current of the second audio tube (approximately 25 milliamperes). The two 4-mfd. filter condensers, the detector bypass condenser, and the i-mfd. loud speaker output condenser are all mounted in a condenser unit installed beneath the output terminal board. The terminals on the power box are arranged for connection to the cable leads coming downward from the receiver. The pilot light receives its illumination from the rectifier tube and thus an extra lamp for this duty is dispensed with. The rectifier tube socket is mounted in such a position that the majority of rectifier tubes fit in it with the long axis of the elliptical plate in a vertical position, so that no danger of a sagging filament touching the plate will be encountered USING A LOOP Hp HE radio circuit of the EA receiver, as has A been stated is of the neutralized tuned radiofrequency type, consisting of two such stages, a detector, and two stages of transformer-coupled audio-frequency amplification. The receiver is designed for use with antenna and ground, and these, in as good a form as possible, should always be used. In locations, however, where it is impossible to put up an outdoor antenna, an indoor antenna or a portable loop may be used. A loop may be connected directly across the antenna and ground binding posts and should be tuned by means of the radiofrequency volume -control knob (left-hand small knob) which operates a series tuning condenser. In some localities, where many powerful broadcasting stations are located and interference is troublesome, a loop may be preferable to an outside antenna. By tuning the loop, and by virtue of the directional effects obtained by turning the loop, selectivity of a considerably higher order is obtained over that of an outside antenna. The range, however, is reduced. When an antenna is used, the series antenna condenser is of great service in controlling the radiofrequency input to the receiver. It is also possible, by means of this condenser, to change the electrical length of the antenna within wide limits so as to change the overall selectivity of the receiver. When the condenser is all in, it is shortcircuited so that the full antenna pick-up is available. As its capacity is decreased, the energy transfer is decreased, and the selectivity of the receiver is increased, in the same manner as if the actual physical length of the antenna itself were reduced. Coupled to the antenna through the first radio-frequency transformer primary is the tuned input, or grid circuit, to the first radio-frequency tube. A bypass condenser of 0.006 mfd. is connected between the low-potential side of this grid circuit and the filament for the purpose of bypassing radio-frequency currents which otherwise would be compelled to flow through various portions of the receiver, including particularly the grid biasing resistance common to the plate circuits of the receiver. The plate circuit of the two radio-frequency tubes is bypassed by a 0.01-mfd. condenser to the filament circuit for the same purpose as the grid bypassing condenser. The radio-frequency transformers consist of low loss bakelite tubes upon which are wound the primary and secondary coils of the transformer. In the operation of the vacuum-tube filaments on a. c, it is necessary to use negative biasing on the grids to eliminate a certain type of disturbing hum which would otherwise be produced. The biasing necessary for this purpose, and for the normal biasing required for the audio tubes to prevent distortion, is obtained from the voltage drop across the resistors in the negative