Radio Broadcast (May-Oct 1925)

5& Radio Broadcast amplification. In changing wave bands, the coils in each of the five mounts are replaced by different sizes. By selecting the proper values, any frequencies used to-day for transmission of radio telephony or telegraphy can be received. Returning to the circuit, switch St is the usual series-parallel switch which adds considerably to the tuning possibilities of the antenna tuning-condenser Cr. d is preferably a forty-three plate variable condenser. C2 and C3 are secondary tuning variable condensers each having a capacity of .0005 mfd. C4 is a .006 mfd. Micadon by-pass condenser. C3 is a .0025 mfd. bypass. C6 is the usual .00025 mfd. grid condenser. C7, C8, and C9 are the isolating-coupling condensers of the resistance-coupled amplifier, all being Micadons of .006 mfd. capacity. C10 is an output bypass condenser, capacity .006 mfd. which may or may not be necessary in individual receivers. Rr is a three hundred to four hundred-ohm potentiometer which stabilizes the r. f. circuit. R2 is the conventional 2-megohm grid leak across the grid condenser. R3 is a General Radio ten-ohm rheostat. R4 is a dismantled twenty-ohm rheostat placed in series with the small three-volt pilot lamp, PL. This lamp is located behind a colored glass jewel on the panel and is an effective and attractive signal that the tubes are burning. It is not, of course, essential to the operation of the receiver. R5 throughout the resistance-coupled amplifier represents the coupling resistors of one hundred thousand ohms resistance. R6, R7, and R8 are amplifier grid leaks, having respective values of 1 megohm, \ megohm, and } megohm. The coupling resistors, coupling condensers, and amplifying tube grid leaks are combined for efficient compactness into three Daven Resisto-Couplers. Daven resistors are used throughout the amplifier. The initials on the diagram represent the initialing on the couplers. Jx is a standard closed circuit jack, placed in the plate circuit of the first audio frequency tube. This is preferable to plugging-in on the detector. Jack j2 is an open circuit jack with filament control. Switch S2 turns on all filaments when the loud speaker plug is in jack J2, and the first three tubes with the plug is in jack Jx. ...... The 4.5-volt C battery while not altogether necessary, is desirable. Particular note should be taken of the amplifier grid leak connections. LIST OF PARTS THE circuit diagram, Fig. 1, represents the following parts used in the construction of the receiver: One three coil Branston Mounting Two Cotocoil mounting brackets 5 Na-ald Sockets 2 .0005 mfd. variable condensers 1 .001 mfd. variable condenser 1 Midget vernier condenser (across C3) 1 series parallel switch 1 Cutler-Hammer battery pull switch 1 General Radio ten-ohm rheostat 1 400-ohm General Radio potentiometer 1 .00025 mfd Micadon 5 .006 mfd. Micadons 1 .0025 mfd. Micadon 4 Daven grid leak resistors, 2 meg., 1 meg., \ meg. and \ meg. 3 Daven 100,000-ohm coupling resistors, 1 7-inch x 2 1 -inch bakelite panel 3 Daven Resisto-Couplers 8 Eby binding posts 1 Pacent closed circuit jack 1 Pacent open circuit, filament control jack These parts represent an approximate cost of thirty-five dollars. To this price must be added the expense of whatever honeycomb coils are selected for reception of various frequencies. CONSTRUCTION THE constructional details of the all-wave receiver are clearly suggested in the panel layout, Fig. 3, and in the photographs of the completed receiver, Figs. 2, and 4. Referring to the back of panel photograph, Fig. 2, the Cotocoil single honeycomb coil mountings are screwed to the baseboard near the right hand (rear view) end. Coils Lt and L2 are plugged into these receptacles. Partly hidden and to the right of the coils a resistance strip from a rheostat can be discerned, fastened to the baseboard. This is placed in series with the small three-volt pilot lamp as described in reference to the circuit diagram. The pilot lamp itself is screwed into a small miniature socket from which the porcelain shell has been removed. It is placed beneath the antenna tuning condenser, and the glass jewel can be seen in the lower left of Fig. 4. The large dial controls, in the panel photograph Fig. 4, are, left to right, tuning condensers, Cr, C2, and C3. The lower left is the series-parallel switch. The upper right hand knob is the midget vernier condenser across the tuning condenser C3. Below the vernier is the potentiometer. In wiring the receiver, particular care should