Radio Broadcast (May 1928-Apr 1929)

96 RADIO BROADCAST DECEMBER, 1928 Loop Aerial C 4f>2 V. FIG. I. COMPLETE SCHEMATIC DIAGRAM OF THE "SKYSCRAPER" RECEIVER The values of resistance used in the filter circuits are such that both of the r.f. tubes and the first audio tube may be supplied with 180 volts from the power supply, and the filter resistance will reduce this voltage to the correct value for the operation of the particular tube. An example will make this clear. Consider the first audio stage, in which socket is recommended the use of a ii2A-type tube. The d.c. plate resistance (not the a.c. plate resistance) of this tube is about 20,000 ohms. This 20,000-ohm plate-circuit resistance is in series with a 20,000ohm filter resistance, R8, across 180 volts. Therefore, half the voltage will appear across the tube and the other half across the filter resistance. Therefore, there will be about 90 volts on the plate of the 1 i2A-type tube. Volume is controlled in this receiver by varying the potential applied to the screen grids of the r.f. tubes, this adjustment being accomplished by Rio, a 6000-ohm potentiometer. In series with this 6000-ohm potentiometer is placed a fixed resistance, Rn, with a value of 20,000 ohms so that not more than the rated value of 45 volts can be applied to the screen grids. On the front panel of the receiver are four controls, besides the on-off switch. The two small knobs at the lower-right and left-hand corners are the filament rheostat and the volume control, respectively. The drum dial on the left tunes the antenna condenser, C2, and the righthand dial tunes the other two condensers, C3 and C4, which are ganged together. For the past several months development work on the "Skyscraper" receiver has been in progress in the laboratories' of the Ferranti Company. For this reason the writer feels certain that those who construct the set will obtain as satisfactory performance as he has from the various models which have been undergoing tests. Optimum performance can be assured only by following as closely as possible the arrangement of apparatus illustrated in the pictures on these pages and in the wiring layouts supplied with the construction booklet. For this reason it is recommended that set builders, who contemplate building the receiver, send for this booklet. If, after the construction has been completed, the set does not perform in an altogether satisfactory manner the wiring should be checked carefully and the various tubes should be tested. It is also a wise plan to examine the B power-supply device, as poor results may be caused by incorrect plate voltages. LIST OF PARTS TP HE apparatus used in constructing the * model of this receiver illustrated in these pages is given below. The total cost of the parts listed is $95.00. The builder may substitute electrically equivalent parts. The last four items in the list, and also the tuning coils, are especially designed for use with this receiver, and may be obtained by writing directly to the Ferranti Company. Those who desire to construct this set can also obtain a booklet giving complete constructional data from the Ferranti Company for $1.00. The list of apparatus follows: Ci One Tiny-Tobe fixed condenser, 0.0001mfd;. C>, C.3, C4 Three Remler SLAV condensers, 0.0005-mfd.; C5, C6, C7, Cs, C13, C14, C15, Ci6, Cn, Cis Ten Tobe by-pass condensers, 0.5-mfd., type 300; C9 One Tobe condenser, 4-mfd., type 240; C10, Cn, C12 Three Tobe condensers, 2-mfd., type 202; Li, L2-L3, L4-L5 Three Ferranti tuning coils, " Skyscraper"-type; L.6 One Hammarlund r.f. choke coil, type RFC 85; Ri One Tobe fixed resistor, 50,000-ohm, 2-watt; Ji One Yaxley jack, open-circuit; J2 One Yaxley jack, closed-circuit; R2, R3, Rs Three Tobe resistors, 10,000-ohm, 2watt; R4, R5, Rg, Rn Four Tobe resistors, 20,000-ohm, 2-watt; R6, R7 Two Tobe resistors, 50,000-ohm, 2-watt; Rio One Carter midget potentiometer, type MVV-6M; R12 One Carter rheostat, 6-ohm, type 506; . R13, R14, Two Carter resistors, 15-ohm, type J-5-I5; Ti One Ferranti transformer, type AF-5; T2 One Ferranti transformer, type AF-5C; T3 One Ferranti transformer, type OP-8C for magnetic speakers, type OP-4C for dynamic speakers; Two Remler tube shields, type 56; Three Hammarlund shaft couplers, type FC; Three Na-Ald sockets, type 424; Three Na-Ald sockets, type 428, for plug-in coils; Three Benjamin sockets, type 9040; One pair of Benjamin brackets, type 8629; Two National drum dials, type VF with type-28 illuminators; Two Carter screen-grid connectors; type 337; One Yaxley mounting plate and cable, 7-wire; Four Eby binding posts, insulated; Ten Lynch resistor mounts; Acme flexible wire in colors to match Yaxley cable, for wiring; One Aluminum base, 10-gauge; drilled; One Bakelite panel, 8" X 24" X tY', walnutfinish, drilled; Three Aluminum Co. of America standard shields, drilled; Thirteen rubber insert rings, for feed lines through base. THIS PICTURE SHOWS THE EXACT ARRANGEMENT OF PARTS ON THE CHASSIS