Radio Broadcast (May 1928-Apr 1929)

JUNE, 1928 A THREETUBE A. C. OPERATED ROBERTS RECEIVER 101 wiring the balance of the set Celatsite or similar hook-up wire is recommended. CONVERTING YOUR D. C. ROBERTS FOR the conversion of a storagebattery-operated Roberts set to complete a.c. operation, the 'wiring diagram and the instructions already given are perhaps sufficient. The substitution of a B power unit for a B battery needs no explanation. Obtaining grid bias without C batteries, though sometimes confusing, should be clear from the diagram. A C-battery in the grid circuit puts the grid voltage below the filament voltage, whereas a resistance between the filament circuit and ground places the filament voltage above the grid voltage. The effect is the same in either case. The C batteries may first be removed and connections made in their place as shown in the diagram. Next, the filament leads of each tube in the d.c.-operated set may be increased in size if necessary to carry the heavier current taken by a.c. tubes. [Wire tables showing current carrying capacity will give this information. Or see Radio Broadcast Laboratory Information Sheet No. 141, November, 1927, issue. — Editor.} Care must be taken in this changeover to a.c. not to remove any leads which serve some purpose other than lighting the d.c. filaments. A "uy" socket may now be substituted for the ux socket in the detector position, and the same grid and plate leads that were connected to the ux socket go to the uy socket. If an output transformer or filter is not in use, one should be installed as shown. In the list of parts given here, some names of equipment are mentioned because of successful experience with these makes. Perhaps other makes will give equally good or even better results, but some experimenters often like to have something definite in mind when they do their purchasing. In the case of the variable conden THE ALL IMPORTANT CONTROLS ARE HERE sers, a rugged low-loss construction such as General Radio is strongly recommended. The parts required for conversion of the d.c. set to a.c. are indicated separately at the end of the list. The values given for fixed condensers, resistances, etc., will generally be found satisfactory, but they may be experimented with after the set is in operation, for possible improvement to suit the taste of the user. LIST OF PARTS USED Ci, C2 — General Radio 0.0005-mfd. variable condensers. C3 — Hammarlund 9-plate neutralizing condenser. C4 — Dubilier 0.00025-mfd. fixed condenser with clips. C5 — Dubilier 0.0005-mfd. bypass condensers. C6 — Dubilier 0.002 5-mfd. bypass condenser. C7, C8 — Aerovox 1.0-mfd. bypass condensers. Ri — Clarostat volume control resistor. R2 — Tobe Tipon grid leak (about 2.0-meg. al although various values should be tried). R3 — Hardwick-Field C-bias resistor, 2000 ohms. R4 — Ward-Leonard C-bias resistor, 1000 ohms. R5 — General Radio center-tapped resistor. Ti — Amertran AF-6, 5:1 audio transformer. T2 — Thordarson R-200 audio transformer. T3 — Acme Apparatus AC-2 filament transformer. T4 — Bremer-Tully output device. • — Frost open-circuit jack. ■ — General Electric tumbler switch. 5 — Spiderweb coil forms and 2 coil mounting arms. — Wire for coils: ;-lb. No. 22 d.c. c; £-lb. No. 26 d.c.c. 2 — Benjamin ux sockets. 1 — Benjamin uy socket. 2 — Marco dials. 5 — Eby binding posts. — No. 14 lamp cord for a.c. circuit. — 25 ft. Celatsite hook-up wire. Test clips, screws, nuts, baseboard, and Westinghouse Micarta panel. ADDITIONAL PARTS I — B-power unit supplying the following voltages: 45, 90, 135, 180 1 — Type 227 a.c. tube 1 — Type 226 a.c. tube. 1 — Semi-power tube, either 112 or 171 type The coils used in the set described are home-constructed according to directions . in the article. Sickles or HammarlundRoberts coils may be used if the builder prefers to use manufactured coils. The circuit and building instructions deal with the 226 and 227 type of a.c. tube although as indicated in the article, the Sovereign or Kellogg heater type a.c. tubes can be employed equally well if the constructor makes the necessary filament circuit and mechanical changes required. If one has on hand a d.c.-operated Roberts receiver which is to be changed over to a.c. operation, it will of course be possible to use in the revised set most of the parts contained in the d.c. model. Only the following items in the above list of parts need be purchased, in changing a set from d.c. to a.c. operation : C7, Cs, R3, R4 R6, T3, one Y type socket and a line switch. THE BASEBOARD ALLOWS ROOM FOR A B-POWER UNIT This provides complete a.c. operation with maximum simplicity