Radio Broadcast (May 1928-Apr 1929)

14 RADIO BROADCAST MAY, 1928 frequency current is bypassed to filament through a i-mfd. Tobe condenser, Cs, which in the photograph can be seen mounted on the baseboard beneath the regeneration condenser. The copper box is grounded to the positive A binding post. No rheostat, filament switch, or voltmeter is provided on the receiver because it is designed primarily to be used with the screen-grid tube super-heterodyne described in the article following this one in this same issue, and common A and B batteries are used, the A battery connections for the receiver being taken off the filament terminals of the second detector socket of the superheterodyne. In this way the filament switch on the super-heterodyne controls the filaments of all the tubes, and the voltmeter reads their filament voltage, which is adjusted by the master rheostat. The plug-in coils are mounted between §" bakelite strips provided with General Radio type 274-p plugs. The socket, or base, into which the coils are inserted, is a General Radio 274-B base, which retails complete for $1.00. The plugs on the coils must be so spaced that they will plug into this standard base. The tickler is mounted next to the antenna coil when it is of the same diameter as the latter, but in the larger coils, to save space, it is mounted within the secondary coil. In any case, the tickler should be at the filament end of the secondary coil. The coils are wound on a bakelite tube which has been cut in half diametrically and held together by Secondary ( ^OIL Tickler Coil No. oj Turns Diameter (Inches) Size of Wire (D.C.C.) No. of Turns Diameter (Inches) Sim oj Wire (D.C.C.) Frequency (Megacycles) Wavelength (MetersApprox.) 4 9 U 2) 37 54 1 1 1 j 18 18 18 18 18 18 18 22 3 4 6 10 13 16 ■9 22 22 22 22 22 22 22 16.7 -26.7 11.5 -18 6. 9.4 4.2-6. 52 2.87 46 2. 1 3.33 1 .37 2. 17 18 -1 1 . 2 26. 1-16.7 36 2 — 22 6 50 -3I.-9 71-4-46 1 04 . 5-65 . 2 143 -90 210 -138 :n!NIi;i:m:i: ■ i,-:w:i ri-riiinnru'ii-iiniri !:■■■ i-i .: ■ nrM A TABLE OF COIL SIZES Comp ete details of the number of turns for the secondary coil, Li, and the tickler coil, L», for various frequencies are incorporated in this table Brass Ring drilled & tapped Machine Screw if CM nd ■TO | "■*' * 5" -Bakelite Tubing "s Brass end Ring FIG. 2 Constructional deta Is for the coil form AN INTERIOR VIEW OF THE RECEIVER The parts are numbered for cross reference to the list of parts in the column immediately to the right brass end rings. Fig. 2 shows the construction of the coil form. The desired number of turns is wound on the form and given two light coats of collodion. When the coil is dry the screws are removed from the end rings, the latter are slipped out, and the bakelite form is pushed together so that the coil can be slipped off. The inside of the coil is then given a coat of collodion to add to its mechanical strength. The accompanying table gives data on the number of turns, size of wire, diameter of the coil, and the frequency band covered with the tuning condenser used. In the front view of the receiver are shown the 175-meter coil, the 80-meter coil, and the 20-meter coil, while the interior view of the set shows the 40-meter coil in place. Elaborate constructional details have not been given in this article for it is felt that those interested will be sufficiently versed in the art of amateur set construction to build the receiver from those data presented. The layout of apparatus can be distinctly followed by reference to the photographs, and the circuit diagram is so simple as not to require an elaborate explanation. The receiver as described is complete and ready for operation with any audio amplifier. It was primarily designed, however, for code reception, and particularly to be used in conjunction with the super-heterodyne unit described in the article beginning on the next page. The following parts are required for the singletube receiver described: List of Parts Ci — General Radio 15-Mmfd. Midget Variable Condenser C3 — Cardwell 50-Mmfd. Variable Condenser C2 — National 250-Mmfd. Variable Condenser C4 — Sangamo loo-Mmfd. Fixed Condenser C6 — Tobe i-Mfd. Fixed Condenser Rj — Tobe 8-Megohm Grid Leak Li, L2 — Plug-in Coils (Specifications in Text) L3 — Samson No. 85 Choke L4 — Samson No. 3 Choke General Radio ux Tube Socket Six Eby Engraved Binding Posts General Radio No. 274-B Coil Mount General Radio Rubber Socket Cushion Two 4" National Velvet Vernier Dials 7" x 12" x TV Bakelite Panel 10" x 1 1 \" x rV Wood Baseboard iOj" x 85" x T%" Wood Baseboard Copper Box with Cover iof " x g%" x 6" Brass Angle, Brass Strip, Angles, etc.