Radio Broadcast (May 1928-Apr 1929)

ceo? $6 Our Readers Suggest 99 OUR Readers Suggest — " is a clearing house for short radio articles. There are many interesting ideas germane to the scioice of radio transmission and reception thai can be made clear in a concise exposition, and it is to these abbreviated notes that this department is dedicated. U 'bile some of these contributions are from the pens of professional writers and engineers, we particularly solicit short manuscripts from the average reader describing the various "kinks," radio short cuts, and economies that he necessarily runs across from time to time. A glance over this "Our Readers Suggest" zl'H1 indicate the material that is acceptable. Photographs are especially desirable and will be paid for. Material accepted will be paid for on publication at our usual rates with extra consideration for particularly meritorious ideas. — The Editor. An A. C. Screen-Grid Booste IN THE August Radio Broadcast there appeared the description of "An Extra R.E. Stage For Any Receiver," employing a d.c. screen-grid tube. I was very much interested in this arrangement, but as 1 desired to use it in conjunction with an a.c. receiver, 1 thought it logical to redesign the circuit for the Arcturus a.c. screen-grid tube. The altered diagram is shown in Fig. i. The following is a list of the parts used in the construction: 2 — Frost ux-type sockets: i — broadcast-range plug-in coil; i — Variable condenser, 0.0003 5-mfd.: 1 — Frost fixed condenser, 0.00025-mfd.; 2 — Silver-Marshall r.f. choke coils: 3 — Tobe by-pass condensers, i-mfd.; 7 — Eby binding posts; 1 — National dial; 1 — Front panel, 7" x 0": Baseboard or sub-panel: Corwico Braidite for wiring: .Miscellaneous hardware; 1 — Power Clarostat; 1 — Step-down transformer, 15-voIt; 1 — Frost strip resistor. 500-ohm; 1 — Arcturus screen-grid tube, 22-A.C-type. The unit is wired to antenna, ground and receiver as indicated. The output post is led to the Arcturus A C 22 antenna post on the receiver. An Transformer Arcturus 22-type tube is plugged into the socket. The ground is connected both to the receiv er and to the booster. The dial on the booster functions as an additional tuning control. The power Clarostat is placed in series with the primary of the stepdown transformer and is employed to regulate the output to fifteen volts for the Arcturus tube. The voltage is best determined by means of an a.c. voltmeter, however, if this instrument is not conveniently available, the voltage may be adjusted by noting the time lag between the turning on of the current and the heating of the tube to its normal operating temperature. When the voltage is correctly adjusted, the lag will be approximately 30 seconds. Peter L. Jones. Boston, Mass. Work Bench Clamp mfd. To Choke Loud Coils Speakei THE presence of wires indiscriminately strewn about the test bench is hardly conducive to efficient work, to say nothing about the possibility of short circuits, wronsr connections and general i-l I — O To An!. Post on FIG. 3 lack of order. I have found it decidedly worth while to equip my test bench with several simple clamps designed to hold the various wires to the table. A simple clamp for this purpose is shown in the picture, Fig. 3. It was assembled from odds and ends out of the junk box. A small strip of thick wood provides the base. Near one end a recess is filed out for the cable. The latter is held in position by a narrow strip of ebonite which acts as the upper jaw. Slightly off the center, a countersunk hole is drilled in the wood block for a iVinch screw, and the ebonite strip is forced down by a small wing nut. Although a spring washer is used between nut and ebonite strip, the latter will have a tendency to turn when the nut is tightened. To prevent this, the other end of the ebonite strip has FIG. 2 been slotted; in this slot lies a stop — a nail driven into the wood block and snipped off about \" above the surface of the latter. C. A. Oldroyd, Lanes, England. Push-Pull with Standard Transformers LTOR clear and undistorted output, with abun* dant volume a well-made push-pull amplifier is the standard of comparison. The transformers specially made for such circuits have a center-tapped secondary winding in the first stage, and a center-tapped primary winding in the output stage. Yet a standard audio transformer may be used with excellent results if an external center-tap is provided, as indicated in Fig. 2. Two 100,000-ohm resistors are connected in series across the secondary. The grid bias is applied at the central junction of the two resistors. The value of the resistors is not critical, and slightly higher or lower values may be used if the experimenter happens to have them at hand. The only other special part needed for a push-pull amplifier is the output transformer or a center-tapped output choke through which the B current reaches the plates of the power tubes. A commercial choke may be used in this position, but the same results can be secured by using two standard chokes connected in series. The inner turns of the two chokes are joined by a connecting lead, to which the B-plus lead is wired. Two fixed condensers are connected as shown in Fig. 2 to keep the loud-speaker windings at a low potential. C. A. Oldroyd, Lanes, England, Home-made Soldering Lug O UNNING out of soldering lugs in the midst l^ of a radio construction job, I twisted the loose ends of the stranded hook-up wire together and bent them in the form of a hook. To reinforce the stiffness of the lug it was tinned thoroughly. While the solder was still soft, the hook was flattened with pliers, thus providing a better contact surface. This lug is no harder to make than it is to solder the wire to a conventional lug— and it costs nothing. Edward Piranian, Philadelphia, Pa.