Radio Broadcast (May 1928-Apr 1929)

272 RADIO BROADCAST SEPTEMBER, 1928 Choke Bell Trans FIG. 4 upon the tube and the plate potential. These voltages should be varied until best results are obtained. Using a 201 a tube, with 45 volts on the plate, a 4.5 volts C-bias will be correct. Joseph A. Gardner, Los Angeles, Cal. A Cheap B Battery Substitute THERE remain many receivers operated from B batteries due to the relatively high cost of power supply devices. A cheap but quite satisfactory socket power device can be built up in accordance with the diagram of Fig. 4, using two bell -ringing transformers having 6 and 12-volt secondaries. The 6-volt winding of one transformer is used to light the filament of the rectifying tube (a 201 a with the grid and plate strapped) while the 12-volt secondary is connected to the 6-volt winding of the second transformer, which will then output 220 volts from the primary, acting as a step-up transformer. If the drain to the receiver is comparatively low, not exceeding twenty milliamperes, the primary of an audio-frequency amplifying transformer can be used as a choke coil. The values of the condensers are indicated on the diagram. Resistors Ri and R? are conveniently combined in a Duplex Clarostat, while R3 is a fixed 5000-ohm resistor. C. H. Meissner, Woodcliff, N. J. Noisy Reception and Soldering Flux THE soldering flux that is used in the construction of radio apparatus is generally considered a relatively inconsequential item in the building of a receiver, regardless of the fact that the use of rosin is recommended in the majority of constructional descriptions. Asa service man whose endeavors are divided between the ailments of home-made and commercially built receivers, I do not think this consideration has been adequately emphasized. Too much publicity cannot be given to the detrimental effects of the average soldering flux. Unnecessary radio noises form fifty per cent, of radio troubles, and in my own experience no less than nine out of every ten cases of noise can be traced directly to the use of an acid flux or paste. Static, poor connections, run down batteries, and antique grid leaks do not produce more than ten per cent, of the -O150V. O90V. 022.5 V. noise difficulties encountered in home-made receivers. Nothing but rosin should be used to facilitate soldering. While the use of rosin necessitates careful scraping of joints, a very clean and hot iron, the extra trouble is more than compensated by permanently quiet reception. Once noise has developed in a receiver an attempt, occasionally successful, can be made to clean the suspected parts, such as socket and transformer terminals and especially cracks adjacent to soldered connections. The terminals and surrounding territory should first be cleansed with an alkali, such as household ammonia. This in turn is washed away with water. A generous application of wood alcohol completes the ablutions. This sometimes works — not always. As usual an ounce of prevention, in the way of rosin-core solder, is worth a pound of cure. A. G. Woodward, New York City A New Use for Burned Out Tubes A BURN ED out tube may be used as a neutralizing r.f. circuits condenser in many where the neutralizing capacity must be approximately the same as the grid-plate capacity of the r.f. tube. No connection is made to the filament. The difference in capacity between tube in the lighted and unlighted condition is so small as to be of no importance when used as a neutralizing capacity. A great advantage of this stunt is that no adjustment is required. Of course, the neutralizing tube will have to be of the same type as the r.f. tubes. R. Wm. Tanner, Berkeley, Cal. Mounting Single Hole Condensers With Special Dials T ENCOUNTERED difficulty in mounting a *■ single hole type of condenser with a Marco dial so that the rotor would turn without binding. This particular make of condenser would not fit the mountings that came with the dial. By adding an extra bracket to the frame, I was able to mount the condenser in a manner altogether satisfactory. This bracket is made of tV' brass strip f" wide and 3" long. Two holes are drilled near each end with a f" drill. The size of the center hole varies with the make of condenser. A f hole is drilled in the frame of the dial at A. A slotted hole comes drilled in the frame of the dial on the other side of the shaft. The condenser is mounted on the bracket and the shaft inserted in the condenser hub as is made clear in Fig. 5. The bracket is then adjusted to fit and is bolted to the frame with two small bolts. The rotor is positioned so as to give the correct reading on the scale and the set screw is then tightened. The condenser is then ready for mounting on the panel of the receiver. Stuart E. Moody, Rochester, N. Y. Two Receivers From One Antenna TWO radio receivers may be efficiently operated from one antenna if connected as shown at A in the diagram, Fig. 6. The size of the radio-frequency choke is not critical but it should have low distributed capacity. The one 1 am using is one inch in diameter wound for four inches with number 26 d. c. c. wire. Both sets must be of the non-oscillating type. Either common or separate ground connections may be used. C. H. Campbell. Bridgeport, Conn. STAFF COMMENT THERE are several other simple ways of operating two or more receivers from a single antenna. Different systems work better with different receiver combinations. Two additional systems are shown in B and C of Fig. 6, B being the better of the two methods. h 2 _j c 1-T-^ An Extra Power Stage of ResistanceCoupled Amplification AN ADDITIONAL stage of power amplification can be added to practically any receiver without touching the wiring of the set itself. A resistance-coupled power stage is assembled exterior to the receiver and is merely plugged into the output jack of the set (or wired to the output binding-posts, as the case may be). Additional connections are, of course, made to the A, B, and C batteries, while the loud speaker is plugged into the output of the power amplifier. The power tube may be left in the receiver, or a lower power tube substituted for it, and the bias changed accordingly. The usual power tube arrangement prevails in the added stage. Archie H. Klingbeil, Ashtubula, O. STAFF COMMENT THIS additional stage of power amplification is not limited to resistance coupling, but may be employed with single transformer, impedance and push-pull circuits. In cases where the receiver is equipped with a choke coil output device, the coupling condenser and the resistor can be dispensed with, only the grid lead to the extra tube being necessary. An external stage of amplification is not practicable on receivers already employing three steps of audio, or with sets provided with output transformers.