Radio Broadcast (Nov 1926-Apr 1927)

AFTER SOME CHANGES HAVE Radio Broadcast Photograph BEEN MADE By incorporating the changes in the Hammarlund "Hi-Q" receiver as outlined in the accompanying article, it is possible to operate the receiver with a loop antenna Improvements Substituting Dry Cell Tubes — How a ize This Popular Hammarlund Receiver Technical Editor IT WAS Lincoln who said : "You can fool some of the people all of the time and all of the people some of the time, but you can't fool all the people all of the time." Before asking oneself what this has to do with radio, and the Hammarlund "Hi-Q" receiver in particular, pause for a moment to hear all of the story. Of the many receivers which are offered to the home-constructor for home assembly, the Hammarlund Roberts "Hi-Q" enjoys the greatest popularity. Even the fellow who buys a commercial type of receiver has the urge to tinker with the set so that it will fit in with his particular ideas of what a set should be. How much more so this is true of the builder of a receiver such as the Hammarlund-Roberts "Hi-Q" is a question the answer to which is reflected in this article. Applying the mania for improving, altering, changing, to the quotation above, it might be made to read: "You can satisfy some of the set builders all of the time, and you can satisfy all of the set builders some of the time, but you can't satisfy all of the set builders all of the time." Pursuing this line of reasoning with respect to the "Hi-Q" receiver results in the bringing out of these salient points: 1. How may dry cell tubes of the 199 type be used with the "Hi-Q" receiver? 2. How may a loop be employed, particularly by the city dweller who requires unusually fine selectivity to tune-in to the locals without experiencing broadness of tuning? 3. How may the sensitivity of the receiver be increased? 4. How may neutralization t>e made simpler? Whether or not any of the changes as outlined above are seriously considered, it is recommended that at least the simple changes under classification No. 4 be made, thus making for better and more complete neutralization of the receiver. In the original circuit, no provision was made for a bypass to the negative filament for the r.f. currents. This sets up the possibility and probability of inter-coupling between adjacent tube circuits through the battery leads. When the bypaths are provided, the r.f. currents are kept out of the battery leads and the possibility of inter-coupling is greatly reduced, and in many cases completely eliminated. Two o. i-mfd. fixed condensers are required to make this addition. The condensers are mounted, one in the second r.f. stage can. and the other in the de At Radio Broadcast Laboratory, an investigation into these various problems was instituted with results which are most gratifying. Radio Broadcast Photograph CHANGES IN THE FILAMENT CIRCUIT A filament ballast replaces the four-ohm resistance strip which was formerly connected in the first r.f. filament circuit. In this illustration the additional filament ballast is that one at the extreme left of the sub-panel tector stage can. To mount them, it is only necessary to unscrew one of the rear wood screws of each can which holds the can bottom to the wood base, and insert one of the feet of each condenser under the head of these screws. One contact of the condenser connects to the B-plus terminal of the primary coil in its respective can. The other contact connects to one of the wood screws holding down the can, and thus completes the bypass circuit direct to ground. The circuit diagram for this addition, step No. 4, is shown in Fig. 1 . USING DRY CELL TUBES I N EXPERIMENTING with dry cell tubes, ' such as 199's, it has been found advisable to control the filament of each tube with its own Amperite because, when neutralization of the receiver is attempted, disconnecting the filament wire from the first socket applies an over-load to the filament of the second r.f. tube since both the filaments of the first and second r.f. tubes are connected in parallel. Controlling each tube with its own filament ballast will solve this problem. Incidentally, this change might even be applied to receivers employing the 5-volt type of tubes because it is much simpler to remove a ballast from its clips than it is to unsolder a wire from the socket when engaged in the process of neutralizing the receiver. In the detector and audiofrequency circuits, filament ballasts are already provided, but for drycell operation, the value of the ballast differs from the value used for storage-battery tube operation. Type 4V Amperite is satisfactory for all but the last audio stage, wherein a 120 tube is recommended. This tube requires the type No. 120 Amperite. To incorporate the additional filament ballasts in the circuit, first unsolder the fixed resistance strip