Radio age (Jan-Dec 1926)

10 RADIO AGE for January, 1926 shown in the list of parts, and is a handy thing to have around the experimental bench, its use in the set will be dependant upon the type of tubes used and the difficulty of preventing the first tube from oscillating. In using the series resistance in the plate circuit of the r. f. tubes a nice control of undesired oscillation may be obtained, but if any difficulty crops up it would be a good idea to make use of the type N for neutralizing the grids of the first two tubes. It is very difficult to lay out hard and fast rules for these things since there are so many variations in the application by the individual set builder. Better have a couple of the type N on hand anyway for use at odd-times. In mounting the Thordarson audio transformers, the 2 to 1 is in the first stage and the 3 to 1 in the last stage. The electrical connections are shown in Fig. 1, the schematic. For the two filament connections on the audios a piece of bus bar wire may be run between the two binding posts and a piece of flexible wire attached to this bus wire, so it may reach upto the left front of the panel where the C bai tery is located. Tapping this flexible wire on different values of C battery will give the set maker an idea of the amount of C bias he wishes to apply to the grids of the audio stages. With 120 volts about 6 volts minus may be used. Lower B battery voltages will need a drop in C bias so as not to have the audio tubes drawing too little plate current with a consequent thinning out of the signal. Change the C battery bias until you get a full rich tone. To secure control of oscillation by means of right hand rotation (clockwise) of the Centralab variable resistor it will be necessary to make the left hand resistor connection the B battery line, while the right hand one is the common wire from both the double-toroid r. f. primaries and one side of the Dubilier 1 mfd. bypass condenser (whose other side goes to the negative filament). When the pointer is at the extreme left the total resistance of the unit will be in series with the plates of the r. f. tubes and no oscillation can occur. As the pointer is turned to the right the resistance will be decreased until a point where the Ugher B battery voltage will cause the tubes to go into oscillation. The point just before the tubes go into oscillation (accompanied by a howl) is the proper place for the set to be worked. The resistor w ill have to be changed slightly from wave change to wave change. The maximum resistance will be in during use on the extremely low waves while on the higher waves the resistance is decreased bit by bit until on KYW at 77 degrees on the second dial there will be no resistance in the circuit at all and the tubes will be securing the full value of the B battery voltage. To Get Going Assuming you have checked and double checked the wiring to make sure you have made no errors in the assembly of the apparatus, you are now ready to tune in stations. Insert your tubes one at a time and see they light properly. The Daven filament resistors take care of the quarter ampere tubes without any need for a rheostat and since we are not using a critical detector tube like the UV 200 it will not be necessary to have a rheostat for the detector tube. This means a saving in the form of one control at least and considerable peace of mind. Put the antenna clip on the antenna tap nearest the top end of the doubletoroid (see photographs Figs. 2 and 4) and the ground on the left hand filament connection of the first tube. Use the second condenser for finding your stations, the one on the left being not so critical as the tandem condenser used on the second r. f. and detector stage. The trimmer on the rear section of the Bremen-Tully tandem should be set at zero, that is, its plates should be disengaged. The front trimmer should be used to equalize the capacity discrepancy between the first ahd second sections due to any difference in tube capacities, wiring variations and other causes. The Magazine of the Hour Tune in a weak signal with the second condenser, setting the first one at approximately the same dial reading as the second one. Then rotate the front trimmer condenser on the tandem( which has a little knob on the front of the panel) until you have built up the signal to its maximum. During this operation to get the balance perfectly it will be necessary to do a little changing of the resistor in the plate circuit. When this is done it will be possible to get the signal strong and clearly defined on a single split-hair on the second dial. The first dial will be nearly the same reading, but its tuning will be slightly broader than the second one. In our operation of the set on an admittedly poor attic antenna in one of the suburbs of Chicago, 226 meters came in at zero on the second dial. This wave is allocated to WBBM and WIBO. From this point all through the scale up to 77, within one hour, the stations shown in the box in this article were picked up with good volume, on some extraordinary volume all on loud speaker. However to get best selectivity we put the bias of the r.f. tubes at. 1 }/£ volts. The volume dropped some from that obtained when there was no bias on the tubes, but it permitted a little sharper tuning for stations in the local area which had a tendency to overshadow some of the out of town stations. For the city dweller this type of a set would be better if it were to be thoroughly shielded with copper. The cabinet maybe wood with copper sheet shielding. If you intend screening, do a good job and shield the shole thing. You might even (we would do it if living in the city) make use of a copper panel, mounting all apparatus on the panel with bushings where no contact is desired, and letting the parts like condenser rotors touch the panel. This would allow you to operate within the shadow of a number of the local boadcasting stations without interference from their transmitters. For those who live in the country and are (Turn to page 12) Top view of set showing all parts, but urxwired. Refer to Fig. J, schematic, for correct electrical wiring diagram. 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