Radio Broadcast (Nov 1926-Apr 1927)

308 RADIO BROADCAST ADVERTISER -NEW PREEMINENT among fine radio receiving units is the Nathaniel Baldwin line. PREEMINENT in this line of units is Nathaniel Baldwin's latest triumph — now available in THE BALDWIN "99" Loud Speaker $28.50 LIST Attached to any receiver whether a three tube set em' ploying forty volts or a ten tube set employing four hundred volts. It will correctly and euphoniously render all tones — low or high — to the utmost pleasure and satisfaction of the trained or untrained musical ear. ORDER TODAY DEALERS AND JOBBERS Can secure detailed information regarding territory and distribution available by writing today. J.W.andW.L.WOOLF Eastern Distributors, Exporters for NATHANIEL BALDWIN, Inc. 227 Fulton St., New York City Tel. Cortlandt 6.V9 B Supply Device Relay UPON installing a B supply device in my home it was most convenient to place it in the basement where the wet B batteries had formerly been. This necessitated either going into the basement to turn the unit on and off or installing a switch for the i io-volt current near the set. I went both of these alternatives one better ToSel A Battery TollOV. AC. Supply HWIr Line supply unit FIG. 5 by constructing from an old automobile reverse current generator relay an automatic switch for the supply unit. Now all I have to do is to pull the A-battery switch on the set and the B supply device cuts in with no further attention. The sketch, Fig. 5, shows the schematic wiring. The core was stripped of both its original series and shunt windings and was rewound with No. 18 d.c.c. (the number of turns will be governed by the number of tubes and current consumed by the set). I used about 30 turns for a set pulling 2 amps. The armature was insulated from the frame with sheet mica. There is practically no arc at the points, and no condenser is needed. H. N. Streit, Boone, Iowa. Keeping Radio Frequency Currents from the Audio Circuit THE statement is often made that the primary of a good audio transformer has such a high impedance to radio frequency currents that they must use the bypass condenser or be blocked. It is the writer's experience, however, that a healthy radio frequency impulse can somehow get through such a rough passage and cause distortion. The author has hit on the scheme, not of trying to choke off these impulses, but of balancing them out. From Fig. 7 it is apparent that there are present in the audio transformer intentional or parasitic capacities, Cp across the primary, Cs across the secondary, and Cc between the primary and secondary. The transformer is drawn in the usual fashion to call attention to the fact that one end of the primary is nearer the I I cP zi= I I — i B + — i 1 ! I ,JL_ 1 -4 Antenna Mast Installation HAVING occasion recently to install a 15-foot antenna mast on the flat roof of an apartment building, I devised a method of securing the mast without having to fasten the lower end with screws. The roof was the usual combination of tar and gravel, and, as it was impossible to fasten the mast to a chimney or other projection from the surface of the roof, I finally decided to guy the mast both top and bottom. After getting the mast in position and securing the guy wires from the top, I simply ran additional guy wires from the lower end of the ' pole to such points as I could find, three in all, thereby securing both ends of the pole and making it impossible for it to work loose in a high wind. The sketch, Fig. 6, illustrates the method used. The pole (in this case 1 inch iron pipe was used) should be screwed to a wooden block about 8 inches or 10 inches square, so that its end will not wear a hole in the lar of the roof. W. T. Mithoff, Detroit, Michigan. if Examined and approved by Radio Broadcast it FIG. 7 filament end of the secondary than the grid endIn a good transformer, the secondary capacity is kept small but, compared with the inter-coil capacity, it is larger. Fig. 8 shows how, by the addition of a plategrid capacity Cn, a balanced or Wheatstone Bridge effect is obtained. The value of this condenser must be found by trial and is not very large. The author found one of 0.0002 mfd. was best with an Amertran De Luxe. Remember, if you have two condensers of say 0.0001 and 0.00025 mfd., that in series they have a value of 0.00007 mfd. and in parallel a value of 0.00035 mfd., so that two condensers make four values available for trial. Connect the end of a condenser to the grid of the first audio tube and listen with the wire to the other end of the condenser alternately on and off the "P" terminal of the first transformer. It will no doubt be noticed that quality is unmistakably improved. H. D. Hatch, Boston, Massachusetts.