Radio age (May 1922-Dec 1923)

RADIO AGE— "THE MAGAZINE OF THE HOUR" One of the greatest troubles experienced in radio frequency or reflex work is the tendency of the tubes to oscillate and howl. Even though radio frequency transformers are very carefully designed and constructed, they cannot be made to give the same results on one type of tube that they will give on another, consequently a particular tube must be decided upon, and the particular radio frequency transformer which works best with it, should be used. The Acme transformers were used in the set described when UV-201A amplifying tubes were used in combination with a UV-200 detector. This combination has been found to work best, while with other types of tubes, other makes of transformers seem to give better results. This does not mean that this particular type of transformer is better than the others, but merely that this arrangement gives a happy combination which works very well. In order that the construction may proceed step by step, and a check up of the conditions may be made as the work advances, the radio frequency part of the circuit should first be connected up and tested. When this part of the circuit is wired up and found to work, then the audio frequency part may be added, but it would be useless to add the audio frequency until this part is found to function properly. To aid the builder in this test. Figure 3 is shown. It will be noticed that the arrangement for making each tube do the same amount of work makes a circuit somewhat different from an ordinary radio frequency circuit. The upper part of Figure 3 (shown in light lines) represents the radio frequency component of the circuit. In order that it may be tested as straight radio frequency first, we must temporarily forget about the lower part of the drawing (shown in heavy lines) and make temporary connections as follows: No. 9 to No. 1, No. 2 to negative "B" battery, No. 3 and No. 6 to the 80 volt positive terminal of the "B" battery, and No. 4 to No. 5. No. 7 is connected directly to the 22>^ volt terminal (positive) of the B battery, and the phones are placed between No. 7 and No. 8. These numbers are shown in the circles in the center of Figure 3. Remember that during this test, these numbers simply represent the terminals of the wires from the radio frequency component and that none of the wires represented by the heavy lines has as yet been put into the set. When the connections have been made as described, place a UV ■foo OHM ® ATT -I|l|l|l|l|l|lh: /6 22 ^ nnyj v^Tcri, OAC/C ® -aMMM mmm ■^ llliillllllll^ 60 "TO Po fZa-TS AfT 4--1 Figure 2. The finished hookup of the Grimes Circuit. 200 detector tube in the last socket (to the right of drawing) and a UV201A in each of the other sockets. This is now an ordinary radio frequency circuit, composed of two steps of radio frequency and a detector. This should now be tried out and if signals come in well, it may be assumed that this much of the circuit is O. K. Before making this test, or in fact, before connecting up the "B" battery, each of the .001 fixed condensers should be tested with a buzzer to prove that they are not short circuited. If no buzz is obtained then they are all right and the "B" battery may be connected. The condenser which is connected across the terminals No. 4 and No. 5 will give a buzz when tested, as this is shorted by the connection between these terminals, but all the others must test clear, or the filaments will be burned out by the B battery current getting through them. After testing the radio frequency circuit and it is found to function, then the temporary connections are all disconnected and the audio frequency component is added. This is plainly shown in Figure 2, which is a duplicate of Figure 3 with the exception of the numbering system used for the temporary connections. In connecting up the radio frequency transformers, the terminal marked "P" should go to the plate terminal on the socket, and the terminal marked "G" must be connected to the grid of the next socket as shown. However the proper connections to the audio frequency transformers may require some experimenting as to the direction in which these currents pass through. This merely means that the two connections on either the primary or secondary coils may have to be reversed, but they must not be changed from one coil to another. The principal requirement in the building of this set is plenty of time and patience. Much howling and squealing will be encountered until one learns just how to adjust the set. This can usually be remedied by turning down the rheostats on the amplifying tubes, but one will soon learn just how to take care of this. All connections must be soldered carefully, and if paste is used as a flux, it should be wiped off^ with a little alcohol after soldering. Most any kind of a loop will work well with the set, but to get the best results a loop 16 to 18 inches across each side is recommended. This should be wound with from 8 to 12 turns of wire, spaced about yi of an inch apart. Either solid or stranded wire may be used. No. 18 wire is very good, but single strand lamp cord affords a much more flexible winding. The exact number of turns must be found by experiment, but the number will be somewhere between 8 and 12. Of course the smaller the loop, the less energy it will pick up, but results have been obtained even on a 4 inch coil, such as is used for the primary winding of a variocoupler. Send $1 to Radio Age, 500 North Dearborn Street, Chicago, and receive this leading hook-up guide for six months. Regular subscription price, $2.00 a year.