Radio Digest (July 1924-Apr 1925)

January 31. 1025 RADIO DICES T— Illustrated Construction of the Six Tube Hetduogen ON looking over the preceding article of this series the writer notes that a very important item was omitted tooth from "the list of parts and figures 10 and 11, the wiring layouts. This is unfortunate for those in rural districts who have some difficulty in obtaining parts locally as it takes several days to secure parts by mail. The item in question is a .5 mfd. fixed condenser indicated in figure 2, and connected across the oscillator B+ lead and A+ lead. We are indebted to Mr. Michael Diller of Buffalo, N. Y., for calling this to our attention in his letter of January 15, in time to be included in this concluding article. As to placing it, refer to figure 11. Just to the right of holes 4 and 5, and above binding post Z, is a space on the underside of the baseboard. The condenser can be secured there with its terminals to the left. One terminal connects to post Z, the other to the +A bus where it goes up through hole 8. We can all feel indebted to M. Joseph Murry, of Titusville, Pa., for a first-class suggestion on soldering tinfoil. To quote his letter of January 12, "I find the operation is simplified by having the foil pressed against a cold metal surface, when, after a little practice, a drop of solder may be applied from the point of the iron without melting the tinfoil." The writer tried this today and it facilitates the operation greatly. Try it first on some foil scraps and then on the foil segment of your filter condenser. We have left to this chapter two points which some might say more rightly belong to the wiring explanation. These are the use of two extra binding posts shown, and the high resistance unit at the right of the sub-panel. The two binding posts are shown so that the builder may use C battery grid bias on the audio amplifier tubes, if it should be found to be of benefit. As to whether the C battery is a necessity or not depends a great deal upon the particular transformers used and, while it is considered good practice to use the C battery, the writer has found that , when using certain transformers, the third stage of audio amplification has been more easily controlled without it. The use of three stages of audio has usually been the occasion for the insertion of some method of doing away with the howl which is almost invariably produced in the third stage bulb. Several methods of connecting-in condensers and high resistance leaks are prescribed for this condition, any one of which may produce the desired result. Controlling Three Stages The method we will recommend in this instance is to connect the variable high resistance directly across the grid and plate of the third audio tube. One side of the unit is to be connected to the grid post of the socket, and the other side of the unit should go to the plate post of the socket. Referring back to the use of the C battery, it has been found that with some transformers, this method will not produce as good a result as if the C battery is not used. Therefore, we advise leaving out the C battery for the initial try-out, and if it can be found to be of advantage during later trials you may make, the C battery can be connected in. Now assuming that the set is completely wired, and ready for the first tryout, the first thing to be done is to connect Hill Part VI — Adjusting and Operating By C. E. Brush up the batteries, aerial and ground. Reading from left to right, the binding posts at the rear edge of the sub-panel' are as follows: Aerial, ground, A battery negative, A battery positive and B battery negative, B positive 45 volts, B positive 90 volts, last two for C battery. The aerial recommended for this set is a single wire, 50 to 75 feet long, exclusive of lead-in. The ground should be the usual water pipes, radiator pipe, or rod driven into moist earth. Hook up aerial, ground and batteries as above and with the loud speaker plugged in at the second stage jack, and the filter coil primary as close to the secondary as possible, we are ready to pull the filament switch and get into operation. It is assumed that no errors have been made in wiring, and that from the mechanical point of view we are actually ready to adjust and tune the set. Pull the battery switch, and turn the rheostats full on. The dials are assumed to have been placed in position such that when the condenser plates are entirely out, the dials are at zero. First Regeneration Starting with the center dial, we find that as we rotate this dial there is no noticeable result until it is turned in to about 15 to 20, when a change is noted in the sound coming from the horn. If followed closely, this change will be noted to take place with a distinct thud, produced when the dial reaches a certain position. This thud denotes the fact that the first detector has been thrown into the oscillating condition, by result of over-sufficient feedback or regeneration. This is as it should be, and shows that we will be able to control the amount of regeneration (and volume) with this 3-plate condenser. Now with the 3-plate condenser turned in far enough to produce the oscillating condition in the first detector, if we turn either of the other two dials we will note a series of distinct clicks as the dial is rotated, which in slow motion are noted to be individual whistles. These whistles are the result of the oscillations from the oscillator tube mixing with those present in either or both of the detectors, by reason of over sufficient feedback, or regeneration, in either of the detectors. These are not carrier wave whistles and cannot be tuned in, and before any station can be tuned in, it will be necessary to adjust at least the second detector feedback to a point just below the oscillating condition. The special condenser built into the filter coil is for controlling the feedback or regeneraton in the second detector, and the adjustment of this condenser is the next thing to be explained. By setting the center dial on the panel at zero, we know that we are not producing oscillations in the first detector, and in this position we can make the adjustment on the regeneration in the second detector. Adjusting? Second Regeneration Move the copper plate section of the special condenser until the copper plate is covering the tinfoil plate completely. This is the maximum capacity position, and the amount of regeneration produced by this setting of the condenser has brought the second detector to the oscillating point, which is evidenced by the series of whistles noted when the right hand dial on the panel is rotated. We must reduce the feedback in the second detector until we get the tube just out of the oscillating position. By adjusting the special condenser a little at a time towards the mini RADIO STAMP ALBUM Includes list of broadcasting stations in United States, Canada and Europe, also handy Radio Log. Spaces for stamps issued by broadcasting stations for each state. Finished on high grade paper and handsomely bound with cloth cover. A future, lasting record for your radio. If your nearest dealer is unable to supply you we will ship C. O. D. subject to satisfactory inspection. Price $1.60. Start Your Album Today DES MOINES PUBLISHING COMPANY Hubbell Building, Des Moines, Iowa Introducing Mr. E. T. Jones IN THE next issue, Radio Digest will publish the first of a series of articles on Radio theory and practice, written by Edward T. Jones of New Orleans. The author of this series is a practical Radio man, much of whose knowledge was gained in the "ham amateur" days before 1912 and in service as a "brass pounder'' on the United Fruit boats. His explanations and suggestions, as contained in these articles, can therefore be taken as facts, not theory, and may be followed without hesitation. During the war he was in charge of the Naval Radio School and later transferred to research work on underground antennas. Mr. Jones has written technical articles for nearly all the Radio publications and is the author of several books on Radio. Like other old timers, his favorite sport is experimental work. Among the many attributes attached to Mr. Jones we might mention that Leatrice Joy, well-known motion picture star, is a cousin, and has set a high goal for Mr. Jones to live up to. mum capacity position, we gradually reduce the regeneration, until at a certain point the tube will cease oscillating, which condition can be known by the loss of the whistles which were found when turning the oscillator condenser dial, and which are now absent. Having adjusted the feedback in the second detector to the point where the tube stops oscillating (a few trials will show the point of best operation for this setting), we can forget this particular part of the apparatus and the rest of the tuning will be done with the dials on the panel. Dials Tune Together As is the case with other regenerative sets, the feedback control should always F» THE SILENT m »» RANC£ n Up tol20 Volts of "B" Battery Can Be Charged Without the Usual Wire Changing! 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With this dial set at a point near to the oscillating point of the tube, we begin our hunt for stations on the other two dials. These two dials run approximately even (or together) for a particular wave length or station, and should be rotated slowly in unison until a station is heard. If you are within a few miles of a broadcasting station, you will have no trouble in tuning it' in, but even on the nearby stations you will note immediately the sharpness of the tuning, (Continued on page 20) Super-Het] Builders! "Radio" and other leading publ ica tions recoram end in highest terras the Thordarson 2:1 ratio transformers for the Best SuperHeterodyne — Take no others 1 fine set builder! Ttf>#«p TRANSFORMERS Troqf they're Best! Chosen— after stiffesttests— by themajority of Quality set builders. Not surprising, however, when you remember they 're made and unconditiondTlv guaranteed by the world's oldest and largest exclusive transformer manufacturer. 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