Radio age (Jan-Dec 1925)

RADIO AGE /or June, 1925 The Magazine of the Hour 39 tube is reflexed back to the first tube (Tl) by means of the audio frequency transformer (AFT-1), the latter being in the grid return circuit of the first stage. This can be any make of transformer having a ratio varying from 4-1 to 6-1, but as we wish to gain every inch possible, I have shown the miniature Premier Hegehog transformer in the picture diagrams. In some cases, a 0.00025 mf fixed condenser (K4) improves results when connected across the secondary coil of (AFT-1), and again, this seems to have but little effect. It seems to be a matter of experiment with each individual set to determine whether (K4) should be used. Its effect is principally on DX rather than on volume with local stations, so that we should try for distance in making this adjustment rather than to experiment for volume alone. The output of the reflexed tube (Tl) now passes to the primary coil of the second audio transformer (AFT-2), and this latter transformer is a part of a straight audio stage that is not reflexed. Connections are made to (T3) in the usual standard manner, and the total output of all three tubes passes out through the output jack (Jl) to the phones or loud speaker. This is not a complicated circuit to hook up, but it requires some readjustments as with any reflex circuit, particularly in regard to the values of the bypass condensers. For the smallest portable set, three 4.5 volt "C" batteries are used for the filament current as at (A1-A2-A3), the cells being in parallel and connected to the circuit through the battery cutout switch marked (SW). 22.5 Volts for Detector T)LATE or "B" battery connections * are tapped according to the require # ments of the various stages. A voltage of 22.5 volts generally proves best for the detector circuit under all around conditions, although 45 volts may give greater volume and selectivity on local stations. A potential of 45 volts is most effective on the radio frequency tubes on distance, hence a 45 volt tap is indicated for this stage. The audio stage requires 90 volts for the best performance, and 67 volts gives nearly as good results with one less block of "B" battery. The set can also be operated with 45 volts on the audio stage, but with greatly diminished volume on all stations. I do not recommend placing the full 90 volts on the radio frequency stage, and never on the detector stage, and after experimenting extensively I find that the best all around results will be found with the plate battery connections as indicated. Four small B batteries (90 vo'lts total) can be used for this set and will last most of the season. The smallest B batteries are the 450 milliampere-hour cells which measure 2" wide, 3-3-8" long and 2-9-16" high. The next largest size take up very little more space and give much longer service. This is the 1200 milliampere-hour size which is 2-9-16" wide, 4-1-6" long and 2-3-4" high. The latter size are the more practicable, especially with three tubes, and are shown in the assembly diagram. With three tubes kicking out from 10 to 12 milliamperes, the 450 m. a. h. type does not last very long before the voltage runs down and the volume falls off. In the table on page 38 is listed all of the material required for building this set, each item in the list being preceded by a letter corresponding to the letters on the diagram. All of these parts are standard and the majority are built by a number of radio concerns so that it will not be difficult to pick up all of the parts at your dealer's. The only special parts are the cabinet, which must be built to fit the job at hand, and the tuning coils and RF transformer, which can easily be wound up at home. A neutroformer or tuning unit can be purchased for use in place of RFT-1, but RFT-2 is special and is not stocked. Outdoors or Indoors — You Will Find All Your Radio Needs Satisfied In RADIO AGE Every Month. Another Blueprint Hookup In July RADIO AGE— Out June 15 Fig. 2 shows all of the parts connected up in "picture" form for the benefit of the novice who does not understand conventional or symbolic diagrams. Either Fig. 1 or Fig. 2 can be used in making the actual connections, for both show the same circuit and the parts are lettered with corresponding letters. Fig. 3 is a rear elevation of the set with the back panel removed and shows how the parts are assembled, ready for wiring. The three tubes and sockets are mounted on the top of the shelf (E) while the audio transformers are hung underneath. This not only saves room but it also shortens and simplifies the wiring. Of course, machine screws must be used for this assembly instead of the more usual wood screws, as all parts are fastened to bakelite. The shelf is attached to the panel by means of the small brass angle brackets (S) which can be made at home or purchased at almost any radio store. I wish to call your attention to the fact that the two radio frequency transformers or tuning coils (RFT-1) and (RFT-2) must be placed at right angles to one another, as shown, to prevent coupling back between stages and to prevent oscillations being set up by induction. The coils are supported by brass lug connections to the terminals of the variable condensers so that their weight is substantially supported. It should be particularly noted that the stator or staBlueprints of the 3-Tube Portable Reflex on Two tionary plates of the variable condensers (C1-C2-C3) connect to the grids of the tubes to prevent the body capacity effect from being carried out to the front of the panel through the shafts of the condensers. The grids are at a high potential, and anything connected to the grids is easily affected by the capacity of the hand and is detuned by this capacity effect. ■ All of the batteries are carried in the bottom of the cabinet, very closely packed together to prevent movement, and connections are made with the circuit above through the flexible cords (X). Connections are more certain and more easily made if "spade" type clips are soldered to the ends of these cords. Do not use solid wire for this purpose or simple cotton covered wire, as such wire is likely to short-circuit. Flexible fixture wire has a rubber covering which is further protected by a cotton braid, making a short circuit unlikely. The Aerial Wire ' I 1HE aerial wire can be a temporary -* affair run around the picture moulding of the room, strung up temporarily from room to room, hung between trees or other supports. It is connected to the antenna post (ANT) with the other end left free and unconnected. For indoor service about 60 feet of annunciator will be sufficient and no supporting insulators will be needed, as the waxed cotton cover will be sufficient insulation when laid along the plaster of the walls or along wood surfaces. For outdoor work, or where it is likely to be damp, a wire with rubber insulation should be used, such as flexible fixture wire or lamp cord. Lamp cord is excellent for this purpose, as it has a low R. F. resistance and is sufficiently flexible to allow winding up in a small coil. After cutting the batteries into circuit by means of the battery switch (SW), the detector rheostat (R2) and the equalizing condenser (C3) are adjusted until all whistling and howling stop. There should be a slight hissing or frying noise which will indicate that the tubes are functioning, but the adjustments should not be much above this point. Next, turn the wavelength adjustment condensers (CI) and (C2) very slowly and at about the same rate of speed until a "station whistle" or voice is picked up. Juggle the condenser dials until the signal is at a maximum, and then manipulate the detector rheostat (R2) just under the point where it is about to break down into free oscillations and where the signal is at a maximum. Working the detector rheostat in connection with the condender (C3) will give the maximum volume. Remember that (C3) is for the purpose of checking oscillations in the radio frequency circuit, and that this controls the radio frequency circuit in about the same way that the detector rheostat controls the detector. Howling can be checked by either (C3) or (R2) depending upon whether the trouble is in the radio frequency or detector circuits. _ The selectivity depends upon the (Turn to page 42) Pages Following