Radio age (Jan-Dec 1925)

RADIO AGE for Aueust, 1925 The Magazine of the Hour 79 Uniformity of Tubes Vital in "Super Circuit Diagrams Fig. 1 is the schematic circuit drawing of the super-heterodyne developed by Mr. Posth of the Radio Doctors, Inc., Chicago, and which has been built by a number of amateurs with excellent results. It is not a radical departure from conventional practice, but is a simple, compact layout which is easily constructed by the novice in "dyne" circuits with an excellent chance of getting results immediately after the completion of the set. It is the result of nearly a year's continual experimenting by one who has alternately added and then eliminated various experimental features which have been brought up from time to time in heterodyne development until the present circuit was arrived at. Fig. 2 is a picture diagram of the hookup which will be of service to those who have not yet delved into the mysteries of conventional diagrams. Here each part is drawn out as it actually appears in the proper proportion, with the wiring runs located at the most advantageous points. The letters and figures on Fig. 1 correspond to those marked on Fig. 2, so that the relation between the two drawings can be easily traced out. For convenience, Fig. 2 is divided into two parts. The lower half of the drawing represents a plan view of the baseboard and apparatus as it appears to the observer on looking straight down on the set. The upper half is the rear view of the panel as seen from the rear of the assembly. Between the upper and lower views we see the connecting wiring drawn in heavy lines, which connect the apparatus mounted on the baseboard to that attached to the rear of the panel. Arranged in this way, the connections are easily followed. Fig. 3 is a front elevation of the panel which shows the controls and the center to center dimensions between the various units mounted on the panel. As will be seen from the blueprints, the panel is 8°x32 "xl/4", a reasonable size for a super-heterodyne and a panel not much longer than that used with many tuned radio frequency outfits. The circuit is arranged exclusively for use with a loop aerial, and with this arrangement the loop plays no small part in gaining absolute selectivity in districts where there are a number of broadcasting stations located within a short distance of the receiver. Starting in with either Fig. 1 or Fig. 2, depending upon the experience of the reader, we note the two variable condensers (CI) and (C2) mounted on the panel at the right. Both are of the low loss type with external vernier adjustments, and both have a maximum capacity of O.OOOS microfarad or the capacity of the standard 23 plate. Condenser (CI) controls the oscillation frequency of the oscillator tube (1) and the oscillator coil (OS). Condenser (C2) tunes the loop and the grid circuit of the first detector tube (2). These are the only tuning controls used and therefore the actual operation of tuning is much simpler than with the usual tuned radio frequency set. Further along the panel we have four filament control rheostats (R1-R2-R3) with resistances varying according to the number of tubes that they control. Rheostats are based on the use of 201A tubes throughout both for the amplifiers and detectors. The three rheostats marked (Rl) have a resistance of 20 ohms and control respectively the oscillator tube (1), the first detector tube (2) and the second detector tube (6). Rheostat (R2) controls the three radio frequency tubes (3-4-5), and because of the greater current has a resistance of only six ohms. Rheostat (R3) has a resistance of 15 ohms for the control of the two audio tubes (7-8). Watching Potentiometer A T (PO) is a 400 ohm potentiometer or -^* stabilizer which controls the grid potential of the radio frequency stages. In actual operation the potentiometer has a marked influence on the volume and selectivity but is not frequently used after the set is set into operation in the same sense that the condensers are used. A potentiometer of lower resistance is not recommended, as it does not give sufficiently accurate control of the grid potential. A voltmeter (VM) is desirable for indicating the potential across the filaments of the radio tubes, but it is not absolutely essential. By means of this voltmeter (0-10 volt scale), the tubes can be kept accurately to the point of greatest sensitivity. An ammeter, shown by (AM) gives the total current consumed by all tubes in the circuit. A battery switch as at (SW) is very convenient and is an insurance against the accidental burning of the tubes after leaving the set for the night. It makes the complete readjustment of the rheostats unnecessary when the set is used the second time. All of the above apparatus is mounted on the panel as shown by the . upper view of Fig. 2. Three output jacks are provided. Inserting the plug into jack (JD) gives reception from the tubes up to and including the first detector tube (6) and this corresponds to the detector tube circuit of the ordinary regenerative circuit. Plugging into jack (JI) gives the addition of one stage of audio amplification, while jack (J2) includes all of the tubes or two stages of audio. Experience has shown that one stage of audio is all that is required for loud speaker operation on all but the faintest and most distant stations. An oscillator coil (OS) of the fixed winding type is located between the first detector tube (2) and the oscillator tube (1). The functions of this inductance coil have been described before. This coil is very compact and requires no adjustment. The inside bakelite tube is 1.5 inch in diameter and carries about four turns of wire near its center which corresponds to the "pick-up coil" of the usual heterodyne oscillator coil. The outer tube is 2.5 inches in diameter and carries both the grid and plate coils of the oscillator circuit. The grid coil carries 20 turns of No. 26 D. S. C. wire and the plate coil consists of 40 turns of the same size wire. The latter is spaced about one half inch from the grid coil. A detail of the oscillator coil is shown in Fig. 4 where the external plate and grid coils are clearly seen. Theinnerand outer tubes are mechanically connected by short pieces of small fiber tubing through which brass screws are run. When tuned by the 0.0005 mf condenser (CI), this oscillator will fully cover the ordinary range of broadcasting wavelengths. At (2) we have the first detector tube with the grid condenser (K2) of 0.00025 mf capacity and the grid leak (GL) with a resistance of two megohms. As will be seen from the plan view in Fig. 2, the oscillator coil and the two tubes (1) and (2) are located well back on the baseboard, so as to clear the variable condensers (C1-C2) indicated by the dotted lines. The outline of the baseboard is indicated by (F). A bypass condenser (Kl) has a capacity of 0.005 mf. Next in order come the radio frequency stages consisting of the tubes (3), (4), (5) and the long wave radio transformers (RD1-RD2-RD3-RD4). All of the transformers are of the iron core 45 kilocycle type and are tuned to work in agreement with the oscillator by means of the fixed condensers (K1-K4). (RD1) is the input and (RD4) is the output transformer. Any iron core of 45 kilocycle type can be employed. Condenser (K4) has a capacity of 0.00025 mf. Owing to the body capacity which is sometimes in evidence, it is frequently desirable to ground the metal cases of the transformers as indicated by the dotted line (g). With the particular transformers shown in Fig. 2, the tube sockets and transformers can be set very close together, about 2 7/8 inch centers. The transformers are of the metal shielded upright cylindrical type, which lend themselves nicely to compact formation. The grid post (G) of the output transformer (RD4) goes to the grid condenser (K5) and grid leak (GL) of the second detector tube (6). The grid condenser (K5) has a capacity of 0.0005 mf, while the grid leak has a resistance of 2 megohms. 201A Tubes Used BY using 201A tubes throughout with a current consumption of 0.25 ampere per tube, the total current is only 8x0.25 = 2 amperes, the exact amount of current taken by a five tube neutrodyne when a soft detector tube is used. As a soft detector of the "200" type take^ about one ampere and introduces a certain amount of hissing tube noise, its use is not recommended in this set. Owing to the high potentials on the grids of the tubes, it is necessary to use the highest grade of sockets to insure against leakage and internal capacity effects. For the same reason, the bottoms of the sockets should be raised well above the