Radio age (Jan-Dec 1925)

80 RADIO AGE for August, 1925 The Magazine of the Hour face of the baseboard by means of spacers or liners, say about 1/4 to 3/8 inch above the board. For the best results all tubes should be carefully matched by the dealer before delivery, for all of the tubes in the radio frequency stages at least must have exactly the same electrical characteristics. When so many radio frequency tubes are connected up in cascade (series), and when the transformers are exactly matched as they should be, any small difference in the tube characteristics will cut down the output to an alarming extent. Matched tubes may cost slightly more than tubes taken out of stock at random, but they are well worth the money. Any one who has constructed a neutrodyne set knows how greatly tubes of the same make and type vary among each other, and how difficult it is to get dissimilar tubes to act together. Large bypass fixed condensers must be used to shunt the radio frequency currents around the windings of the potentiometer and across the resistance of the "B" batteries. This is even of more importance with long wavelengths than at broadcasting frequencies and the capacities of the condensers must be correspondingly greater. Condenser (K3) has a capacity of 0.5 microfarad and is used to shunt the R.F. current around the potentiometer windings. Fixed condenser (K6) has a capacity of 1.0 microfarad and shunts the "B" battery. Smaller condensers should not be used. Last are the two audio frequency stages at the extreme left of the board. Tubes (7) and (8) are the first and second audio tubes respectively, while the audio frequency transformers will be seen at (AT). In general, these two audio stages are the same as any audio stages but owing to the nature of the super-heterodyne, it is necessary to filter the output by means of certain fixed condensers so that the second stage can be worked without noise and distortion. To use these stages "straight" without filters means trouble as soon as the output is taken from the second stage through the jack (J2). Any high grade audio frequency transformer can be used for this purpose. The ratio of the first stage should preferably be from 3/1 to 4/1 while the ratio of the second stage transformer can be 5/1 to 6/1. Higher ratios are general not advisable. Grid biasing by means of the "C" battery is most essential to the proper operation of the set. It at once promotes clarity of tone and effects a saving of "B" battery current in the audio frequency tubes. For a plate potential of 90 volts, a three cell 4.5 volt "C" battery will give the best results with the 201A tubes. There is no current drain to speak of on this battery and it can be the smallest type of three cell battery procurable. While most high grade audio transformers are well shielded, yet it is safest to place them at right angles to one another as shown in Fig. 2. This eliminates any danger of noise or interference. Two fixed condensers are connected across the primary and secondary of the first stage audio transformer (ATI). Condenser (K4) has a capacity of 0.00025 mf. while (K8) is a 0.001 mf. size. Another filter fixed condenser (K4) is connected between the grid (G) and the (— ) post of the "C" battery at (K4) and has a capacity of 0.00025 mf. This completes the audio frequency stages except for the three jacks (JD-J1-J2) which are interconnected with the stages as shown. The Loop Connections At the extreme upper right hand corner of the panel in Fig. 2 are the two binding posts for the loop connection. It is best to use binding posts and to avoid the use of a jack at this point as a jack introduces objectionable capacity into the circuit and also permits of some leakage of the already weak radio impulses. As explained, the set is somewhat more sensitive and selective if the lower binding post is grounded, or if the (+A) line is grounded. This ground can be made directly from the post or from some more convenient point in the circuit as at the (+A) binding post. This effect is particularly noticeable in cities where the radio traffic is congested and where the utmost in selectivity is necessary. In regard to the "B" batteries it must be noted that the demand for plate current is very heavy and that for the best service a storage "B" battery is highly desirable. If a storage "B" is out of the question with the user, then only the largest size of dry batteries are advisable. The eight tubes will run down a small or medium size "B" battery in a very short time and in the end, the smaller dry cell batteries will prove much more expensive than storage batteries or large dry batteries. A full 90 volts should be maintained at all times for the maximum output, and much of the trouble experienced with superheterodynes can be traced to exhausted "B" batteries which have been allowed to outlive their usefulness. At the left of the baseboard will be seen the terminal strip of bakelite on which the battery binding posts are mounted. This is 1 1/4 inch wide and 6 inches long with a thickness of 3/16 inch. Wires to the connections run off through the side or back of the cabinet, and this makes a much neater arrangement than with the binding posts mounted on the front of the panel as we sometimes see such sets. Spacers are placed beneath the terminal strip to raise it well above the bottom board and so that the screw heads will not make contact with the wooden bottom board. Wood is not a perfect insulator and therefore we should avoid placing any current carrying parts in contact with it. While spaghetti can be used with profit on all "A" battery and ground wires, its use is not advised on wiring which carries radio frequency currents, except at points where a short length is necessary to prevent actual short circuits. Spaghetti has a high dielectric value and increases the capacity of the circuits with attending losses. 1 T GOES without saying that all joints ■* must be soldered and that particular care must be taken where soldered connections are made to the jacks. Rosin flux must be used exclusively (no acid) and in using the rosin one must take care that the parts are actually soldered and not simply stuck together with the non-conducting rosin flux. After soldering, shake the wire vigorously to make certain that the parts are soldered. In such a complicated set, it is exceeding difficult to trace trouble when due to open joints, hence we must be vigilant during the wiring operations. Tubes should be matched by the dealer so that all of the radio frequency tubes are electrically identical. If this is not done, then it will be impossible to secure maximum amplification in the radio stages. Much of the success with a super-heterodyne circuit depends upon the accuracy with which the transformers are matched and their agreement with the tubes. When the transformers are successively numbered from the input through to the output transformer, they must then be arranged in numerical order as shown by RD-1, RD-2, RD-3 and RD-4. For the convenience of the builder the "A" and "B" battery connections are made according to two different systems. In Fig. 1 the negative "B" ( — B) is connected to the positive "A" (+A), and in general this will give the best results. The connections can be, seen at the extreme right of Fig. 1 at the terminals. However, under certain conditions it is better to connect (—A) to ( — B) as in Fig. 2. Either connection is easily had without labor by changing the cross connection or "jumper wire" at the terminal board. In the first case, the jumper in Fig. 1 runs from the ( — B) terminal to the (+A) terminal. In Fig. 2 this is switched from (+A) so that the (— B) terminal is connected to the (—A) terminal. This is simple, and we should try out to find which is best. In connecting the ammeter and voltmeter, we must observe the polarity marked on these instruments; that is, the wire from the positive bus must go to the positive terminal of the instruments. If these connections are reversed, then the instruments will have the needle come to rest on the zero stop and will not indicate the current or voltage. In connecting up the transformers, the marks on the transformer posts should be observed, the grid (G) on the transformer being connected to the grid (G) of the socket as shown in both Figs. 1-2. Particular care should be taken to connect up the variable condensers so that the connections between the grid of the tube and the stator (stationary plates) are always observed. If the grid is connected to the rotor or movable plates, then we will have trouble from body capacity effect, as the full grid potential is then carried out to the hands through the condenser shaft. The proper connections are clearly shown in Fig. 2. In order to absorb undesirable vibrations from the radio stages, a grid leak (1 megohm) is connected across between the negative of "C" and the grid of the last audio tube so that the leak (GL) and the condenser (K2) form the conventional grid leak and condenser. This has a notable effect in reducing noise when the second stage of audio is thrown in. The negative of the "C" battery must go to the grid (G) as shown.