Radio mirror (Jan-Oct 1923)

hut 14 RADIO DIGEST July 7, 1923 SINGLE TUBE SUPER GOOD FOR PORTABLE THE problem of a good circuit for a portable set using a loop aerial can be very satisfactorily solved through the simplified type of a single tube super regenerator. A typical circuit of this type is presented as RD-88. The loop should be about 2 feet square with about 12 turns spaced % inch apart. The wire can be about No. 20 gauge single or double silk covered. Any of the standard type of variocouplers can be used. The variable condenser gives best service if it has the vernier adjustment feature. The two honeycomb coils are connected in series with a coupling adjustment. A standard two-coil mounting should be used. The fixed condensers should be of the mica type, as the paper condensers are too unreliable in capacity values. This circuit will give satisfactory service with practically any type of tube. With hard tubes the plate voltage can be increased. ,^^^^^^_^^____ FIRST STEPS IN RADIO (Continued from page 11) Thus when a difference of potential across this condenser was varied by variations in the plate voltage, the changing values were impressed upon the grid circuit, giving a feed-back effect and regeneration. This circuit is used in a somewhat different form at present, shown at B in Figure 36. The variable condenser in the aerial circuit will be found to have a positive potential applied to one set of plates and the other plates being connected to the negative of the B battery through the phones. Thus when the plate current is varied by the original signal wave current, the potential across this condenser will vary and current impulses will be impressed upon the grid circuit to assist those from the wave, thus giving regeneration. The action is assisted by the inductive effect of the variometer in the aerial lead, the self induced current in it increasing the variation in the potential differences across the variable condenser. With this circuit the grid leak is very critical, for the positive of the B battery reaches one side of the grid condenser and if too much escapes across through the leak, the grid will be unable to assume an appreciable negative charge from the incoming wave and the set will not be sensitive. Modification of the Circuit A modification of this circuit that is somewhat superior to that just described is shown at C in Figure 36. The action is identical but the regeneration is easier to control and the signals are somewhat louder by reason of the tuning inductance being connected to the grid and plate. The application of regeneration to double circuit tuners is identical in principle, the only difference being that the energy is transferred to the secondary tuning circuit instead of to the aerial circuit, although the latter method is occasionally used, the aerial circuit in turn transferring it to the secondary. The simplest form of inductive feed-back in the two circuit tuners is the three-coil honeycomb or spider web circuit. In this arrangement, three coils are arranged close together, the center one or secondary being fixed in position, the other two being so mounted that they may be swung to the side. The coil on the left is usually made the primary or aerial inductance, while that 'on the right the tickler. In operation, the left hand coil is used to vary the coupling between the aerial and secondary circuit for best selectivity. Then by moving the coil on the right the proper amount of current can be transferred from the plate circuit to the secondary circuit to get good regeneration without distortion. Sometimes a variable condenser is shunted across the tickler coil to help control the regeneration, which is highly critical. A better method is to connect the variable across the phones, in which position it acts as a throttle to control the amount of Radio frequency currents flowing in the plate circuits and thus assist in controlling the regeneration. As a rule, a variable condenser across the phones instead of the fixed condenser used there in most regenerative circuits, will permit better control of the operation and may overcome the squealing and howling that cannot be completely cleared up in many sets. There are numerous methods of inductively coupling the plate circuit to the secondary, but as a general rule no circuit should be used where a change in tuning will vary the feed-back effect. For instance, at B in Figure 37 is shown a circuit used by many where one inductance is made to act as the tickler and aerial tuner. It will be very apparent that any attempt to vary the coupling to obtain selectivity will alter the feed-back, or in attempting to control the feed-back the selectivity of the set must be sacrificed. Various other arrangements of inductively coupled methods are described by the technical press, but a study of the above will show the principle used and will assist in the selection of a suitable set. Feed-Back in the Reinartz Set The well known Reinartz tuner is a form of capacity feed-back with a special untuned or aperiodic aerial circuit. In Figure 38 is shown a modification of the Reinartz tuner that is somewhat easier to build and will be found very simple in operation. As shown, a tap is taken off one variometer between the stator halves and grounded through a variable condenser. Another variometer is connected to the plate and the other terminal to a .0005 mfd. condenser, which is connected to the aerial lead. This set functions as a closely coupled two circuit tuner. Half of one variometer stator and the aerial circuit condenser form the primary while the balance of the variometer forms the secondary circuit. Regeneration is obtained in the following manner: The variometer being connected to the plate, the small fixed condenser and the aerial tuning inductance form a tuned circuit between the positive and negative terminals of the B battery. Should a change of potential on the grid cause a variation of the plate current, the voltage applied to the tuned circuit will vary and the currents will flow therein at Radio frequencies. TVnen this circuit is tuned to the same wave length as the secondary circuit, the currents therein build up rapidly and flowing through the aerial inductance part will be transferred to the grid circuit, and being in phase with the currents therein, will assist them and increase the changes in grid potentials, giving greater signal strength. This circuit is recommended for trial by those not familiar with it, for it has given excellent results wherever used. The action in the regular Reinartz circuit is similar, except that the inductances are varied by taps and the condenser in the plate aerial circuit is made variable. The Three Circuit Tuner We come now to the three circuit tuner which is exclusively regenerative and the earliest type of feed-back circuit discovered by Armstrong. The circuit is shown in Figure 38 and is usually known as the variocoupler-two variometer circuit. The action here is rather difficult to explain. When the set is operating the plate current is varied by the changing potentials on the grid. Each change of the plate current is opposed by the inductive action of the plate variometer. Thus when the plate current is decreased by a negative charge on the grid, the plate variometer tends to keep the current flowing through the tube and builds up the plate voltage. The increase of positive potential on the plate induces a greater negative charge on the grid by electrostatic attraction, which in turn reduces the plate current still lower, giving a greater change in plate current than would result from the original negative charge on the grid. The plate circuit is tuned to the incoming wave and thus the inductive action of the plate variometer is properly timed to act in synchronism with the Radio frequency currents in the grid circuit. The proper operation of this circuit demands the adjustment of three different circuits to obtain maximum signal strength and therefore is more difficult to adjust. As a rule, it is unsatisfactory to the beginner for its great selectivity and difficulty in tuning a station, but once the THE NEW B-T VERNIER TUNER Gives 100% Improvement on REINARTZ CIRCUITS Does away with soldered taps, switch points, etc Easy to change to other circuits if desired. Photo Diagrams furnished — ask your dealer, or write for circular. '■IT'S A BEAUT" BREMER-TULLY MFG. CO. 532 South Canal Street CHICAGO PORTABLE RECEIVER (Continued from page 13) apparatus so as not to interfere with the wooden strips in the carrying case. This also holds true when wiring the set. mounted on the side panel in this layout, and the sub-panel eliminated. The one peep hole is dispensed with also. Two binding posts take the place of the loop jack. The Radio frequency transformers are mounted on the sub-panel IA1 T LOOP i ", v|m ! 1 , Layout for Reflex Number Three Reflex Number Two The panel layout for the three-tube circuit is slightly changed from that of the four-tube set. The control apparatus is the same, except that the rheostat for the separate audio frequency tube has been eliminated. The loop and phone jacks and the battery switch have also been eliminated. The audio frequency transformers are also knack of handling the tuner is mastered, it is second to none for long distance reception with amplifiers. Body Capacity In most regenerative tuners the approach of the hands to adjust the dials untunes the set. This is termed body capacity and is due to the fact that any conductor brought near the set forms one plate of a condenser and thus changes the tuning of the circuits. This is prevented by shielding the set by fastening sheets of conducting material to the back of the panel and grounding to the ground terminal or to the negative filament. The wiring of a regenerative set should be such as to prevent any feed-back to the grid circuit other than that provided for in the instruments themselves. It is apparent that any lead in the plate circuit which lies parallel to the grid circuit lead would induce currents in the latter either by inductive or electrostatic action. Therefore separate all wires carefully and let them cross only at right angles. (TO BE CONTINUED.) alongside of each other. Reflex Number One The panel layout of the two-tube circuit becomes very simple with plenty of room for mounting of apparatus and accessibility. One of the sockets and one of the potentiometers are eliminated, otherwise the top panel presents no variations from the three-tube layout. In the side panel layout, just one transformer of each of the two sets is eliminated. (TO BE CONCLUDED.) Electric Soldering Iron A. C. OR D. C CURRENT DEALERS — Attractive Discounts NOTE: We are the Lamest Exclusive Radio Jabbers in the Middle West .1M.1M! 123 W.MadironSi, Chicago pioclrip PACIFIC COAST BRANCH: 329 Union League Bldg. LOS ANGELES "Built First to Last This little beauty was approved by our engineers only when convinced that they had produced the one socket that com' bines all the essential features of a good socket. Positive Cotogrip Contacts. Hard Rubber Insulation. Rugged Construction. Compact Design. Concealed Mounting. It is impossible to show the unique mechanical action of Cotogrip contacts in a photograph. It is different from any other socket you have ever seen. This socket will interest you in every way. You ought to have one. Ask Your Dealer C0T0-C01L C0.£Rovipem* ■■ ■■ -■ .,-.... . . ; — ===ssea