Radio mirror (Jan-Oct 1923)

August u, 1923 RADIO DIGEST — Illustrated 11 First Steps for Beginners in Radio Chapter XII — One Tube Reflex Circuits By Thomas W. Benson, A. M. I. R. E. p.EGINNERS will find the accompanying " series by Mr. Benson very helpful in learning the rudiments of the popular science of Radiotelephony. The articles appear are: yet to Chapter XIII— Multi-Tube Reflex Circuit Operation. Chapter XIV — Headsets and Loud Talkers. Chapter XV— Filament Batteries. Chapter XVI— Plate Batteries. Chapter XVII — Using Alternating Current on Tubes. Chapter XVIII— Testing Radio Instruments. Chapter XIX — Locating Trouble in the Set. Chapter XX — Useful Information and Formulas. HE advent of reflex circuits seemed to promise something radically new, 1 but a consideration of their principle of operation •will show that there is really nothing new in the phenomena. "We have seen from previous chapters that a tube can be used to amplifly at both Radio and audio frequencies. Since amplification in both cases is accomplished in a similar manner, it should be possible to amplify both frequencies simultaneously, the real problem being to keep the frequencies separate to prevent interaction and a jumble of sounds instead of music Luckily this is readily done; the simple reason is that they differ so greatly in their frequency. To handle the two frequencies in the same circuit use is made of two other principles that should be familiar to the reader. The first is that a condenser will permit a high frequency current to flow through it; the other is that a large inductance will choke a high frequency current but permit direct current to flow through. By using these two instruments we can devise a circuit that will handle both Radio and audio frequency currents without interaction. "When only one tube is used we can then have one stage of Radio frequency amplification and one of audio. For detection, use must be made of another tube or a crystal detector. For the reason that a crystal detector gives clearer reception, is cheaper in construction and maintenance and in the fixed types requires no adjustment, crystal detectors are usually employed. Assembly of Parte on Single Tube Reflex Let us see, then, how these various instruments may be assembled to use a single tube for both forms of amplification at the same time. Referring to Figure 51 we find a loop aerial with a variable con They must be of the mica dielectric type to prevent loss of current or variation in capacity. It has been said that the small condensers used, usually .002 mfd., will not pass audio frequency currents. As a matter of fact the amount of current a The detector is likewise important; it is necessary to employ a type of detector that will not burn out with strong signals passing through it. Some form employing iron pyrites will give good results. In Figure 52 is shown a circuit embody O— -t m POTENTIO METE' PLATLRESISTANCE * .ooz. Figure 52 — One-tube reflex with refinements for best operation condenser will pass depends on the frequency of the current; so some current will flow at audible frequency but not enough to make much difference as far as short circuiting the phones or secondary of the audio frequency transformer. So when a circuit of this type does not function properly it is advisable to try different capacities at these points. The capacity should be such as to pass all the Radio frequency current and little or none of the audio frequency. Nothing definite can be said about the transformers; some seem to function perfectly and others give little or no results. They should be of the shielded type to prevent feed backs and howling. Often a plate voltage too low will cause reflex sets to work improperly; a high plate voltage should be used to obtain good results. Since hard tubes are used in these circuits exclusively the voltage may be pushed as high as 120 volts without harming the tube. Too much voltage on the plate will be indicated by the tube turning blue. The very nature of the circuit using as it does a feed back phenomenon for its operation, makes it very prone to selfoscillation. Self-oscillation of the circuit can be prevented to a large extent by adding a potentiometer. This instrument is also valuable in that it serves to bias the grid to a proper amount to put the operating range of the tube on the steepest part of its characteristic curve and thus give the greatest amplification. Better Way to Bar Oscillation A better method of preventing oscillations is to add a variable resistance in the plate circuit to stabilize the tube. This *^ .002. c Figure 51 — Simplest form of one-tube reflex to show principles of operation denser across it to tune to the waves desired. The received currents are fed directly into the tube as in a Radio frequency amplifier, connections being made to the grid and to the filament through the consenser C 1. This condenser will pass the Radio frequency currents without difficulty. When signals are being received the plate current will be varied in accordance, with a step up in intensity, but still inaudible by reason of their frequency being above audibility. These currents flow through a Radio frequency transformer and the condenser across the phones in the plate circuit. The transformer then induces currents in the circuit containing the detector which is required for detection and rendering the signals audible. This circuit includes a condenser and audio frequency transformer; according to the operation of the detector, currents at audible frequency will flow into the primary of this transformer. The output of the transformer is now fed back into the grid circuit of the tube by its connection across the condenser in the grid circuit. This condenser will not short the low frequency audible currents; therefore the audible currents are impressed on the grid circuit and again amplified. The audible currents in the plate circuit will not flow through the condenser across the phone; hence the signals are made audible. Condensers Must Be Mica Dielectric There are, however, numerous little details that make or mar a circuit of this type. Take, for instance, the condensers. resistance should have a range of 500 to 2000 ohms. Oi e of the old type B battery potentiometers serves the purpose nicely, or one can be made using the lead from a medium hard lead pencil for the resistance and arranging a slider to move over the lead and thus vary the resistance. The effect of the resistance is to damp out any oscillations in the circuit, since a circuit will not oscillate when its ohmic resistance is four times as great as its radiation resistance. TWO SUPERSENSITIVE CIRCUITS (Both Copyrighted) My Highly Improved Beinartz brines in all important stations on both coasts and Mexican border, loud, clear and without distortion. We dance to music from Atlanta received on one loud Baldwin unit. Build one of these wonderful sets from my blueprints and specifications, price 50c, or with a perfect and complete double wound spiderweb coll, J3.00 by mall. No other windings used. Fkoto of my set on a glass panel with every order. This copyrighted circuit Is the most successful of any Beinartz modification yet produced, and is imitated the most. Thousands are in use. My W. D. 1 1 Circuit is especially designed for use with the "Pickle" tube and brings out the roll value of that little rube as no other circuit can. Stations 1000 miles away come in clearly on one tube. This set is small, complete, portable. For the man who wishes the highest efficiency, this is tke set to build. Price of blueprint and specifications, 50c. or with complete and perfect windings, $3.00. Photo of set with every order. Either set is easy to build, easy to operate. Everything clearly shown. Sets built from these plans will receive all broadcasting stations operating under the new laws. Their wave length range is from 140 to 670 meters. S. A. TWITCHELL 1925 Western Ave. Minneapolis. Minn. ing the refinements mentioned above. It will give very good results when its operation is mastered. A potentiometer is shunted across the A battery to bias the grid, a fixed condenser being connected from the contact arm to the battery terminal to prevent change of tuning when the arm is moved. The resistance is also shown in the plate circuit to prevent oscillations. Any type of hard tube capable of standing 60 to 80 volts on the plate can be employed in this circuit, but the tubes using 1.5 volts on the filament will not function very well, particularly in reflex circuits containing two or more tubes. This is due to the fact that the plate current in these tubes is limited by their construction; when the tube is required to do double work the maximum signal intensity Is not as great as in a hard tube capable of carrying larger plate currents. In single tube reflexes the dry cell tubes give fair results, but where amplification is carried further in two and three tube sets they do not function well. In beginning to experiment with re flex circuits the experimenter is advised to mount the instruments temporarily in the position they will occupy in the finished set and to test the circuit thoroughly before assembling the set. When good results are obtained the set can then be permanently wired. It should be remembered that the selectivity of the set depends entirely on the tuning apparatus used with it and the height and length of the aerial. The circuit shown employs a loop aerial because this is the simplest arrangement; good work can be done with this device. It gives freedom from static, and selectivity due both to its small size and the ability to utilize the directional effects of this form of aerial. "Where greater range is desired with an outdoor aerial it will be necessary to use a variocoupler to obtain selectivity with condensers in both aerial and secondary circuits to obtain close tuning and selectivity. Having covered here the principle of the operation of the reflex circuits, the second part will consider the application of the same principle to two and three tube sets. (TO BE CONTINUED.) Electric Soldering Iron $2.50 A. C. OR D. C. CURRENT DEALERS — Attractive Discounts NOTE: We are the Largest Exclusive Radio Jobbers in the Middle West ■ HMM-'BcM* 123 W. Madixon Jt.Chicaqo Our Latest Triumph The Michigan <<Hil> Midget" $27.00 Long-Distance Wonder Worker That Tunes Oat Local Broadcasting The most dependable long-distance Receiver ever assembled in so small space. Convenient for campers and tourists, yet equally suited to home use the year-round. Handsome mahogany finished cabinet 14% m long, V/$ in. high, 9f^ in. deep at base. Operates with any of the dry cell tubes as well as with standard 6 volt tubes. Cabinet will hold three No. 6 dry cells and 22 volt "B" Battery. Levers in place of dials make tuning easier and accurate. Wonderfully clear, pure-toned reception through headphones — add two stage amplifier for loud speaker reception. A Radio Engineering triumph. Price, without tubes or battery, F. O. B. Grand Rapids, $27.00. Ask your dealer. If he cannot supply you, remit to us and send his name and address. Send for list of Michigan Quality Radio Receivers and Parts; variometers, variocouplers, all-range couplers, special rheostats — 50c, potentiometers— 200 ohm and 400 ohm — 60c, etc. Dealers, the set you have been waiting for to make complete package "over the counter sales." Grand Rapids, Michigan