The "Beginner" Three-Valve Set

Radio Record, Volume II, Issue 53, 19 July 1929, Page 34

The "Beginner" Three-Valve Set

A Small, Powerful Receiver

HE. set described here was-de signed by Mr. C. Walter PalNii mer especially for those read4 ers who are as yet not well nee SS versed in the art of. set convf struction, who are using either a crystal receiver, a small one or twovalve set, or no set at all. There are, we know, a great number of crystal and one-valve sets being used at present, despite the reduction in the price of larger apparatus. Many beginners believe that the construction of a larger set would be beyond their scope, but it will be found (by those who try) that the construction of a good, if simple, receiver is quite easy. This "Beginner’s Three’ has been designed with several points in view. In the first place, the cost of the parts is an important consideration; for most beginners are rather’ sceptical about spending very much as they. are not sure that the set will work properly. This hesitancy can be overcome by careful choice in the matter of the circuit and the apparatus. Another question, equally important with the cost, is that of the efficiency of the receiver; and for this reason, in the vital parts of this set, the best available apparatus. should be used. Such parts as the new-style dials, while they add much to the appearance of the receiver, have very little effect. on its actual operation; and those of the older style, which can be obtained quite cheaply from your local dealer, are quite suitable for our purpose. Should the constructor desire to "dress up" the little set later on he may readily do so. Many readers who would be interested in building a, ‘set of this type will have some paris already in their "junk-box,"’ and, of course, these will help to. Keep the cost at a, minimum. (Ns

Simple and Compact. H® "Beginner’s Three" contains one stage of radio-frequency ‘amplification, a regenerative detector and one stage of audio-frequency amplification. The first thing to do when building any receiver is to collect all the necessary parts, The apparatus used in the original ‘set is appended: :-

The coils may, be constructed on the general lines described by "Megohm" in Volume II., No. 51. The secondary

of each coil contains 86 turns of No. 28 8.8.0. or 8.0.0. wire. . The turns should be space-wound, unless D.C.C, by placing a piece of thread beside the wire. The primary of each coil contains 15 turns-and the tickler on the detector coil 20 turns. Some constructors may have on hand smaller. condensers, such as .00035 mfd., and in this casé the outlay for the set will be reduced by using them. In this. case the secondaries should be arranged so that 115 turns are left for each cf these coils. For further details regarding the use of other wire see Listener’s Guide, "Table of Relative Inductances." Mounting the Coils. ‘AFTER the coils have been arranged, the next point is to mount them. The strips for mounting the coils are cut from bakelite, hard-rubber.or fibre

panels. They are each 3 inches long, 4-incly wide and 4-inch thick. That for the aerial coupler (the one with two coils) has a-hole drilled in each end to hold the mounting screws. The other is drilled at one end only. While the strips for the coils are being made, it may be well to make the bindingpost strip also. -This strip is also cut from }-inch panel material, and is 34 inches long by 4-inch wide; in this, however, six holes are drilled. The two at the ends are for the mounting screws; four others are then evenly spaced over the remaining space, for the aerial, ground and two ’phone binding posts. After the strip$ have been prepared, they should be cemented to the coils. Collodion ‘(or liquid court plaster, as it is sometimes .called) is a very convenient material for ‘this purpose. When the cement has dried, the coils are ready for mounting on the baseboard. It may be well to explain how this is done, even though we. are not quite ready for assembling ‘all the parts. The antenna coil is secured to the wooden base by two long’ woodscrews, passed through the holes in the coil mounting strips, and. through a suitable ‘washer -which raises the coil above the base. ‘The washer may be made from a piece of metal tubing; or any other suitable method which sug-

gests itself to the reader may be used. In the original set, two old bindingposts of the metal ‘type used ‘several years ago (those with a screw in the top and a-hole in the side for the wire to pass through) were drilled out so that the wood-screw would ‘pass through. _ ; The detector coupling coil (the one with the three windings) is mounted by & small angle-piece which is sctewed to the coil strip and is secured to the baseboard with a small wood-screw. A 6-32 machine-screw and nut are used to fasten the coil strip to the angle, which is one of the small brass brackets found in the local.5-an-10-cent stove. Other Assembly. AFTER the coils have been prepared, the next point is to drill the panel for the tuning condensers, oscillationcontrol resistor R38, and the filament switch. The templates supplied with the condensers will ‘provide the most convenient ‘way of laying out the holes for these instruments. The resistor

and switch are centred between the two condensers, as shown in the illustrations. The pictorial lay-out (Fig. 2) shows the positions of the other parts of the set. The coils are placed at right angles to each other, so that there will be the minimum of. interaction between them. They are also so located- that the wiring between the grid end of the coil and its condenser and tube-socket terminal is as short as possible. The radio frequency choke, which is equipped with a machine screw for *subpanel mounting, is fastened to a small brass angle; which is in turn fastened as shown, to the plate (P) terminal of: the detector tube’socket. ‘This is done for convenience only, as the mounting bracket is not connected in any way to the terminals of the choke.: The "Phasatrol" stabilises the radiofrequency amplifier, and the resistor R83 is used to control the detector as mentioned before. The latter-resistor is shunted by an 0.5-mf. by-pass condenser which serves to carry the audiofrequeney. currents in the primary circuit of the transformer, so ‘that. they will not have to pass through the high resistance. In-order to make the set as low in cost as possible, a cable has been improvised from the hook-up wire. This reduces the cost of the cable and still permits the convenience of the

eable leads; though a manufactured cable, the cost of which is low, will enhance the convenience of connecting pnd disconnecting the battery. The set is wired: with the convenient insulated wire. Point-to-point wiring is used, so that the leads will be as short as possible, and also to keep them from running parallel. The wires which connect to the batteries are arranged with an excess length of several feet, and are brought together at the back of the baseboard. These wires are ised as a cable, and they may be braided together if desired. A Staple, driven into the base, will serve to keep thé wires of the cable together. Choice of Valves. HIE valves used in the set are chosen , to give the best and most stable results; The standard 201A-type is \uséd as the radio-frequency amplifier; this stage serves the triple purpose of increasing the volume, the sensitivity and the selectivity of the receiver, ‘while preventing the oscillation in the . detector. st.ge from causing interference in neighbouring sets. The. second valve is of the. specialdetector, high-amplification type, since this valve is far more sensitive than thé 201A type, when properly used. . It will be noted that the grid return of the detector is connected to the negative side of the filament, contrary to’ the usual procedure with hard valves. Since this might. cause confusion in the minds of some of our readers, it is well to mention that this is done solely because of the special type of soft valve used in this socket (200A). If the regular 201A valve is uged in this socket, the grid return should .be connected to the positive side of the valve filament. In any case it is well worth changing over to ‘ obtain the best results. -- The valve in the audio-frequency stage is a 112A semi-power type. This was chosen becatise of the better quality resulting from its use. This is especially true if a second audio stage is to be employed and, sinc we are building the set with the thought in mind that a power amplifier and "B" power unit may be built later to give good loudspeaker volume, we have ‘incorporated the 112A in the original set. In the above it will be noticed we have specified types. Any good dealer will be able to advise fhe constructor the valve corresponding to the type he ‘requires, in the make he fancies. _ ." Testing and Operation. Av TER the set has ‘been completed, the next point is to test the wiring to be sure that no mistakes have been made. This is done by connecting a "OC" battery and a pair of headphones in series und applying this test unit across the variou$ cable connections in turn. With. the valve out of the sockets, no click should be heard between any of the wires; either in the filament line or the wires to the plate | supnly. If no clicks are heard, the batteries are connected as shown in the |

schematic diagram (fig. 1). and the valves then insertéd in the sockets. The headphones should be connected to the terminals marked "I.S.’; ‘and the aerial and ground connected to the other two terminals on the bindingpost strip. The°set is then ready for operation, and the filamént switch may be closed.

It is not important which side of the primary coil is connected to the aerial and which to the ground. In some cases, however, the operation of. the receiver may be improved by connectaw

ing the "-" (negative) -side of the "A" battery to ground. If this is found to effect an improvement, the wire may be incorporated directly in the receiver itself; it is then important to have the ground connected to the correct terminal. The additional wire is merely connected between the negative filament. "H'-*" terminal, of the first or

radio-frequency valve, and the "Ground" binding post. It will be found that the set will whistle when the resistor knob. of R3, on the panel, is turned too far to the right. While tuning the set, the re-

sistor should be turned back until the whistle stops, and the dials should be turned. again. If a soft whistle is heard, the "Phasatrol" is turned too far, and its screw should be turned to the left until the whistle stops. When the "Phasatrol" is adjusted and the resistor is turned to the correct position, no whistles will be heard (except those caused by other sets in the neighbourhood, which unfortunately, may be heard in any set). When the set is first placed in operation, it is advisable to turn the screw of the "Phasatrol’ all the way to the right; so that trouble will not be encountered in picking up signals until the receiver is adjusted properly, " If an amplifier or B power unit is to be built later, it is more economical to buy very small B and © batteries, so that the cost.of the set,can be kept low. The filaments of the valvés are lit by a storage battery of the usual type, and a good one should be obtained. If it is desired to operate the set entirely from the electric-light lines, an A power unit may be purchased instead of the battery. A Few Pointers. VERY receiver, however simple, has ° a number of points at which trouble may be encountered ; and those not familiar with "trouble-shooting" might find them difficult to locate, if the set does not work right at first. The first thing to suspect is the valves; take them to the dealer from whom they were purchased and have them tested. If these are all in satisfactory condition’ look over the aerial. This is a very critical point; especially if the aerial was erected some time before the set is built. The insulators should be checked, and the lead-in should be looked over very carefully. .A poor contact in the wire, or poor insulation at the point where the lead-in enters the building, often causes a good set to operate poorly. : If the testing of the valves and the inspection of the aerial and ground are satisfactory, the batteries should be tested with a voltmeter. Again test all the wires and connections with the C battery and phones mentioned above, to be sure that they are all tight. In this test, with the phone and battery, a coil should give a loud click through the winding and none between windings (this is between primary and secondary). An audio transformer gives a weak click in both the primary and secondary, but in most cases the click from the secondary is weaker than that from the primary. It may be better to use a B battery for testing the audio transformer, so that the click will be louder. If no click cah be heard, the transformer is defective. In testing condensers, the click is very weak and no continuous noises are heard. In some cases, as in the variable condensers, it is necessary to disconnect the part from the set; because the coils or other apparatus are shunted across the condenser and it will appear to be short-circuited. . (Concluded on page 36.)

. (Continued from page 35.) If the trouble is not found by these tests, it is advisable for the novice to cal} some one in who is more familiar with sets and their troubles, in order to locate the difficulties. It is well, in fact, to have any installation checked by an older head. The above suggestions should not keep any one from undertaking the "Beginner’s Three"; as the possibilities of encountering trouble in a set of this type are very slight, if the instructions are followed exactly. They are merely given so that if the omission of something causes trouble, the reader will have some means of locating it before calling for outside assistance.