The "Home-Made" Three

Radio Record, Volume III, Issue 8, 6 September 1929, Page 28

The "Home-Made" Three

(Constructed from. the Junkbox)

(Bv

CATHODE

T is a common: ‘complaint among newcomers to the art of radio that, having spent their all in the purchase of a receiver capable of giving loudspeaker reproduction, the wherewithal -to: properly equip #t, The writer has made it a practfee to recommend the purchase of the necessary accessories and the construction of a. loudspeakers receiver. It is astonishing what ¢an be done in the direction of cutting costs even where every single thing has to be bought. In describing the construction of an "economy set" it has seemed advisable to design it in such a manner that the makeshift components employed can gradually and without difficulty be replaced .by commercial apparatus as the novice’s purse recovers from the inroads made upon it by, valves and batteries. This gradual replacement will be found desirable, as some of the "made for nothing" apparatus is. not fitted to stand up to prolonged use, and will gradually deteriorate. The resistances, in particular will become noisy . in time, but will be eminently satisfactory unitl the purse begins putting on weight again after its operation. The old hand doubtless recognises some of the modes of component construction as being popular among the initiated many years ago. The intention is that the receiver in its original form will comprise three valves: a detector without reaction, followed by two audio amplifying valves. Audio transformers can obviously neither be readily made nor procured for nothing, so that our receiver must nerforce be resistance-capacity coupled. | Nevertheless, baseboard room adequate for the accommodation of a transforraer to couple the last stage will be left, so that the final receiver will pos-

sess an r.c.c, first audio and a trans-former-coupled second audio, Provision.. will also be made forthe subsequent incorporation of reaction and a neutralised high-frequency valve; if the constructor has no intention of eventualiy incorporating these latter features, he will be able to shorten the baseboard appropriately, If the purse is very thin, even before its operation, and the local statian is near and reasonably powerful, it will be permissible to dispense with the third valve, thus reducing the initial expenditure quite appreciably, A study of the circuit diagram of Figure 1 discloses the following apparatus as being necessary :- One baseboard 174 inches by 9 inches, equipped with, strips of three-ply (or other material 3-1G6in. or jin. thick) at, the extreme ends of the underside (to permit of some.of the wiring being 1located beneath the baSeboard) ; 1 variable condenser (see text); 3 valveholders; 1 tuning coil; 9 terminals; 2 fixed condensers of capacity somewhere between .006 mfd. and .015 mfd.; 1 fixed condenser about .0003,. mfd.; 2 coupling resistances about, 150,000 ohms each; 2 gridleaks about 2 megohms; 1 gridleak about 1 megohm; eonnecting wire (if uninsulated, sleeving will also be necessary) ; in addition, some small pieces of ebonite or other insulating material will be required for mounting the tuning condenser, the terminals, and the resistances and gridleaks; the construction of some of the components will involve the purchase of a number of tiny screws and nuts for assembling. vuniuieimpenaiiogm SS OT na -- SS. _. a

What ean be Made, TPAKING these components in turn, ; let us differentiate between those’ ‘we can economically construct and those we should buy. The baseboard will not usually involve, any expenditure, although it is worth while to go to some pains to finish it nicely. A satisfactory variable: condenser cannot readily be made, so that, unless the constructor happens to have one on hand, this had best be purchased; variations in the running coil can be made to accommodate any condenser which happens to be on hand, but where 'a purchase is contemplated a condenser having a maximum. capacity of .Q003 mfd. should be chosen if available. Valve-holders can be made, but they are so very cheap that, in view of the labour involved in making them, a purehase is, in the writer’s opinion,

justified. Do not purchase expensive spring-mounted holders, as they are unnecessary. If one is on the constructor’s hands it can be most usefully employed in supporting the detector valve. It has been assumed that valve-holders for the American-base valves will be employed, and the practical wiring diagram depicts these; the experienced constructor will be able to visualise the necessary changes in wiring to accommodate Pnglish-base valve-holders. The novice will be well advised to use. the American-base holders depicted and adhere religiously to the practical. diagram. . The tuning coil will in all cases be not only because expense can be avoided. thereby, but also because’ this course will result in a component of superior efficiency, as compared with the commercial article. Moreover, the task of construction will | be found to be one of almost ridiculous ease, — ° The terminals required will have to be bought. It may be pointed out, however, that the terminals shown for battery connections can be dispensed with if a seven-wire connecting cable is used, the flex leads being brought to appropriate points ou the internal wiring; it is questionable whether the economy is worth while, as a terminal strip will figure in the receiver in its: fina] form. Small screws equipped with two nuts might, however, quite well be used as a substitute for the terminals,

Constructing Condensers. "THD two coupling condensers are mounted on the neat little nine‘penny mounting bases, with which many constructors will be familiar, The second coupling condenser will eventually be mounted in parallel with. the first and on the same base, its discarded base being then used to mount the grid condenser, which in the meantime is supported on the wiring. Soldering can be, avoided, if so desired, by looping the connecting wire round tiny bolts inserted through the holes at either end of the grid condenser. The same hrocedure is adopted. in making tion to the coupling condensers, but these do not depend on the wiring for their support. . .Fixed condensers can be fairly readily made, if the constructor does not object. to the: work involved. The method of construction is illustrated in fig, 2, from which it should be clear that two pieces of 8-16in. .ebonite are utilised as clamping pieces to hold the assembly together, the clamping screws being heavily countersunk on ‘the underside to avoid making contact with i

the baseboard. Suitable sizes for the clamping pieces for the grid condenser will be: Top, 14in. long by i#in, wide; bottom, 2}in. long by 1l#in. wide; for. the coupling condensers the widths must be increased to 1#in. The grid condenser is a specimen of the simplest possible form of condenser, comprising, as it does, two conductors, each connegted to a terminal point, and separated by 4 dielectric or insulator. For the conductors we will use coppetfoil or tinfoil, and for the insulator mica, .002in, thick; mica is usually sold in thicker sheets than the this ness specified, but may be very rea. ily split down. : Lacking the means for precise measurement, it may be assumed that when the mica is split down so thin as to be almost. glass-like in its transparency, this is near enough to correct. Any _ sheets showing. signs of faults should have the faulty pieces removed at once to avoid any possibility of these being included in the condenser. As a temporary expedient where it is intended to eventually replace the home-made condensers by commercial products, each sheet of mica may be replaced by: two sheets of uncreased waxed paper, such as cigarettes are wrapped in. Hach sheet’ of copper-foil or tinfoil is 13 inches by % inch, ‘the mica sheet separating them being 14 inches by 1 inch and so arranged as to, protect beyond the area where the two pieces of

"foil lap over one another (1 inch by $ Jinch in extent) by 4+ inch on all sides, \A veference to fig. 2 will make the matter clearer. The coupling condensers, being of considerable capacity, would involve too bulky a component if only two conductors were used, so here we use @ total of 21 foil "plates," the width of these being # inch instead of 4 inch, the width of the mica’ (or paper) sheets separating them being likewise increased to 1} inches sa as to avoid any possibility of two adjacent "plates" touching. Starting from the bottom of the’assembly,. plates 1, 3, 5

7, 9, 11, 18, 15,"17, 19 and 21 will be connected to one end of the condenser and plates 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20 to the other end; each plate must, of course, be carefully insulated from -both its neighbours by sheets of mica arranged in just. the Same way as in the case of the grid , condenser, ' A portion of the assembly is shown in fig. 2, this also serving to show: the manner of clamping all the projecting ends of foil at each end with a little bolt to hich the connecting wires are ultimately also joined; a washer is placed immediately above the foil (this

is particularly necessary if tinfoil is used) to prevent its-being torn when the nut is tightened. It will no doubt be obvious that, although holes for screwing the coupling condensers flat on the baseboard’ are shown in. the. diagram, the exigencies- of space: will necessitate their being mounted edgewise, in which position they may be supported by the connecting wires; the mounting holes will probably come in handy eventually when the condensers are used in some other piece of apparatus, ‘ Resistances and Grid Leaks, "THE making of the resistances and | grid .leaks is even more simple, d since wire-wound resistances such as will éventually be used for coupling the first audio stage are expensive, their construction is. advised in all cases. Both resistances and grid leaks are cut out of blotting-paper which has previously been drenched with Higgins’ Waterproof American India Ink and dried; inasmuch as this . jnk is a trifle expensive, it will be well to stage, either in person or by a friendly proxy, a slight accident at the office whereby the bottle of India ink is spilt over a nice clean sheet of blotting-paper which is subsequently surreptitiously dried and secreted. As a last resort, Is. 6d. may be expended on. -a small bottle of the ink, but be sure to obtain’ the kind specified. — Fig. 3 gives the dimensions of the pieces of prepared blotting-paper required for the various components also showing the method ° of mounting on a ‘f-inch strip of ebonite; a two-inch length of the strip will suffice to mount a gridleak, a resistance necessitating a three-inch length. As a very temporary job, the mounting strips may be dispensed with altogether, the strips of prepared paper being simply screwed down to the baseboard (which must be thoroughly dry and not of totara), with small brass woodscrews, under the heads of which the connecting wires are secured, a washer being disposed between the wire and the prepared paper to protect the latter. It will be found that the paper is rather apt to tear, some little care in handling it being necessary. |. Tuning Coil. . "THE only component remaining to be . dealt with is the tuning coil. The constructor can please himself whether he cuts costs on this or makes a permanent job of it. A little expenditure in making a good job will not be wasted, as the original coil can then be shifted so as to tune the grid circuit of the high-frequency stage when this Is subsequently added. & . If it is intended to construct the coil, once and for all, a 33-inch length of ebonite or formica tubing of 3-inch: diameter should be. procured; the thin purse should be temporarily stimulated by using a similar piece of cardboard tubing or by making a tube by winding several layers of varnished paper on a suitable formér*the paper tube is subsequently baked in: a warm oven, for which dperation a time should be chosen when the rest of the household is away from home. The winding is the same whether the coil is temporary or permanent, but depends on the capacity of the variable . condenser with which it is to be tuned. Where the condenser is to be bought, it has been already suggested that it should be of .0003mfd. maximum capa-

1. Projecting ends of tinfoil. 2. Clamping bolt. 3..Hole bored through both pieces of ebonite for screwing to baseboard. _ 4, Serew and nut for connections, washer under nut to protect tinfoil beneath. Centre Diagram.-Plan of grid condenser before positioning of top clamping piece. . 5. Area where tinfoil sheets overla with mica sheets bétween. _ : 6. Tinfoil (bottom sheet) under mica. 7. Mica separating tinfoil sheets. 8. Tinfoil (top sheet) above mica. . Lower diagram showing the method @ assembling ‘the .oupling condensers with mica sheets, insulating the tinfoil "plates" one from the other. Only four of the total of twenty-one "plates" are shown. ‘

city, for which a coil having 68 closely wound turns of 24 8.W.G. double cotton covered wire will be suitable. It is desired, however, to make provision for those who have a condenser on hand of some other capacity, so the following little table has been provided to acquaint them with a suitable winding for their particular condenser :- Condenser. 0005 mfd. ... Winding 54 turns 22 D.C.C. 00035 mfd. ... Winding 64-turns 24 D.C.C. .0003 mfd. ... Winding 68 turns 24 D.C.C. 00025 mfd.\... Winding 70 turns 24 D.C.C, All windings are close wound cand will occupy some 2 inches-on the former. The’ winding should be. started £ inch from the bottom of the former -after first securing the bared end of the wire under the nut of a small screw-and continued until about the fifteenth turn, when a tapping loop is passed through a hole in the formér and secured under the head of another screw, the portion of the loop’ in contact with the screw being, of course, first bared. The winding is Athen continued until the required number of turns is reached, when the bared end of the wire is passed under a third screw, this one being mounted. at the top of the coil instead of the bottom. , Mounting the Components. TIXHE mounting of the components wilt present no difficulty. The practical wiring diagram shown in Fig. 5 is drawn accurately to: scale and -considerable care should be taken to mount each component precisely in the position shown, making use of the small seale printed below the diagram. The main terminal strip is 6 inches long, the terminals being centred 7-8in, from each other, while for the other terminal strips.and the condenser mounting any ‘scraps of ebonite which happen to be handy may be used. It is hoped that the many constructors who are, unable to wire a receiver from a theoretical diagram will have: ‘no difficulty in doing so from the practical wiring diagram shown in Fig. 5; certainly no effort has been spared to ensure this. Just to make absolutely. certain that no mistakes are made, the constructor should check his wiring from the following list of connectioris :- (1) Aerial terminal to tapping on coil. . -(2) Terminal on frame of variable condenser to bottom connection of coil, thence to negative filament terminal of detector valve-holder, thence under baseboard to earth terminal, thence to "A_* terminal, thence to "B-" terminal, thence to "C plus" tertninal, thence under baseboard to negative filament terminal of first audio valveholder, thence under baseboard to negative filament. terminal of second audio valve-holder. (3) Low potential end of detector gridleak under ‘baseboard to "A plus" terminal, thence under baseboard to ‘positive filament terminal of detector valve-holder, then under baseboard to the positive filament terminal of the first audio valve-holder, thence under baseboard to positive filament terminal of second audio valve-holder. (4) "B plus" terminal: under baseboard to one end of first place resistance, thence under baseboard to one end of second plate resistance, thence to "Loudspeaker" plus terminal. (5) Other end of first plate resisfence to one end of first coupling con-

denser, thence to plate terminal of detector valve-holder. (6) Other end of second plate resistance to one end of second coupling condenser, thence to plate terminal of first audio valve-holder, rar (7) Grid terminal of. first audio valve-holder to other end of first coupling condenser, thence to high-potential end of first audio’ gridleak. ' (8) Other end’ of second coupling | condenser to high-potential end of second audio gridleak, thence to grid terminal of second audio valve-holder, *(9) Low potential end of first audio gridleak to "OC" minus 14 terminal. (10) Low potential end of second audio gridleak to "C" minus 4} to 9 terminal. ‘.(11) Plate-of second audio valve to "Toudspeaker" minus terminal. (12) Insulated: terminal of variable condenser to top connection of coil,

thence to one end of grid condenser. (13) Other end of grid condenser to grid terminal on detector valve-holder, thence to high potential end of detector gridleak, .. wo Walves Only. WHERE only ‘a two-valve version is contemplated, the continuation of the wiring to the last valve-holder and associated .apparatus may be omitted, the plate terminal. of the first audio valve-holder being connected direct to the "Loudspeaker" minus terminal. Wiring may be carried out either with insulated wire, such as Glazite, which is‘bared where it is desired to make a connection, or with tinned copper wire of about 18 or 20 8.W.G., over which insulating sleeving is. slipped’ wherever there is a danger of two wires touching or of a wire touching any piece of apparatus. ‘The methnd of doing part of the wiring under the baseboard involves drilling a fair number of holes to pass the wires through, but this is not difficult.

Valves to Use. NLESS, the constructor is. already in possession of an ‘accumulator or valves of a different voltage it is reeommended that a -4-volt accumulator and valves of the 4-volt class be purchased. Whatever the filament voltage of the valves the first should have an impedance somewhere between 7000 and about 20,000 (eg. Mullard P.M.4 D. or P.M, 3 Marconi or Osram D.E.L. 410, Philips A415 or A409, Cossor’ 410 Is.B.). ‘The first audio valve should have similar characteristics, any . of the foregoing valves being suitable; while the third valve should have a rather lower impedance of somewhere about 5000 to 8000 ohms (e.g., Mullard P.M. 4, Marconi or Osram D.E.P. 410, Philips B409, Cossor 410P: Care must be taken to obtuin the type of base for the valyeholders used.

Batteries. . "THE "B" battery should be at least 90 volts and preferably more, up to a maximum of, say, 1385 volts. The "QO" battery must be of the type having a tapping at 14 volts negative. The maximum negative potential required from the "C" battery is: determined by the type of valve used in the last audio stage and by the voltage applied to its plate (ie. the voltage of the "B" battery). ° Information regarding the negative bias necessary at different plate voltages will be found on the maker's in-. structions acgompanying the valve, Note, however, that notwithstanding any instructions. given by the valve manufacturers, the negative bias applied, to the first audio valve is not to exceed 14 volts. , [zt will bé noted that no on-off switch is fitted. to the receiver. -This omission will be remedied so soon as we describe the fitting of a proper panel, but in the meantime it will bé necessary to disconnect the "A" negative lead from the accumulator when it is desired to "switch off" the set. If a battery clip is used this will entail no hardship. . The-batteries should be connected up for the first time before the valves are inserted. If a vicious blue spark is ‘encountered on connecting any of the batteries, a mistake in wiring has

been made, and all batteries should be © at once disconnected and the mistake, traced before going any further. .f On inserting the valves the local station should be tuned in. without the slightest difficulty, and the writer ventures to prophesy that the constructor will be agreeably surprised .by both volume and tone. If results are not what they should’ be, and home-made eondersers are used, feel the first and second audio valves in turn. If the first is appreciably warm or the second uncomfortably warm, it is probable that. a mistdke has been made in assembling the coupling condenser preceding the offending valve. . The "B" battery should be: hastily disconnected and the condenser taken out and tested by connecting it in series with a grid bias battery and a flashlamp bulb; if. the bulb lights, there is obviously no insulation bey.

tween the two sets of plates in the condenser, and a reassembly is indicated. es ; ’ The only other likely source of trouble, provided the valves are sound, is a mistake in wiring, and every precaution has been taken to ensure that the constructor will not be guilty of this. The receiver in its present form is unsuitable for distant reception, so nothing should be expected of it in this direction; the disability will he remedied in due course. . The speaker should be connected that the flex lead having a red tracer is connected to the terminal marked "+." and the other lead to the "-" terminal. In closing, it may be noted that if care is taken to buy valve-holders and other components having terminals, no soldering at all is necessary, so that the constructor who dreads the solder‘ing iron need have no fears on this score. ' The next article will deal with various small improvements, these including the fitting of a proper panel and the wiring into: circuit of a filament switch.