Crystal and Amplifier to Operate from A.C. Mains

Radio Record, Volume II, Issue 12, 16 November 1928, Page 26

Crystal and Amplifier to Operate

from A.C. Mains

By

Pentode

=e N glancing down the list of "what is wanted" one will be sure to run across an amplifier that will derive its A and B power Stas from the mains., To the or purchaser of shop-made apparatus, who possesses a radio set purely for the musical entertainment deriyed, the various batteries, etc., that form a necessary: part of the equipment, are an endless expense and worry, Having made an "all from the mains" crystal set and amplifier, the writer considered it of sufficient interest to describe in these columns. Excellent results have been had with one built up as here described. The volume is quite sufficient for general domegtice use. During a lull in the programme, just a suspicion of a hum is noticed, but not enough to produce any discomfort. All that is necessary at the beginning of a programme is to switch .on the electric light or power.. There are no accumulators to charge, no batteries that run down in six months, anc if a valve ever does burn out, the amplifier and rectifier are adapted to take two ordinary three electrode valves, ard not the expensive A.C. valves. Particulars are given for designing the transformer for either two, four, or six-volt valves, or tappings can be made ta adapt the whole to either voltage. Appended is a list of the components NACESSATY, with their approximate cost.. . ‘THE most ticklish job is the transformer for stepping down the voltage from the mains, and this will be described first. The one mention-ed-is designed for the 230 and 50 cycle power. Although by following the directions given it is quite simple to conptruct it, so that either 110 or 230 volts can be used for the input. ‘By applying. to a sheet metal worker, about 60 pleces of black-iron, not more than 1-64 inch in thickness, can pe cut to a size 5hin. x 3 i-8in. If diftieulty is experienced in obtaining the shéet iron of this thickness, kerosene tins can be cut into rectangles of this size to form a pile 13 inches high.More pieces will be required as the tin is much thinner than the black iron. Out of the middle of each piece a window is cut 8 inches long ‘by 5-8 inch wide. The best way to do this is to cut a piece of thin cardboard the size of the oblong with the window cut out according to the size given. This can be placed over each inilividual piece of-iron, and with a sharp file point a seratch can be made on the iron underneath to indicate the correct size. Each piece has now to be cut in two pieces along the dotted line, If the two halyes are numbered with white chalk, 1-1, 2-2, and so on, they can be ‘assembled in pairs, however irregularly the dividing line has been cut. With a sharp pair of snips cut along _ the two sides of the window of each half, and by bending down, as shown, the ends can be eut out quite neatly. This is rather a tedious job, but it should not be hurried, or when assembling it will be found that the separate laminations will not fit evenly inside the coil. The pieces should

when piled up and pressed . together, form a heap 14 inches high, . The bend which has been made to cut out the ends of the windows, should be hammered flat on a flat surface. . , Now give each side: of each individual piece a_ liberal coating of shellac varnish. Shellac is easily obtainable from the chem-

ist or hardware store, Dissolve in methylated spirits, apply, and allow to dry. The chalked numbers show through the’ shellac, and should be distinguishable ready for assembling, Winding the Transformer. EXT comes the windings. A piece of wood just over 14 inches square is cut to a sufficient. length over 3 inches to fit in the. winding jig. The spool ends are 23 inches square, with a hole cut in the centre 1.3 inches square to make a tight fit over the square, spindle round which four or five layers of brown paper have been wountl. The distance between the two outside ends is three inches, and if the spool ends have been cut out of 1-8 inch fibre, the distance inside will be 24 inches. All the joints are given a liberal coating of secotine, and allowed to dry. Do not warm to dry as the ends are liable to warp and twist so that difficulty may be. experienced in getting the layers of wire on evenly. The primary winding consists of 2500 turns of 32’s s.w.g. énamelled wire. ‘An

average will be 200 turns per layer and 15 layers will be required. Do not wind right close up to the face of each. end or the pressure will tend to bend the end pieces outwards and the lower layers will fall down. | For those constructors who desire to work off the 110-volt mains, the wire. should be cut half way at the 1250th

turn and the lead brought out at the side. Then begin again and wind on another 1250 turns, so that the primary is divided into two distinct halves with separate leads brought out from each half. | As soon as a layer has been wound on a strip of paper cut 22in, wide should be wound round the spool ready for the next layer, Do not use shellac on enamelled wire as the spirits will dissolve the enamel and destroy the insulation. The leads are brought out through holes in the ends of the spool and about 8 inches. of wire left for connection to the ter-. minal strip. When the primary has been wound on a layer of empire cloth and another layer of paper is bound round ready to take the secondary "winding, which consists of 2100 turns of 36's s.w.g. enamelled wire. Averaging 300) turns per layer, the correct number of turns can be got by winding on "seven layers. With this extremely fine wire, a piece of thicker wire, say 28 D.C.C., is. soldered on to the be: ginning and end to bring out. for can necting to the strip at the top. About.

8 inches will be enough to leave and this should be tucked out of the way during the process of winding. A It will be thought that the voliXge available will be about 200, but after the resistance of the wire and the voltage drop across rectifying valve and smoothing choke the direct current available will be about 150 volts, Be-. tween each layer is wound a layer of paper which can be obtained cut into strips 2}in. wide, from most printing works, a Care should be taken to see that "none of the leads are brought out at the ends of the spools, where the laminated core will cover, It is perhaps advisable to mark in pencil beforehand two. lines running from the 1jin. centre to the same side at each end. Wrap more empire cloth and paper round the secondary winding and proceed to put on the filament winding. Preparing for the Vaive. for the amplifying valve no centre ‘tap will be required, so that the following table should be consulted for the number of turns for any special valve used :- 2 volt valve, 28 turns, 22 D.C.C, 8,W.A, 4 yolt valve, 46 turns, 22 D.Q.C, 8.W.A. -§ yolt valve, 72 turns, 22 D.C.0. 8.W.C, It will perhaps be noticed that the ratio is apparently not quite correct, Two or four yolt valves usually ¢onsume .1 amperes, while the 6 volt class consume usually .25 amp. The resistance. of the wire inereases with more current flowing, so that if 6 voit valves taking either .06 or 1. amp. are used, then a few iess turns will’ be required. Sixty-eight turns should be enough when the valve is used without a rheostat,

The rectifier valve Hlaments are next and as they are at a high potential with regard to the amplifying valve filament winding, they must he thoroughly insulats 1 therefrom--a layer or two of empire cloth and in this case a coating of shellac, which is allowed to dry before winding on the next. layer. It will have to be known beforehand which valve is being used for rectifying, whether 2, 4 or 6 volts filament. The number of turns and method of winding is the same as for the. previous filament winding except that in this case there is a centre tap brought out. When half the number of turns have been wound on make a loop of wire, pass it through a hole in the. nearest spool end, and continue in the same direction for the remaining half. Round the whole spool wrap two layers. of empire cloth and stick the overlap with a touch of seccotine. Assembly. [DISMANTLE from the jig and tap out the wooden spindle. The whole is now ready for assembly. Sort out the laminations with their -cotregt halves, and start to slide inside, the coil. The lamination cuts are stag- gered. That is, the cut is first at one side, then at the other, forming an over lap, and when all the pieces have been fixed in, the whole will be found to be firm, Wooden clamps 44 inches long, drilled 3% inch centres for brass bolts, 2 inches long, are used, for clamping all the

laminations firm. These are hardwood strips, 1 inch x 3-8 inch thick (oak is suitable). By referring to the diagram it will be seen clearly how these are ~agsembled. Two small brackets can be serewed on the bottom wooden strips to jx the transformer tot’ baseboard, On the sides of the two clamps at the top, ‘wo ebonite terminal strips are screwed. Olbgone side, half! an inch from the top, four’ terminals are arranged in two sets of two. The sets over the coil are arranged for the mains supply, while the other two are for the high voltage secondary. On the other side, five terminals are arranged-a set of two over the coil, and a set of three opposite the high voltage terminals on the other side. -Lengths of spagette insulated sleeving can be used to join the leads to their respective terminals. The group of three goes to the filament windings for the rectifying valve next the middle terminal for the centre tap. The remaining two on the same strip are joined to the two leads from the ampli-

fying valve filament. In the case of the split primary, four terminals instead of two are placed on the side of the mains input, the two centre ones being the end of first half and beginning of the second half. Starting from one end, the four terminals are arranged thus-‘In" of first half. "Out" of first half. "In" of second half, "Out" of second half. Care should be exercised in arranging these correctly, otherwise, if a mistake is made, the primary winding is liable to be burnt out when the current is switched on. Here are the instructions. for using with either voltage. If a reader

is on 110 volts, and is.expecting to be changed to the 280v_ supply, the directions should be copied out, and kept safely for future reference. Tor 110 supply. Join terminals one. to three, and two to four. Supply leads join to one and four. In other words, join the two "ins" together, and the two "outs" together, so that the two coils are in parallel. For the 230 volts supply, join’ the two centre terminals together, and the two outside ones to the mains. In this case, the two halves of the coil are in series. « The Choke. THIS finishes: the description of the power transformer. In the list of components, a low frequency choke is mentioned. If the reader wishes to construct this himself, an excellent .one can be made out of the laminations of a burnt-out transformer. If the secondary is intact, this can be used, although the extremely fine wire, generally 46’s, is liable to break down

after a few weeks’ use. After dismantling the old transformer, a bobbin can be made to fit in the place of the old windings. This can then be mounted in the winding jig, a geared handbrace makes an excellent jig for this job, and the bobbin filled with 38SWE enamelled wire. Flexible leads are taken out from the beginning and end of the winding. The wire is just run on without paper to separate the layers. Ideas will, no doubt, occur to the constructor for arranging a terminal strip for the two leads, and if the transformer fiad a metal shroud before, this can be put on again to effec-

tively prevent any stray magnetic fields. Assembly. : | [y the diagram, a dotted line is shown separating the crystal set from the amplifier. No doubt, some readers will have crystal sets already and only wish to make up the amplifying portion. In this case, the crystal set can be omitted, and a small terminal block made for the two leads that cross the line. If this is done, always try reversing the leads from the crys*. tal set, as it will be seen that one terminal of the transformer primary is earthed. Although it does not run directly to earth, the secondary winding of the power transformer has a capacity to the primary, which is earthed via the mains. The space occupied by the crystal portion is about five inches, so that the baseboard will have to be 11 inches x 7 inches, if the set is omitted. A. baseboard, 16 inches x 7 inches is planed, and two small strips fixed at

each end underneath, These need only be about 1 inch x 4-inch, so that it lifts the base a little, so that the filament wiring can be done underneath, The panel, either three-ply, varnished, or ebonite, is 7 inches high x 11 inches long. The diagram shows the plan, and an idea of the disposition of the parts is easily obtained. The variable condenser is fastened to the left of the panel, with two terminals, aerial, and earth at the bottom. Output speaker terminals are arranged on the same level as the aerial and earth terminals. Between the condenser and the speaker terminals is mounted the semipermanent dector, with the adjusting knob on the front panel. A 200 or 400 ohm potentiometer fixed in the position shown, finishes the panel drilling. It will be noticed on the right-hand side a small fuse block is arranged for the two main leads and the high voltage secondary leads. Two ebonite blocks about ifin. square are drilled

for four small terminals, To the clamping nuts underneath are attached the fiex leads from the mains, and on the other side the. two wires from the input terminals of the transformer. Strips of tinfoil 1-16in. wide are cut, and when fastened between the terminals on the fuse block provide a safeguard in the event of a temporary short. A similar fuse is. provided for the high voltage secondary, and this effectively prevents any damage to the wire in the transformer if the secondary circuit is shorted. NEST to the p.ver transformer is sereened a 2 m.f.d. condenser. Then the choke is screwed into pluce with the direction of the laminations at right angles to those of the power transformer. The other 2 m.f.d. is fixed next to two clips to hold the © battery. It was considered advisable to use an independent C battery in this case, as the cost is very small and, as little current is used, the upkeep of this battery is negligible. A new © battery will generally last about twelre months or more, so it hardly warrants the cost of the extra ‘resistances and condensers that are necessary if the C potential is required from the mains also, The positions of the two valve sockets and intervalve transformer is shown in the diagram. In selecting a transformer one should be chosen with a high ratio between primary and secondary to be able to get as much step-up voltage as possible. A .0O1 m.f.d. fixed condenser across the primary prevents any choking effect the iron cored. coil in the transformer may have. In the amount of 22 D.C.C. gauge wire specified, allowance was made for the winding of the tuning coil, Sixty turns should be wound round the 3in. ebonite former and tappings brought out at the tenth, twenty-fifth, and fortieth turns fer. connections to the spring clip from the aerial terminal.

Wiring. LL the filament wiring can be done ‘underneath the baseboard. From the transformer to the valve sockets

twin flex wire is used, as this prevents to a great extent the field produced when raw A.C. is used in a receiver. The remaining wires are number 18 tinned copper, covered with lengths of insulated sleeving. Here is a point-to-point connecting scheme. Join by twin flex the two filament winding terminals to the plants of the first valve holder. Carry these wires to the two outside terminals of the potentiometer. Join by twin flex the outside terminals of the tapped filament winding on the transformer to the filament terminals on the second valve socket. Of the same valve holder join the grid and plate together, and

connect to one side of fuse block. The opposite terminal of fuse block goes to one of the high voltage secondary terminals on transformer, the adjacent terminal to third fuse terminal, and the remaining terminal of fuse joins to the following points. Centre of potentiometer, C buttery plus, one terminal of each of the two smoothing condensers, the cases and shells of the various components used, the primary of intervalve transformer, the one end of tuning coil, and moving plates of condenser, finally to the earth terminal. Sleeving should be slipped on all lengths where cross wires are likely to occur. The, third end of coil joins to fixed plates’ of condenser, and to one end of the detector, to the opposite end of dector to free terminal on primary of transformer. The secondary transformer marked G joins to grid of first valve holder. The. terminal marked ‘. on this transformer joins to negative of C battery, and the centre tap filament winding to first terminal on smoothing condenser, and also to one

end of choke, Join the other end of choke to next condenser, and carry lead on to the output loudspeaker terminal on the front panel, The other speaker terminal joins to the plate terminal or lug on amplifying valve socket. Lastly, connect up the mains winding to the fuse block provided, and also to the flex for the lamp socket adapter.

Trying Out, THIS finishes the wiring, and all that remains is to wait for the local

station to start operations to give the set a try out. A loud hum will no doubt greet the constructor when first the set is turned on. Turn the potentiometer until the hum disappears altogether or dies away to almost inaudibility. The crystal set operates in the usual way, and if the dector is of the semi-permanent type, do not twist round the knob until you have first lifted the crystal from the face of the other one inside the detector tube. Tt has been found that a radio frequency choke inserted in the lead between the centre tap on power transformer, and first smoothing condenser, will make the rectifier more efficient and further prevent any possibilities of any hum. This can be of the slot wound typé, with about 1000 turns of 86 enamelled wound in a slot made by sticking two 2in. cardboard circles on a half-inch circle of wood or cork. The slot may be about 34-inch wide, and the wire just run in jumble fashion. The two ends can then be brought out to soldering lugs, and the choke inserted where indicated.

3 1 @b: 32 8, S.W.G. Enamelled Wire 1 Ib. 36 8. S.W.G. Enamelled Wire [' 8 4 Ib. 22 S W.G_, DCC Wire 2 Condensers; 2 m.f.d: each ; 9 Holers _ | 3 1 1 Transformer, Ratio 145 17 Black Iron (sufficient, quantity) Crystal Detector 4 inch Length Ebonite Tube; 3 inch dia: 7 Variable: Condenser , .0005 m.fd Potentiometer 8 Fixed Condenser, .001 m:f,dz 0 Baseboard, 16in. X 7in. % gin. Panel (Wood Or Ebonite) . Terminals, Insulated Sleeving, etc:

Components to be Used for Crystal and Amplifier.