A Crystal Set and Amplifier to work from 22½ Volts

Radio Record, Volume II, Issue 22, 14 December 1928, Page 28

A Crystal Set and Amplifier to work

from 22½ Volts

(By

Galena

fiers there are literally hundreds, yet all are modifications of a simple plan. The following is . another to be added _ to category, but should make a strong appeal to all who desire the local station on good loudspeaker strength with a minimum of running cost. A erystal set is described, together with the amplifier, but any crystal set may be used by hooking the speaker terminals to the transformer. _ A small eabinet contains everything, and if carefully constructed, well-

stained and polished, will become a fine piece of furniture. Only one valve is used, but it is of the double grid type, requiring only 224 volts on the plate and auxillary grid, with the usual low filament volt-

age and consumption. Under these conditions the set will work from one small B battery and three dry 13volt cells. The drain is very small and, providing the set is moderately used, the cost of renewing the batteries will be trifling. Prospective constructors should not expect to get beyond the local station,

I that is not claimed, but it is claimed that there are few, if any, sets that can be run on such low voltage and give amplification that will far exceed that of the ordinary one-valve amplifier. The circuit is not original, but an individuality has been given which renders it different from the usual type of set. The Double Grid Valve. HE valve is provided with two grids, and is thus enabled to give in the case of Philips 241 an amplification factor of 4.5 with the low internal

resjstance of 4500 ohms. The low plate voltage, filament voltage and consumption renders it highly suitable for a one-stage amplifier. The 241 valve requires only 2 volts on the filament, and as three volts are provided by ithe batteries a 30 ohm resistance must be placed in the circuit to prevent the filament from being spoiled.

UU EL The writer knows of no other make of double grid valve on the New Zealand market but 241 which can be readily obtained.

The C*'=ct. HF usual procedure in construction is to build a set, comprising a

baseboard panel and components, and connect these by long leads to batteries, either concealed or lying about the . set. A cabinet may be constructed, | but usually not till some time after- " wards, if then. This time procedure is reversed and the cabinet is constructed first, and the amateur will not find this difficult. Diagram (1) will give an idea how this should look when finished. The timber is half-inch, well-season-ed heart; 8 x 4 inch dressed lining is very suitable, and is readily ogtainable from a timber yard for very small sum. About 7 feet will be ampie. Space will not permit full constructional details of the cabinet. Measurements are fully given, and no difficulty should be encountered. Otitside measurements are given in each case. The front portions should be joined as in diagram II. The edges can then be rounded off and polished. It is advisable to fit the bottom and back into the front and sides; that is, like the end of a fruit box. The top forms a lid and is provided with hinges. A small overlap, say haltinch, is preferable in the front. This should be bevelled off to give a neat appearance. When complete, smooth with sandpaper, stain, varnish, and allow to dry. Not until thoroughly dry is it advisable to carry on with the assembly. The Coil. HE coil to be used is of the basket type. Although being perhaps a! little more difficult to constrvict than \. the ordinary plain coil, the writer considers it better in this circuit. Many have been tried, but this was found to be the best. On a piece of timber draw a circle With a radius of 12 inclies, divide this

into 15 equal parts, and drill to take 15 pieces of meat skewer each 23 inches long. Twenty-four gauge double

QHUUAUEIRUVEAVUALUUUUIUUUGUEUOUEIREUOSEOLEALUUELEULONUOEUELUOSUAULEUUAEAS ASG Ee T HERE are four stages in the evolution of the wireless "fanatic," the crystal, the amplifier, the broadcast valve receiver, and the short wave receiver. The advent of Christmas will mark the initiation of many newcomers to the joys of radio. Of these a large proportion will start with the simple crystal. But Christmas broadcasting, with all its cheer, calls everyone to listen-in, but how --on a crystal? To fill that gap this set is described. Both initial and running costs are low, while quality and volume are truly remarkable. e THUPDUCUOUL TODO EE oe eee ee eee eee

cotton-covered wire is to be used for the winding. Making .one end secure to the timber, start winding in and out of the uprights, passing inside one and outside the next. It will be noticed that each lap passes on the other side of the skewer from its predecessor. Thus series of diamond shapes are form- » ed, the centres being the skewer. ‘The whole will somewhat. resemble a ‘basket, hence the name. Wind on from forty-five to fifty turns, according to the length of the aerial: the longer, the aerial the fewer the turns. To secure the meshes cut fifteen pieces of the 24 wire, each four inches long. Pass one end down each diamond shape and up the adjoining series of diamonds. Thus a U shape, with long arms, will connect each series of diamonds. Twist the ends of each wire together to form a firm support for the corners of these cavities, When firmly secured twist each end of the coil to the U nearest and remove from the former. Place this in the cabinet as shown in the lay-out diagram (IIT). Lay-Out. ey selecting components use only the best. They cost more, but amply reward; especially does this refer to

the condenser, the transformer and the rheostat. A list is appended. Drill the upper half of the panel 2iin, from the top and three inches from each end. On the right-hand side fit the condenser, on the left the vheostat, that is when one is facing the cabinet. A chisel may be introduced to make a neat fit. Lay out the remaining components as shown in diagram ITI. It will be noticed that the C battery overlies the B. In the

diagram a large 14-volt cell is shown, but a torch cell would be sufficient. This is to economise space. For safety’s sake the valve base and transformer should be fixed to a piece of timber of suitable dimensions, say, seven inches by four. ‘This would allow both to be removed when an alteration is necessary. Care should pe taken that the letters G and GB (GC) of the transformer should be opposite P and G of the valve base. A permanent crystal has been selected and should be mounted vertically about two. inches fiom the top of the right-hand end of the cabinet. It is shown horizontal in order to simplify the wiring diagram. The terminals marked A (ANT) and G (GND) should be two inches from the top. Two jacks are provided; one will plug into the crystal set alone for ’phone use, the other into the amplifier for speaker use. They may be mounted as shown, or with the speaker jack below the ‘phone jack. All joints should be soldered. Wiring. BY the aid of the diagrams no difficulty should be encounted in wiring. Square 18 tinned wire may be used if covered with spagetti. Ordin-

ary insulated 18 wire will do if the outer ‘insulation is stripped away, leaving the rubber covering. Wherever possible wire along the sides and floor of the cabinet. To the aerial terminal attach two-wires, one. to one end of the coil and the other to the erysta]. and on again to the fixed plates of the condenser. Connect the moving, plates to the free end of the eoil and to B of the transformer. From the soldered joint of the coil

‘pass a wire along the floor of the. cabinet, up the back to the ground terminal, A wire passes from this. terminal to the portion of the jack connecting with the tip of the plug. as shown. Join the free end of the crystal to P of the transformer. From about the centre oi this wire a branch runs to the other prong of the ‘phone jack. This completes the ‘phone winding. Plug in and try out. A round or two might have to be taken from the coil, but the use of a .0005 mfd. condenser would make up for any inaccuracy in number of turns, If, on tuning in, it is found that the. loudest point is 100, more turns should be added. If loudest at O, the number should be. reduced. Amplifier Winding. | (CONNECT G of the secondary of the transformer to G of the valve base and GB (or ©) to the negative terminal of the cell lying on the B battery. Jonnect the positive of this cell (C battery) to the negative of the A battery, that is, to the outside terminal of the two cells on the left back. From this terminal (A-) run a wire to one of the filament (F) terminals of the valve base. Connect the other filament terminal to the rheostat. Do this

‘y running a wire along the bottom f the cabinet ari up the inside of the front. From the other end of the rheostat take « wire round to A plus, and another to i: "inus. Connect the two free terminals of the A battery.

This completes the filament wiring, turn on the rheostat and test by connecting a torch bulb to the F terminals, this should light. To complete connect the terminal on the valve base marked P to the longer prong of the speaker jack. (that is, to the prong connecting with the tip. of the plug). Connect the remiining prohg to B-+- Repeat the, test ‘with the torch lamp and if everything is in order introduce the valve. . On one side of the valve is a terminal, Connect this to B-++ or to the B-++- terminal of the jack, as shown. Plug in the speaker and turn on the theostat only partially-to turn it on full would ruin the valve. | By following the diagrams it will be seen that the wiring follows the sides and bottom of the cabinet ‘wherever possible. Do this for neatness as well as efficiency. Working Hints. ,JLLOWINE are a few hints that should aid th* beginner to look after the set and keep it in tip-top order. Do not let the batteries get too low. batteries tend to burn out slightly more rapidly than B, so keep a voltmeter handy. If toreh batter cells are used for

(’, test frequently, otherwise quality will be lost. Th writer found an improvement effected by placing a celluloid. screen between the condenser and the coil. A (Continued on Page. 30.) a

Components for Crystal and Amplifier CRYSTAL AMPLIFIER alb. 24 S.W.G. D.C.C. Wire. Transformer small ratio (33 to 1). Permanent Crystal. Double-grid Valve (241). 0005 S.L.F. Condenser. Valve Base. Lightning Arrester. 30-ohm Rheostat. Wire for connections. Plug and Jack. Two Terminals, Gnd. and Ant. 222-volt B Battery. Phone Jack and Plug. 3 12-volt Cells. PY nn er ne ee NT e.g. OOO OLDE A Ae a ee

Crystal and Amplifier

(Continued from page 29.) piece of celluloid un’ "neath the coil would guard against leakage in the ease of the wood being green.

When the set is going well, don’t tinker round, sit down and enjoy the programme. Remember, if investigiati: y the set whilst going, you may be amusing yourself, but be propably annoying someone else. The set is designed to give good entertainment from the local st. tion, so don’t strain batteries and valves by trying to get distant stations, you will be disappointed in more ths one sense. ; If desired the jacks may be placht on the front of the panel, but they rook better on the back. To put the set out of action, turn off the rheostat. This automatically cuts out the A, B and C. N commending this set, "Galena" would like to hear of readers havying constructed it, and with a view to helping the amateur will try to help out anyone who gets into difficulty or whose results are unsatisfactory.