A Home-Made Dynamic Cone

Radio Record, Volume II, Issue 34, 8 March 1929, Page 28

A Home-Made Dynamic Cone

Constructional Details of Radio ’s Most Popular Speaker

By

PENTODE

PART II. N the last issue was given details of the construction of the pot magnet of a moving coil speaker. The remaining half, consisting of moving voice coil, cone and mounting, will be discussed . this week. To many the speaker will seem more than half completed at this stage, but by far the most difficult part has to come. It will call for maximum patience and the constructor is advised not to hurry. The more time that is spent over details, the more efficient will be the finished article. The Moving Coil. As one’s patience is better at the beginning of a job, we will, begin on the moving coil, which calls for more of this characteristic. The usual method is to make two or three coils, choosing the one that is neatest. Obtain an accurately turned wooden cylinder 1 18-16in. diameter and about 2in. long. Round this wind a strip of thin drawing paper 7-8in. wide, known as "detail paper." Two turns will be sufficient, between each layer painting on a thin coat of secotine. Allow this to dry, and make sure that the paper cylinder will slip off the wooden cylinder. If too much secotine is used the -paper will stick to the wood and the two will be unable to be separated. Two narrow strips of similar paper 1-8in. wide are now wound round this paper cylinder between which the fine wire of the moving coil has to be accommodated. Two turns will be sufiicient, forming two ridges, one of which has to be flush with one end of the cylinder and the other wound so that a —

space of 3-8in. is left for the wire. Reference to the figure 1 will make this clear. With the paper cylinder still on the wooden former, give the paper a coating of thin solution of celluloid in amylacetate. When dry give another coat, and allow to thoroughly harden before taking off the paper cylinder, after which a coating can be given inside. Take great care that this fragile paper cylinder does not lose its shape, and for this reason it must be kept at a constant temperature until the cement is dry. Next comes the tedious job of winding the moving voice coil. For this. purpose it would be advisable to rig up some form of winding jig. A hand-drill comes in useful here, and if a length of 4 B.A. threaded rod is passed through a hole drilled in the centre of the wooden former, and nuts tightened at each end, one end of the threaded rod can be held in the chuck of the drill which, in turn, can be secured in the vice. By calculating the gear ratio of the drill, the number of turns of wire can easily be calculated. The question now arises whether the coil is to be of the high or low resistance types. The writer seriously recommends the low resistance coil used in conjunction with a suitable step down output transformer, and details are given for this method. Using an output transformer of 25-1 ratio, the moving coil will have to be wound with 105 turns of No. 33 B and S or (36 s.w.g. wire, enamelled. The beginning and ends of this fine wire will have to be soldered to short lengths of No. 34 s.w.g. wire for the external connections, and in no case will the wire have to be wound on irregular. Make the top of the coil as level as possible, and tae

where the thicker wire is joined, part of one of the narrow paper ridges can be cut away. Leave about 6 inches of the free wire for connecting, and give the whole a liberal coating of the celluloid cement, still keeping the whole on the wooden former.

The End Plate. ON a piece of fairly heavy aluminium about 20 gauge, mark a circle, and cut a dise having a diameter of 1 13-16ths of an inch. This is to fit into the end of the paper cylinder, and must be fastened very securely into place. Liberal applications of secotine did the trick in the writer’s case, and fastened it quite firmly. The whole will resemble the bottom half of a pill box. This end plate must be of a strong yet light, material, as it has to bear all the strain of the movement of the coil. An 1-8-inch hole drilled in the centre of this end plate takes a bolt to which is later fastened the cone. Next comes the suspending and centreing device. In moving coil speakers as a rule the paper cone has to be fixed in an exact position, so that the coil can be centred. In this speaker, however, the cone is quite separate, and all adjustments are done before the pot magnet is mounted. The pot magnet and moving coil are as one unit, and are bolted to the cone afterwards. This is far easier than fastening the coil directly to the cone, which necessitates hours of patient work in getting the whole thing central and rigid. Bxamine carefully the various diagrams to see exactly how the moving coil is fixed, so that it ‘floats’ ‘easily in the small gap, and the following descriptive text will be more readily understood. The spider on which the eoil is mounted is composed of material of the thickness of a postcard, and the size required is just a little larger than that of a blank postcard, so, unfortunately, this cannot be used. A circle of cardboard (3 ply Bristol board) of a diameter of 33 inches, is marked out, as shown in figure 2. With a sharp knife cut out the centre, and three surrounding pieces, leaving a skeleton of two concentric rings fastened by three bars, each being j-inch wide. The outside ring is 3-8-inch wide, and the inner ring j-inch wide. Now earefully and accurately fasten the wire-wound paper cylinder to this prepared disc. Do this by moistening one side of the inner ring with secotine, and with the whole on a flat surface, press the moving coil into contact with the aluminium end plate nearest to the cardboard centring piece. When finished it will resemble figure 1, which is a side view. When the secotine is thoroughly dry, give the cardboard a coating of thin shellac varnish, and when drying see that’ everything retains its shape. Thus is finished the moving coil and suspension as one unit, and it can be mounted as a permancy to the pot magnet. Just prior to this, however, arrange the two flexible wires from the moving cojl so that they lay along two of the three suspension strips and retain in place by sticking over thin strips of

adhesive paper, finishing off on the outer ring by joining to two soldering lugs fastened by two small nuts and bolts. Fig. 2 shows this plainly. We have now to find the positions for the three holes through which pass the bolts to hold this unit in position. With the pot magnet standing on one end, the three bolts and collars uppermost, lay the moving coil unit as though trying to insert the coil in the gap. The three projecting bolts wi

preve.t this, and holes have to be cut in the outer ring to accommodate these. Mark and cut holes 3-16in, diameter, so that there is sufficient latitude for adjustment. A washer ou either side will, when bolted, hold the moving coil in position and allow the coil to move in and out of the gap without touching. The movement will be about fin. which is far more than will ever be needed, even in conjunction with a powerful amplifier, especially

_ ~- , with the 9in. cone used. Having | reached this stage successfully, the * eonstructor can "pat himself on the back," as the most difficult part has been passed. : The Paper Cone. HIE construction of the base, baffle board and cone comes next, and a glance at the various diagrams will be almost self-explaining. The material of which the cone is made takes some choosing. In the writer’s case, a piece of Whatman’s heavy drawingpaper was found suitable. This has the advantage of being readily obtainable in this country. There has been a great deal of controversy as to the best shape to make the cone of a dynamic speaker, but undoubtedly the strongest cone is one having an angle &: 90 deg. at the apex. Details are given for cutting the material to shape so when bent a 90 deg. cone will be produced having a diameter of 9in. at the base. An extra jin. is allowed round the rim for fastening to the kid or sheepskin on which the cone is floated. Lay the heavy drawing-paper on a ‘flat surface, and with a pair of compasses, draw a circle having a radius of 6.36 inches. Describe a second circle having the same centre of a radius of 6.6in. If a protractor is not available to measure an angle of 103 degrees from the centre, adjust a rule until a chord in the inner circle of 10 1-10in. is obtained. Allow a further din. on one side of this angle for the overlap when fastening the cone. Use secotine for this purpose and stand a weight on the seam until quite 4 dry. The }-inch rim can now be bent, ' little by little, so that when the cone is stood upright, the rim lies flat. At the apex of this cone a reinforcement has to be made, and the most convenient method is to make two copper cones, arranging one either side of the cone with a bolt running through the two to clamp the whole firm. These copper cones are made in the same way as the paper cone, only circles having a radius. of 17in. are used. When shaped they can be soldered and trimmed with a fine file. An eighth-inch hole is drilled right in the apex of each of the cones through which the bolt passes. ‘This bolt incidentally is the one arranged in the centre of the aluminium plate of the moving coil. Another method (that shown) is to use one cgpper cone with a wooden cone to fit into the apex of le speaker cone. Quite a number of materials are uitable for suspending the paper cone. Thin kid, sliced sheepskin, flannelette, or thin rubber sheet, are quite O.K. Thin kid is perhaps the best, as it possesses a certain amount of elasticity and does not perish as does rubber.

A piece 12in. square will be needed. Lay on a flat surface and smooth all irregularities by stroking from the centre. Give the rim of the paper cone a coating of secotine and press down firmly on the square of kid. When this is firm, with a very sharp knife or razor blade cut out the centre circle of kid inside the cone, or trim off with a small pair of manicure scissors. The Base and Baffle Board. OTHING more can be done until the base and baffle are constructed. A dynamic speaker is no better than any other unless a baffle board is used. To those who are puzzled by the term "baffle," it is a partition dividing the two sides of the moving cone, and upon its size depends the ability of the speaker to reproduc» the low notes. The larger the baffle the better, but to quote in feet and inches, a baffle should be at least 3ft. across. This does not always mean @ flat board 3ft. square, but can be im the form of a cabinet, the distance from the cone, along the front and sides, being at least 18 inches. ‘No build and adjust this speaker inside a cabinet would be decidedly awkward, so that the cone has been fastened to a small baffle, which is in turn fastened to the larger baffle after all adjustments have been made. For the baseboard, a piece of kauri or wellseasoned timber 12in. square and an inch thick is used. Two upright pieces of wood are screwed from underneath, and a cross-piece arranged so that the centre bolt in the aluminium end plate of the moving core is exactly 5in. from the top of the baseboard. If the pot measurements have been exactly adhered to as given, then the bottom of the pot magnet will have to be 2 7-16in. from the baseboard. The reason for this will be apparent later. A strip of brass or aluminium, bent to the shape of the outside of the pot, can clamp over the top of the magnet and screwed to the two upright pieces to hold the heavy pot magnet firmly. Two terminal strips, mounted on either side are used for connecting the leads from the magnet and moving coil to their respective places on the receiver. For the small baflle, use a piece of hard wood 12in. square and #in. thick. Drawing diagonals from each corner, mark the centre and describe a circle 104in. in diameter. With a keyhole saw cut this out. Drill three holes along one end and screw to the front of the baseboard. Take out the screws again and prepare to mount the cone in the hole just cut. For this purpose cut a ring out of guage 20 aluminium, having an inside diameter of 103in. (that is equal to the diameter of the hole in the baffle), and jin. wide. Note.--In last week’s issue a diagram was given

which made this 8in. This was 2 draughtsman’s error. The outside diameter will be 11}in. Hquidistant round this ring drill 12 holes. Now lay the cone on a flat surface, with the kid lying flat, and lay the metal ring over the kid so as to mark, through the holes in the ring, where corresponding holes have to be cut in the kid. Perhaps the easiest way to cut these holes is to use a fin. belt punch on a hard bench. Punch these 12 holes, each tin. diameter, as this allows plenty of latitude for adjusting the cone when mounted. Next screw the ring to the baffle, between which is the kid, the cone being held centrally meanwhile. By

slightly loosening the screws in the ring, the kid can be pulled so that there is an even and very slight tension on the paper cone all round. Final Assembly, All is ready now to assemble. Slide the pot magnet well back away from the front when screwing the baffle to the baseboard. Now, gently pushing the pot magnet forward, see if the bolt on the moving coil will slide through the hole in the apex of the cone. If not, then adjust from the front of the baffle by slackening off the screws through the metal ring, and pulling the kid until the bolt is directly behind

the hole in the cone. Slide on one of the small copper cones, push the pot magnet into place, then put on the remaining small- onner cone on the end of the bolt inside the paper cone. Finally add a in. washer and bolt up tightly. The speaker is now complete, and when trying out don’t do as the writer did, and get the two sets of terminals at the back mixed up, The great point to note throughout the whole of the construction is to make everything absolutely firm. All joints fastened by secotine must be hard and tight. It is nota job that can be com pleted in a night, and if a list of com. ponents was given, the writer would head the bill with "patience." One hint to anyone who may feel competent enough to do it: the gap in this speaker has been made fairly large, and if any reader cares to go to more trouble in the construction of the moving coil, the size of the gap can be reduced to 5-64 inch, with a far greater increase of Sensitivity. The smaller the gap, other things being equal, the greater the vol- ume obtainable with a given input.

A Reminder.-This speaker has a low resistance winding and a 25-1 transformer will have to be used between the speaker and receiver or amplifier. This could quite easily be arranged on the baseboard alongside the pot magnet.