A Power Pack for "250" Valves

Radio Record, Volume III, Issue 24, 27 December 1929, Page 28

A Power Pack for "250" Valves

Suppiies all Current for A.C. Receivers

By

MEGOHM

HIS week we commence the description of the most comprehensive power unit yet described in the "Radio Record." If has been designed by "Megohm," whose description of this fine piece of apparatus may be followed by the novice. Owing to the size of the article, it will be extended over several consecutive issues.

HIS is the description of a band eliminator or A.C, power-puck designed by the writer, and run successfully for several months. The apparatus as described will | deliver up i about 180 mills of . current, but in order to draw moré than this it would be necessary to slightly alter the chokes, as dealt with elsewhere. Most constructors will make a demand of per-: haps 70 to 80 mills at the most, and at this .output high efficiency will be obtained. . The word "super" might well be applied to this eliminator, because it will fill the requirements of a pushpull stage employing any of the "super" power valves at present offering. The transformer secondaries are together capable of delivering up to 200 mills of current at a voltage of 499 to 500, according to the number of turns decided upon. Although the eliminator is capable of heavy work, employing a pair of UX 281’s type rectifiers it would be a good plan for any constructor to build it for immediate use with a Raytheon valve at 300 volts, with suitable taps en the secondaries, and at a later period with the enlargement of the amplifier, the highest plate voltage could be used and a pair of large rectifiers installed in place of the Raytheon. Two variable grid-bias voltages, a high and a low, are provided, so that dry cells are entirely dispensed with, and bias can be adjusted to a nicety. Careful tests have shown that bias 2btained in this way for both stages of the A.C. amplifier gives results quite equal to when a bias battery is used foy the first amplifier valve. This eliminator is in every Way suited for all-electric operation, as several filament windings may be put

on the transformer for either immediate or future use. Most constructors will find it convenient to adopt the idea of making their broadcast outfit in three separate units-eliminater. amplifier, and R.F. tuner. The two features of the exterior design are the placing of the two recti-_

fiers outside the case, so that the heut may be easily dissipated and the covering of the front panel with a metal

flap which is easily raised when adjustment of voltages is required; this idea imparts a neat and finished appearance to the whole. Regarding the actual circuit of tae eliminator, it contains nothing new, but the arrangement of the output voltage control has been specially

planned for convenience, and will give as much variation of voltage us the constructor wishes to provide. The idea of stowing the condensers in u box under the base has already been published in the "Listeners’ Guide," and in use has proved extremely convenient. THE actual construction of the trausformer and chokes will only be briefly dealt with, as reference to the "Listeners’ Guide" will give details of the various operations. Dimensions and any special features will be fully dealt with here. The whole of the materials, exclud. ing rectifiers, should be purchased for about £9, which is a very reasonabie figure for an eliminator of this voitage. The Panel and Container. EFHRRING to the general view of the eliminator, it will be seen that the front flap which usually hangs in front of the panel, has been raised. The left-hand portion of the panel consists of a strip of ebonite or other insulator, drilled to take any required number of flush sockets, one for each voltage provided, including grid bias voltages. This method of tapping voltages is convenient for experimenting, and well worth carrying out, although it would be quite practicable to connect output leads direct to the respective voltage-divider taps, dispensing with the sockets. Some constructors may

favour placing two sockets for the lower voltages, so that two leads may be plugged into the same voltage at the same time. Close to this row of sock- . ets will be seen the various leads for the several stages of the receiver. These leads consist of suitable rubbercovered thin flexible wire, terminating | in a plug, the top of which may be insulated with adhesive tape. . These leads pass through holes in the panel, and continue through the baseboard and along to the output plug. . where they also connect to one sid’ |f the corresponding output condenseT, The larger, or right-hand portion of the panel may be of metal, preferably sheet-iron. At the lower portion of it will be seen two 400-ohm wire-wound potentiometers connected in series to form the two grid-bias outputs, each arm providing a voltage. The one connected to "B-" gives the low voltage,

and the other the high. Potentiometers are used here because they will efficiently carry both the "waste" current and the whole of the last stage plate current as well. This is a point for special notice, and is the reason for not employing composition resistances. . Most voltage-dividers provide for 90, 65, 45, and perhaps 22} volts, in addition to the full voltage ‘for the last stage. Any voltages between these may be provided for by means of a, gariable , resistance of 3000 to 5000 ¢ , one end connected to (say) the 90-volt tap, and the.arm to an additional socket which might be marked 60 volts, though it would be variable from. 90 volts downwards, depending upon the resistance value used. Any output lead plrgged into this extra. socket, would then give » variable voltage. There is~ plenty of room on the panel for any such resistances that may be required. At the left of the container is a ledge upon whieh the rectifying valves are held in suitable sockets. At the left also is the output plug | which allows the cable running to the , receiver to be readily disconnected any time. Should this conveni not be considered necessury, the ¥ rie ous leads could emerge through one hole and be formed into a cable to continue to the receiver. A plug with seven connections is suitable, providing for R.F., detector, audio, power, Ci, ©2, and B-, Such plugs can now be purchased, but in case of dit-

Yeulty they can be constructed, using an old bakelite dial for the pins and a'-square of ebonite to hold the sockets, this being screwed to the side of the case, A solder-tag is placed on each pin, and all wires come through a hole drilled in dial knob, and are soldered to the tags. One pin should be out of regular position, so that the plug may only be inserted correctly. Flexible metal gas-tubing is used to contain the eabled wires leading to thé recejver. This tubing should be earthed, and in case of being unable to procure a seven-way plug, then instead a sixway plug, ¢ould be used, with an extra loosé pin, to connect B- to earth. From the back of the baseboard emerge the twisted flex leads for mains curgpt and A.C. valve heaters. lower portion of the container is merely a wooden box to hold the condensers, the front and back being of 7-8. rimu and the remainder 3-8in. The lid, which forms the baséboard., is strengthened by screwing underneath two strips of lin. by 3. The whole should be firmly screwed together -and may be lined with tin, though this is not essential. The cover and front flap are made of strong tin or thin sheét-iron, the heavier the better, and both metal and wood parts of the container are coated with black Sapolin, If a coat of shellac is first rubbed into the wood, a bright finish will be imparted to the Sapolin. The top of ‘the’ cover should not be much less than one inch above the transformer core, or a vibratory hum y be imparted to the cover. A strip of 24’s copper shéet bent to LL section and bolted inside the top of the cover will cut out any slight hum that may develop. Constructors who have built a small eliminator will find that in handling the higher voltage and greater current there are more chances of trouble that must be watched. The dimensions of the original are given, but may be altered at the outset to suit components. The condensers should all be purchased before deciding upon the size of the lower containers. In thé origina] the condensers used were T.C.C, 4 mfds. of 800 volt test, and Western Blectric 2 mfds. of 500 volts test. Any reliable makes of condensers of not less than the same test aus capacity may be used. The

space occupied by the T.C.C. is 2 x 2x 5 5-8, this latter dimension including the terminals. Some condensers~ have projecting lugs for screwing to a baseboard, but as these will not be required, any that interfere with compact stowing may be cut off. . The baseboard is hinged to the front of the condenser container, allowing. of instant access to the condensers at any time. The total inside height of this container should not be less than three inches, which allows. for the tin, strips under baseboard, and a@ space above the 2in, condensers for leads, All leads from the upper to the lower portion are taken through the baseboard at the front, where the hinges are placed, so that they will not interfere with the free movement of the baseboard when lifted. The baseboard may measure 14} by 10% inches, allowing 1-8in, overhang all round. . The diagreth shows how a wooden. frame is constructed from strips 1 x 3-Sin., and screwed to the front of baseboard. The panel is screwed to the back of this frame, and the front flap is hinged to the top, either by means of brass hinges, soldered to the flap, or by a hinge made by cutting the top edge of the flap, and a strip of tin with corresponding spaces, the projections being curled over a piece of 14’s wire to form the hinge. The flap should project 1-8in, beyond the frame at each side, and about din below top of baseboard at lower edge. Edges of flap and cover should be turn ed in, and a layer of cloth or other insulator secured to the inside of flap, so that when holding it up with one hand and altering tappings with the other, there will be. no chance of re-. ceiving a shock if a connection is accidentally touched. A good rule with eliminators is always to turn off the supply current before making changes. The flap and cover are connected to B --, the cover by resting upon the metal tray, with edges turned up half an inch, forming the floor upon which the transformer and chokes are to rest. This tray may be of brass, copper, aluminium, or any other non-magnetic _ metal, in order to avoid hum from the j stalloy cores. The tray also connects to B-, and is attached to the baseboard with jin. screws, _ When ill connections have been made the baseboard, a sheet of stout

cardboard and then a square of sheet iron are attached by short screws to the clamps under baseboard, leaving a tin, space for connections. Under the transformer an extra piece of din. strip should be secured, so that the board and iron may be attached to it with additional screws in order to prevent any chance of hum from the transformer core. Before putting the clamps on they should be drilled or grooved side-

ways, where required, so that filament and plate leads may be passed through the holes. The container as described makes a very suitable housing for an eliminator purchased in "kit" form, being greatly superior in appearance and convenience to the "breadboard" style of construction. MATERIALS REQUIRED. 7% doz, liin. stalloy strips (3ft.). 121b, 32’s s.w.g. enamelled wire.

1i1b. 24’s s.w.g. enamelled wire. 4lb. 30’s s.w.g. enamelled wire. . 1dlb. 18's s.w.g. d.c.c. 4 lengths 3-16 threaded rod and 16 nuts. .4 iron clamps. * Rectifier sockets (unsprung U.X.). 12 pins and sockets, : 1 voltage divider (sometimes known as tapped resistance). 1 extra do. ot 12,000 ohms resistance. 2 potentiometres, 400 ohmis. 4 4-mfd. condensers, 800 test. 5 2.mfd. condensers, 500 test. i-way plug and cable. Sheet iron, holts, screws, flexible wire, fibre, spaghetti, washers, timber, tape. The Transformer. [Ha core of the transformer is constructed of strips of stalloy 14iu. wide, built to 1 3-8in.,, so that the wooden former will be 13 by 1 3-8 by #in., the latter being the outside length of spool. The 1} dimension must be a shade over, so that the stab loy will slip in without forcing. The spool ends are 34 by 3%in., which gives an ever depth all round. The long piece of stalloy is cut 5 1-8in. and the short piece 2 5-8in., but the latter would be slightly increased if. any constructor finds it necessary to provide a greater number of windings than are , Specified. The stalloy may, of course, be cut up by the constructor, or ordered to be cut by~a supply house, where this service is undertaken. Every piece, should be shellaced on both sides after cutting. Details of transformer building ave fully dealt with in the "Listeners" Guide," so only necessary extra puarticulars will be given here. Before commencing winding, strips of medium weight unereased brown, paper must be prepared to place between every layer of wire. Between every separate winding place not jess than one layer of overlapping tape and two thicknesses of brown paper. The wide strips must be a fit for the inside of the spool, 3 9-16 in the original. About 20ft. length of this will suffice for the primary and he tween other windings. The narrow strips for the secondary will be a bare 1%, which should leave a space of 1-8in. between the two secondaries. About 40 feet of this width will be required. When assembling the laminations, and the centre of the spool is getting full, slip in a strip of tin 1 1-8in. wide,

turned up at one end as shown ina diagram .and. sufficiently long to turn up fin. at the other end when through. This forms a protection ‘to the inside of the spool, and renders easier the process of inserting the last few strips. Iron’ clamps at least 1-16in. thick should be used if they can be procureil, i}in. wide against the laminations, and fin, for the flange, which should have a hole near each end for bolting ‘o. the. baseboard. The length of .the clamps need not exceed 44in., drilled at each end for jin, bolts, 4in. from centre to. centre of holes. The spool ends must be marked with th. approximate position of all leadout holes so that all may be brought out at the correct place as winding is completed. The positions are. given in a diagram, which should be followed if trouble at a later stage is to be avoided. The actual. holes are made with a fine bradaw} or drill, as required during the process of winding. All holes must be marked before winding with title of the winding, and this should bz clear and distinct. Note carefully: the position-of the lamination space when marking out. * Lhe number of primary turns of 24's enamelled wire is 1125, the number having been slightly raised in order to compensate for the | comparatively heavy gauge of wire. The first few layers may be wound to the ends, provided that the papers are. truly cut, and fit well. With careful winding,’ the primary will. just fill eight layers. The following figures give the ‘total number of turns at the end of each layer in the original. First layer 140 turns, second 280, third 425, 567, 708, 846, 996, and at the end of the eighth layer 1125 turns. Tap the primary at 1100 turns. This primary is for 230 volts, 50 or 60 cycles, After a layer of tape and double: paper, the rectifier filament winding is put on. This consists of 47 turns of 18’s s.w.g. d.ae. wire, which almost fills one layer. Shellac well and allow to dry. This may be centre-tapped at the 24th turn: with a thin wire, say 24’s or rad s, from which to draw. the ---- _--_--_-

rectified high voltage current, unless this-is to be .aken from a tapped resistance across the rectifier filaments. ' The latter method is deseribed, but either may be used. Needless to say, this part of the: circuit must be carefully insulated. ' This filament winding will give 73 volts when 24 amps. are drawn. ‘Tape and double paper prepares the way for the secondary windings, the two being put’ on separately. Solder on a 26’s wire for lead-in, and‘ commence winding the left-hand -section, keeping the paper strips close against the spool end, and leave a space ft 1-16in. at each en of the turns in-:each layer. The greatest number that wiil go in ane layer is 130 turns, but the uverage number may be 115 to 120, occupying 20 layers. In the original, jb. of wire ran out at 1025 turns, and 4lb. at 1991, The paper strips may be cut’ to lengths in readiness, the first about 8 5-8 long, each oné increasing by about 1-8th inch, the twentieth being about 11fin. long. These operations are repeated for the second section. A space of about 1-8in. will remain between the two windings, and this is conveniently filled in by runing in thin twine, accompanied by a liberal application of shellac, The number of turns of 32's S.W.g. to be put upon each secondary is 2400 for 400 volts and 2850 for 500 volts. These figures allow 60 to 70 volts for drop in chokes and other losses. Also’ allowing for losses. 1850 turns will give 300 volts. The full 500-volt winding may be put in, and the lower voltages provided for by means of taps. If there are no taps on the secondaries, the end of the first is connected to the beginning of the second, and from this connection the centre-tap is taken. It may interest constructors to know the time actually taken in preparing the stalloy laminations for the origina] transformer. Cutting with snips 1} hours, flattening ends 40 minutes, shellacing all pieces 1 hour,

A fuse panel is provided,: measuring about 6 by ldin. This is drilled near each end- to fit over the projecting threads of the bolts, and is secured by an extra nut on each. See diagram. The pair of 1-8in. brass bolts for each fuse may be. placed in holes with centres one inch apart. For: both mains and high-tension fuses use 40’s wire with the: insulation cleaned off. The Diagrams. O. 1 shows the general arrangement Of panel and case, viewed from the front. The flap in front of panel ‘ts shown raised. The edges of this tin panel are turned in flat to a. width of about half an inch; a square of fabric may be secured with seccotine, and the edges turned down upon it ail round. ‘The object of the fabric, or cardboard if preferred, is to allow of the flaps being held up by the hand without contacting the earthed metal. The narrow ebonite panel is seen on the left, with sockets for various voltages ° ‘ The holes marked P, A, R, D, are those through which the flexible leads with pins on the end are brought from the outputs, power, audio, radio and detector. A similar plug-in arrangement is provided for the grid-bias voltages for convenience in experimenting. Constructors may ‘leave out any of these conveniences that. they do not require, connecting the output leads direct to the suitable tap on the voltage divider. At the top of the righthand panel are shown two variable resistances, should variable voltages be required. The valnes could’. be 5000 ohms for the r.f., and. 500,000 for the detector. ‘ No. 2 shows in plan the positions: of rectifiers, transformer, and chokes. No.3 shows the general arrangement, omitting transformer and chokes for clearness. The metal tray is shown, and should continue under the front upright frame, which is: screwed down through holes in the tray. The panels are screwed upon the back of the frame. The method of bringing in the flexible leads is here shown. A 8-16 hole is drilled in the back edge of the .base’ for each separate wire, grouping. in pairs, ‘another hole being drilled through the metal tray. about an inch from. the edge, to meet the horizontal hole. The ends of. the flex are then treated with seccotine to prevent fraying of the braiding, and when this is dry, the end is pulled through the hole, only a short end being required te solder to the filament leads coming out on the lower side of the spool. Where found more convenient, the flex may be taken..down wuhder the baseboard and there joined to fila-. ment’ leads. The sockets for the rectifiers are screwed under the baseboard, the top coming out through a hole: of suitable size, allowing the socket to appear almost flush with the top of the baseboard, which may be covered with tin: well secured with screws, and provided with. holes to coincide with the sockets. No. 4 shows connections to grid-bias resistors on back of panel, and also connections for. optional variable resistances, should they be required. In this and the previous diagram, the extra resistance is not shown, but the connections appear. The constructor will decide upon the form of this resistance as dealt with elsewhere. Note that B-end of the divider is connected to the beginning of bias resistances,-to the metal tray, one side. of output condensers :(not..to ‘the 4 mfd.smoothing |

condensers) and through the cable earth. The six flexible plug-in leads go down through the baseboard, where each connects to.a cable socket, and all but "power" also connects to one side of its output condenser. The lead marked "SC" ‘poes: down through the baseboard and connects to one side of all the high-test smoothing condensers and to the transformer centre-tap, This lead is well insulated throughout, although it is common to one side of several condensers. No. 5 shows a-section the actual size of a’ portion of the transformer windings, so that constructors may check up the space occupied as winding progresses. The spacing could be marked inside one of the spool ends before

winding. Provision is shown for three layers of 18’s heater windings, but for the average 4.c. receiver, only two layers will probably be required. As these windings are outside, they. can be altered at any time without disturv- . ing the fine wire windings. The theoretical diagram also appearing shows clearly the whole of the connections and output arrangements. (Further constructional details will be. given in next week’s issué.) ff