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A Power-pack for "250" Valves

Continued from last week

. THE one or two dividers may be mounted upright on a small block of wood attached to the baseboard by a screw. If two dividers are ‘used, only one is tapped, the connections being made to corresponding sockets on the panel. ‘The Pilot divider is specially mentioned, because the manufacturers definitely state that two can be used in series to reduce 400 volts. ere is another scheme of resistances i ries to break down 400 volts: B1 (22) 1500 (42) 4000 (90) 11,500 (400 volts); total 18,500 ohms. This would run a high "waste" current, though another 1000 or 2000 ohms would be added for bias resistors. Any scheme can be made up of separate resistances, and by further dividing up the resistance values, a greater variety of voltages may be obtained. The objections to using a number of separate resistances in series are bulk and expense. Nothing but wire-wound resistances must be considered in any part of the eliminator circuit, excepting perhaps the detector and R.F. outputs if additional reduction is required. a) It should be remembered that the greater the amount of current flowing through a resistance, the greater is the drop in voltage, so that when a heavy return plate current traverses the bias resistors, a lower value will be required to give a certain bias than that necessary to produce the necessary drop when the return current is small.

Grid Bias from Voltage Drop. THE simplest method of obtaining grid-bias of two or more voltages is by extending the voltage divider resistance beyond B- and from this extra resistance tapping off the voltages required. The method of connecting up this system is shown in the diagrams. An important point to note when using this system is that the plate current for the whole receiver and also the "waste" current return to the transformer through the bias resistance, so that its carrying capacity must be high, say, about 40 watts dissipation, For this reason composition resistances are useless, as they would burn out rapidly, and only wire-wound types may be considered. Neither will it} be permissible to use composition resistances that have wire contacts in the form of staples, giving the uninitiated an appearance of being wire-wound. In order to provide a variable resistance having good heat-dissipating .qualities, the writer has found the use of 400-ohm potentiometers to be very effective. Any number of these may be connected in series, but actualty it is only necessary to employ one for each voltage ‘required. The low. bias voltage will be obtained from the arm of the resistance connected direct to B- whilst another potentiometer connected directly to the first will give a bias voltage up to about 28 or 380 ~Yolts. Should the power stage require a higher voltage than this, say 40 volts, it is an easy matter to raise the variable values by placing an extra fixed resistance of 500 to 1000 ohms between the two potentiometer resistances. Such extra resistance must have carrying capacity equal to the variable portion. The potentiometers ‘

actually used contain 36’s resistance wire. It is practically immaterial whether the B Gentre-tap is connected to the arm of the last bias resistance or to the end of the resistance-strip winding. Hach varigble arm connects to the corresponding socket on panel, or’ diyect to the output seven-way sockets, if no

panel is used. In either case the output socket at side of the container eonnects to its ‘corresponding output condenser in the base. When the amount of bias is variable it is a simple matter to adjust it by ear for best results whilst the receiver is in overation. Adjustment by means of a milliameter in the power-valve plate ejreuit is better still. : The highest bias voltage obtained in this way reduces the maximum plate voltage by that amount, but where, as in this case, the drop has been allowed for, it is of no consequence.

This method of obtaining grid-bias is quite satisfactory for ordinary conditions, and, being variable, allows of the very best effect being obtained. Grid Bias from Separate Rectifier. OMB constructors may prefer to obtain the grid-bias from a separate tetcifying and smoothing system. Though it mean a little additional cost, this method is the most reliable of all. The chief additional expenditure would be a small smoothing choke, four 2 mfd. condensers,.400-volt test, a rectifying valve, and additional resistance. The additional windings required upon the transformer are very small. The high-voltage winding consists of 860 turns of 36’s s.w.g. enamelled wire. running. into three layers. For the rectifier,half-wave, a 4 or 6-volt powervalve that has gone off in emission may be.used, and a filament winding of 22’s or 24’s d.cc. should be put on aecordingly; 22 turns for 4 volts and 82 for 6 volts. The high voltage may be drawn from the centre-tap of a 50-60-ohm resistance across the filament terminals. The rectifier and choke would be placed in front of the transformer, or better still, place the valve outside, well forward from the plate rectifiers, The condensers would be secured. together to form a compact pile. This type of bias supply was described in unit form on July 12 last, and in the All-Electric Handbook.’ For the present purpose a single’ wirewound resistance should be used. This may consist of two 400-ohm potentiometers as in the preceding .ystem, the ‘only difference being that the total resistance from’ C + to C- must not be less than 11,000 or 12,000 ohms. To effect this, the two variable resistances may still be connected in series with any necessary extra resistance between, whilst in the connection to the return side of the high voltage winding, a 10,000 ohm: fixed resistance will be included. This shoulu be wire-wound, but may be of low dissipation, as only a few mills will be passed. The other side of the high-voltage winding connects to the rectifier grid and plate, which are both connected together. An r.f. choke of 1000 turns of 36’s enamelled wire on a fiat spool should be included in the circuit before ‘the first condensers.

The smoothing choke, which need: not have a gap, may consist of 5000 turns of 36’s or 40’s s.w.g. enamelled wire on a 1-inch stalloy core with window about 1 3-8 X din. Long piece 2 38-8, short piece 1in., if assembled without gap, which is quite permissable when fhe current passing does not exceed 10 ‘mills, and a higher inductance is thus obtained. Radio-Frequency Chokes. A RADIO-FREQUENCY chokes of 1000 turns of 30’s s.w.g. enamelled wire should be placed in each rectifier Plate lead. These chokes give greater selectivity to the receiver, as they prevent radio-frequency picked up by the mains acting as aerials, from reaching the plates of the valves. A flat formation should be adopted, giving a winding about jin. thick ahd 2in. diameter. Two pieces of ebonite or other insulating material 24 inches square; and bolted together with a $-inch piece of wood or ebonite lead-in tube for separation, will serve well. The total thickness should not exceed in. The Diagrams, JO. 11 shows the layout of condensers, which should be adhered to as nearly as different makes will allow. Only the connections to the top of Ceach pair of terminals is shown. The bottom connections of each group are all jointed together, and the common output condenser lead connected to B -. The bottom terminals of the 4 mfds. are all connected together with insulated wire, and connect to secondary centre-tap and C high output. This wiring cannot be shown here without confusion, but the connection is made to the top or output lug of 2 mfd. condenser marked "CT." A 2 mfd. condenser connects to each output except power and B -, the top pair in the diagram. . No, 12 shows the "CT" connection mentioned above. This comes from the connection of the two secondary wind--ings at the front side of the transformer. Good rubber flex should be used for this wire, which connects to lower terminal of all 4 mfds. and to high bias output as alread) given. The two. battens or stiffeners 1 x din. are shown at A and B; the latter must be grooved before screwing on, at each. place where a lead is shown crossing it, so that no wires cross on the outside (underneath). The two high-tension leads from the lower end of the fuses each pass

through the baseboard vat Cc and D and proceed through the’ ‘radio: chokes. to the plate of respective rectifiers, At EB and F the leads for the rectifier filaments appear and pass through the grooves in the batten. These two leads should be kept as equal in length as convenient. Hach lead connects to a filament tag on each socket, and to one end of the tapped resistance, This resistance may take any convenient form-may be made of a strip of mica or fibre about 2 by }in., pierced with three 1-8in. holes, length of resistance wire is wound on to give a resistance of 30 to 40 ohms, connecting to a bolt in the centre, from which B+ current flows to the first choke. Nichrome wire of not greater than 34’s gauge may be used; 26 inches of 36’s or 18 inches of 38’s will answer well. Looking at the diagram, the under side of the valve sockets is seen, and they are secured by screws placed the reverse way in the usual holes. In order to rise flush with the outside of the baseboard, the sockets must be of the pattern with a high raised centre, such as the Alpha, which was used in the original. The best procedure is to solder all connections and resistance to the sockets before putting the latter into place; then all is lifted in one piece and the screws placed in the sockets, Grid connections are not shown, as they are not used. The radio chokes must not be a greater thickness than #in., the thickness of the battens. Keep all these parts clear of the space occupied by the sides of the container when closed. Only the holes for the seven output leads are shown, as these present no difficulty. When all wiring is completed, cover with 4 thin sheet of cardboard and then a piece of thin sheetiron, screwed to the battens. At G, under the transformer, fasten with serews a small piece of batten thickness and put a screw through the iron sheet into this to prevent hum. The iron must be connected to earth; its purpose is chiefly to shield the directcurrent output from the rectifiers and A.C. leads. A strip may be cut away where the output leads come through the baseboard, and here it would be wise to turn back the waste piece in order to present a rounded edge to the leads. No. 13 shows how a seven-way plug may be constructed from a 3in. dial with the knob sawn off, and a piece of ebonite about 2%in. square. It would even be possible to use two pieces of ebonite. Clamp the two together and drill both at the same time to fit the pins. The holes in the ebonite must then be carefully enlarged to take the flush sockets. A hole is cut in the side of the container, slightly smaller than the ebonite, leaving room for screwing at the corners. Solder-tags may be secured under the pins and the flex leads soldered on, leading out through the central hole, where they are bound together with tape to form a cable, whi¢h may be enclosed in metal gas-tubing if near a broadeast station to prevent pick-up by the leads. The metal. tubing is earthed. The bevel of the dial is best placed inwards to form a grip when removing. No. 14 shows the metal cover with turned-in edges, which latter could be dispensed with if heavy iron is used.

Genera] Hints. } MH open portion of laminations on transformer and chokes should be tightly bound with adhesive tape to prevent rattle. A screw clamp to press the laminations together whilst binding is earried out makes the operation easier and more effective. Two clamps are better still, as One may be moved whilst the other maintains pressure. The flexible leads for heater current may be arranged so that the free end is convenient for plugging into the 1eceiver. This may be effected by a pair of pins spaced an inch apart in a smali strip of ebonite, and neatly joined ta the flex, insulating with tape. A similar strip containing two sockef% is attached to the receiver or _amp}°JMer. The flexible mains lead should ‘He fitted with a moulded adapter, and not a wooden one. Where possible plug in to a hot-point wall socket. 68 Potentiometers purchased for use as grid-bias resistors must have good dissipating powers as they will get too hot, on account of the wire being too thin. The wire on these should not be thinner than 86’s swe. A de Jur power potentiometer of .2000 ohms resistance is obtainable, and may suit some constructors. Fixed resistors for given grid-bias values are also obtain-: able, but are not so convenient as a variable arrangement. Tapped resistances for getting the electrical centre-of filament or heater circuits are sold under the name of "balancers," but 80 or 40-ohm rheostats may be used, mounted on the baseboard, each end of the resistance strip \ connected to a filament lead. The arn’\ connects to earth (B -) direct.’ It was stated in the preliminary notice of this article that the powerpack would deliver up to 200 mills of D.C. current. Whilst it would be possible to obtain an output approaching at least near to that figure, the apparatus in its specified form would uot supply it efficiently. The transformer secondaries will jointly deliver 200 mills without any overloading, and the 281 type of rectifier may quite well be made to pass nearly 100 mills each, which, although an overload, they will: stand up to it well. The chokes, as already specified, must be enlarged, and it would probably be necessary to enlarge C3 to 10 or 12 mfds., in order to handle the larger current. © The total resistance to reduce 500 volts to zero is about 30,000 ohms. / Up to 3 amps. may be drawn from the rectifier and power-valve filament windings, using the turns given under the heading "2 amps." In the case of the rectifiers and two sides of the high voltage secondary, the outputs of the two sides add together to make the total output. Constructors are advised to tap the primary winding at 1100 turns, so that in places where the mains are inclined to be habitually low the lesser number may be used. With a plug-in arrange- . ment the turns may be varied at will. There is no necessity to be nervous about somewhat exceeding the rated plate voltage for amplifying valves; the manufacturers generally state these rather conservatively, If in doubt over™ any of the diagrams, follow the theoretical for connections. . With a total output of about 60 mills, the consumption of mains current is exactly 100 watts, or 10 hours per unit of electricity. At Wellington rates this works out at jd. per hour, Where

higher charges are in force the run‘ning cost would not exceed 3d. per hour, In addition to the milliamps output, the filament lighting is included of a pair ‘of 281 type rectifiers, a pair of 210 type valves, and a cathode heater am-

plifier. These are the valves actually in use when the test was made. Be careful in handling the high volt-ages-always switch off the mains before making changes. The smoothing

condensers discharge themselyes through. the voltage. divider. This is a type of apparatus that can be carefully constructed, and works "right away" when connected up, and when that moment arrives the constructor may easily say, "Well, it was. ‘some job,’ but well worth the trouble." For either’ 210 or 250 valves the secondary turns should not be less than 2850. The maximum rated plate voltage of the 250-valve is 425 volis, and at this figure the grid bias is 83 volts, towards which'an extra 200 turns may be put on each secondary, if bias is obtained from voltage drop. Otherwise, bias may be supplied from a separate: rectifier, which has the advantage over voltage drop when using any two valves in push-pull, that double voltage. may be applied, which gives bias to "cut off," or near rectifying point, with the maximum output, which is considerably greater than is possib with the ordinary bias valve. ‘To get this increased output means that the first audio-stage must also be capable of handling the increased volume, and it may be necessary to instal a pair of small valves in push-pull to accomplish this. The grid-bias winding for double bias ‘on 250’s should -be 1250 turns of 36’s wire. Wherever "tape" is mentioned, it is ‘understood to be the black adhesive tape used by electricians; #-inch is a convenient width. Adhere strictly to the gauges of wire specified for the transformer. It is quite feasible to use 32’s s.w.g. for the chokes, for current under 100 ma. The D.C. resistance of each choke would then be 3840 ohms. ‘Three pounds of wire required for the pair. 12, connect to filament, test voltage and add turns until correct. In order to effect this, the core must not be tight against the outside of windings, and it will then be possible after adjustment to thread through a length of adhesive tape overlapping a sufficient number of turus to cover the winding. The spool ends are 3}in. square, and ‘the positions of leads is shown in a diagram. The writer will describe the pushpull a.c. amplifier that works highly successfully with the power pack, either in the "Radio Record" or in next year’s "Listener’s Guide."

This article text was automatically generated and may include errors. View the full page to see article in its original form.I whakaputaina aunoatia ēnei kuputuhi tuhinga, e kitea ai pea ētahi hapa i roto. Tirohia te whārangi katoa kia kitea te āhuatanga taketake o te tuhinga.
Permanent link to this item
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https://paperspast.natlib.govt.nz/periodicals/RADREC19300110.2.56

Bibliographic details
Ngā taipitopito pukapuka

Radio Record, Volume III, Issue 26, 10 January 1930, Page 27

Word count
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2,993

A Power-pack for "250" Valves Radio Record, Volume III, Issue 26, 10 January 1930, Page 27

A Power-pack for "250" Valves Radio Record, Volume III, Issue 26, 10 January 1930, Page 27

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