Crystal Receiver and Two-stage Amplifier

Radio Record, Volume I, Issue 28, 27 January 1928, Page 12

Crystal Receiver and Two-stage Amplifier

Perfect: Reception of Local Station on the Loudspeaker ‘

(By

Megohm

pons EV ERAL requests have ; a| been received from novices for complete instructions dealing with the items assembly of a crystal set with two amplifying valves, and the following is accordingly presented. Very fine reception of the local station is, assured with a carborundum fixed crystal and an amplifier such as this. Some listeners are receiving concerts on the headphones from long distances with such a set, but there is no guarantee for long distance, as much depends upon the location of -the receiver. This set is primarily to give full loud-speaker volume from a near station, and with good components and reasonable care in construction, will give volume and quality equal to many expensive yalve sets, and indeed will excel many in quality of tone. Those who are best satisfied with real music from the local station, _combined with a minimum amount ot attention, will find this set very well suited to their needs. Many listeners, no doubt, who have previously lis-tened-in with headphones and a crystal, will find it an opportune time to convert the set into something larger and more entertaining, so far as the whole family is concerned. Explanation will be given as fully as possible so that the new constructor, who is handy with tools, may suc cessfully build his first set. The materials and components to be purchased are as follows approximate prices being given.

PANEL AND BASEBOARD The panel is best made of ebonite, though some constructors now use three-ply wood for the purpose. Ebonite or formica imparts a smart appearance to any set, and is well worth the extra cost, especially on account of the high insulating properties. The length of the panel must equal that of the baseboard, which is conveniently 14 inches, though it could be slightly less at the expense of unnecessarily crowding the components. Fourteen inches wide and six inches high, then, is to be the size of the panel. The baseboard is constructed of fin. rimu, 14 by 7 inches, with a 1 by gin. strip projecting along under the front edge and a similar strip under each end to serve as battens. A small diagram is given of the method of screwing the panel tc the front edge of baseboard with strip underneath finishing flush at the front. The woodwork may be polished up with shellae dissolved in methylated spirits and rubbed on with a cotton rag.

or cabinet or not, is left to the wishes ‘of the constructor. TUNING ARRANGEMENTS, The question of providing a cover The aerial tuning condenser should have a maximum capacity of .0005 microfarads, and a tuning-coil to suit this will have 54 turns of 20’s standard wire gauge enamelled wire, either wound with turns close together on a 2in. diameter cardboard or ebonite tube, or wound with a slight space between each turn and supported only by strips of celluloid, as has been described in this column several times recently. If the set is not specially for long-distance work the close winding will be quite suita e, Those who already have a crystai set and wish to dismantle it, may have a suitable coil ready for use: The coil is placed behind the condenser, not less than an inch away. ;

t INCHES To wind the coil, two holes are made, zin. apart, in one end of the 83-inch tube and the end of the wire passed through inwards and out again, leaving six inches over. Winding is then proceeded with, the best way being to turn the tube away from oneself on the knees, working from left to right, keeping the wire taut, and steering it on with the right hand. When the 56 turns have been neatly put on, two more holes are made in the tube close to the end of the wind-

ing, the wire cut with six inches to spare, then threaded down through one hole and out through the next. This finishes the operation of making the coil. In connection with the aerial tuning coil, the best results are not always obtained by connecting the aerial to the top of the coil, and it is a good idea at the outset to try connection the aerial terminal to, say, the 18th turn from the bottom or earth end. The crystal connection may also.be tried at this point instead of at the top. The condenser must, in -every case, be connected to the extreme ends of the coil. The above idea is more important if it is’ proposed to attempt long-distance reception. DRILLING THE PANEL, Few holes are required in the panel. The various parts should be placed in about the position shown.

With regard to height, the condenser and rheostat centres should be just about half way. The condenser spindle should not be less than three inches from the nearest end, but the pattern of condenser will decide the position, which must be fixed before the position of the crystal exn be determined, as the latter must be placed to clear the vanes of the condenser when they are at minimum position, or full out. A template is usually provided with condensers as

a guide to drilling in correct position the holes required for fixing. The Igranic rheostats have an outside indicator plate that serves as 9 gauge for drilling the fixing holes. The positions marked 1 and 2 at back of panel indicate two sockets that are let into the panel, and from the back of the crystal near by, a piece of flexible covered wire leads out through a hole in the panel. To \the other end of the flex a split pin

is attached that plugs into either of the sockets 1 and 2. The object oi this is to provide a switch so that the ceyatal alone can be used when desired. To use the crystal the plug is inserted in the socket marked C on panel, and ’phones or speaker are connected to panel terminals 4 and 5. When not required, the A battery may be turned off at the rheostats, and the B battery it a dry one, by removing the wander-plug. Holes for the three panel terminals are drilled not less than 1% inches apart. Having made the baseboard and drilled the panel, the latter may be serewed in position: by four halfinch brass screws in the holes along the bottom edge. Brass and not iron screws must be used in all radio work. It is a good idea to arrange | Ln

a strut of some kind at one end of the panel to assist in holding it upright. A short strip of wood can he eut at an angle at each end, screwed to the baseboard and_ the panel through a hole in the ebonite near the condenser. This having been done, components not already in the panel can be put into place. The two transformers Tl and T2 (Fer-

ranti) are now screwed in place and between them are screwed down the two valve holders, noting carefully the position for the connections P, &, and positive (plus) and ‘negative. (minus). Constructors are recom-" mended to adopt American valve holders only, because British and other valves can all be obtained with Yankee bases, but if British holders are adopted, American valves cannot be used without an adaptor or the changing of the holder to one of the American type. The Klosner UX is suitable for amplifiers, but requires the connections soldering. Along the back of the baseboard there is a strip of ebonite one inch wide, about eight inches long, holding six terminals. These strips should be raised above the baseboard by a small block of wood where the hold-ing-screws are placed. Sometimes it is possible to purchase such strips with the terminals ready in place,’ otherwise it must be made. A simi-' lar strip 34ins. long holds the ter-. minals for aerial (A) and earth (E) connections,

Audio transformers vary as to the way in which the terntinals are marked, but the Ferranti are as shown on the diagram. Some are marked "IP, IS, OP, OS," standing for "in primary, in secondary, out primary, out secondary." In the case of the first transformer (TI) however, it is marked, the two primary terminals connect to socket 2 and the earth terminal respectively. WIRING-UP. All is now ready for making the various connections, which may be made with square bus-bar wire or with tinned 18’s round wire. Thewhole of this receiver may be wired up without soldering, unless the valve holders are of a pattern without terminals, in which case no attempt must be made to dispense with solder-° ing. A pair of round-nosed pliers will be required in order to neatly twist a circular loop at the ends ofeach wire to be connected. This loop is then slipped over the top or bottom screw thread of the terminal and secured by the head or nut, as the case may be. Commencing at the crystal, which will be a carborundum cartridge, if minimum trouble is desired, a loon is made on the end of the connecting wire, slipped over the thread, which is then placed in hole in panel and secured outside by the nut. The wire now continues to the outer endl of the condenser and a Idop is made to go over the screw thread connecting to the fixed plates. This having been secured by the nut, the wire continues to the back of the board and is bent downwards to be cut off and looped to the underside of A terminal, and at the same time the wire from the top end of the coil is cut off and the end well cleaned to loop over the same terminal,’ the nut then being screwed on very tightly. A wire is now run from the back of panel terminal 5 to the moving plate connection of condenser, then continues under the coil or round the inner side, keeping it low, to loop round the E terminal under strip, and continue to one of the primary connections of transformer 1. The lower end of the tuning-coil is now connected to the E terminal and the securing nut tightly fastened, when the ebonite strip may be finally fixed

in place. This wiring is shown on the back view of receiver, as well as on the full wiring diagram. Other wiring could not be clearly shown or the small back view, which is only intended to show the position of components. There are six connections to be made to terminals on the ebonite strip, and the most convenient method would be to cut off all wires sufficiently long and at the last make all connections to the strip at the same time. Now run a wire from socket 1 on panel, round the back screw of terminal 4, and on to P or plate con nection of valve holder 2. From ql or grid connection of this holder( run a wire to G or grid connection (secondary) of transformer 2, the wire standing up clear of others, and clear of both valves. Now one secondary of transformer 1 connects to the end terminal on strip and the other to G or grid of first valve. P of this valve-holder is connected to P of transformer 2 and this wire must rise over the top of grid wire at least one inch. The other primary connection of transformer 2 runs to the positive B1 on strip, and should keep to the outside of the other wires, and not between P and G as shown. The last connection of the second transformer is GB to C2 terminal. This and all other wires shown dotted should be .passed under the board, holes being drilled in a suitable position. From terminal 3 on panel a wire is run to positive B2 round the side of transformer as shown. The transformers may be placed an inch or so further from the ebonite strip than is actually shown, but T1 should be kept nearly two inches from either tuning-coil or condenser. Transformers marked "IP, IS," etc., are connected as follows:-IP to positive B battery, OP to plate of preceding valve, IS to grid-bias, or C battery, OS to grid of next valve. This will not refer to the primary (IP, OP) of the first transformer, which will be connected, as shown, to E and socket. The filament, or A battery, wiring is now to be carried out, the wires to pass under the board where shown dotted. From one side of the rheostat, a wire runs to the negative connection of both valve holders. The positive of each holder is connected together, and in this case the wire may be passed up through a hole in board to positive of holder 1, looped to the terminal or soldered, then passed down through the hole to continue to positive A terminal, this and the next wire to be kept close to the under side of board to alluw grid bias wire to cross with a good space btiween it and filament wires. It jis an advantage for wires under the board to be insulated. The remaining connection is from the other side of the rheostat to negative A terminal. There is the short piece of flexible wire to be attached to the rear end of crystal cartridge, passed through . hole in panel and attached to the split pin by twisting the bare wire round the screw and securing with the nut provided. | The Ci and C2 terminals connect to the negative taps of grid bias, or C battery, and in the event of only one C voltage being used, both these terminals would be permanently connected by a short-piece of wire, only one wire then would run to C battery negative. C positive is connected to the same terminal as the negative of A battery.

If ‘only one B battery voltage is used, positive 1 and 2 would be connected together by a short wire or "jumper," and only one wire’ would connect to B positive, whilst in either ease negative B connects to the same terminal as positive A battery. VALVES AND BATTERIES Up to the present stage, every re-. ceiver built from these specifications should be practically identical, but now we come to the choice of valves, and to some extent the valves to be used depend upon the battery power that is to be available, so the battery question must be settled first. ’ If dry batteries only are used good results will be obtained, but the amount of volume will be rather less than is consistently obtained from wet batteries, The advantage of? dry batteries is an apparent financial gain, but the gain is only in first outlay, as the continual instalments paid for renewals will in time equal the amount that would be laid out on wet batteries. For dry-battery operation valves requiring not more than 4 volts on the filament should be used. This will allow the employment of three dry-cells giving 1% volts each, or a total of 4% volts when connected "in series." Whilst the cells are new, the extra voltage is kept down by the rheostat, and this matter should be carefully atended to, as any excess voltage on dull emitter valves shortens their life, though with ordinary care there is little chance of trouble. ¢ For dry-cell working then, the most suitable valves would be a pair of UX199’s (4 volts), or a UX199 in first stage followed by a UX120 in the secon" both these valves taking the same filament voltage. In the Mullard series, two PM2’s (2 volts, requiring only two dry-cells), or two PM3’s (4 volts), or a PM3 followed by a PM4 (4 volts). In the Philips series, two B205’s (2 volts, 2 drycells), or a pair of B406’s (4 volts), oe an A409 (4 volts) followed by a On the above valves grid-bias will be from 1% to 8 volts on the first stage, and when the B battery voltage is the same on both stages with the same yalves, will be the same on each. When a higher B voltage is used in the second valve, grid-bias will be from 4% to 6 volts in most cases, so that a grid-bias battery capable of giving’ 6 volts will suffice. Exact particulars regarding grid-bias are supplied in leaflets packed with the valvas. Many people are inclined to dispense with gridbias, but for the small cost of the battery it is worth while, for two good reasons. Firstly, correct gridbias improves the tone of the amplifier, and secondly, its use lessens the drain upon the B battery, making it last longer, which is important when dry batteries are in use. There is no actual current taken from the grid-bias battery, so that it will run for many months without need for renewal, ° WITH A ACCUMULATORS In the case of adopting accumulators for the A supply, -whether the B battery is dry or not, the position is improved. Listeners who have to take accumulators some distance to be re-charged, may keep down the consumption of current by using the economical valves mentioned above for dry-cell use.. Where the accumulators can be quickly replenished, there is less need to study consumption so carefully. The four-volt valves mentioned above are quite suitable for use with two of the small glass-container "Exide" type of accumulators which are not at all expensive to buy, will run two valves with a charge once a fortnight, at small cost. Three such cells will run 6-volt valves, but some valves of this voltage require a good amount of current (expressed in amperes) which would cause the accumulators to need charging too frequently, unless an accumulator of large capacity (30 to 60 ampere hours) were employed. The small Exide cells are rated at 10 ampere-hours which means that they will supply one am-

pere for 10 hours, but as some of the valves quoted above only require one-tenth ampere or less, the time an accumulator will last is greatly extended. It is not good to run an accumulator until it will give no more current, but appoint a regular period for recharging when about threequarters of the total capacity has been used. A four or six-volt accumulator of thirty or more ampere-hours capacity would snit’ this amplifier very well, voltage according to the valves to be used. Very good results are obtained on four-volt valves. Where a "wet’’ B battery, that is an accumulator, is to be used, full advantage may be taken of the use of a good power-valve in the first stage, and a super-power in the sec‘ond. the latter requiring a plate voltage of anything from 120 to 170 volts to give full efficiency. SIX-VOLT VALVES. The valves usually classed as "sixvolt" require a 6-volt accumulator supply, but-the actual voltage re‘quired by the filament is considerably less than six volts, and usually | from 5 to 52 volts. . | The UX201A may be used in both stages, or may be followed by a UX112 or UX 171 in the last stage, if a wet B battery is employed. The PM6 may be used in both stages, or followed by a PM 256 power valve with B accumulator. A Philips A609, followed by a B605, would be a good combination. Two de Forest D401A’s may be used, or in the ‘second stage a D412 or D471. CHOOSING VALVES. In selecting valves for dry-cell work the filament voltage must not be more than four volts. Regarding the B battery supply, the plate current is what must be studied, and this is measured in "milliamps,’"’ or thousandths of an ampere. The more milliamps taken, the shorter time will the B battery last. The higher the B voltage, the more milliamps will be passed. An ordinary generalpurpose or small power-valve will take from one to three milliamps at 60 to 75 volts, and such consumption is suited to a dry battery. Superpower valves require a high B voltage and will consume as much as 20 milliamps, which would soon exhaust a dry battery, and that is the reason why. they cannot be used. In the "Record" of January 13, there was a list of-characteristics of the three leading makes of valves. This should be cut out and kept for reference by every constructor. B BATTERY VOLTAGE. Amplifiers will stand any voltage up to 100 or 120, but the higher the voltage, the greater the drain on the battery, so that if dry batteries are used, moderation is best, so that 60 or 90 volts will be very suitable. OPERATION. The operation of this receiver is very simple. With the batteries connected to their proper terminals, and the aerial and earth connections made, the rheostat is used to turn on the amplifier filaments, the rheostats only being turned a small amount at first. The loud-speaker is connected to the two right-hand terminals on the panel and the plug, or split pin, is placed in the socket marked A. Now turn the tuning knob or dial slowly until maximum volume is heard, then turn the rheostat slowly a short spece further to find the point where greatest volume is obtained. Let the rheostat remain fixed at that point, as any increase is unnecessary. The aerial Tuning will practically remain always the same for the local station, and will not require frequent adjusting. The positive or marked ’phone or speaker cord must always be connected to terminal 3 on panel, which should be marked. The numbers are not intended to be marked on panel, but the sockets A and C should be so marked. It should be noted that resistancecoupling amplification cannot follow immediately after a crystal, as this method relies upon impulses from the plate of a preceding valve. Probably the majority of constructors will run this amplificr with dry cells, and quite good results will be obtained, Where high B voltages are used with a super-power valve in the last stage, an output filter or output transformer is necessary to protect the speaker wind-

ings. This is also a good addition even when only moderate power is used. Next week an article will deal with this matter in connection with amplifiers generally, In order to provide for all requirements, two voltages are included for both B and C batteries. ‘If only one. voltage is used for cither of these, positive Bl and positive B2 are connected together by a short bit of wire, and the B battery connected to one of the terminals, or Cl and C2 (both negative) are both connected together in the same way for one voltage. _ Constructors should watch the next issues for further details in reply to queries. Next week there is to be a short resume of the Browning-Drake receiver, and in that constructors will find derer of the construction of space-wound coils,

8-16in. Ebonite Panel, 14 by GINS 6. ee ee eee eee iVariable -0005. Dial for same..........65 Carborundum Cartridge Cryscr ‘ Ferraati AF3 Transformer.. Audio Transformer, large, good make, 5 to 1 ratio Rheostat, 30 ohms ....... ° 2 Valve holders, American .. 8 Terminals .......6..66 ee 8 Pin Terminals for panel... 2 Sockets, 1 Pin ......... : 4lb. 20’s enamelled wire.... Baseboard, connecting wire, screws, etc .....-e..60. C2OSOTNOAMD AOA Won nolo

MATERIALS REQUIRED.