A Good Crystal Receiver with Amplifier

Radio Record, Volume I, Issue 51, 6 July 1928, Page 12

A Good Crystal Receiver with Amplifier

Full Details to Help Beginners

(By

Megohm

HIS is a receiver that is easy to build, and that will give high quality siznals. At short distances of a few miles from a ------_ s main station, a small may be used, the voltage obtained being according to location dimensions of aerial, distance, and general efficiency of the receiver, which latter will be good if the directions are carefully followed.’ Good headphone strength for two or three pairs of headphones will be obtained at 100 miles, or more from a main station, depending upon the factors giready mentioned, but no exact statement can be made as to the strength of signals in a given locality. The best way for intending erystal .operators is to ascertain from crystal .users in their own district, what station. may be cousidered within range, aud how they are received. Radio enthusiasts are usually only too pleased to assist others with information when the request is made, CARBORUNDUM UNIT RECOMMENDED, "CARBORUNDUM $ erystal unit is shown in the diagrams because it is recon. .:ded for this receiver. Th? writer is quite aware that many crystal users have purchased such a unit, and on account of expecting from it much more increase in volume than 1s possible, have been disappointed. The writer has experimented with one of these. units, and with the small bias supplied from a single dry-cell, finds in operation that after a station is tuned in, the potentiometer control is moved until a definite point is reached where great clarity and slightly inereased volume are noticed. This slight inerease in volume is good aft any time, but when it is to be amplified. ft is greater, especially as finer clarity if also gained. The wording of a letter recently received from 4 corre--pondent showed that he expected the unit to give amplification equal te —

that gained by a valve. Some may wish to cut out the unit to save expense, in which case any kind of erystal may be put in its place. Im such event, the two wires connected to the unit, and to each side of the rheostat would be dispensed with. ‘ PANEL AND BASEBOARD. THE panel may be of ebonite or three-ply. The former gives a smart appearance, but oregon three-.

ply finished with shellac varnish rubbed in, makes a neat, inexpensive panel. On the front of the panel, which measures 12 by 7 inches, there will be the condenser dial in the centre, to the left the erystal unit imob, and to the right the rheostat teountrol. Tht panel is attached to the {

front edge of the baseboard by driiling three or four holes along the lower edge, and countersinking to take the heads of the brass screws used. The baseboard should be of 3-8in, rimu, finished with shellac dissolved in methylated spirits and rubbed on with a cotton rag. Battens may be serewed underneath to. prevent warping. Dimensions are 12in. long by 7 or inches deep. The position of components as shown works out well,

put need not be strictly adhered to. The main thing to avvoid is haying the connecting wires unnecessarily long where it can be avoided. Wires shown dotted in the wiring diagram are to pass through holes inthe baseboard and run underneath, and on this account if battens are put under the board, it is best to place one at each extreme end. Wires under the board may be run close together where pos-. sible, but should be well insulated. THE TUNING COLL. ,

TE tuning-coil or inductance is to be wound on a former 8 inches in diameter. This former may be purchased ready-made of bakelite, with ribs which keep the wire out of contact with the surface, making a "low-loss" coil. A colvern coil and pase, costing 6s., would make a neat former, or a plain cardboard tube could be used, or a strip of celluloid cemented together on a former to make a tube of the desired diameter. The wire to be used is 24's s.w.g. double cotton-covered, of which 56 turns must be put on, occupying a space of about two inches. At the certre of the coil, that is the 28th turn, a "tap" is to be led out. This is done either by scraping off the insulation and soldering on a Gin. length of the same wire, or by twisting a small loop in the wire whilst winding, afterwards cleaning of. the insulation at the loop and soldering on the tap

wire. Wind the wire tightly and evenly. This is best done with the spool on the floor and the coil on the knee, turning it and finding the wire with the hands. A pair of holes Is made at each end of the winding, and through these the beginning and end of the wire are secured by merely passing through the two holes, in and out again, The former may be any length, but as the wire oceupies only tv, inches, need not be longer than 3 inches, Having completed the 56 turns which is the secondary coil, we have the primary aperiodic or untuned aerial coil to provide. This consists of from ten to twenty turns. of 28's Siv.g. double cotton-covered — wire, found on top of the 24's, half the turns being on each side of the centre tap. Let the 28's rest in the hollows formed by the 24’s. The fewer turns, the more selective, but with reduced volume. A good plar is to put on 20 turns with a tap ar the 15th by means of a loop, then by actual trial the best position can be found, depend- | LI cunnections are clearly shown ing to some extent upon fhe distance from a main station. he coil is fastened to the bonrd by a strip of wood cut to fit the inside (diameter, fastened to inside of former, and secured to the base by & screw through the centre of the strip. THE WIRING DIAGRAM, on this diagram. The wires shown dotted are battery wires and are taken through holes in the board and pass underneath out of the way, giving the set a tidy appearance. Starting at the rheostat, from one terminal a wire runs to filament of yalve, from the other rheostat terminal Red peeern sper st ie under the board to A negative, and

continues to B negative, a branch cornnecting to the transformer secondary marked C negative or A negative. The other secondary counection marked "erid" connects direct to G on valveholder, which should be American UX type, as all makes of valves may be obtained with UX base. From each rheostat connection at convenient points under the board, eonnections are soldered on to run to the two.prongs on the carborundum

unit. These prongs are intended to hold a single small dry-cell for bias, but instead we are getting the bias by the drop across the filament rheostat. One ’phone terminai connects to the yalye plate terminal and the other to the full B battery voltage terminal, 45 volts.

The moving plates of the variable condenser connect to the lower end of the 56-turn coil, and to one primary contact of the transformer in passing. The fixed plates of the condenser connect only to the top end of the tuning coil. The remaining primary contact of the transformer is connected to one side of the crystal and other side of erystal to the centre tap of the secondary coil. The remaining filament eonnection on yalye-holder connects to A positive terminal,

Notice particularly that the 20 turns outside the eoil connect to aerial and earth only, the top to aerial, There is no direct connection between this coil and the rest of the circuit, the energy being transferred by ‘induction." The 20-turn connections "re shown by heavier lines in the diagram. Connecting wires should be of 18's or 20's, preferably the former, underboard wires well insulated. Wires above board may be insulated, bare tinned, or sauare bus-bar wire. Where terminals are not provided, wires should be soldered on rather than adopt any unreliable method of twisting on. _ It should be noted that in the generul view all wires are shown above the board in order to illustrate the connections as much as possible, but those indicated in wiring diagram should be put under the board. BATTERY POWER. NOTHING very extravagant is vequired in the way of batteries, and with an economical valve these should last several months. A. 45-volt B battery block is required, and the two extreme ends of this are connected to B positive and negative terminals, as shown. One of these connecting wires must be diseonnectedt at either end when the set is not in use. A small switch could be included between the B positive terminal and *phones. Three ordinary cylindrical dry-cells giving 14 volts each will constitute the A battery, to be connected exactly as shown. The central terminal of each cell is the positive, and connects to the negative of the next cell. A total of 43 volts is thus available, when. the cells are new, and as the suitable valves only require three volts on the filament, a good portion of the rheostat must be kept in the circuit, There is a certain point that can soon be found by experiment, beyond which the rheostat can be turned without any increase in amplification, and this point should on no account be exceeded. Excess of current shortens the life of a valve, but with care it should last well over a year. Great excess of current, say, a full volt over the rated maximum, miy ruin a valve in a few moments, so nothing is te be gained by pushing valve filaments.

TWO AMPLIFYING STAGES. SoMzE constructors will prefer to enlarge the baseboard, and add 2& second stage of amplification, which can easily be done, using the same batteries. The only extra parts required are a transformer, valve holder, and valve. (Continued on second column of next ‘(aged

A GOOD CRYSTAL RECEIVER (Continued from page opnsite ALTERNATIVE CONNECTIONS... , WHERE great selectivity-ability to _ cut out unwanted stations-is not required, the experiment may . he tried of connecting the aerial to the top of the 56-turn coil and earth to the bottom. In some cases greater volume may be obtained. | Sometimes the crystal cirenit works | best without earth connection when the 20-turn coil is"in nee. To conne t | the crystal to earth fer trial, couneecr. the IZ terminal, as shown by the small! crosses, to test the effect. } Another test is to place the erystal connection to the top of the coil instead of to the centre tap. The best arrangement to suit the location will thus be found, and may be permanently kept. VALVES TO USE. ‘ REFERENCE to the valve guide in the "Listeners’ Guide" shows the most suitable 3-volt amplifier to be the UX199, taking .06 ampere, which is economical on hattery current. Using two dry-cells instead of three. we nay use a 2-volt valve, Phillips Azo. There are other suitable Valves, | but they use np more current, PARTS REQUIRED. Variable condenser, 005m... Dial for do, . Audio transformer 5 to 1 ratio. Rheostat, 30 ohms, Crystal or detector unit. Valve holder, UX type. Six terminals, two ’phone do. Wire, 24’s and 28's. Headphones, A and B batteries, Panel, baseboard, connecting wire, screws, ete, 5