Crystal Set, Wave Trap and Detector for Valve Sets

Radio Record, Volume II, Issue 11, 9 November 1928, Page 26

Crystal Set, Wave Trap and Detector

for Valve Sets

Will Cut Out the Local ‘Station

By

Pentode

mma AN you cut out the local » al station? "That is the question asked everyone who jj has a multi-valve receiver. , | Answers vary, and it does not always follow that because a man has six valves, one’ after the other, he can .cut out the local station and "drag in the Aussies." The prospective buyer then asks, "Why the need for so many valves when all one hears is the home programme?" More often than not, the nearby station is somewhat distorted by having to use so many valves and depending on de-tuning to cut down the volume. People say that reception on a crystal is so much clearer. : It is not generally known that a valve used in the proper way can give less distortion on a powerful signal than a crystal. However, that is beside the -point. In -the. majority of cases the distortion that arises when using a ‘powerful receiver on a nearby statioi -ig due to overloading of the high-fre-quency and detector valves. And so.it is that many people blame the broad‘easting station when the trouble is right at home with them. The writer has often thought of the need for an efficient wavetrap ¢ombined with a crystal set. The one now to be described, however, has many salient points in its favour. In itself it is a long-range crystal receiver, and ean be used with the phones or coupled to a single or two-valve amplifier. When the ’phone plug is disconnected it forms an efficient wavetrap for use in the aerial lead of any valve set. There is also another use to which it can be put. By making tlie special adapter, described here, the detector or first audio valve of any set can be taken out and the adapter plugged into their places. When arranged this way, your three, four, five, or six"valve set is converted to a crystal detector with one or two audio stages. The high-frequency and detector valves PRPC

can then be turned out. Besides a great saving in battery current, there will be a decided improvement in the tone of the local station. It can be used in three different ways, and being easy to construct, the assembly should present no difficulties to the merest novice. Making the. Coil. JPIRST there is the coil to be made. This is of the space wound, lowloss type, wound on celluloid. Procure a sheet of clear celluloid 10in. x. 6in. and cut two strips half an inch wide from one side. The remaining piece, five inches wide, is wound round a 3inch former (after winding 2 or 3 layers brown paper to prevent celluloid from pinding) and a touch of celluloid cement spread along the overlapping edges. If there is more than din. overlap, the surplus should be cut off before cementing, If the wire is inclined to be twisted or kinked on the reel, sufficient should be wound off in an open space (about 48 feet will be required) and stretched slightly until it is quite straight. Two small holes are made at one end of the celluloid former and the end of the wire threaded through. Spacing is done with the aid of thin string, or better still, a length of 22 or 24 gauge wire. Whichever is used for the spacing, it is wound on at the same time as the straight wire already mentioned. When twelve turns have been put on, make a twist or loop in the wire on the opied

posite side to the cemented seam. Afterwards~tliese twists are cleaned and are used to make variable tappings from the aerial. A similar twist is made after the 18th, 27th, 38th and 50th turn; These loops are made in a straight: line all on the same side of the coil and project about jin. Wind both the spacing wire and coil wire on fairly tightly and when 60 turns have been put on fasten the coil wire to the celluloid. A good plan is to wind on a few more turns than necessary, drill a hole through celluloid and former and fasten the wire tightly at the end. OW carefuliy unwind the spacing wire or string, taking care not to disturb the even spacing of the wire

on the coil. The whole should then _ be given a liberal coating of, celluloid | cement. A. good formulae for making celluloid cement. is to get a chemist to mix equal parts of amyl acetate and acetone, into which should be placed a few strips of clear celluloid. Amyl acetate alone takes too long to dry, while acetone dries quickly but tends to peel off as a consequence. Put the ) coil in a warm place to dry as moisture ‘makes the. cement go milky. When quite dry slide out the centre former. Unwind the few turns extra and leave about six inches of wire at each end.

to the wire with a small pair o scissors. The two strips tin. wide ar now cut to a length about one inch longer than the coil, and a quarter of an inch from each end an #in. hole is drilled. When put one on each. side of the coil seam and = screw down to the baseboard on two blocks of wood, about din. high, this makes an excellent holder for the coil. So much for the hardest part of the construction. Both ends can now be trimmed off

[HE plan below will give an idea of the disposition of the components. The ebonite panel is drilled and screwed to the front of the baseboard. In the centre of the panel is mounted the variable condenser, while on either side at the bottom are the detector and single circuit filament jack. At the two top corners holes are drilled for two terminals. ‘Just behind the variable condenser the choke is screwed. The secondary winding of a burnt-out transformer will do quite well in place of this choke. A 1 m.f.d. condenser is fixed in place immediately behind the jack. By following the wires on the wirin, diagram, no " difficulty should be eo. ienced in the wiring up of the par ts. Thick. wire, square busbar or No. 18 S:W. and tinned copper, should be used in this receiver, as when used as a wave-trap. The whole tuning unit has to have as little high-frequency rv istance as possible. Here is a. point to point wiring list. Join left-hand free end of coil to back holding nut of detector and also to the moving vanes of the variable condenser. The other end of coil winding is con- nected to the following points: Moving plates of cond. .er, one terminal of choke coil, one terminal of fixed condenser and then to the earth terminal at the top of nel on left. The other end of choke joins to the centre lug of the jack. The top lug of the jack runs to the other end of the detector. The third or frame of the jack is connected to the free terminal on fixed condenser. On the aerial terminal is joined a length of flexible wire with a small metal clip fastened to the other end so that the clip can be moved along the variable tappings on the coil until the best position is found. When completed thus far we ure all ready for a trial run. A plug ‘vill have to be attached to the ’phone cord and inserted into the jack. . Join up the aerial and earth to their respective terminals. By tuning the variable condenser the local station should come in strong and elear.

AS &@ Wave-iLrap. WHEN used as a wave-trap the aerial is still connected to the aerial terminal, but the earth terminal is joine}l, by a length of wire, to the aerial tgrminal of the cet. ‘The ground or earth is connected to the receiver as usual. When it is being used in the capacity of a wave-trap the ‘phone plug should always be taken out. This automatically breaks the detector circuit and. leaves just the coil and condenser free. To operate, set the receiver dials to-the readings of a distant station. The local Avill be heard in the background more or less stronger than the desired station. Now turn the variable. condenser

on the trap, and it will be found that the interference is cut out. and distant stations come in without a trace of the local. It will be noticed .in some cases that the aerial condenser on the wteceiver has a different reading than Myhen used without the trap. This is lue to the effect of another tuned cir-cuit-the aerial lead. For Use With a Valve Set. TPHE next part to be constructed is the connector, for use when using the last one or two valves in the set. Care must be taken to see that all the following connections are made the correct way round. It wouldn’t do to say what might happen if a mistake was made, especially if the receiver was earthed as well as the crystal set. It is assumed that the reader has at least.one valve with a burnt-out filament! Break away all the glass and clean the solder from the tips of the pins. A small hole can be seen running down the yalve legs, and it is through these that connections are made. About a yard of twin flex is obtained. Clean the wires at\each end, and tin the ends to facilitate soldering. Now, look down on the valve base, with the legs underneath, and the pin at the back, push one wire down the’ plate leg (the right-hand top corner), the other wire, push in the hole of the

filament positive leg (the right-hand bottom corner). Take a note of the colours. In some plugs it is quite easy: to see which is the tip and which the sleeve connection. (Igraniec pacent plug is recommended.) The wire fastened to the plate leg in the valve socket is soldered ( soldering is preferable, as no mistakes are made), to the sleeve connection of the plug. The filament positive lead joins on the tip of the plug. If coloured flex is used it is quite simple to follow these. instructions. So much for this.part of the business. (CONNECT up the crystal set as though the phones were to be used. Now push in the adaptor plug, and take out the detector valve of the big set. In its place insert the adaptor socket. Switch on and listen. The R.F. valves can then be either turned out or taken out. of the set all together. If the volume is too much with two audio valve, this adaptor can be plugged into the first audio socket, and have merely one amplifying valve on the crystal. If, in any of these constructional articles there are any difficulties. eneountered by the builder, the writer will be pleased to help to make it clearer, and would appreciate any reports of the results of the constructor’s efforts. .

using two cartridges, would give two amps. without overloading. For full wave the centre tap is negative. The case of each cartridge is connected to a respective outer end of the secondary. To each small end of cartridge is connected a 10-amp. fuse, the outer ends of which are connected together to form the positive output. Yor half-wave the cartridge is similarly joined to one side of secondary, its thin end being positive, whilst. the other lead includes a 10-amp. fuse, and is the negative. Connecting B to C. [* the construction of a receiver the B battery negative lead is usually connected to the A plus terminai. The main object in this is to obtain a few volts higher potential on the B battery. Less risk of "blowing" the valves would be run if B minus was connected to A battery negative. The

few volts lost would be negligible, In the majority of sets the A minus is direct to earth, If by any chance the B plus terminal touched the earth or aerial wires or fell over, catching the C battery, then the B. battery would short circuit, but the valves would still be whole, which would not be the case if B minus was on A plus and the filament switch was turned on. New Linen Diaphragm Speaker. Y the time this issue is out of the press the new Linck diaphargm speakers will be on the market. These will be available as kit sets, including unit, linen, with wooden frame, and "dope" to treat the material after fixing it on the frame. Units alone will also be obtainable, and these will prove eminently suitable for the home construction of cone or double roll speakers.

List of Components Needed For Combined Crystal and Wave Trap

ib, 20 s.w.g. wire d.c.c. Strip of celluloid, 10 inches x 6 inches. 1 Variable Condenser, .0005 up. 1 Semi-Permanent Crystal Detector. 1 single filament Jack (Igranic No. 65). 2 Terminals. 1 Choke (or secondary of burnt-out transformer). 1 Condenser, 1 m.f.d. 1 Spring Clip. Ebonite Panel, 9in. x Gin. Baseboard, 9in. x 8in. 2 Terminals,