Crystal and Valve with Three-Valve Performance

Radio Record, Volume II, Issue 20, 30 November 1928, Page 26

Crystal and Valve with Three-Valve

Performance

By

Pentode

mmm) OW many -listeners live too far away from a {| broadcasting «station to | fh} be able to hear the programme ona loudspeaker, using a vavie detector with one stage of audio amplification? This little receiver was designed for speaker reception of stations from 20. to 100 miles from. a broadcasting station, and ‘phones’ reception of ‘the remaining three Néw Zealand stations under favourable conditions. By using a second stage of audio amplification all the main broadcasting stations can be heard on the speaker on good nights, and many nights the-writer has heard Australian stations, using but ‘two valves and a erystal. The theoretical diagram will show that this is not a reflex set, and is very inexpensive to build. Only one tuning condenser is used, with a reaction or volume control, so that it is quite an easy set to operate. Like so- many sets for which super claims are made, this is not a tricky receiver, It can be built by a novice in an evening, and is always certain of results, providing the instructions have been carefully followed. N the ordinary crystal receiver, the erystal detector has a great ‘damping effect on the tuning system. Space winding with thick wire is used to cut down the resistance of the eoil, brit there is still the detector. In this receiver reaction is used, which still further, reduces the resistance of the tuning coil. In fact, it goes further than reducing the resistance of the coil. It makes ‘up for the added damping of the crystal detector. In the usual reflex set, two or more tuning coils and condensers. are used. These are a source of expense, very often producing a set that is tricky to operate. Beyond the price of the components for a crystal and one-valve amplifier, there is only a small fixed condenser and resistance. and the writer considers the whole cireuit eminently suitable for Kew Zealand conditions. : Arranging for Reaction. MPHE coil will be described first. For . economy. vurdbourd tubing of the diameter specified would do quite well, but this must first be dried thoroughly in the oven and given a liberal coating of shellac varnish or solution of celluloid in amylacetate. This prevents absorption of moisture in damp weather In one end of the 3-inch tube, the reaction coil has to be able to be turned rotnd by the spindle from a knob on the front panel. Various methods of assembly will suggest themselves to the reader. but the method employed by the writer was quite easy to construct and has caused no trouble by slipping, etc. In opposite sides of the larger tube, about half-inch from the end, twa holes were drilled large enough to take a length.of tapped brass rod. In each side and in the middle of the Smaller piece of ebonite tube two holes were also drilled The rod can then be threaded through both tubes, the sma!lcr inside the 3-inch dinmeter tube, sothat the inside tube will revolve round without touching the sides of the outer

tube. When wound, four nuts and a few spacing washers can be threaded on, so that the smaller tube is clamped tightly to the spindle, and will turn freely inside the outer coil. In order that the reaction knob wil) be in the middle of the panel,. three inches from top-and bottom, this coil will have to be arranged to stand one

inch from the baseboard. Two ebonite strips 53in. long by. din. wide can be used to clump the bottom of the coil. These will project din. over each end and can be drilled so that when an fin. screw is passed through these holes and two blocks of wood lin. high the whole will be firmly fixed to the baseboard, one inch above it. Winding the Coils. OW the coils‘must be wound. Dis: mantle the reaction coil arrangement. Beginning at fin. from the end opposite to the reaction coil, wind ov 40 turns of the 28 dec. wire. Make two small holes in the former to pass the beginning and ends of the wire in and. out to hold fast. Beginning Hin.

from this aerial coil, wind on 60 turns of the 22 s.w.g. d.ae wire and fasten the ends to the former. A nut and bolt with a smail soldering lug makes a very neat way of beginning and finishing off the ends of the wire, and it is easy to solder on the various leads when connecting up. This should bring the end of the wire about tin, from the

holes through which the reaction spindle passes. Both these coils should be wound in the same direction. The tickler coil’ deserves attention next. This is wound in two sections, one on each side of the spindle and enough room will have to be left for the clamping nuts. If necessary, these may have: to be filed thinner one way so that they will not press on and cut the wiring. Another way is to slip a small insulating washer between the nut and the ebonite former round which is the coil. The number of turns on -this ‘coil will vary, "and the amateur will not be surprised if he has to ~ wind on a few more turns or take a few off after the receiver has been tried out,

It depends to some extent on the crys: . tal detector, the amount of coupling between the two coils, etc. The writer uses fifty turns, and this number ¢hu | be wound on as a ‘first trial, This is . wound with the No. 28 s.w.g., d.ce¢." wire, and if ‘there is not enough room can be wound on in two layers, separ- . ated by a layer of paper. If the set oscillates too readily when finished: a _ few turns can be wound off. If-the reaction effect is insufficient to overcome | the damping, then more turns will have to be employed. To each end of this coil, two flexible leads are sgoldered and fastened to two small terminals or soldering lugs on the larger coil. Arranging the Components.

So far, so good. The components should now be arranged on the ‘baseboard «and = screwed down, after the front panel has been drilled for the variable condensers, etc., aud screwed to the front edge of the baseboard, A far neater job can ‘be made if the base is ‘first planed, sandpapered, and given a coating of stain and varnish. Spirit soluble analyne dyé-can be dissolved in methylated spirits and brushed evenly . on. After it is dry a.coat of shellac varnish will bring the colour up and give the whole a professional touch. The front panel has just three dials: the variable tuning condenser, the reaction control, :and the rheostat filament switch. ‘Aerial, earth, speaker, and battery terminals are al] arftanged’ on strips of ebonite along the back of the baseboards. Two clips should be | made tor the grid leak -and permanent: detector, and atranged on ebonite’ bases so as not to have any par ts touching the baseboard, The fixed condensers, too, should be arranged to stand clear of the wooden bases unless square, stiff ‘busbar is- used, wheu they will be selfsupporting. Very indiffent — results often occur if grid condensers, ete., are serewed directly to a wooden surface, especially if the wood is at all damp. Arrange the components aceording to diagram, and: this brings all connect Ing wires fairly; .short,.on which 75 per cent. of the éfficiency depends. It will be seen that’ the filament and theostat"is in the negative lend. and when the audio transformer is connected as shown, it provides grid bias automatically by the voltage _ drop across the rheostat. Always. work with the filaments as low as possible without introducing distortion, as this not only prolohgs the life of the valve, but Rives more © bias and consumers less B bat- . tery current. at the same time clearing up the tone. The Wiring.

No difficulty should be experienced in following out the wiring, aud each lead should be checked off as it is put on. Begin at the earth terminal, abd join to the following points:-A bay tery negative terminal, inside ‘end of aerial coil, moving plates of the variable condenser, left end of secondary coil, one terminal of rheostat, B+ and . ¥- terminals of transformer, and one +- side of a .001 mfd. fixed condenser © across the primary windings. © Join the remaining rheostat terminal to I terminal-of valve holder, Plus filament terminal on holder to A+ and B-- terminal on strip of.ebonite. Join the .fixed plates of the variable con-

denser to tickler end of secondary coil, Aone end of detector and to the fixed condenser. .0001 capacity. Free end of detector joins to the P terminal of transformer primary also to free end of .001 bridging condenser. Terminal G\on transformer to one end of grid le The other end of grid leak to of valve and free end: of .000 mf.d. condenser. Plate of vive to one end of reaction coil (found by experiment). Free end of reaction coil to one loudspeaker terminal, also to ome side of the .001 mf. . by-pass condenser. The other speaker terminal joins to by-pass condenser and B + terminal. Finally the aerial terminal is connected tothe end of the aerial coil on the left. This completes: the wiring, and after checking over the wiring the batteries should be connected and phones used for a try-out. Turn the reaction coil round; if it does not oscillate do not jump to the conclusion that more turns will have to be added. Try reversing the reaction coil connecions; as this may be connected the

ee ee eee eee eee eee eee eee ee eee oe Oe OOM ESSE ERE S UPS VOUS SRS POSS eee l yr oseserureves wrong way round, and can only be found by experiment. As the valve is acting under the capacity of high and. low frequency amplifier, the anode voltage can be up to 90 volts, above which outside grid bias will have to be used,

If the reader has a high frequency choke on hand, this can be: used quite efficiently in the place of the #-megohm grid leak, Concerning the, Crystal Valve. Any general purpose type will do, but preference should be given to one. with a fairly low impedance consistent with an amplification factor of eight or over. A note about the type of detector to be used. This is preferably of the fixed or semi-permanent type. If the catswhisker variety is used, the receiver bursts into oscillation every time. the crystal is jarred, and necessitates different reaction settings every time the crystal is adjusted. The property of a detector is to be able to pass more current in one. direction than the opposite other, and here is a valuable hint before inserting’ a fixed crystal. in any set: Give it a rough test by the aid of a small 44-volt C battery and a pair of phones. Fasten one phone tip to one end of the C battery, and with the free

tip touch alternately one end and then the other of the detector, with the opposite end of detector touching the free ‘terminal of the C battery. A _ loud cliek should be heard when connected one way round, and nothing or a much fainter click when connected the oppoon ~-

site way round. If both clicks are of the same intensity, then the detector will not rectify, and is very little use in any set. Some commercial detestors, when unable to rectify, can be improved by giving them a sharp tap or literally dropping them on the fioor, although the writer does not recommend this remedy for glass-enclosed types. | As has been mentioned before, it is quite a simple matter to add a further stage of audio amplification by connecting the output leads to the primary of another audio transformer and using another valve. A _ point worth noting is the following: If the builder is using a very long aerial the number of turns in the aerial coil can be reduced to 25, as, if the aerial and coil are in resonance to the station’ to be received, diffienlty may be experienced in getting the receiver to oscillate. In any case, various tappings can be brought out at the twenty-fifth, thirtieth, and thirty-fifth turns, and the ~

most suitable one by experiment connected to the aerial terminal.

DUG OLLIE LLLL ULL LULL LLORES Components for Crystal and Valve Variable Condenser .0005 m.f.d. 2 Fixed Condensers .001 m.f.d. | 1 Fixed Condenser .0001 m.f.d. 1 Valve Holder 1 Permanent Crystal | 1 Fixed Grid Leak 4 meg. : or H.F. Choke. "~ 1 Intervalve Transformer i Rheostat, 30 ohms. Ebonite etc, db. 22 D.C.C. Wire. 2ozs. 28 D.C.C. Wire. 3in. dia. Ebonite Tube, 44in. long. Zin. dia. Ebonite Tube, 1jin. long. Panel Qin. x Gin. x 3/16in. Baseboard Qin. x 7in. x }in. Gin Brass Rod, 4BA, with Nuts, etc. Strips, Terminals,