CRYSTAL TO VALVE

Evening Post, Volume CVI, Issue 108, 3 November 1923, Page 22

CRYSTAL TO VALVE

HOW TO CHANGE OVER

SOME PRACTICAL ADVICE.

In the course of the past few weeks three or four young amateurs have consulted the writer of these notes in regard to changing over from crystal to valve circuits, and the nature of their problems suggests that a few notes on | ..the subject will be of general utility. The fact that valves lighted by dry cell batteries instead of accumulators are available is no doubt responsible for an increased interest in valve' circuits; and there is every reason for advising the use of these tubes, which 'are not only economical to run; but--Just as efficient as the standard types.' It appears that quite a. large, proportion of users of crystal sets-use single coil tuners—a simple' inductance coil ■with either a single slider., or in some • cases with, two • sliders^. Others, no doubt, have obtained variometer tuners. The first step in progress towards using a valve is. to discard suclv a tuning sys- . tan,, though the coil need not be thrown away or dismantled. In the first place, a '-'loose coupler,".'having. one coil in the aeria.l circuit and another connected with the ;-crystal detector'"'and f hones, is preferable, valve or no valve, t gives less interference from undesired signals and-atmospherics; and louder . signals in the phones when properly ad- : justed. When a valve is used, with Tegeneration, a single circuit tuner 'will almost inevitably create interference, and'itsuse'is; •forbidden. ■-. -. ■ - MAKING COIL FORMERS. ■ - The fitting up of a loose coupler can be .done in a- great variety of ways, and the'-«mateur who -wishes to construct his own'can find plenty of designs and take his 1;; choice. But for broadcast ..reception there is no need to go in for honeycomb coils, or indeed anything but plain cylindrical windings though the ease with which spider-web coils can be wound encourages.many* to "adopt them. The writer; for various reasons, prefers and Tecornmends'xylindrical coils: of generous diameter." The formers for the coils cant easily be built up. of brown paper, with or without .thin cardboard, pasted round a tin of suitable'size. Obviously the;: paper must;,not;, be pasted to the tin.: An excellent device in making these formers is to wind the' tin -first ■with a thin fishing line, the ends of ■which are firmly fastened down. Then over this is placed a.single thickness of paper, and the. building up of the former.Ao a thickness 'o£_«ay, an"; eighth of «ir;inch, using ordinary flour, paste, followi. If the':tin-is" water-tight"it' can the^..>A v fi}]e^ i^th.- .bpUing,,,,water, and by the time it ie cold the former will be practically dry. The use of the string now becomes apparent. If the pasting has-teen properly done, the former will be too tight to move, but it can easily; be taken off by releasing one end of the string and pulling it out. FOR A BROADCAST TUNER. To make a tuner suitable for the broadcast: wave-lengths, and capable of responding well up to 600 metres, one former four inches in diameter and one 3£ inches in diameter will be needed. For the larger one a:two-pound treacle tin makes a suitable foundation.. Neither need be more'-'fhayutwo 'inches l long.^ Tho next step-isrW*rJm"the.ie<lg£s with a sharp knife, -making- as neat .-ffvj'ob -as :- possible. The formers" should be"'baked" in a slow oven for,.ah hour or so until they are thoroughly dry, and then varnished with shellac-- varnish.- - The ed?es .can"th"enibe;'fu?tlier' dressed- with" a_ finely-.set"p.lane, or a~ fiie| after rwhich they may need to ; be varnished "again, *t ;ih k'-^portanl' that .the '■■material should b>_danip-proof: Any."holes .subsequently made in thb-formers should be' dressed with 'shellac, for-the. same .reason. Before the coils are iwound, a decision should be "come to-as to how they are to ba mountedVbut this will depend a good'deal upon the circuit they are to ;be used in ■ Treating them merely as a general utility loose-coupler, howover, all that is necessary is that one should be capable of being inserted into the other to varying degrees, or with- - drawn well away from it, and a typical simple arrangement is to fix one coil ■flat upon a boiTd and fix the other upon a.leyer which in turn is pivoted .at the level of the top of the fixed coil. The windings for the purpose in view will 5?: onV* 1?, 1" 56 former > 25 turns of JNo. 22 double cotton-covered wire; for ™c smaller, 45 turns of 24 or 26 wire -The gauges of the wire are not really important, and the smaller size may be used on both coils. The 25 turn coil is used m the aerial circuit, and the: other in tne detector circuit. - TUNING METHODS. Tfre builder can please himself aa to how he tunes the aerial, Hs may remove a atrip of insulation and" fit a slider; or use taps or a variable condenser. The tuning for the aerial-coil is not after all, very critical; but that of the eecondary is. This should have its ends of the wire taken by the shortest route to the terminals of a variable • condenser of .0005 mfd. The reason for not using a. larger condenser is firstly that this is plenty large enough; and secondly^ that, especially with a valve circuit it is difficult to tune accurately with a bis condenser . If a crystal detector is used the connections are the. same as those to the single-coil tuner. A lead is taken from one condenser terminal to the detector, a lead from that to the phones and another from the phones to the second terminal of tho condenser. If .preferred,. tho. secondary can be tuned with a variometer instead of a condenser, and. this is a very populrmethod among American amateurs. The secondary coil may then be reduced to say 60 turns, and the variometer may be found subsequently to require some alteration, though jf j t ; s a well-designed one it may tune to a, fi hort enough wavelength. With variometer tuning one terminal of the variometer and one of the secondary coupling coil are connected and the two are treated as a eingle coil so far as connection with the detector is concerned: that is, one of the free terminals is joined to the crystal detector and the other to the phones A variometer may also be used, also by joining it in series, for tuning the'pri: mory, but, if 60, a condenser of small capacity will be needed in "series, or the -wave-length of the aerial, system will be too long. •

Further detail will be given later of the procedure in converting this circuit for use'with a valve. to the meantime, the directions given so far may be found useful to beginners; the materials described will give excellent results with a crystal detector with a minimum of; apparatus, and they require little in fhe^way of special insulation.