THE USE OF VALVES

Evening Post, Volume CIX, Issue 101, 2 May 1925, Page 22

THE USE OF VALVES

CAUTIONS FOR BEGINNERS.

. Valves, or "vacuum tubes," are the vital instruments in all wireless receiviiifr sets, except those simple ones-which utilise a crystal detector.. There are a number of simple rules , which . should, be carefully'kept'in mind by till who use theni. ...- , . ..-. '■ ■,' : .-■ There V only one really delicate thing about a wireless valve—the", lilamerit. Those.designed to worlc with large cuiren'iA arid to be Jit by "means of an accumulator • have, filaments' 'winch " aro "fiUidyr, st-ixin^,,.aiid. are Viible' to.: stand fairly, rou-' Ii iisu<»e • but 'in I'siirife 'of the dry-cell tubes tii«y are extremely delicate, and a fall which leaves the glass container intact may break the filament. The strength of the other type, however, does not justify rough usage. A. severe shock can easily result in the internal structure of the valve being deformed, so that too filament may come into contact, with, the ",<;rid.". which siiVrtninds il. .with ruinous\'results. . Kven if the "Vid- or the plate is merely displaced, llip'lbph.'ivirmr of tlie valve ma,y. he scriuusly affected, usually .tin- tli'e worse. '"".'■" ' ' i The filament of a valve simply -will . not stand .grossly . excessive current be- ' ing...passed through, it,, ..Every, care muft b« iuk.cn, therclore, tUul- Uie platg of

B battery is not connected in such a way as to pass its current through the filament. This will not happen if the current is properly connected up and thoroughly checked before the valve is put into the socket. Probably the majority of burn-outs are caused not by wrongly connecting the circuit but by "messing about" with wires or screwdrivers, pliers, or other metallic instruments while the batteries are in circuit and the valve in position. Adjustment ol the circuits, or of screws, or anything of the kind should be made without first removing the valve or valves. This is a lesson which some people find it is hard to learn until it has been rubbed in by the sudden loss of a valve.

There is no need, when "closing down," to disconnect the B battery circuit, as some operators do. A moment's thought will show that the circuit is broken, between the filament and the plate, when the valve is turned out. It is only completed, when the valve is alight, by the stream, of electrons emitted from the hot filament. Moreover, in a single valve circuit, another break in the circuit is.made by the customaryremoval of the telephones. When inserting a valve, see that its pins or "prongs" make good contact v.jth the metal springs of the socket. Ihere is no need for the contact to be so firm that it is difficult to get the valve on and out of the socket- the essential thing is to have the contact points clean. The springs can be polished, if necessary, with a bit of fine sandpaper on the end of a pencil. The tips of the valve pins are usually covered with solder. This is vem- easily cleaned, if it gets oxidised, witn a bit of fine sandpaper or a very fine ,file or by scraping with a knife. Treat it gently ; the vibration of hard filino- may break a fine filament.

As a general rule, it is a good plan to design a set so that the valves are enclosed. Some receivers are built with the valves projecting their full length. Inis gives an imposing appearance, but the only benefit it confers is on the va.ve manufacturers when an unguarded movement brings about a catastrophe. . In choosing a valve, the beginner may be strongly advised against selecting a soft valve as detector. Soft valves, by wi uc h 18 meaut tll contain j nn . a little gas have a hi<rh reputation" as sensitive detectors, and they deserve it. i ,-^ ? lve their fu!' results on'y when skilfully handled, and for Roneral purposes the added sensitiveness is simply not worth the trouble of keeping them in accurate adjustment. That a hard valve is quite good enough can be gauged from the fact that the first Jinghsh amateur to .receive New Zealand amateur signais used one as his detector. The hard valve as detector "ill give perfectly, good and reliable results -if the grid-condenser is a <rood one, and the grid-leak nicely adjusted. There is no need whatever to fiddle about with the plate voltage or the filament rheostal; if the reaction coil is properly chosen, the valve's operation can bo beautifully controlled with the reaction coil alone. ,