Radio Broadcast - 1927, May - 61 Pages, 4.9 MB ... - VacuumTubeEra

''Measuring TubeCharacteristicsA Paper Delivered Before the Radio Club of America Discussing the SeveralBridge and D. C. Systems for Use in Obtaining Tube CharacteristicsBy KEITH HENNEYDirector, Radio Broadcast Laboratoryr T IS neither the desire nor purpose of the size and relativewriter to burden position, tubes differ inthis characteristics.There grid voltages thearticlefactors.with anAt low eulogyon the vacuum must, then, be tube. some amplificationmeansNearly every articleby factor falls offwhichsomewhat,that has engineers can rising toappeared on the tube incompare tubes just as a maximum atthe popularradio rate they zero grid voltage, andgenerators and press hasmotors doneor remaining constant thereafter,orother electricalthat, pointing out that it isfalling gently in some tubes.a mostapparatus.wonderful device, the modernThe Aladdin'splate impedance depends upon a lot ofLamp, and a number of other TUBE superlatives thatCONSTANTSthings, the filament efficiency, the amplificationfillup space. There can be no doubt constant,thatandthetheIN TUBE grid and platetube isimportant. Witness our engineering, there voltages. Noare two present broadcastingstructure, ourvery one curveimportant factors which, when or graph can show how it varies.known, define To long-distance telephone the properlytube representservice, our communication in it wouldexactly the same require a threedimensionalby telephone acrossmodel, such asmanner that weused has beenthe Atlantic, and to say in constructedschool that the United States isby high frequencies and exceedinglylow powers to all parts of the north by Doctor Chaffee and others. Somebounded on the by Canada, on photographsthe eastworld.bythe of All of these Atlantic very beautiful models of this natureOcean, etc. The two may beconstantsthings depend upon theseen in the tube.which Proceedings of the I. R. E.No really are not study of theconstants at alltube can beare the After the complete withouttube is sealed and a amplification factor placed inandknowledge oftheits varied services. For plate impedance, the operation!impedance example,changes with each change in in-',it is possible with a tube to convert directstantaneouscurrentplate or grid voltage all of whichfrom a set of batteries to alternating currentmakes the oftheory of the tube more or less com-all frequencies from as near zero as one likesplicated.upto 60,000 or more kilocycles. It is thenThese three possible,elements, the filament, the grid.with another tube exactly similar to theand the first, toplate, cause any current flowing in theconvert these extremely high frequencies backplate circuit to change, making itgo throughto direct current. It is also possible to very wide fluctuations.amplifyThe plate current is definedby the equation:both direct and alternating currents, and henceamplify power. It is also possible to = separateIP f (E P + jAEg)what is placed on the input of the tube intoBy maintaining constantdirect andany one of the threealternating currents ofpracticallyvariables in this equation andany ratio desired. varying the otherAll of these varied functionstwo, we arrive at theare carried out without relation between the plateany moving parts, withoutgrid orcurrent and the voltage on thenoise, with plate thatpractically no loss of power, andwe usually know aswith so little fuss that tubes now characteristic curves, and itexist that areisby means of these curves thatcapable of the giving serviceimportantfor 20,000 hours, a lifetube factors are defined. Forgreater than that of example, boththe gridcircuit in which they areand plate potential have some effect on theused. There can be no doubt gridabout the importanceof the tube is relatively morevoltage, but the gridin the importantfield of electrical engineering.In itthan the plate. In Fig.iaddition, tubes and may be seen that attheir associatedzero grid bias, changing thecircuits are now plate voltage frombeing used to measure the of rate90 to 135 changes the plate currentgrowth ofby 5.2 milliamperes,while changing the grid biasplants, to measure extremely smalldifferences of by 5 voltsthickness, the strength and rapiditywill do the same thing.of a man's pulse, and from this latter, to determineThe amplification factor is then defined as thewhether he is a lover, a thief, a liar.ratio ofThe tube the changehas been in plate potential to theharnessed and trained to do anda every function that thevast number tubeof performs andchange in grid potential which produces the sameinteresting tricks.itsNow efficiencythis in doing little assembly of glass and so may bemetaldiscovered by aeffect in plate current. In this case the amplificationknowledge of these factors and factorthe constantsperforms its ofis 9:multitudinous functions with the the circuits into which the tubeaid of threeworks... _ AEp l35-oo4i;elements. TheH-first and most importantof theseT-p = = =Another factor is the mutual conductanceAEg Q5-0 5elements is the filament. This which, contraryfilament has to popular opinion, is not so The other factor of importance, theundergone rather remarkableplateiimportant as itchanges since tubes may seem. The term isfirst somewhat impedance, is the ratio betweencame into the change inexistence. difficult toThe picturefirst ones were made physically. It has the dimensionsofplate voltageof to the resultant tungsten whichoperated with a change in platea conductance, i.e., a current divided current. In thishigh temperature, thenbycase it is45 voltsgoing to a voltage, but divided bythe current exists inlow one circuit 0.0052 amperes, or temperatureroughly oxide-coated8700 ohms:filaments manufacturedby a in another, with the tube ascomplicated and difficult process, the Rpand the voltagethence connecting link. It is due = AE P= _J15-oo_ ._i5_ =8toto our mostProfessorAlp 0.0116-0.0064 0.0052recent filament, the thoriated Hazeltine.wire. The measure of efficiency of the Now, as has beenfilament is These indicated constants, orby the Greekits emission per watt factors, are variable letter expended inA in theheating it, within definitions, these factors arerather wide definedby changes, andand limits.the Fornewest thoriated wire isexample, theexceedingly amplification factorefficient. Pure may range from for accuracy the changes3 to tungsten filaments 30, mustoperate at a whilevery high temperature. Oxide the be small.impedance varies, as someone has The filamentsmutual consume said, from conductance, defined as theHell to Peru. The ratioconsiderable current at low amplification factor between voltage and at amuch lower is pretty well determined when a change in piate current andthe tube the changeis sealedtemperature. Thoriated filamentsare somewhere and in grid voltagepumped; that that produced it, is also the ratiois, itdepends to withinbetween.very between thenarrow amplification factor limits and theThe other upon thetwo elementsgeometry of platethe tube.are the grid and impedance, as can be seen plate, The from themesh of the mathematicsand grid andbecause these elements can be its spacing with respectto the other comparing tubes underbelow. Forchanged inelements are the exactly thegoverning same conditions, this factor is somewhat import-