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Show Amplifying Tubes and Selectivity Negative Grid Bias on R. F. Amplifiers Will Reduce Interference. By HERMAN BERNARD Aisoc!1 tnltut at HJlo EpeIicitO With selectivity ihe outstanding consideration of the moment, because stations are so numerous und close together to-gether In frequency, much attention has been given to circuit designs that will afford high selectivity. Yet little attention has been paid publicly to the ' proper use of radio-frequency nmplify-j nmplify-j Ing tubes so that their contribution to ! selectivity may be adequately capitalized. capital-ized. The characteristics of such tubes, I ns well ns those of super-sensitive spe-j spe-j clal detector tubes, are such that Im-I Im-I proper use may linpnlr selectivity, j Selectivity Is something most usual-! usual-! ly associated with receiver circuits. colls and condensers, but the tube Is J entitled to a Just share of the honors, j Selectivity may be regarded as the j ability of an operating receiver to re-I re-I Hrlct reception to only one frequency. I or to a very narrow band of deslrcv j frequencies, at a lime. As tli? piob- lem arises usually In connection with ware lengths or frequencies quite close together, selectivity enables discrimination discrim-ination among these wave lengths or frequencies, the ucceidunce of one of them to the exclusion of nil others Thug Is iDter-stntlon Interference avoided. That goal Is very Important today. A circuit Is sclectiw because Its re-eistance re-eistance to a desired wave length Is very low when the correct dial settings set-tings prevail, while the resistance to other wave lengths, ut tho;-e same bettings, bet-tings, Is very high. Now, to make things easy for the desired signal, while maintaining barriers bar-riers against nil other frequencies, you may Introduce a negative grid bias on the radio-frequency nm;lif. Ing tubes. If the uual G"Vi to !- volts nre applied ap-plied to the pli.tes of those tubes. Absence Ab-sence of such bins Is equlvuleut to putting put-ting a resistance across the coll or variable coiid;:isor In the tuned circuit. cir-cuit. The te- Imknl term used to de-eribe de-eribe such n coi.dlt'on In the tuned circuit Is that when the tube Is used wdthout bias the "luput Impedance" lb low. ' The Effect Stated. Tha negative grid bias, besides reducing re-ducing the plate current dr..iti on the "P" supply, whether that '.iji;.!y be butteries or eliminator. Incr, aes the Input li:ri-i!.it!',i result 'ng In de creahed i!a::'.i li:g of the Input circuit (grid to fouir.cnO. A low Irput Impeilcnce. therefore. It equivalent to adding a re-l.tr.nce In parallel with the tui.lr.g ronden-w-r. thus broad, nlt'g the tuning a:id re '.-wring the o!t:ge built up acrusji the mnder.ser by a given Induce! s'gr.al voltage. Ti ls efTert in.iv be reduced to a minimum by the negative grid l!a. Kvn o ftuall n bias as one o'.t helps considerably In ln?pro ing piv j lectlvlty. although reducing t!:v plate current only s'il.t'v. For Instance. the CXfVilA. n r:sd'-frejiiener nu.p:l fylr; tube (which Is ni.so a iV!iii.,r and nndio-nmpiilh-r) draw 0 miillae.i-pcree miillae.i-pcree of plute current at 1' ) voits. w ith the grid o-iinecteil to !!, n,.it:o r.Iamint, I. e.. nt rero i-rl, lila". If the jrld Is in ,-:.tlcy Wa-i! only one volt, ns by i oiinei ling the grid return to the battery side of the rh-. !nt (to minus A). ln-tea, of to minin fllunieii. the plate rur.-i-nt Is re.iu.i-1 to ." lellllnnipercs. ThU Is due to the one volt nriitUe grid hi:n reuniting fr mil tl. drop of oil tl In t!i rLst-!aL rLst-!aL Put nt tolts negathe gib Ida, again assuming He faine piatc io;,i'" of liO, the plate (iinnil dr..ps to '' mllMiunpercs. n reduction of i'j'.S per cent. However, for rinho frequency amplifying purposes oonie may pivfer to use lees than 4.." M.'t negative him, the minimum in-r:n'! grid bias rw ciiuuended for y plnte voiia-.-e of '.si, because of Ihe somewhat leiM-mM ml lime resulting from iieathcly biasing Ihe radio-frequency iiinpllfylug tubes to the nllounble limit. Th Rclationahlp. On the score of ie. tlvltv the -iln even from a single volt of negative grid bins Is relatively nnoh iruger than Ihe saving In plntK current drain, hecnufe Ihe I nerec.se, Input Impedance la prouonniN'd. An Indirect relationship hrtu-n-n the tube and selectivity exists ln regard In the CXOixU. the 4-elnl detretor that outperform Its pi-eilec,.s M s without living critical ns to lllainent or plnte voltage. This tube, vvhb h works on Ihe a kall vapor principle. Is tome thing new In tube d. .ign Its rrmm mended operntlnj voltnges are ,'i volts lllainent and 4.1 vnlis nn l(1. j.iate. I'lefernhly Ihe grid fhoubl be retnrne.l to negative diriment TliU tub., great I l.v Increase the volume of weak si,; linls. for Instance, vv h n distant Ma tlons nre belli received, ns etmipnred with llrt- results obtnlncj when m.lng I a CX.'MHA us dotei tor. tin slroni; slgtuil" the i-ar does not noi,-, n,s particular effevt so rendllv. Ilenee, If one ileslres greater fr lectlvlty It Is pii-slble to snerlller the volume gain t i nec.uq,ish Ihe po-;Sil,iv more tinpiirtii.it end. l inger rouplln 'f Ihe antenna may be employed. One popular method Is trt plm-c n I1;e, r. ileniier, i.ey of .l),si i,,f,. enpaclty or smaller. In ules whh the aerial. An ilhcr Is to remove turns fiuin the n.I1 I thr l Is III the nerlul clrculi, t oi.s CoupllnQ. Tl.n n-rlcs coiineeiloii of tl.v fixed ciuidarer (i.nvUta of Joining mm side to the HUtennn binding post of the re ceiver, nnd the other hide of Ihe fixed condenser to the nntennii lead In the wire that hits been removed from tins I antenna post of the set. The reduction reduc-tion of the number of turns ou Ihe antenna coll Is a substitute for the series condenser remedy, and Is not auxiliary. I The use of loose coupling reduces the detrimental eiTect of the antenna resistance, hence increases selectivity. Less energy Is Induced, and volume Is . comparatively less, where one compares com-pares results with a given type, of detector de-tector tul. Put If the UA Is being used os detector, replace It with the :;iK.iA and the methods of Increasing selectivity may be resorted to, the previous volume still being retained, nit hough the selectivity has been materially ma-terially Increased. |