US2047019A - Grid structure - Google Patents

Description

Inventor- FERRIS Warren fiFerr-Ls y Hts AttOThgg.

GRID STRUCTURE Filed May 29, 1950 July 7, 1936.

Patented July 7, 1936 r 1-:

' "UNITED/STATES" EATENf FFicE New York The present invention relates to electrondischarge apparatus, more particularly to those devices in which the electron stream is intercepted by a plurality of gridmembersi 5 The usual form of thermionic amplifier com-:

prises-a filament, one or more helical grids. surrounding the filament and an anode. There may be used two, three, four or more grids depending on the tube characteristics desiredand theamount of electrostatic control necessary, and for other reasons. The gricl, arrangement generally includes a membercalled a control grid whose sole function is to offer a variable control of the electron stream andanother electrode termed a screen grid which serves to reduce .or neutralize the electrostatic. capacitynormally existing between the control gridand' the anode and to increase the amplification'of the tube. The turns of the screengrid whichis positioned between the control grid andjthe anode, are usually wound closer together, i. ,e. havingless pitch and more turns per inch of length, than those of the control grid. andit often happens that theratio of the respective number of turns is two, three or other multiple. .Under an'yf of these conditions, a certaini-numberof turns of. the outer or screen grid will lie between. the adjacent turns of the inner. orjJcontrolfgrid' as viewed from the outside. of. .the.1 tube looking toward the .filament, and" so be subject to'the electron impact of the .high v,vgalocity electron stream. However, iotheri portions ,of .the screen grid will lie directly inlfront of the "inner grid wires or directly in bad; of each controlgrid wire considered from the direction of electron traveLand hence will be shielded to a large ergtent from electron impactf It has been found that as the screen grid current is increased, for example, by raising the voltage applied thereto,

40 the screen grid becomes heated along those lengths of wire positioned'in the unobstructed beam Lof electrons but is relatively cool at" the remaining portions: of. the grid, along'those l ths which. ar i th e tron shadow-of e control r d w s, e cqnd i efii qq i e heating produces in the =screen grid alternate lengthsof heated and unheatedv portions which may afiect the capacityneutraliz ing function of the member and cause-I many. other defects such as variationsin the space current flowing tojgthe anode, fusing nd wa ping'er, th grid s'tructure, also the liberation of :deleteriousjgases; The condition of unevenness of peep ng, infaddition, may reduce the maximunifpower output of the tube which ordinarily wo'uldbe-derived fronitlie use of an additional space charge grid because the greater the density of. therelectron stream the" more pronounced is the localized heating. effect. c I

An object of the present invention is to im- 5 provethe 'grid construction and arrangement in thermionic devicesto 'the endv that each of the grid members will be heated uniformly throughout its length. This object is attainedxin brief, by reducing that portion .of the grid subject to 10 the direct-impact of the.high. velocity electron stream and-to allow the grids to shield one another in the greatestpossible degree without sacrificing the efficiency of the tube. Accordingto my. invention, the improved arrangement 15 consists in winding the successive grids in opposite directions so that the differences in pitches ofthe respective grid membersbecome additive and the shortest lengthof each gridis presented to the. direct action. of the electron stream. 20 1 The invention will'be better understood when reference isl made to the following description and the accompanying drawing in which Fig. 1 represents an'elevational view partly in'section of a tube improved in accordance with my invention;TFig. 2 is aiplan VieW'lOoking down on the electrode structure; Fig. 3 shows the relation between 'a" pair of-gridsand the 'filament; Fig. 4 151a diagrammatic viewdepicting in rather an exaggerated form by way of comparison the 30 priorlart; method 10f arranging the screen grid andicontrol grid wires in the righthand view, and on-"the -left,: the technique as improved in accordance iwith my invention is illustrated; Fig. 5riS aview useful in explaining the invention, and showinggtypical electron paths which intercept the;grid s, whi le Fig. 6 is a diagrammatic view of a pentode in which my invention finds application, also associated circuits.

- Referring to Fig. 1, numeral I designates an 4 evacuated envelope of the tipless type so-called, terminating a combined re-entrant stem and press: and containinga plurality of electrodes. The electrode organization includes. an M shaped filament}; supported at the'bottom by three 5 stiif wires A, the two outer of which serve as leading-.infconductors. "The upper apexes of the "n "merit; are supported in the usual manner by n ansof twq uprights" 5 which are secured in a beadfoffglass (not shown) and supported from 50 the stem 2. Arranged symmetrically about the filament, there are three grids fi, 1, 8, also an 'apoeeaas shown moreclearly in Fig. 2,the in- {nerr'iiostgrid f6 functioning as the electrostatic control member; the intermediate grid 1 as the 5 screen member and the outermost grid 8 serving to reduce or suppress the secondary emission from the anode. Each grid is supported between respective pairs of vertical rods I secured in the press 2, one rod per grid except in the case of the outermost grid being extended through the stem to constitute a leading-in conductor. These conductors, in addition to the filamentw leading-in wires, may be connected to a plurality ofcontact pins ll, molded in a base [2. The upper ends of the grid rods are held in position by a mica disc or plate I3 bridged between the rods l4 which are secured in the press 2. The anode; or

plate member 9 takes on a rectangular shape and is provided with rectilinear extensions i5 which embrace the rods I4. The outermost grid or secondary emission suppression member 8 is conveniently formed as a coarse mesh preferably off; nickel, while the inner grids 6 and "I are. wound as flat helixes in order to accommodate their formto the M shaped filament. The controlngrid 6 is preferably. made of nichromeand. the screen grid. of. molybdenum. The: present invention is directed more especially to the manner of winding the inner grids 6 and I, as will; be explained hereinafter although it will, be understood. that the principles of my inventionapply-equally well to any pair of wound. gridsiin. an electrode organization.

Fig. 6 shows in diagrama. typical. circuit in which the improved tube fin'ds application. In this figure, numeral Iii designatesanzinputtransformer connected between they control. grid 8 and filament 3'. The plate battery or othersource of electromotive force I] serves to energize the plate circuit and the screen. grid circuit in paral: lel while the secondary. emission suppression grid 8 is connected effectively to the mid-point of the filament as indicatedby reference character I So; for well understood reasons; this connection is conveniently made within the tube. The output current of the transformer may flow through transformer I8 to a load circuit'. Thecontrol grid circuit is generally provided-.witlr anegative bias battery l9. 3

From the circuit diagram, it is evident that. a relatively high potential is: applied tothe screen grid! under operating conditions and it has been found that when the turns on the latter greatly exceed thoseon the control grid; and in amentreme case bear a multiple or a-near multiple ree lation. therewith, certain lengths of the screen grid wiresare excessively heated during opera-.- tion. The reasons for this effect will be clear from an inspection of Fig. dwhichindicates the electrostatic fields surrounding the electrodes. In this figure, the filament 3- isshown as-emi'tting electrons which move toward the plate 9 under the. potential gradient established by-the plate battery, which electrons are intercepted by the control gridG andthe screen grid. The wires 6 are surrounded by anegativeelectrostatic field indicated roughly by the dot and dash lines, a which repel the electrons shown as dottedlines b and converge the same into a highly concen trated mass. sweeping throughthe space between the wires 6. The greatestden'sity of. the ereetron stream occurs mid-way betweenthewires. It is apparent that those portions 11) of. the screen grid which'are inthe direct pathof the electrons, and particularly in the path mid-way between the wires 6; will be subject to greater electron impact thanrscreen grid wires To which are in linewith. the control; grid wires, hence; shielded the electrons.

by the latter. In this figure the secondary emission suppression grid has been omitted for the sake of clearness although it will be understood that such a grid may be employed in combination with the other grids and may also be improved in accordance with the principles of my invention.

In order to minimize the heating effects of the wires. which areexposed to the full impact of the electron beam, it is apparent that only such wire as is absolutely necessary should be presented to In accordance with my invention, this end is accomplished by winding the screen grid; helix in a direction opposite to that of the control grid, as indicated in the lefthand view of li'i-g. 4... This manner of winding serves effectively to add the. pitches of the helixes as measured from a common datum line and to increase the angleatthe point of intersection between the Wiressor that the length of each screen grid wire positioned between the turns of the control grid is reduced tothe minimum. When the wires are wound in the'same' direction, according to the prior art, thepitches, as measuredfrom the common datum line, are subtractive and the angle between intersecting wires is very acute. The

advantage winding the grids in opposite di rections is readily appreciated from the comparison of the twoviews, in Fig. 4. In the prior art arrangement, at the right, the wires 1bof the screen grid are exposed over their entire length between thesupport rods lllto the full impact of the-electron stream and hence will-seriously heat, while the adjacent wires-1c are'fo'r the most part shielded by the control grid'wire 6 and will not heat to any appreciable extent. However; when the wires cross inopposite directions, asindicated in the lefthand view,the effective length of screen grid wire inthe space between the turns of the control gridis materially shortenedand the heatingv efiectl reduced in like: degree. Thus; in the improved construction the length of wire "1 whichdiagonally spanseachpair of control wires ET is; much less than the distance between the inner supportrod's Thence less than the length of screen gridwire of the prior art arrangement whichwoul'dnnormally be exposed to the full impact oii theelectrons. It. will also be noted that thewires. 12:, which in the prior art construction werecompletely shielded by the control grid, are now exposed to the electrons-in substantially the same amount asthei wiresJb, hence, subjected, to substantially thesame heatingeifect'.

While-my invention. has been explained with particular reference to a pair of grids having turns-in a. one-to-tworelation it; is evident that theimprovement.applies-to all turn ratios and more.especially; to those cases inwhich the number of;turns on; the; screen grid is slightly'less-or greater; than aamultiple of the number on the controligri'dz one type of, tube improved in accordance with: my inventionv the control grid had I55 turnsper; inch and the screen grid 34 turns-per-inch and itwas foundthat the screen grid was heated'uniformly over'its' entire length. Ontheother handj whenxthe grids were wound in they same" direction according to the teachings of the prior art, the wire became heated in lengths approximately /3 7 apart. v Thev advantages ofieredlby my invention are seeuredwithout. 'detracti'nginany. substantial, degree; from. the control. exercisedby the various grid; members and therefor-ado not affect the electrical ;characteristics ofthe tube.

What I claim as new and desire to secure by tional grid comprising a wire wound in a. plu- Letters Patent of the United States is: rality of turns in the opposite direction.

1. An electron discharge device comprising a 3. In an electron discharge device, the combicathode, an anode, a plurality of helically wound nation of two grids each comprising a wire wound 5 grids positioned therebetween, a pair of said in a plurality of turns in one direction, and an 5 grids being wound on the same axis and in opadditional grid between the two first mentioned posite directions. grids comprising a wire wound in a plurality of 2. In an electron discharge device, the combiturns in the opposite direction. nation of a grid comprising a wire wound in a 10 plurality of turns in one direction, and an addi- WARREN R. FERRIS. 10

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