US2939988A

US2939988A - Electron tube

Info

Publication number: US2939988A
Application number: US699663A
Authority: US
Inventors: Robert D Culbertson; Jr Jackson W Kendall; Corbit M Wesenberg; Paul D Williams
Original assignee: Eitel Mccullough Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1957-11-29
Filing date: 1957-11-29
Publication date: 1960-06-07
Anticipated expiration: 1977-06-07
Also published as: NL98313C

Description

June 7, 1960 R. D. cuLBERTsoN ET AL 2,939,988

ELEcTRoN TUBE Filed Nov. 29, 1957 2 Sheets-Sheet 1 IN VEN TORS. Rober! D. Cu/berfsan Jackson M Kendall, Jr. Carb/'f M Wesenbsrg Paul D. Will/'ams A TTU/MIE Y FIG.

June 7, 1960 R. D. cuLBl-:RTsoN ETAL 2,939,988

ELECTRON TUBE Filed Nov. 29, 1957 2 Sheets-Sheet 2 ATTORNEY United States Patent 2,939,988 v ELEcrRoN TUBE Robert D. Culbertson, San Jose, Jackson W. Kendall, Jr., Los Altos, Corbit M. Wesenberg, Mountain View, and Paul D. Williams, Menlo Park, Calif., assignors t0 lltel-McCullough, Inc., San Bruno, Calif., a corporanon of California Filed Nov. 29, 1957, Ser. No. 699,663

11 Claims. (Cl. 313-250) This invention relates generally to electron tubes and particularly to electron tubes capable of generating or controlling large amounts of power at high frequencies and with good linearity.

The design of an electron tube for high-power, highfrequency operation presents somewhat of a paradox since the desirable design features for high power are undesirable for high frequency operation and vice versa. For example, the various electrodes of the tube will tend to be heated in direct relation to the amount of power handled by the tube. Thus, in high power tubes the heating of the various electrodes will be high and they must be capable of withstanding such heating, or of rapidly dissipating the heat. This has been accomplished iu the past through the use of large, heavy electrodes. However, in high frequency electron tubes it is desirable to have the electrodes closely spaced to reduce the electron transit time through the tube and avoid Barkhousen oscillations and other space charge eiects which tend to occur at high frequencies. Furthermore, the electrodes and supports should be small and light in order to enable close spacing and reduce losses due to inductance and interelectrode capacitance. Thus, it will be seen that high power and high frequency tend to be incompatible from the standpoint of design.

it is an object of this invention -to provide an improved electron tube capable of generating large amounts of power at high frequencies.

It is another object of this invention to provide an electron tube having electrodes which are closely spaced and yet are capable of dissipating large amounts of heat.

lt is still another object of this invention to provide an improved electron tube capable of producing large amounts of power per unit volume thereof.

It is a further object of this invention to provide an improved electron tube capable of generating large amounts of power at high frequencies and with low feed through capacitance.

lt is a still further object of this invention to provide an improved grid structure for an electron tube capable of supplying increased electrostatic shielding with low electron interception and high heat dissipating characteristics.

Another problem often encountered in prior art tubes is that of providing an efficient cathode structure having a large area emitting surface.

lt is yet another object of the present invention to provide an improved cathode assembly which has relatively high thermal eciency and which includes a large area emitting surface.

The foregoing and other objects of the invention will become more clearly apparent from the following description when read in conjunction with the accompanying drawings.

Referring to the drawing:

Figure l is a side elevational view, in cross-section, of

an electron tube constructed in accordance with theA invention;

ICC

Figure 2 is a view in cross-section taken along the line 2-2 of Figure l;

Figure 3 is an enlarged view of the portion 3 3 of Figure 2;

Figure 4 is an exploded view in perspective showing the heater and ceramic Washers forming part of the cathode assembly; and

Figure 5 is an exploded View in perspective of the locating and centering pin at the upper end of the cathode structure in Figure l.

The embodiment of the invention shown in Figure l is a tetrode capable of handling substantial amounts of power and may be of ceramic construction, for example. The tube shown has an evacuated envelope of generally cylindrical construction and an external anode.

The side walls of the envelope comprise stacked

ceramic rings

11, 12, 13 and 14, with

metal terminal rings

17, 18, 19 and 21 interposed therebetween. A cupshaped external anode 22 forms the upper portion of the envelope and is supported by a metal sealing ring 23. The sealing ring 23 is brazed at one end to a metal ring 24 which surrounds and is brazed to the lower portion of the anode 22. The other end of the sealing ring 23 curves inwardly and is bonded to

ceramicrings

14 and 26. It is observed that the anode 22 is not directly bonded to the ring 26. Thus, it may move relative to the ceramic ring durinfy thermal expansion and contraction of the parts which have different co-eicients of expansion and contraction. A plurality of spaced channels 27 are formed at the lower end of the cup-shaped anode 22 and serve to connect the space 28 enclosed by the ringV 23 with the interior portion of the envelope. Consequently, a pressure differential is not produced between the space 28 and the interior of the envelope when the tube is heated in an oven, evacuated and the various parts thereof brazed together. The upper portion of the anode 22 carries an exhaust tubulation 29 which is preferably covered by a cap (not shown).

The

ceramic rings

11, 12, 13 and 14 which form the lower portion of the cylindrical envelope are preferably of high refractory material such -as alumina and are metallized at both ends by a suitable metallizing procedure, such as by the molybdenum manganese powder sintering process. The metallic rings are thin and have high electrical conductivity. The rings may, for example, be made of'nicltel. The sandwiched parts are brazed together using high temperature brazing alloys such as copper-gold eutectic.

The metal bonds of the brazed joints form strong mechanical connections and provide a vacuum seal so that the side walls of the envelope form a solid impervious cylinder of rugged construction. The

terminal rings

17, 18, 19 and 2.1 provide desired lead-.in conductors through the envelope to the tube elements carried within the same.

The electrodes disposed within the envelope include a cathode assembly 31 having a cylindrical emitting surface 32. The cylindrical emitting surface is preferably of the oxide coated type and is heated by means of a lilamentary heater 33. The cathode assembly has a large emitting surface which is capable of handling large amounts of power, is of rugged construction and has a relatively high thermal etiiciency. v

The cathode assembly comprises a cup-like metallic member 3l?. An oxide coating on the exterior surface of the side wall of the cup provides the emitting surface 32. Y

A tubular center post 36 is mounted within the cup-like member 36 and coaxial therewith by means of a metallic disc 37 spot welded to one end thereof, which disc 37 is, in turn, spot welded to the end wall 3S of the cup-like member 3%. It will be seen that the end wall 38- of the cup-like member is dishcd inwardly to. provide for the mounting. of thermal insulation whichfwill be described Y hereinafter.

41, as 'will' be presently described. In general, theV straight portions are considerably longer than the bight portions 40. The lamentary heater 33 is supported with- Y inthe cathode cup by means of a pair of spaced ceramic Vwashers 39 and 41,. The ceramic washers have an annular groove 42 formed' on one surface near the periphery ofV the same. The annular groove serves to receive the adjacent bight portions 40 of the iilament.

In assembly, the cup is oriented with the end wall 38 down and the first ceramic washer 41 is slid over the post 36 and seats itself against the end wall 38 of the cup. The Vitlamentary heater 33 is then lowered into the cup and the bight portions 40 are seated in the accommodating groove 42 formed in the ceramic `washer 41. The second ceramic washer 39 is then inserted over the post 36 so that the groove 42 therein engages the adjacent bight portions 40 of the filamentary heater 33. A ring43 is then placed 'over the extending end portion of the post 36 and seated against the washer 39. A flaring tool is then inserted in the post 36. The daring tool includes an elongated dieA portion which extends through the post and cuts an opening in the end wall ofthe cup so that it may receive a pin to be presently described. The flaring tool also includes a flaring means whichrserves to dare the lower end of the post outwardly to engage the ring 43 and hold the abovementioned parts in assembled relationship. Y Y

. The. free ends of the iilamentary heater 33 extend through openings 44 and 45 (Figure 4) formed in the lower ceramic'washer 39 to provide means for electrical connectionV thereto.

It will Vbe seen that the heater-cathode assemblyY above described will have considerable mechanical strengthY due to the mounting of the ilamentary heater 33 between the

ceramic washers

39 and 41 and the mounting of thev washers within the cup-like cathode member. The

ceramic washers

39 and 41 not only insulate the heater austriaca:Y 1 e j' -e- VVon'the support. 54. 'Ehe heat, insulatingassembly 56 inhibits the radiation of heatV from within theY cup-like cathode member to other 'parts of the tube thus completing the oven-like enclosure for the ilamentary heater 33. An electrical insulatingmsleeve l59 extends through the heat insulating materialnjralignment with the hole 45 in the ceramicgwasher39 andiserves toV electrically insulate one end 61 of the lamentary h eater A33V from theA heatinsulating material 56.l ,The Yother end 62fof Viilame'utary heater 33"ex'tends directly throughV a hole in thewheatV insulatingassembly, 56V and a hole in the shoulder 57 of the support memberV 5.41 in alignment with the other hole 44 in the ceramic Washer 39 and is electrically connectedfto-V the. support member'. 54.

The complete heater-cathode assembly. above describedk thus forms a cylindricalV ovenilike enclosure, the heat losses from which are relatively low. Thus, it is seen that a largearea emittingsurface cathode assembly is provided which isrelatively efficient. lAnother feature tobe notedis thatY the heater-cathode. assembly isV of strong construction. Relatively large filament wires zigzag` between. a pair of ceramic washers spaced atopposite ends of the post 36. f Thus, the heater is not sub'- ject to breakage under. mechanical vibrations and shock.

The heater-cathode assembly including the cathode support 54 is mounted on a stem comprising a rst hollow, truncated, support cone 66. TheV support cone 66 has a radially extending ange at its lower enlarged end which provides the terminal ring 18 previously Vdescribed. The support 54 is suitably attached and electrically connected to the upper end of the cone 66. Since one end of the lamentary heater 33 is electrically connected to the. cathode support 54 the terminal 18 serves 'both as a cathode terminal and as one terminal for the heater33. A second hollow, truncated, Ysupport cone 67 is coaxially mounted withinthe iirst cone and-in spaced relation thereto. AV flange on the enlargedend ofthe cone67 `providestlieterminal17, and the ceramic 33y from the cathode but also provide thermal insula-Y tion Vat both ends of the heater which tends to increase the Vheating of the side walls of the cathode where the emission surface32ris located. The thermal efficiency of the heater cathode structure may be further increased by providing additional thermal insulation, as follows. Thermal insulating material 47 Vis mounted withinthe dished end wallv 38 of the cup-like cathode member. The insulating material 47 may, for example, comprise a plurality of circular sheets 48 having corrugated sheets 49 interposed therebetween. insulating material and be held in place by means of tabs S2. which may be formed at the outer periphery of the dished end wall 38 of the cathode member.

A ring 51 may overlie the The open end of the cup-like cathode member is provided with a thin metal extension which acts as a heat dam tending to reduce the conduction of heat away from the cathode member. For example, a ring 53 of thin metal may be suitably attached to the inner surface of the: cathode member and extend beyond the end of such member. Y The extending end of the ring 53 may be suit- Y Vdescribed is carried in the annular shoulder 57 formed ring 1.11 is Vsealed between the terminal 17 and the termi.- nalV 18 on the first cone as previously described. A center post 64 is electrically and hermetically attached to the inner surface of thesecond coneV 67 and extends downwardly therefrom. Thus, the second cone 67 and the center post 64` serve as an end wall of the evacuated envelope. The center post 64 serves also as a centering device. and to rigidity the sternV structure as will be described hereinafter. rv.A exible lead 63 is electrically connected between the second cone 67 and theother end 61 ofV the iilamentaryk heater' 33 whereby the terminal 17 along with the terminal 18 provide for the application of heating power Yto the rilamentary heater 33.

j The control grid is of conventional vertical wire cage construction. The grid` is mounted coaxially with respect tothe cathode emitting surface 32, the wire elements 71Y thereof beingV attached at their ends to a support cylinder 721 which has a ange at itsY lower end extending outwardly to Vform the terminal ring 19. The upper ends of the control Ygrid elements 71 are attached toa control grid cap 73 having an aperture through the center thereof into which is suitably brazed a Vceramic post or pin, 74. V'I'he cap is secured to the ceramic post 74 by forming a metallized'rin'g 75 orrthe ceramic post and then brazing the capf thereto (Figures l and 5). The end of the pin is then inserted within the cylindrical post.36. Y The pin serves to' center the elementswhereby they are rigidly maintainedv in coaxial relationship, Due to this construction, the control gridY may be mounted relatively close to the cathode surface without danger of contact therebetween in the event of mechanical shock or .vibration Y A novel feature of the present invention is the barlike or ribbon-like constructionof. the Vscreenv grid elcments. which` permits-closeV spacing of'V the control grid and screen grid, and yet maintains low; feed through ca pacity and electron interception. Referring to the dra-wing, the screen grid 77 includes a plurality of bar-like or ribbon-like elements 78 which extend vertically with respect to the cylindrical axis which they dene (Figures l, f2 and 3). The ribbon-like elements are attached at one end to a cap 79 which is suitably secured to the post 74 as, for example, by applying a metallized ring 81 to the post 74 and suitably brazing the cup thereto. The elements are attached at their other end to the upper end of a ring-like member S3. The complete assembly is supported by mounting the ring-like member 83 on a cylindrical member 34 which has its lower end secured to the terminal ring 21. The ribbon-like elements may be supported at their intermediate regions by means of a plurality of circumferential members 88. The ribbonlike elements are arranged with their narrow face 89 facing the electron stream, each screen grid element 78 being radially aligned with a control grid element 71 whereby a minimum electron interception is performed. In general, the complete screen grid assembly is formed including the cup 79 and the ring S3 and then the assembly is lowered on the pin and suitably secured to the pin and to the support 84.

The screen grid is placed as close as possible to the control grid and each bar of the screen grid is aligned with an element of the control grid. This is done in order to take advantage of the electron beaming action performed by the control grid and results in a reduction in the number of electrons which will impinge upon the elements of the screen grid. It will be seen that at least that dimension of the screen grid bars which is trans- Verse to the direction of flow of the electrons should be smaller than the diameter of the control grid elements since if it is larger than such diameter the advantages of the electron beaming action mentioned above will be lost. if circular wire elements were used in the screen grid it will be seen that they would have to be of Acornparable size to the elements of the control grid and that their cross sectional area would thus be limited. However, due to the use of the rectangular or ribbon-like elements the cross-sectional area may be comparatively long. it will be seen that the thickness of the ribbonlike elements is approximately equal to the diameter of the control grid elements, thus the portion of the ribbon which is transverse to the direction of electron flow is such that minimum electron interception is obtained. However, the width (or cross-sectional length) of the ribbon is such that the cross-sectional area is much greater than that of the control grid elements. The heat dissipating capability of the ribbon-like screen grid elements is correspondingly high.

The ribbon-like screen grid elements have the further advantage of providing more screening action than could be provided by circular wire screen grid elements. The increased screening action results in greater linearity in the operation of the tube and in less control grid to anode capacitance. The electrostatic field of the anode cannot penetrate through the spaces between adjacent bar elements due to the width thereof and thus the control grid has more linear control over the electrons emitted by the cathode. Furthermore, the bar-like elements decrease the angle through which the anode can see the cathode and thus the capacitance between the anode and control grid is reduced.

The factors described permit closer spacing of the tube elements whereby the tube occupies a smaller unit volume. Yet, the closer spacing of the elements does not decrease the power handling capacity of the tube because of the novel arrangement of the bar-like grids and the relatively large cathode emitting surface. The closer spacing also reduces the power requirements for driving the tube.

In generm, one might say that the perveance of the tube is considerably increased. Perveance is herein dened as the constant of proportionality in a vacuum tube aesaess that compares to conductance in a resistor. It is, to some extent, a measure of the ability of the tube to handle high powers at relatively moderate voltages. The improved construction of the tube gives a tube having a relatively high perveance.

The tube illustrated includes heat radiating ns 91 which are mounted on the outer surface of the anode and which provide a large heat dissipating area.

The lower portion of the envelope includes a disc-like support 92 which has its peripheral edge brazed to the ceramic ring 93. The ceramic ring is added primarily to reinforce the braze between the ceramic ring 11 and terminal ring 17. The ceramic and metal parts have dilerent coeicients of expansion and without the ring 93, the joint between 11 and 17 would not be strong and gas-tight. Holes 94 may be provided in the lower member 92 since it does not form a part of the air-tight envelope. A guide sleeve or socketing member 64, previously described, is adapted to receive an associated socket guide pin and is suitably brazed at its upper end to the member 67 as previously described. At its lower end it is secured to the ring 92. It is, therefore, rigidly held in the tube.

It is apparent that although the vacuum -tube described is of metal ceramic construction, the novel cathode assembly and grid structure may be employed with other types of tubes, for example, glass tubes.

Thus, it is seen that an improved vacuum tube is provided. The tube is capable of handling large amounts of power per unit volume with improved linearity. The driving power required is relatively low in comparison to the prior art tubes. The screen grid is so constructed that it has relatively `high thermal dissipation without sacrifice of electrical characteristics. The cathode, because of its novel construction, has a relatively high efiiciency with a large area emitting surface.

We claim:

l. An electron tube comprising a cathode assembly having a center post, spaced ceramic washers carried at the ends of said post, a heater in which the heating elements dene a cylinder and extend lengthwise thereof, said element being supported between said washers and in spaced relationship to said center post, heat insulating means disposed adjacent the outer face of said ceramicV washers, a cylindrical cathode surrounding said heaters and washers, and means for holding said cathode assembly in assembled relationship, said washers, insulating means and cylindrical cathode forming an oven-like enclosure which serves to increase the thermal eiciency of the cathode assembly.

2. An electron tube comprising a cathode assembly having a center post, spaced ceramic washers carried at the ends of said post, an annular groove formed on one face of each of said washers, a heater supported between said washers and in spaced relationship to said center post, the heating elements of said heater defining a cylinder and extending lengthwise thereof, said elements being supported between said washers with the end portions accommodated within said annular grooves, a cylindrical cathode surrounding said washers spaced from and adapted to be heated by said heater, and means for holding said cathode assembly in assembled relationship, said ceramic washers and cylindrical cathode forming an oven-like enclosure which serves to increase the thermal eiciency of the cathode assembly.

3. An electron tube comprising a cathode assembly having a center post, spaced ceramic washers carried at the ends of said post, an annular groove formed on one face of each of said washers, a heater of flamentary material which is zigzagged with straight portions substantially longer than the bight portions shaped to dene a cylinder and extend lengthwise thereof, said elements being supported between said ceramic washers with the bight portions accommodated inthe annular grooves, a cylindrical cathode surrounding said Washers spaced from and adaptednto'be heatedbyithe heater, and means" forhnlding"l said cathode assembly in assembled'relationshig.said ceramic washers andJcylindrical' cathode forming Aan oven-V like enclosure which serves to increase the thermal eciency of the cathode assembly. Y Y

4. An electron tube'cornprising a cathode assembly'including spaced ceramic washers, a heater of filamentary material Supported between said Washers, a cylindrical cathode surroundingv said washers spaced from and adapted to be heated by said heater, means for holding' said i cathode assembly in assembled relationship, Ysaid ceramic washers and cylindrical cathode forming an ovenlike enclosure which serves to increase the thermal efficiency ofthe cathode assembly, an anode surrounding said cathode, a cylindrical control giid structure surroundingY said cathode and a cylindrical screen grid interposed betweensaid anode structure and said grid, said screen grid comprising a plurality of elongated ribbon-like elements arranged to denne a cylindrical surface and present their to the cylindrical cathode, andV a cylindrical screen grid interposed between said anode structure and said' grid, said screen grid comprising a plurality of elongated ribbonlike elements arranged to lie on a cylindrical surface and to present their 'narrow edges to the electron stream, said screen grid elements being radially in line with the control grid elements. Y

,6. An electron tube having an envelope comprising atV least first, second and third spaced metallic terminal rings,

' a cup-shapedY anode, a plurality` of ceramic rings sand- Y wiched between said terminalY rings, a ceramicvring, and

a Vsealingl means connecting-said anode to said ceramic` ring, a cathode assembly carried by-said first terminal ring and-including spaced ceramic washers, a heater supported between said washers, and a cylindrical cathode surrounding said'washers and adapted to be heated by the heater, means for holdingrsaid cathode assembly in assembled relationship, said ceramic washers and cylindrical cathode forming an oven-like enclosure which serves to increase the thermal efiiciency of the cathode assembly, a cylindrical control grid carried by said second ring and a screen grid carried by the third ring, said screen grid' comprising a plurality of elongated ribbon-like elements arranged to denne a cylindrical surface, said elements' arranged to present their narrow edge to the electron stream. Y i

7. An electron tube having an envelope comprising Vat least first, second and third spaced metallic terminal rings, a cup-shaped anode, a plurality of ceramicV rings sandwiched between' said terminal rings, a ceramic ring, and afseah'ng meansrconnecting said anode to said ceramic ring, a cathode assembly carried by said first terminal ring and'including a center post, spaced ceramic washers` carried atthe endsV of said post, a heater having heating Velements which define a cylinder and extend lengthwise thereof, said elements being carried between said washers and spaced from said center post, heat insulating means disposed adjacent theA outer face of each of said ceramic washers, and a cylindrical cathode surrounding said heaters and washers, said washers, insulating means, and cylindrical cathode forming an oven-like enclosure which servesgtoincrease the thermal efiiciency of the cathode assembly.

Y, 8. Anf electron tube Vhaving `an envelope'comprising atV least ii'rst, second and third spaced metallic terminal rings,

naar Y Y arrcup-shape'd anode, a plurality of ceramic ringa saudwiched between said terminal rings, Ya ceramic ring, a sealingrneansV connecting said anode to said ceramic ring,V

Va cathode assembly carried by said'first terminal ring and including spacedV ceramic washers, an annular groove formedkon one face'of each of said washers, a heater of tilamentary material wound to define a cylinder and extend lengthwise thereof, said heater beingk supported between said washers andhaving its end portionsy accommodated within the annular grooves, a cylindrical cathode surrounding said washers and adapted to be heated by the heater, means for holding said cathode assembly'in assembled realtionship, `said Vceramic washers and cylindrical cathode forming an oven-like enclosure which serves to increase theV thermal ecie'ncy'of the cathode assembly, a cylindrical grid structure surrounding said cathode and supported on said second terminal ring, said grid structure including a plurality of elongated elements ar-` ranged to lie on a cylindrical surface, and a screen grid carried by said third terminal ring and interposed between said anode'andjcontrol grid structures, said screen grid comprising a plurality of elongated ribbon-like elements' arranged to denne a cylindrical surface and to present their narrow edge to the'electron stream, rsaid elements of the screen grid being radially in line with the elements of the control grid. Y l

9. An electron tube comprising a cathode assembly having a hollow center post, spaced ceramic washers carried at the ends of said post, an annular groove formed on one face of each of said washers, a heater supported between said washers and in spaced relationship to said post, heating elements of the heater defining elements of a cylindrical surface `and supported between said Washers with the end portions accommodated within said annular grooves, a cylindrical cathode surrounding said washers spaced from and adapted to be heated by Ysaid heaters, a cylindrical control grid structure spaced adjacent to the cylindrical cathode,V -a cylindrical screen grid, and a ceramic pin adapted to register in said center post and serving to hold the control grid and screen grid structures coaxial with respect to the cathode assembly.

10. An electron tube having an envelope comprising. at least first, second and third spaced metallic terminal rings, a cup-shaped anode, a plurality of ceramic rings sandwiched between said terminal rings, a ceramic ring and a sealing means connecting said anode to said ceramic ring, a cathode assembly carried by said first terminal ring and including a hollow center post, spaced ceramic washers carried at the ends of said post, an annular groove formed on one face of each of said washers, a heater supported between said washers and in spaced relationship toisaid center post, the heating elements of said heater defining elements of a cylindrical Asurface and supported between said washers vwith the end portions accommodated within said annular grooves, a cylindrical cathode surrounding said Washers spaced from and adapted to be heated by said heater, and means for holding said cathode Vassembly in assembled relationship, said ceramic washers and cylindrical cathode forming an ovenlike enclosure which serves to increase the thermal elhciency of the cathode assembly, a cylindrical control grid carried by said second ring and a screen grid carried by the third ring, said screen grid comprising a plurality of elongated ribbon-like elements arranged to define a cylindrical surface, said elements arranged to present their narrow edges to the electron stream, a ceramic pin accommodated within said hollow center post and extending upwardly, said pin serving to receive the upper portion of the cylindrical control grid and said screen grid and a cup-shaped anode, a plurality of ceramic rings sandwiched between said terminal'rings, a ceramic rng'and a sealing means connecting said anode to said ceramic ring, a cathode assembly carried by said first terminal ring and including a hollow center post, spaced ceramic washers carried at the ends of said post, an annular groove formed on one face of each of said washers, a heater supported between said washers and in spaced relationship to said center post, the heating elements of said heater defining elements of a cylindrical surface and supported between said washers with the end portions accommodated within said annular grooves, a cylindrical cathode surrounding said washers spaced from and adapted to be heated by said heater, and means for holding said cathode assembly in assembled relationship, said ceramic washers and cylindrical cathode forming an oven-like enclosure which serves to increase the thermal eiciency of the cathode assembly, a cylindrical grid structure surrounding said cathode and supported on said second terminal ring, said grid structure including a plurality of elongated elements arranged to lie on a cylindrical surface, a

screen grid carried by said third terminal ring and interposed between sad anode and control grid structure, said screen grid comprising a plurality of elongated ribbonlike elements arranged to dene a cylindrical surface and to present their narrow edges to the electron stream, said elements of the screen grid being radially in line with the elements of the control grid, a ceramic pin having its lower end received within the hollow center post and its upper end serving to engage the upper end of the cylindrical grid structure and the upper end of the screen grid structure and serving to maintain the same coaxial with respect to the cathode assembly.

References Cited in the file of this patent UNITED STATES PATENTS 1,941,661 Crowley Feb. 2, 1934 2,130,280 Knoll Sept. 13, 1938 2,577,239 Eitel Dec. 4, 1951 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTTN Patent No., 2,939,988 June 'ZV 1960 Robert Da Culbertson et aL7 It is hereby certified that error appears in the printed specification of' the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 42, for "element read elements me; column 8, line 36, for "heaters" read heater lo Signed and sealed this 15th day of November 1960o (SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON .Mieming Officer Commissioner of Patents