US3419743A

US3419743A - Electron tube having a tensioned cathode

Info

Publication number: US3419743A
Application number: US513957A
Authority: US
Inventors: Donald B Kaiser; Roth Robert
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1965-12-15
Filing date: 1965-12-15
Publication date: 1968-12-31
Anticipated expiration: 1985-12-31
Also published as: GB1141517A; FR1505101A; DE1299079B; NL6617565A

Description

Dem 17968 I D. B. KAISER ETAL 3,

ELECTRON TUBE HAVING A TENSIONED CATHODE Fiied'Dec. 15, 1965 Sheet of 2 Inn/mien.- flamw 62 611.95? Rams-fir Ra m V Adorned United States Patent 3,419,743 ELECTRON TUBE HAVING A TENSIONEI) CATHODE Donald E. Kaiser and Robert Roth, Lancaster, Pa., assignors to Radio Corporation of America, a corporation of Delaware Filed Dec. 15, 1965, Ser. No. 513,957 6 Claims. (Cl. 313278) ABSTRACT OF THE DISCLOSURE A power tube having two coaxial cylindrical metal blocks provided with spaced registering slots for receiving an elongated cathode filament. The filament is sutficiently long so that end portions thereof extend beyond the slots. Clamping rings are provided for closing the open ends of the slots. The end portions of the filament extending beyond the slots are melted to spherical shape having a larger cross sectional area than the space defined by the slots and the clamping rings, for confining the filament in the slots. Spring means urge the metal blocks apart thereby tensioning the filament and causing the spherical ends to bear against the clamping rings.

This invention relates to electron tubes and particularly to an electron tube having a filamentary type cathode that expands and contracts axially in response to temperature changes.

In some types of electron tubes of relatively large power output, the cathode comprises a plurality of rectilinear filaments engaged at opposite end portions and having intermediate portions extending through opensided slots or grooves in a metal block. Each slot serves to form into a beam the electron emission from a cathode filament therein with reduced interference from the electron emission from adjacent cathode filaments for increased efiiciency. In such structure, the spacing between a cathode filament and the walls of a slot in which it is positioned, is relatively small, so that a slight buckling of the filament in response to axial expansion thereof at elevated temperatures, may cause the filament to contact an adjacent slot wall. Such contact is objectionable because it will drain heat from the filament that is necessary for its emission and adversely affect the electrical characteristics of the tube.

Any attempt to avoid this difiiculty by suspending each contact filament under tension to preserve its rectilinear shape at elevated temperatures, is accompanied by the further difiiculty of determining the type and location of the suspension means. Where the suspension means involves direct spring engagement with a filament, the proximity of the spring to the cathode may result in an appreciable heat transfer to the spring from the cathode at its operating temperature. The resultant heating of the spring may anneal the material thereof and reduce its flexibility to such degree that it cannot keep the filament in satisfactory tension. Furthermore, a spring engagement with each of a plurality of cathode filaments employed in a tube may render it difficult to insulate electrically the cathode from other tube elements in order to pass to each filament the requisite electric current for attaining its operating temperature.

Accordingly, it is an object of the invention to provide an electron tube wherein a rectilinear cathode is preserved from deformation in response to temperature changes.

A further object of the invention is to provide a cathode suspension system wherein a tensioning means is disposed in a location appreciably spaced from the cathode for isolating the tensioning means from cathode heat.

Another object is to facilitate electrical energization 3,419,743 Patented Dec. 31, 1968 of a cathode for heating the cathode to its operating temperature.

One example of a structure wherein the foregoing objects are attained is an electron tube having two generally cylindrical metal blocks axially spaced. The outer sides of the two blocks are provided with a plurality of circumferentially spaced grooves or open-sided slots extending parallel to the axis of the blocks. Elongated cathode strips or filaments are received in the slots and extend in spaced relation with respect to the walls of the slots and parallel to the axis of the blocks. End portions of the cathode filaments are anchored in retaining means provided on the sides of the blocks adjacent to their remote ends. Means are provided for urging the two blocks apart against the tension of the filamentary cathodes.

To allow for heat losses in end portions of the cathode filaments engaged by the retaining means Without affecting the operation of the tube, the active regions of the filaments may be limited to intermediate portions thereof appreciably spaced from the filament ends.

Further" objects and features of the invention will become evident as the present description continues.

In the drawing:

FIG. 1 is a sectional elevation of an electron tube having an improved structure in accordance with the teachings of this disclosure;

FIG. 2 is an enlarged sectional fragment illustrating the structure for retaining the cathode filaments in axial tension; and

FIG. 3 is a view in elevation of apparatus that may be used in axially compressing the two metal blocks of the tube and fixing the cathode filaments thereto for preserving a desired axial compression of the blocks in the completed tube.

The electron tube shown in FIG. 1, by way of example, includes an anode 10, a cathode in the form of a plurality of directly heated filaments 12 and a two-part structure 14, 16 defined by peripheral portions of two

metal blocks

18, 20. The structure parts 14, 16 comprise opensided slots formed in the periphery of blocks .18, 20, each slot receiving a cathode filament 12. I

The anode 10 and the two blocks 18:, 20, may be made of copper, preferably the OFHC type. Each of the cathode filaments 12 comprises a base which may be made of any suitable metal such as an alloy known as Hastelloy having a melting point of about 1475 C. This alloy comprises by weight 2.5% cobalt, 4 to 6% iron, chromium, 26% molybdenum, 1% silicon, 1% magnesium, 0.05% carbon and the remainder nickel. The base may be suitably coated for desired electron emission by sintering thereto particles made of a suitable metal such as nickel or tungsten for example. The matrix formed by sintered metal particles is suitably impregnated with an electron emitting material such as barium. The sintered coating may extend axially along the active portion of a filament which is the portion received in slots 14, 16 of the

blocks

18, 20 as viewed in FIG. 1.

The cathode filaments may be fiat in cross section, i.e., ri=bbon-like, with one of their flat sides adjacent to the bottoms of slots 40 (FIG. 2). Two clamping rings 44 surrounding the array of filaments at each end thereof serve to keep the cathode filaments in this position.

The block 20 is provided with an axially-extending recess 22 open at its top end as viewed in FIG. 1 and having a closed bottom 24. The block 18 also has an axially extending recess 26, of substantially the same diameter as recess 22 and with an open end adjacent to the open end of recess 22. The other end of recess 26 is provided with an inwardly extending annular shoulder 28 having a function to be described.

Within the

recesses

22, 26 is an alignment and spacing member 30 made of insulating material such as aluminum oxide, for example. One end of member 30 bears against the shoulder 28 and the other end bears on spring means which for example may be in the form of Belleville springs 32, 34, each of which is made of a suitable spring material capable of retaining its spring nature at a temperature at least as high as 400 C. Each of springs 32, 34 has a generally concavo-convex shape. Springs of this type are well known in the art. If desired, a metal plate 36, which also may be made of suitable spring material, may be interposed between the spring means 32, 34 and the adjacent end of member 30. Plate 36 may be omitted if the adjacent end of the member 30 has a sufiiciently large bearing surface.

The

grid blocks

18, 20 are held in compression against the force of spring means 32, 34 by the cathode filaments 12. To enable the cathode filaments 12 to perform this compressive function, each filament is engaged at its ends by mutually similar structures comprising metal support rings 38, each having an array of open-sided slots 16. The periphery of the rings 38 are stepped to provide shoulders 42 (FIG. 2) thereon. The metal rings 38 may be fixed to the

blocks

18, 20 as by brazing. To confine the filaments 12 in the slots 14, 16, the meta1 clamping ring 44, made of copper for example, is seated on the shoulder 42 and serves to close the sides of portions of slots 14, 16 above the shoulder 42 as viewed in FIGS. 1 and 2, and to restrain displacements of the filaments from the slots 14, 16. Each cathode filament 12 has enlarged

end portions

46, 48, which may be of spherical shape and which have a volume of appreciably larger transverse dimensions than the space between the bottom of a slot 14, 16 in ring 38 and the clamping ring 44. In this way, the filaments 12 serve to keep the

blocks

18, 20 in compression against the force of spring means 32, 34.

The length of the cathode filaments 12 is preferably of a magnitude to provide a spacing 50 between the

blocks

18, 20 of about 15 mils when the tube is cold. During operation, the cathode filaments 12 are heated to a temperature of about 800 C. This results in a lengthwise expansion of the filaments of about 15 mils and an increase in the spacing 50 to about 25 mils. This is well within the compression limits of the spring means 32, 34. The characteristics of the tube are determined when the tube becomes heated during operation. It is to be noted in this connection that the coefiicient "of expansion of the Hastelloy alloy of which the cathode filaments 12 are made is appreciably smaller than that of the

copper blocks

18, 20. As a consequence, the 15 mils elongation of the filaments 12 when heated results in an increase of only about mils in the spacing 50.

The anode 10 is associated with a structure that provides suitable compensation for expansions and contractions of the cathode filaments 12 and relative axial movements of

blocks

18, 20 in operation. This structure as shown in FIG. 1, comprises an appreciable portion of the tube envelope and includes two metal rings 51, 52 fixed as by brazing to opposite ends of the anode 10 and to adjacent ends of two insulating rings 54, 56, made of aluminum oxide for example. Two metal rings 58 and 60 are brazed to the other ends of the insulating rings 54, 56. A welding ring 62, brazed to block 18 may be fixed as by welding to ring 60. Between ring 58 and a metal ring 64 is an expansion compensation structure 66 suitably fixed as by welding to rings 58 and 64. Since the compensation structure 66 forms part of the tube envelope, it is hermetically closed as by an annular weld 68. The compensation structure 66 permits the envelope structure associated with the anode 10 to follow the relative movements of the

blocks

18, 20 caused by expansion and contraction of the cathode filaments 12 during operation of the tube.

To permit evacuation of the envelope formed by anode 10 and its associated structural elements referred to in the foregoing, one block 18 is provided with an axial channel 70 in communication with a transverse channel 72 extending to the interior of the envelope of the tube.

An opening 74 defined by shoulder 28 affords communication between

recesses

22, 26 in the

blocks

20, 18 and channel 70 so that these recesses may be suitably evacuated. An exhaust tube 76 is brazed to the walls defining channel 70 and after evacuation of the tube envelope, the exhaust tube 76 is closed as by a pinch 78.

Cooling means for the

blocks

18, 20 may be in the form of

radiators

80, 82. The anode 10 may be cooled by a radiator 84.

'In assembling the tube shown in FIG. 1, a technique may be employed as shown in FIG. 3. In practicing this technique, the two

blocks

18, 20 are assembled with the insulating alignment and spacing member 30 disposed in the

recesses

22, 26 of the axially aligned blocks and with the spring means 32, 34 and plate 36 disposed between recess bottom 24 and the adjacent end of alignment member 30.

In each block, the slot 40 in the support elements 38 is in alignment or register with a slot 14 or 16 in such block. This alignment is effected when fixing the support elements 38 to the

blocks

18, 20. In a completed tube it is desirable that the aligned slots 40, 14 in one block be in alignment with the aligned

slots

40, 16 in the other block as shown in FIG. 1. This is accomplished by axially rotating the

blocks

18, 20 relative to each other until the

slots

40, 16 in one block are aligned or in register with the

slots

40, 16 in the other block. The cathode filaments 12 are then extended through the aligned slots in support rings 38 and in the two blocks until the filaments abut against a ring-shaped stop 90. The two blocks are then urged toward each other against the force of spring means 32, 34 until the spacing 50 is reduced to 15 mils. The vertical posit-ion of stop is adjustable by means of a sleeve 92 and a lock screw 94. Initially, the stop 90 is spaced about 125 mils below the slots in the adjacent cathode support member 38. The filaments 12 have such length, for example 1.500 inches, that when the lower ends thereof, as viewed in FIG. 3 abut against stop 90 their upper ends extend above the slots in the upper cathode support ring 38, a distance of about 125 mils. The assembly is then enclosed in a bell jar 96 which may be made of glass, and an inert gas such as argon is introduced into the bell jar through a tubulation 98 to a flow of about 60 cubic feet per hour.

The

compression elements

86, 88 on which the assembled parts are supported is rotatable either by manual or automatic means, not shown, in a path that brings successive end portions of the filament 12 in the upper end of the assembly as viewed in FIG. 3, in close spaced relation with respect to an electric torch 100. In one example, the spacing between the torch 100 and an adjacent filament 12 was about 30 mils. An electric current source of about 30 amperes was used and the

blocks

18, 20 were axially rotated at a velocity of one revolution per 20 seconds. In another example in which a mechanized apparatus was used, the blocks were rotated at a velocity of one revolution per five seconds, the spacing between the torch 100 and an adjacent cathode was 5 mils, and an electric current source of 75 amperes was used.

As each cathode filament end portion is brought into proximity to the torch 100, the entire end portion of the filament becomes molten and assumes the shape of a sphere. The melting proceeds to a region close to that in which the filament emerges from the upper end of slot 40 as viewed in F-IG. 2. Melting of the cathode filament 12 below this region is prevented by the cooling effect imparted to the filament by the support ring 38 and the clamping ring 44. When a sphere so formed cools down to room temperature, it is in actual bearing contact with respect to the two

rings

38, 44, as viewed in FIG. 2.

When all of one group of adjacent end portions have been formed to spherical shape in accordance with the foregoing, the assembly comprising the

blocks

18, 20 and their associated parts including the filaments 12, is inverted and repositioned between

compression members

86, 88, and axially compressed. The stop 80 is then raised to engage and urge the previously formed spheres- 46 in the filaments, against the adjacent ends of the support ring 38 and clamping ring 44, as viewed in FIG. 3. The assembly is then rotated under the conditions previously described to form spheres 48 (FIG. 1) at the upper ends of the filaments 12 as shown in FIG. 3. In the assembly so formed the spring means 32, 34 serve to keep the cathode filaments 12 in tension at all temperatures to which the filaments may be subjected.

To facilitate the incorporation of the resultant assembly in the completed tube, the welding ring 62 is fixed as by brazing to the upper portion of the block 18 as viewed in FIG. 1, and the welding ring 63 is similarly fixed tothe lower portion of the block 20. The three parts of the envelope assembly comprising anode and

envelope parts

60, 54, 50, 52, 56, 58, 66 and 64, are suitably joined as by brazing to form a tubular structure that can be telescoped downwardly upon the cathode assembly as shown in FIG. 1, until the flanges on welding ring 62 and metal envelope part 60 are in register. The envelope parts referred to are so dimensioned that when such register occurs, the lower end of metal envelope part 64 will be flush with a flange on welding ring 63. The two adjacent flanges on welding ring 62 and on envelope part 64 are then joined as 'by welding, and the lower end of envelope part 64 as viewed in FIG. 1 may be fixed to ring 63 as by welding. I

The radiators '80, 82 may then be aflixed to

blocks

18, 20 respectively, as by being brazed thereto. The anode radiator 84 may be attached as by brazing, to the anode either before or after completion of the tube assembly. The interior of the tube envelope is then evacuated through exhaust tubulation 76 which is appropriately closed as by the pinch-off 78, after evacuation has been completed.

While the space 50 between the blocks 18, extends between regions of the active portions 14, '16, its dimension is so small, as indicated in the foregoing, that it exhibits no appreciable adverse effect on the operation of the tube.

What is claimed is:

1. An electron tube having:

(a) two metal blocks movable in relation to each other,

(b) a plurality of filamentary cathodes,

(c) means fixing said filamentary cathodes to said blocks, said fixing means comprising two support rings fixed to said blocks and having registering slotlike passages in mutual registration, said filamentary cathodes extending through said passages and having enlarged portions bearing against remote end surfaces of said support rings adjacent to said passageways, and

((1) means urging said blocks apart against the tension of said filamentary cathodes,

(e) whereby said filamentary cathodes are preserved from deformation during temperature changes thereof.

2. An electron tube according to claim 1 and wherein said enlarged portions are spherical.

'3. An electron tube comprising:

(a) an elongated structure having axially movable end portions,

(b) elongated cathode filaments having end portions fixed to said axially movable end portions to provide 6 an assembly,

(c) means urging said axially movable end portions apart against the tension of said cathode filaments,

(d) an envelope enclosing said assembly,

said envelope being evacuated and including one portion fixedly sealed to one of said axially movable end portions and another portion fixedly sealed to the other of said axially movable end portions, and

(e) compensating means joining and sealed to said envelope portions to permit relative axial movements of said envelope portions, whereby said envelope is preserved from harm and the evacuated condition thereof is unimpaired when said axially movable end portions move axially in response to expansions and contractions of said cathode filaments.

4. An electron tube according to claim 3 and wherein said axially movable end portions comprise metal blocks having relatively thick walls defining axially registering recesses therein, one of said recesses being relatively deep and having a bottom and said urging means is disposed only in said one of said recesses adjacent to said bottom, whereby heat sensitive portions of said urging means are shielded from heat from said cathode filaments.

5. An electron tube having:

(a) two metal blocks movable in relation to each other,

(b) a plurality of filamentary cathodes,

(c) means fixing said filamentary cathodes to said blocks, said fixing means comprising a metal clamping ring confining end portions of said filamentary cathodes, and

(d) means urging said blocks apart against the tension of said filamentary cathodes,

6. An electron tube having:

(a) two metal blocks movable in relation to each other,

(b) a plurality of filamentary cathodes,

(c) means fixing said filamentary cathodes to said blocks, and

(d) means urging said blocks apart against the tension of said filamentary cathodes, said urging means comprising an insulating member and spring means between one end of said insulating member and an inner surface of one of said two blocks,

References Cited UNITED STATES PATENTS 2,385,435 9/1945 Werner et al. 313-278 2,542,639 2/1951 De Walt 313--278 X 2,570,121 10/1951 Harbaugh 313-278 X 2,726,349 12/1955 Wing et a1. 313-278 X JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner.

US. Cl. X.R.