US2980984A

US2980984A - Art of fabricating electron tubes

Info

Publication number: US2980984A
Application number: US678790A
Authority: US
Inventors: Merrald B Shrader; Fred G Block; Morris R Weingarten
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1957-08-19
Filing date: 1957-08-19
Publication date: 1961-04-25
Anticipated expiration: 1978-04-25

Description

April 1961 M. B. SHRADER E'l'AL 2,980,984

ART OF FABRICATING ELECTRON TUBES Filed Aug 19, 1957 2 Sheets-Sheet l awn rams ]\/[ER'RALD B. SHRADER Munms R. WEINEARTEN FRED E. BLDEK April 1961 M. B. SHRADER ETAL 2,980,984

ART OF FABRICATING ELECTRON TUBES Filed Aug. 19, 1957 2 Sheets-Sheet 2 i 4 i "la-an 0 as k m i/vrmi MERBALD B. SHBA'DER Munms R.WEINEARTN BY FRED 55mm United States Patent ART OF FABRICATING ELECTRON TUBES Merrald B. Shrader, Mount Joy, and Fred G. Block and Morris R. Weingarten, Lancaster, Pa., assignors to Radio Corporation of America, a corporation of Delaware Filed Aug. '19, 1957, Ser. No. 678,790

6 Claims. c1. 29-2544 This invention relates to the art of fabricating election tubes of the type having a plurality of thin apertured electrodes wherein the apertures of one of the electrodes are to be aligned in a desired direction with the apertures of another of the electrodes. I

In the design and tabricationof electron tubes having a plurality of aperturedelectrodes, particularly those which are to be operated at high power levels, it is important that the electrodes be mounted and maintained so that their structural elements are in accurate desiredalignment with each other along the cathode-anode electronstream path. For example, in the case of a tube having a control grid and a screen grid, if the conductive elements of the screen grid are not perfectly shielded by, or aligned with, the conductive elements of the control grid, electron impingement upon the screen grid will result. Because of this impingement, some electrons emitted by the cathode fail to reach the anode and are thus not utilized to produce output power. This means not only an inefficient use of the electrons available from emission, but also an inefiicient use of input power. Such impingement also means screen grid current, which causes an undesirable heating of the tube and consequently a heat dissipation problem. Both of these place a distinct.

limitation on the operational ratings of the tube.

In spite of this consideration, plural grid electron tubes fabricated according to the prior art have consisted of grid units which were separately fabricated and then mounted together and roughly aligned to form the device. Such fabrication involves many operations, any one of which if improperly performed could impair the quality of the assembled tube. For example, ina tube with cylindrical gridsthe control grid and. screen. grid.

units. must be fabricated with identical configurations but withthe screen grid somewhat larger. Even assuming. that a pair ofgrids couldbe so constructed according.

bly fixed to' maintain this alignment. In the prior art. it has been the practice to manually align each pair ofgrid wires separately. This has been a time consuming and. costly procedure. equipment, such as'shadow graph apparatus and the like,

such manual alignment has actually been a matter of' a' finite degreeof misalignment.

It is therefore an object of our invention to' provide an iinproved method of fabricating an assembly of apertured electrodes wherein'the apertures of'eaeh'el'ectrode Even with the help of precision 'ice are in substantially perfect alignment with the corresponding apertures of the other of such electrodes.

Briefly, according to our invention, two or more thin electrode blanks, each having an imperforate portion, are first permanently mounted in their desired predetermined spaced relationship suitable for incorporation in. an electron tube. Aligned apertures are then cut through the imperforate portions of all the blanks in asingle. continuous operation by a method which involves substantially no deformative force on the blanks.

The requirement of a non-deformative cutting operation results from the desire to use the improved method in forming apertures in thin, fragile, electrode blanks. For example, the thickness of the walls of the blanks may be of the order of a few thousandths of an. inch. A thin walled blank is desired because of the resultingability to achieve a mutually close spacing between all portions of the electrodes so formed. Such a closespacing results in better high frequency operation. Suitable ways of performing the cutting depend upon the rigidity of the blanks to be cut. The thinner the blanks, the less the permissible force thereon. Cutting methods which are suitable include: ultrasonic machining, wherein. a,

shaped tool is vibrated at an ultrasonic frequency toward. and away from the work while a thin film of abrasive;material suspended in a liquid vehicle is maintained between the tool and the work; electrical discharge machining,- wherein an electrical discharge is repeatedly established between a shaped tool and the work toerode away the work over an area corresponding to the shape of thetool; and abrasive spray, whereinv abrasive material is. sprayed, e.g. in air, against the work over a predetermined area to cut away the work over that area. Actually, if the blanks to be cut were thick and rigid enough, then an ordinary milling operation might be used. However, this is not the case in the manufacture of thin electron tube parts to which this invention relates.

In the drawings:

Fig. 1 is an elevation view in partial cross-section of an electron tube made according to our invention;

Fig. 2 is an enlarged plan view in cross-section of a portion of the tube of Fig. 1 taken along line 2-2 of Fig. 1;

Fig. 3 is an elevation view in partial cross-section of a grid blank assembly'being formed in accordance with, our invention by one method of cutting;

Fig. 4 is an enlarged plan view of the grid assembly and cutting tool of Fig. 3 taken along line 44 of Fig. 3;,

Fig. 5 is a longitudinal cross-section view of a jig being, used in the practice of our invention;

Figs. 6 and 7 are views similar to those of Figs. 3 and 4 respectively except a modified method of forming, the grid apparatus by another cutting method is shown;

Figs. 8 and 9 are elevation and enlarged plan views in partial cross section respectively of the cathode and grid assembly of another electron tube made according to our invention; and

Figs. 10 and 11 are elevation and enlarged plan views in partial cross-section respectively of another modified method of forming the grid apertures according to our. invention.

Referring to Fig. 1, an electron tube 10 comprises a cylindrical cathode 12, a control grid 14, a screen grid 16, and an anode 18, all arranged in nested coaxial array, The cathode 12 includes a tubular sleeve member having at one end a cylindrical electron activeportion 20 coated with an electron emissive material, and at the other end a radial flange portion 22 having at the outer periphery thereof an axially extending tubular portion '24. The ifiange portion 22 and, the tubular portion 24 are adapted to be seated in a cathode mounting cup 26 so that the cathode may be inserted in the tube in predetermined f a "menace spaced relationship to the control grid 14 after assembly of the vacuum envelope portions of the tube. The control grid 14 and the screen grid 16 comprise, respectively, electron active cylindrical perforated

portions

28 and 30, conical

tubular support portions

32 and 34, radial flange portions 36 and 38, and outer tubular terminal contact portions 40 and 42. The anode 18 comprisesa cup-like member disposed over, and surrounding, the cathode and grids. A metal tubulation 43, brazed to the top of the anode 18, communicates to within the anode and serves as a means of evacuating the tube when its fabrication is completed. After evacuation the tubulation 43 is pinched off by a cold weld operationto seal the tube 10. A heater element is provided by a coil 44 of wire disposed inside the cathode sleeve adjacent the sleeves coated cylindrical portion 20. One end of the coil 44 is connected to the cathode sleeve 12 and the other end to a heater lead-in assembly 46 which comprises a heavy conductor 48 and a terminal cup ,50 disposed coaxially within the cathode sleeve. The heater lead-in assembly 46, cathode 12, control grid 14, screen grid 16, and anode 18 are all held in a predetermined insulative spaced relationship to each other in the order named by a series of separating annular ceramic disk 52-58 sealed respectively to the radial flange portions of those members. A heat radiator 60, comprising a cylindrical member 62. and a plurality of annular disk flanges 64 attached thereto, is fitted tightly over the external cylindrical surface of the anode.

As an example, the electron tube 10 of Fig. 1 has been fabricated with an overall length of approximately 1.9 inches and with approximate grid and cathode data as follows: an outside diameter of 0.322 inch and an axial length of 0.435 inch for the electron active portion 28 of the control grid; an outside diameter of 0.338 inch and an axial length of 0.425 inch for the electron active portion 30 of the screen grid; 72 grid wires of about 0.004 inch square cross-section for each grid; and an outside diameter of 0.290 for the coated portion 20 of the cathode.

Electrons emitted from the coated cylindrical portion 20 of the cathode travel in a generally radial direction to the cylindrical surface of the anode, passing through aligned perforations, or apertures, in the control grid and the screen grid. In order to obtain the highest possible operating efiiciecny from the tube, screen grid current must be kept to a minimum. As is well known in the art, this can best be achieved by obtaining an accurate alignment of the conductive elements of the tube grids so that electrons which pass through the control grid apertures will be presented with corresponding openings in the screen grid rather than a conductive element upon which they may impinge and result in screen grid current. Fig. 2 best shows this alignment between the elec: tron active wire portion 28of the control grid 14 and the electron active wire portion 30 of the screen grid 16.

In Fig. 3, according to our invention offlfirst mounting imperforate grid blanks in their ultimate spaced relation to each other and then forming sets of substantially prefectly aligned apertures therein, the

imperforate grid blanks

14a and 16a comprise solid one piece metal members shaped identical to the finished grids 14 and 16 shown in Figs. 1 and 2 but having solid imperforate

cylindrical portions

28a and 30a instead of the

perforate portions

28 and 30 of the finished grids. As assembly consisting of at least an imperforate control gridblank 14a, an imperforate screen grid blank 16a, and a separating annular ceramic disk 56 is first fabricated to provide a a series of aligned longitudinal slots equally spaced around the peripheries of the two grid blanks is shown and described. Such a cutting results in a pair of grids having an equal number of longitudinal wires disposed in cylindrical array with the wires of one grid in substantially perfect radial alignment with the corresponding wires of the other grid. Fig. 3 shows the cutting operation of a grid assembly approximately one-half completed with the cutting being performed longitudinally of the axis of the grids from top to bottom.

An ultrasonic metal cutting (or grinding) machine is provided which includes a pan 68 in which the work is A positioned, a vibration velocity transformer 72 onto which a predetermined shaped cutting tool 74 is attached, and a transducer 76 for vibrating the transformer and tool. The transducer 76, transformer 72, and cutting tool 74, are fixedly mounted above the pan 68. A pan support and, vertical feed mechanism (not shown) below the pan serve to advance the pan work piece contained therein upward toward the tool 74. A magnetic chuck 78 is fixedly attached to the bottom of the pan 68 for securely holding the work in place during cutting.

To produce a finished pair of grids, a grid blank assembly 66 is first tightly seated in a jigging socket 80. The

. chuck 78 such that the grid blank assembly 66 is axially aligned with the tool 74 and both may be fitted into their respective recesses of the jig 82. The magnetic chuck 78 is then energized to hold the jigging socket 80 and grid blank assembly 66 in proper position for cutting. The

., tool 74 is raised, and the jig 80 is removed. The tool 74 may then be advanced for cutting.

In operation, an abrasive material is suspended in a liquid vehicle and flowed over the cutting area of the work such as by hoses (not shown) played thereon. At the same time the transducer 76 is repeatedly energized to vibrate the tool 74 in a vertical direction, as illustrated,

toward and away from the work. As a result, as the vibrating tool and the work, i.e., the grid blanks 14a and 1612 are brought close to each other, the work is worn away over an area directly oppositely and corresponding to the shape of the tool.

. According to a preferred embodiment of grid structure and a preferred method of cutting that structure, the tool 74 comprises a thick walled hollow cylinder having longitudinal teeth 90 internally thereof. The tool 74 is shown more clearly in cross-section in Fig. 4. The diameter of the tool 74 at the base of the teeth 90 is slightly larger than the outside diameter of the outer grid blank 16a of the grid blank assembly 66. The

a teeth 90 of the tool 74 are sufiiciently long to grid blank assembly 66. Because of subsequent assembly extend radially inward to a diameter smaller than the inside diameter of the inner grid blank 14a of the grid blank assembly. As such, when the tool 74 is vibrated and then brought close to the grid blanks and advanced downwardly over the grid blanks as shown in Fig. 3, an equal multiplicity of longitudinal slots is cut in each of the grids in such fashion that a slot in the control grid 14 is as perfectly aligned with a corresponding slot in the screen grid 16 as the particular tooth of the tool 74 which out those slots is uniformly true throughout its length. In effect, this amounts to a substantially perfect alignment.

According to a preferred practice of our invention as shown in Figs. 3 and 4, the control grid wires 28 and the screen grid wires 30 are the same width; and the screen grid slots, or apertures, are centrally, radially aligned with, but larger than, the control grid apertures. Such a structure is preferred over a grid structure having equal width control grid and screemgrid-apertures because it provides better shielding of the screen grid wires by the control grid wires and thus-results in less-electron impingement on the screen grid. Another advantage of such a grid design obtains from the shape of the tool teeth 90 necessary to out such a grid, and the'shape of a broaching tool suitable for producing such va tool 74. Since the tool teeth 90 adapted, for forming such a grid assembly are tapered, being thicker at their base portion than at their-[end portion, a stronger. toothed tool is provided. Also a broach suitable" for forming such a tool 76 has parallel-sided teeth which results in a stronger broach than would be the casewere the broach teeth of reverse taper, beingv thicker at their ends than at their bases.

Figs. 6 and 7 describe an alternative method of prac-,

ticingour invention. The method ofFigs; 6 and 7 differs from the method of Figs.-3 and 4 in that the cutting is performed by electrical discharge machining and in. that a cylindrical tool 92 with external teeth 94 rather-than a hollow cylindrical tool 74 with internal teeth 90 is used.

In cutting by electrical discharge machining. a tank 95' in which the Work'is positioned is filled to a level above the cuttingarea of thework with a dielectric oil 96. The tool 92 is supported in a tool holderp97 which'is fed downward,'as illustrated, from a feed mechanism (not shown) mounted above the tank 95. A pulsating DC. voltage 98-is applied between the tool .92 and. the

grid blanks

14a and 16a by connecting between the tool holder 97 and a grid blank assembly clamp 99. The. clamp 99 serves to maintain the grid blank assembly 100 in fixed position during the cutting operation. When the tool is brought close to the work, a series of electrical discharges occur which cause the work piece. i.e., the

grid blanks

14a and 16a to be vaporized or eroded away over an area directly oppositeand corresponding to the shape of the tool.

In contrasting the method of Figs. 6 and 7 with that of Figs. 3 and4, the too] is advanced from the opposite end of the grid blank assembly such that the tool moves inside the control grid blank 14a rather than down over the screen grid blank 16a. The diameter of the tool 92 at the base of the teeth 94 is smaller than the inside diameter of the control grid blank 14a; and the diameter of the tool 92 at the outer ends of the teeth 94 is larger than the outside diameter of the screen grid blank 16a. This is shown in Fig. 7 where the tool 92 has already advanced to partially form the electron

active wire portions

28 and 30 of the control grid 14 and screen grid 16 respectively. As such, a longitudinal advance of the tool 92.to within the control grid blank 14a produces a series of substantially perfectly aligned longitudinal slots in both grids somewhat similar to that produced by the method of Figs. 3 and 4.

Since the tool 92 must pass through the central openings of all members of the grid blank assembly 100, the assembly is fabricated to include only the two

grid blanks

14a and 16a and the single separating ceramic disk 54. These parts must be dimensioned to accommodate the tool. This may necessitate a slightly dififerent shaped supporting portion 32 of the control grid blank 14a. But, on the other hand, this method finds particular advantages in the fabrication of a grid assembly for a double-ended tube. The term double-ended is meant to describe a grid having a tubular supporting portion and a radial flange portion on both ends of a central electron active wire portion rather than having one cupshaped or closed end. Such a structure is known in the art and is particularly adapted for high frequency operation.

It will be appreciated that the electrical discharge cutting method can be used with a cylindrical tool having internal teeth such as is described in reference to Figs. 3 and 4. Likewise the ultrasonic cutting method can be used man-a cylindricai toolhaving external teeth H tion to an electron tube similar to the tube of Figs. 1

and 2but-with the" addition of an apertured beam-former shield electrode 101 around the coated portion 20 of the cathode sleeve 12. Suchan addition could be provided for thetube 10 of Figs; 1' and 2 by a modification of the cathodemountingcup 26. ,Fig. .8 illustrates such a modification. The modified mounting cup, constituting the apertured shield electrode "101, includes an elongated tubular portion-102 attached to or integralwith a cup portion-104, which: is identical to the cathode mounting cup26 of Fig; 1. As "such, there is provided in addition to the imperforate

cylindrical portions

28a and 30a of the two grid blanksofthe grid blank assembly, a third imperforatecylindrical: member 102 insulatively mounted coaxial therewith. The tubular portion 102 is adapted to fit closely around the coated portion 20 of the cathode 12; s

By using aitool similar to the tool 72 of Figs. 3 and 4 but with slightly longer-teeth, aligned slots can be cut through not only the

imperforate portions

28a and 30a of the control grid and screen grid blanks but also through the imperforatet. tubular portion 102 to form the apertured shieldl 98.. Such. a structure serves to permit a copious emission of electrons from the cathode over only those areas bounded by thetslots or apertures in the shield member 985 Thus, electrons are beamed toward the anode through the. aligned. grid apertures rather than being emitted,- at. all points around the cathode and being attracted directly toward the grid wires as well as through the grid apertures. This even-more elfectively reduces electron interception by the grids, particularly by the control grid.

We havedesoribed= the preferred practice of our invention;as. applied to. the; cutting of. elongated, longitudinal slots insa series of spaced cylindrical walls by the axial advance of acylindrical tool having a plurality of longitudinal teeth However,-a variety of modifications of my invention: arepossible.

For, example, referring to Figs; 10 and 11,. aligned apertures could be cut in the cylindrical grid blank assembly 66-by cutting one set of radially aligned aperturesat a time.- A- single toothed tool 1061s advanced radiallythrough the imperforatewall portions 30a and ZSa-of the' grid blanks-16a and14a, then the tool vvith drawn,- the grid blanle assembly- 66 angula-rlyrotated and indexed, and another set of apertures cut. This procedure would be repeated until aligned apertures were provided completely around the periphery of the grid blanks. In Fig. 11 a number of such sets of aligned apertures are shown as already cut with the tool not yet Withdrawn from the cutting of one set.

According to this method of cutting, any shape apertures could be cut, e.g. circular, square, or even circumterential slots part way around the periphery.

Another modification of our invention resides in the general configuration of the electrodes themselves. The imperforate portions of the electrode blanks need not be cylindrical. A planar electrode assembly can be provided with apertures by the same method but with the use of a flat tool having one or more elongated teeth, each of which cuts through all of the planar blanks of the electrode assembly.

These and other modifications of the broad concept of our invention, such as electrode shape and use, aperture shape, cutting method, tool configuration and the like will be readily suggested to one skilled in the art of electron tube fabrication.

It should be apparent from a reading of the description of our invention, that several alignment problems hereinbefore mentioned, which exist in the prior art, are solved according to our invention. It has been earlier stated herein that in order to chain a reasonable alignment of, for example, cylindrical grids" according to priorart methodsof tube fabrication: (l) the control grid and screen grid units must be fabricated with identical configurations but with'the screen grid somewhat larger, (2) that they must be coaxially'rnounted with neither shifted nor canted axes; and (3) that they must be oriented with respect to each other. According to our invention 'the third of these requirements is achieved to perfection by a cutting ofapertures with a single tool subsequent to mounting of grid blanks in their ultimate spaced relation. And, while we do not claim that our invention provides a way of achieving the first two of these requirements, it does even better in that these requirements cease to exist insofar as aperture alignment is concerned when tubes are fabricated according to our invention. Even if the grid blanks 14aand 16a are not made with their imperforate portions 28a'and30a identicaliin configuratiom'and even if they are not mounted exactly coaxially, nevertheless, when the: apertures are formed according to our invention they are still in perfect radial alignmenL' Thus, insofar as aperture alignment is concerned, our invention provides an essentially perfect solution to the problems of the prior art.

. What is claimed is:

a 1. The method of fabricating an electron tube subassembly comprising the steps of first mounting in fixed relation to each other a plurality of relatively thin electrode blanks having imperforate longitudinal portions in parallel, facing and spaced relationship and in register along a line passing through their thin dimensions, and then cutting apertures simultaneously in said imperforate portions of said blanks in 'a direction normal to said line and longitudinally of said blanks in a single continuous operation by successively removing particles from said portions of such small size as to be substantially free from exerting a deformative force an said blanks.

2. The method of fabricating an electron tube subassembly, comprising the steps of first mounting in a concentric array and in a radially spaced relation a plurality of relatively thin walled hollow cylindrical elec trode blanks to dispose imperforate portions thereof in radial register, fixing said blanks in said spaced array, and then simultaneously cutting at least one set of radiallyaligned apertures in both of said electrode blanks in a single continuous operation and in a direction parallel to the axis of said array by eroding from said imperforate portions relatively small particles only, of the material of said portions, whereby said blanks are substantially free of any deformation force during said apertureforming step. I

'3. The method of fabricating an electron tube sub-assembly, comprising the steps offirst mounting a plurality of relatively thin walled imperforate hollow cylindrical electrode blanksin concentric array, mechanically connecting saidblanks to each other in insulated and fixed relation, and thencutting a plurality of sets of radially-aligned axially-extending slots uniformally spaced around the periphery of said blanks in a direction parallel'tothe axis of said array and in a single continuous operation, while'substantially refraining from exerting a deformative forceon said grid blanks.

4. The method of fabricating an electron tube subassemb'ly comprising the steps of first mounting a plurality of electrode blanks, each having a relatively thin imperforate hollow cylindrical wall portion, in permanent insulative spaced relationship with their said imperforate wall portions coaxial. and coextensive, then cutting a multiplicity of sets of radially aligned, longitudinally extending slots in said electrode blanks, by simultaneously eroding a plurality of longitudinally extending portions of said wall portions in'a direction axially of said wall portions. 5. The method according to claim 4 wherein the slot cutting step isaccomplished by ultrasonic machining.

6. Method of fabricating a sub-assembly for an electron tube comprising the steps of first mounting in fixed, spaced and insulated relation, a plurality of electrode blanks having thin spaced wall portions, to provide a sub-assembly .wherein a predetermined plane intersects said wall portions; and then cutting one set of apertures through said portions and in said plane by advancing an erosion -zone,through.said portions and in said plane while preservingsaid portions from contact with material of sufficient mass to deform said portions.

. References Cited in the file of this patent UNITED STATES PATENTS 3 I OTHER' REFERENCES 1 Machinery, published April 1954, page 244.