US3436585A

US3436585A - Electron tube planar grid electrode

Info

Publication number: US3436585A
Application number: US445012A
Authority: US
Inventors: Hiromi Murakami
Original assignee: Nippon Electric Co Ltd
Current assignee: NEC Corp
Priority date: 1964-04-20
Filing date: 1965-04-02
Publication date: 1969-04-01
Anticipated expiration: 1986-04-01
Also published as: NL6505023A

Description

HIROMI MURAKAMI ELECTRON TUBE PLANAR GRID ELECTRODE April 1, 1969 Sheet Filed April 2. 1965 a II n (III/ll y e n w m MA MA R H v. B

I n n m w p L a i m April 1969 HlROMl MURAKAMI 3,436,585

ELECTRON TUBE PLANAR GRID ELECTRODE Filed April 2, 1965 Sheet 2 of 2 Inventor Attorney United States Patent ELECTRON TUBE PLANAR GRID ELECTRODE Hiromi Murakami, Tokyo, Japan, assignor to Nippon Electric Company, Limited, Minato-ku, Tokyo, Japan, a corporation of Japan Filed Apr. 2, 1965, Ser. No. 445,012 Claims priority, application Japan, Apr. 20, 1964, 39/215,106 Int. Cl. H01j 1/46, 19/46, 19/28 U.S. Cl. 313348 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an improvement to electron tubes and in particular to a high-reliability planar grid electrode for use in an electron tube. Specifically, this invention relates to a planar grid electrode for use in discsealed tubes and other planar-electron tubes which operate in the ultrahigh-frequency range.

The electrodes in disc-sealed tubes and other ultrahighfrequeney tubes, have generall been formed heretofore in a planar shape to minimize the reactance thereof. As is well known, however, such a grid is not reliable. In conventional planar grid electrodes the grid ring is usually formed of tungsten, molybdenum, or other similar material and a number of the grid wires of tungsten, etc. are stretched along one of the surfaces of the grid ring under mechanical tension and are brazed at their end portions to the grid ring. In such a structure, it is not possible to subject the grid wires to sufiicient tension and consequently to prevent them from deforming or from sagging due to: the electrostatic forces produced between the electrodes when the electron tube is in operation; the stresses produced during assembly of the tube, the thermal expansion produced by the temperature rise, etc. In particular, if the thermal expansion of the grid wires is smaller than that of the grid ring (which will occur when the grid wires are tungsten and the grid ring is molybdenum), then the contraction of the grid ring (after brazing) will be greater than that of the grid wires. As a result, the sagging of the grid wires becomes more serious. Moreover, if a grid of this construction is used in a highpower electron tube, the grid wires are more likely to sag because the grid wires increase in length and receive more heat and because the potential difference increases between the electrodes. Sagging of the grid wires results in a partial narrowing of the electrode spacing which in turn unnecessarily increases or decreases the anode current and further causes a deviation of the tube characteristics from the preferred ones. If the sagging is more serious, a discharge can occur between the grid and the neighboring electrodes to shorten the life of the electron tube.

In one proposed planar grid electrode proposed to re- 3,436,585 Patented Apr. 1, 1969 move the above-mentioned defects, after the grid wires have been brazed to one surface of the grid ring, a V- shaped groove is formed in the grid ring on its other surface along a diameter which is perpendicular to the grid wides so that the grid ring will extend in the direction of the grid wires to tension the grid wires. In another prior art device, prior to the brazing of the grid wires, local corrugations are formed radially or concentrically and after the brazing of the grid wires have been completed, the corrugations are stretched by mechanical means to tension the grid wires. In another prior art device after the grid wires have been brazed to the horizontal surface of a grid ring whose cross section is in an L shape, the grid ring is deformed by mechanical means so that the cross section is provided with a V shape to thereby tension the grid wires. In all these prior art planar grid electrodes, it is true that the required tension is given to the grid wires to prevent their deformation. However, deformation of the grid ring by mechanical means after the brazing of the grid wires, not only substantiall complicates the manufacturing process but also frequently deprives the grid ring of evenness thereby making the grid surface uneven. Moreover, the partial and severe deformation renders it next to impossible to use tungsten as the material for the grid ring, because tungsten is brittle and is not readily wroughtable. Unfortunately, tungsten is the most suitable material for grid rings and in fact is indispensible when the grid ring is to be used in a highpower tube where it is requred to tightly extend relatively thick grid wires across a large ring.

An object of this invention is to provide an electron tube having an improved disc-sealed electrode.

Another object of this invention is to provide a planar grid electrode where the desired and required tension is readily given to the grid wires and is maintained almost constant without the need during manufacture, to mechanically deform the grid ring after the grid wires have been brazed.

With this invention, it is possible to provide and maintain desired tension to the grid wires by using a metal having a coefficient of thermal expansion (to be explained hereinafter) for the support of the grid ring so that the difference in the thermal expansion between the support and the grid ring (originating with the brazing) may positively be utilized to cause relative deformation between them automatically without any external mechanical force.

The above-mentioned and other features and objects of this invention and the means for attaining them will become more apparent and the invention itself will be best understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings in which:

FIG. 1 is an enlarged top plan view of an embodiment of this invention;

FIG. 2 is a further enlarged axial sectional view of the embodiment, taken along line 24' of FIG. 1 and seen in the sense shown by arrows, with the central portion omitted;

FIGS. 3, 4 and 5 are similarly enlarged axial sectional views of the planar grid electrode of this invention shown in FIGS. 1 and 2 and are provided to illustrate a process for manufacturing this device; and

FIG. 6 is a side view of a disc-sealed tube, partly shown in axial section, with a portion of the envelope removed,

3 for illustrating one possible practical use of the planar grid electrode of FIGS. 1 and 2.

Referring to FIGS. 1 and 2 wherein the same parts are designated by the same reference numerals, a planar grid electrode 10 of this invention includes an apertureddisc-shaped grid ring 13 made of tungsten or similar material and a plurality of substantially parallel very fine tungsten grid wires 11 which are brazed by means of brazing material 12, such as pure gold, to the peripheral portion of one of the surfaces of the grid ring 13. A gridring support 17 is provided which has an apertured-discshaped flange portion 15, to which the other surface of the grid ring 13 is brazed by means of brazing material 14, such as gold or silver solder which has a lower melting point than the previously-mentioned brazing material 12. A cylinder portion 16 is provided which is extended as shown in FIG. 2 from the periphery of the flange portion 15 in a direction perpendicular to the grid plane incluing the grid wires 11. Cylinder portion 16 has substantially the same or a somewhat larger diameter than the whole diameter of the grid ring 13. Portion 16 may be made of Invar, molybdenum, or other suitable metal which has a considerably larger coetficient of thermal expansion at temperatures around the melting point of the brazing material 14 and preferably has substantially the same coeflicient of thermal expansion at the operation temperature of the planar grid electrode 10, when compared with the grid ring 13. With the planar grid electrode 10, the difference in the thermal expansion caused by brazing, and in the thermal contraction after the brazing between the grid ring 13 and the flange portion 15, deforms the grid ring 13 and the flange portion 15 in such a manner that the inner peripheral edges of both will be displaced by a larger amount towards the grid ring 13 than the outer peripheries thereof and that the circurnference 18 of the connection between the flange portion 15 and the cylinder portion 16 will be protruded outwardly. In FIG. 2, for clarity the deformation of these portions is illustrated on an exaggerated scale.

Referring to FIGS. 3 through 5 inclusive, wherein the same reference numerals indicate the same portions as in FIGS. 1 and 2, the processes of manufacturing the planar grid electrode of FIGS. 1 and 2, will be explained in conjunction with a practical example with a view toward providing a complete understanding of this invention.

Referring to FIG. 3, homogeneous and gold-plated tungsten grid wires 11 having a uniform diameter of 0.1 mm. are stretched with some tension provided thereto in accordance with known manufacturing methods for conventional planar grid electrodes, so that the wires are parallel with one another and so that they do not sag. About thirty wires are stretched in this manner per centimeter, on one of the surfaces of a grid ring 13 which is a flat tungsten disc which has a diameter of 29 mm. and a uniform thickness of 0.5 mm. Moreover ring 13 is provided with a concentric round aperture of 22 mm. in diameter by a press. The wires 11 are then brazed at both end portions by pure gold whose well-known melting point is 1063 C. As illustrated in FIG. 4, the grid ring 13 having the grid wires 11 brazed as indicated in FIG. 3 is then placed with the surface on which the grid wires 11 are stretched, facing upward on a grid-ring support 17 which may be formed of Invar. The flange portion 15 of support 17 has outer diameter of 29 mm., an inner diameter of 22.4 mm., and a thickness of 0.5 mm. The cylinder portion 16 of support 17 has an outer diameter of 29 mm. and thickness of 0.5 mm. Ring 13 is then brazed over the contact over the entire area between the lower face of the grid ring 13 and the upper face of the flange portion 15 by gold solder 14 having a melting point of about 890 C. Inasmuch as the coefficients of thermal expansion of tungsten and Invar at the temperature of brazing are about 5x 10'/ C. and more than 15 x 10-/ C. respectively, the thermal expansion of the grid-ring support 17 is greater than that of the grid ring 13 during the brazing. As shown in FIG. 5, this causes the outer diameter of the grid-ring support 17 to become somewhat greater than that of the grid ring 13 just after the brazing. With a decrease in the temperature of both, the outer contour of the grid-ring support 17 is subjected to a greater force of contraction than that of the grid ring 13. As a result, the grid ring 13 is subjected to axially upwardly directed forces, which result because the grid-ring support 17 and the grid ring 13 have already been rigidly fastened to each other by the gold solder 14. This force causes the previously-mentioned deformation (illustrated in FIG. 2) of the grid ring 13 and the support 17 which increases the distance between the outer periphery of the grid ring 13 (as measured in the direction of the diameters thereof and along the outer surface of the grid ring 13) and increases the tension applied to the grid Wires 11.

The planar grid electrode 10 manufactured as indicated hereinabove, may be sealed as shown in FIG. 6, within the envelope 30 of a disc-sealed tube 20 between a planar anode 21 and a planar cathode 22. The surfaces of the anode and cathode are in parallel, and the plane of the grid in turn is positioned to be parallel to the surfaces of the anode and the cathode. Tube 20 may be energized by attaching clip leads to the disk electrodes thereof. The giglfi6disk 31 is shown generally in a cut away form in Thus, with this invention it is now possible not only to maintain the tension applied to the grid wires 11 to prevent sagging (by suitable choice of the materials and dimensions of the grid ring and the grid-ring support) but also to accomplish such tensioning without any outside mechanical process applied to the grid electrode. AS a result the grid electrode of this invention can be manufactured with fewer steps than conventional electrodes. Also, the grid ring 13 is so Subjected to deformation that its annular portion is uniformly bent or tapered into the form of a conical surface. As a result, the evenness of the grid plane determined by the grid wires 11 or the plane defined by the inner edge of the grid ring 13 is not at all disturbed. Furthermore, the deformation is limited by the choice of materials such that the grid ring 13 made of tungsten can not be damaged. Also, the deformation to the grid ring 13 and the grid-ring support 17 does not adversely affect the electric characteristics of the grid electrode 10 because, as seen from FIG. 6, the current flowing from the cathode 22 through the grid electrode 10 t0 the anode 21 does not pass through the deformed portion.

This invention is not only applicable to a large-sized grid electrode for high-power service but also electrodes for low-power service. By suitably selecting materials for the grid ring 13 and the grid-ring support 17 the electrode of this invention can be made to meet the intended service.

In the drawings, the grid wires 11 are shown to be arranged in parallel relation to one another on the grid ring 13 and the brazing between the grid wires 11 and the grid ring 13 is illustrated as being carried out for each grid wire at a portion of the grid ring surface near the outer periphery thereof. It should, however, be understood that the grid wires 11 may be arranged in a mesh-like form or any other form on the grid ring 13. Moreover, the brazing between the grid wires 11 and the grid ring 13 can be performed along the whole contact area therebetween (although the brazing shown in the drawings is preferred). It should be understood that the brazing may be accomplished by preliminary placing the brazing material 12 on the entire surface of the grid ring 13 and by arranging the grid wires 11 thereon for brazing. Also, it should be noted that although the drawings show a gap in the thickness of the brazing material 12 between the grid wires 11 and the grid ring 13, no gap need be formed by the brazing material 12, when relief is provided by scraping off those surface portions of the grid ring which are not subjected to brazing. Also, it is preferred to bevel the edge of the grid ring 13 where the inner short cylindrical surface thereof meets that surface thereof on which the grid wires 11 are brazed. This serves to prevent the grid wires from being damaged by the unbevelled sharp edge. The grid ring 13 can be made either of the same material as the grid wires (as heretofore explained in conjunction with the embodiment) or it can be formed of a metal having a smaller coeflicient of thermal expansion than the grid wires. In the illustrated embodiment, brazing operations were completed first between the grid wires 11 and the grid ring 13 and then between the grid ring 13 and the grid-ring support 17. This may, however, be accomplished simultaneously by using the same brazing material. Insofar as the flange portion 15 of the grid-ring support 17 is concerned, its width which is illustrated in the embodiment to be somewhat narrower than the width of the annular portion of the grid ring 13 (in order to achieve better results), may be greater than the width of the grid ring 13. Also, the coefficient of thermal expansion of the material of the grid-ring support 17, which is preferably nearly equal to the coeflicient of thermal expansion of the material of the grid ring 13 at the working temperatures, need not necessarily be so, pro vided the difference between these coefficients is smaller at the working temperatures than the difference at the melting point of the brazing material. Furthermore, the effective area of the grid, which is illustrated in the embodiment to be circular (the most common form) may be made to be elliptical, square, or of any other form. This of course will require a change in the cross sections of the accompanying components.

It should be understood that the embodiment described hereinabove does not in any way restrict the technical scope of this invention but that the material, the dimensions, and the shape of the individual component const1- tuting the planar grid electrode of this invention can be varied fairly widely.

While I have described above the principles of my 1nvention in connection with specific embodiments, it is to be clearly understood that this description is made only by way of example, and not as a limitatlon to the scope of my invention as set forth in the ObJBClZS thereof and 111 the accompanying claims.

What is claimed is:

1. An electron tube comprlslng:

(A) a sealed envelope;

(B) an anode electrode and a cathode electrode positioned in spaced relation within said envelope;

(C) a planar grid electrode spaced between said anode and cathode, said planar grid electrode including:

(1) a tubular support member having a first thermal coeflicient of expansion,

(2) a planar grid wire holding member having a central aperture therethrough, one planar surface of said holding member being brazed to one end of said support member such that at least a portion of said central aperture is in registration with the hollow defined by said tubular member, said planar grid wire holdmg member having a second thermal coefficient of expansion,

(3) a plurality of grid wires fixed to the other planar surface of said holding member such that a grid plane is defined over said central aperture and parallel with said other surface,

(4) said holding and support members being formed of materials having predetermined different coefficients of thermal expansion in the vicinity of the temperature at which said members are brazed to each other with the first thermal coeflicient of said support member being greater than said second thermal coefficient of said holding member, to normally place the entire periphery of said holding member in radially outward tension;

(D) and connecting means connected to each of said anode, cathode and grid electrodes for supplying energy to said electrodes.

2. An electron tube as set forth in claim 1 wherein said support member includes a flange portion extending over one end thereof, which flange has an aperture therethrough and wherein said holding member is brazed to said flange portion such that the aperture in said holding member is in at least partial registration with the aperture in said flange.

3. An electron tube as set forth in claim 2 wherein the grid wires are brazed to the holding member.

4. An electron tube as set forth in claim 3 wherein the anode, cathode and grid electrodes are all positioned within said envelope in parallel relation with each other.

5. A planar grid electrode for use in an electron tube having a sealed envelope housing an anode, a cathode, said planar grid electrode being positioned between but spaced from said anode and cathode electrodes, said 0 planar grid electrode comprising:

a tubular support member, having a first thermal coeflicient of expansion a planar grid wire holding member having a central aperture therethrough, and having a second thermal coeflicient of expansion one planar surface of said holding member being brazed to one end of said tubular support member such that at least a portion of said central aperture is in registration with said hollow defined by said tubular support member; a plurality of grid wires fixed to the other planar surface of said holding member such that a grid plane is defined over said central aperture and parallel with said other surface; said holding and support member being formed of materials having predetermined different coeflicients of thermal expansion in the vicinity of the temperature at which said members are brazed to each other with the first thermal coeflicient of said support member being greater than said second thermal coeflicient of said holding member, to normally place the entire periphery of said holding member in radially outward tension.

6. A grid electrode for an electron tube having a sealed envelope housing enclosing an anode, a cathode and a grid electrode spaced therebetween comprising:

a tubular support member substantially concentrically mounted with said cathode and provided at one end thereof between the cathode and the anode with a radially inward extending annular flange, with said flange having a first thermal coefiicient of expansion,

an annular grid wire holding member substantially concentrically and circumferentially bonded at one axial surface to said flange, said holding member having a second thermal coeflicient of expansion,

a plurality of grid wires fixed to the other axial surface of said annular holding member, to form a planar surface of grid wires between said cathode and anode,

said flange and holding member being formed of materials having different thermal coefiicients of expansion, with said first coeflicient being greater than said second coefficient with both coefiicients being selected to normally place the entire periphery of said grid wire holding member in radially outward tension.

7. A planar grid electrode as set forth in claim 4 wherein said tubular support member has a flange portion having an aperture therethrough, extending over one end thereof and wherein said holding member is brazed to said support member such that said central aperture is at least in partial registration with said aperture in said flange.

8. A planar grid electrode as set forth in claim 5 wherein said grid wires are brazed to said holding member.

9. The device as recited in claim 6 wherein the grid wire holding member is made of tungsten and wherein said support member is made of a material selected from the group consisting of Invar and molybdenum.

(References on following page) 7 8 References Cited 3,297,902 1/1967 Beggs 313-348 BI'OWH Ct a1 8/ 1948 Eitel 313348 X JOHN W. HUCKERT, Primary Examiner. 1/1950 laclfson 313 348 X A. J. JAMES, Assistant Examiner. 7/1962 Wc1ssfloch 313---348 12/1962 Katz 313 348 US. Cl. X.R.

7/ 1965 Lewis 313-348 313 349, 350, 269, 283; 29 2s.13