EP0456550B1 - Electron tube with cylindrical grid - Google Patents

Description

The present invention relates to cylindrical concentric power electrode tubes. These tubes are, for example, triodes or tetrodes.

A triode tube mainly comprises a central cylindrical cathode emitting electrons when it has reached a sufficient temperature, a control grid around the cathode, an anode surrounding the control grid. The electrons emitted by the cathode pass through the grid and reach the anode, if the potential of the grid and the anode have appropriate values. The tetrodes have an additional grid, called a screen grid inserted between the control grid and the anode.

The cathode is often made from two layers of emissive metallic wires which are crossed in order to obtain a mesh. The assembly thus produced has a cylindrical structure. Each end of the cylinder is fixed on a support. These cathodes are called cages.

The grids are also meshed. They can be made from sheets of son of a refractory material which is crossed in order to obtain a mesh. The wires are welded together at each intersection. The assembly thus formed has a cylindrical structure and its ends are connected to supports.

A second way of making a grid is to take a sheet of refractory material, in the shape of a cylinder, and to pierce it with regularly spaced openings in order to obtain the mesh.

As refractory material, pyrolitic graphite or molybdenum is commonly used. Each mesh is defined by a succession of bars connected by their ends and the intersection between two bars is a knot.

The cathode and the grids, due to this very cut structure, are subject to vibrations affecting their mechanical stability. The distance between the cathode and the control grid is small, generally less than 1000 micrometers, and the vibrations which may occur cause appreciable variations in this distance. These vibrations are detrimental to the proper functioning of the tube. The same remarks apply to intergrid distances in the case of tetrodes or other multigrid tubes.

The importance of mechanical stability will be measured, adding that the cathode can operate at a high operating temperature (of the order of 1700 ° C.) and that it must also have good resistance to deformation. The grids will reach a lower temperature (around 1200 ° C) but must also have good resistance to deformation

Another condition to integrate, in order to obtain a good functioning of the tube, is the transparency of the grids. The bars and the mesh nodes form a barrier for the electrons coming from the cathode. The interception of a large number of electrons by a grid, causes a high grid current, especially in high power tubes. This grid current causes additional heating of the grid and requires the use of a relatively powerful grid supply. The transparency of the grid depends on its geometry.

To make the grid, it is also necessary to take into account the distribution of the grid potential between the bars. The potential must be distributed as regularly as possible. This is important for the control grid which is used to regulate the potential around the cathode. This last condition also depends on the geometry of the grid.

An ideal grid in terms of potential and transparency would have an infinity of vertical and very fine wires. The gate current would be very low and the gate potential distributed very regularly around the cathode.

On the other hand, this grid would have extremely poor mechanical strength, especially if it had large dimensions.

We therefore had to cross the wires to increase the rigidity of the grid.

The grids used frequently have meshes in the form of quadrilaterals: square, rectangle, rhombus or parallelogram. From a knot of mesh leave four bars. In patent DE-C-868 320 the grid is produced by stretching a strip provided with slots arranged in staggered rows. The finished grid has parallelogram type meshes.

In high-power tubes, a grid of this type is deformed and we have been forced to strengthen it by adding bars, we have made triangular meshes. This is described in application FR-A-2 276 681. From each node, six bars now leave. The surface of the nodes is larger and the current also grows.

Hexagonal mesh grids are known in linear beam tubes. In patent FR 2 358 011, the grid is flat or in a cap. In US patent 4,684,994, it is a grid for camera tube. In US Patent 4,767,964, it is a grid for cathode ray tube.

The present invention aims to remedy these drawbacks and proposes a grid tube operating with a reduced grid current. For this, we sought to minimize the interception surface of the electrons, without harming either the mechanical stability or the distribution of the gate potential around the cathode.

The present invention relates to an electronic tube with concentric cylindrical electrodes including a central cathode and at least one grid of the mesh type, a mesh being defined by several bars in contact at their ends. The cathode has two groups of substantially parallel wires, the two groups being crossed. The grid meshes are hexagonal and the intersection between two cathode wire is aligned with the central part of a grid mesh.

Preferably, the meshes are substantially identical. Preferably, the hexagons are substantially regular. Preferably, when a grid rod covers a cathode wire, the rod and the wire are perpendicular to minimize the overlap area.

Preferably, the meshes are produced from a sheet of refractory material in the form of a cylinder pierced with hexagonal orifices.

The material can be pyrolitic graphite or molybdenum.

• les figures 1a et 1b, un maillage respectivement en parallélogramme et en losange d'une grille d'un tube électronique selon l'art antérieur ;
• la figure 2, un maillage triangulaire d'une grille d'un tube électronique selon l'art antérieur ;
• la figure 3, un maillage hexagonal régulier d'une grille d'un tube électronique selon l'art antérieur;
• la figure 4, un maillage hexagonal irrégulier d'une grille d'un tube électronique selon l'art antérieur;
• la figure 5, une grille de tube électronique selon l'art antérieur;
• la figure 6, la superposition d'un maillage de cathode et d'un maillage de grille d'un tube électronique selon l'invention.

Other characteristics and advantages of the invention will appear on reading the following description, illustrated by the appended figures which represent:

• FIGS. 1a and 1b, a mesh in parallelogram and in rhombus respectively of a grid of an electronic tube according to the prior art;
• Figure 2, a triangular mesh of a grid of an electronic tube according to the prior art;
• Figure 3, a regular hexagonal mesh of a grid of an electronic tube according to the prior art;
• FIG. 4, an irregular hexagonal mesh of a grid of an electronic tube according to the prior art;
• Figure 5, an electron tube grid according to the prior art;
• Figure 6, the superposition of a cathode mesh and a grid mesh of an electronic tube according to the invention.

FIG. 1a represents a mesh in parallelogram of an electron tube grid, of the triode type for example. Figure 1b shows it, a diamond mesh.

Each of these meshes can be produced from two substantially parallel layers of wires 1,2, which are superimposed by crossing them. The wires 1 of a ply are then welded to the wires 2 of the other ply, at all the crossing points. We obtain meshes 4 delimited by portions of wires 1, 2 or bars 5. Each crossing forms a knot 3. From each node 3 leave four bars 5. A mesh 4 consists of four bars 5.

In FIG. 1a, a mesh 4 has the shape of a parallelogram, it consists of four equal bars, two by two.

In Figure 1b, a mesh 4 has the shape of a rhombus, it consists of four equal bars 5.

The wires 1,2 used to make these grids are made of refractory metal, for example molybdenum.

A grid of this type can also be made from a sheet of refractory material of graphite or molybdenum for example. The sheet is pierced with openings by any known means, machining, sandblasting or EDM, for example. The openings are preferably regularly spaced and have an appropriate shape to obtain the mesh.

An electron tube grid, for example a triode grid, is cylindrical and it is mounted around a cathode emitting electrons. The electrons pass through the grid when the latter is brought to a negative potential compared to that of the cathode. The bars 5 and the nodes 3 form an electron screen. Certain electrons are intercepted by the structure of the grid when it is brought to a positive potential compared to that of the cathode. The intercepted electrons cause the appearance of a gate current. A high grid current causes an excessive increase in the temperature of the grid and requires the use of a powerful grid supply.

In a medium power tube, it is possible to use a mesh grid as shown in FIGS. 1a, 1b. The gate current which is established due to the interception of electrons is acceptable.

But when we want to make a high power tube, the grid then has larger dimensions and we see that it lacks rigidity.

We were led to strengthen it by giving it a structure as shown in Figure 2. We made a triangular mesh. Two plies of crossed wires 22,23 were used as above and a third ply of wires 21 substantially horizontal was added at each intersection or node 25. We made 24 stitches, triangular, defined by three portions of wires 21,22,23 or bars 26, the ends of which are in contact. From each node 25 there are six bars 26.

With such a mesh, we have gained mechanical stability and resistance to deformation. But on the other hand, we also increased the gate current, because we increased the interception surface of the electrons, in particular at the level of the nodes 25.

FIG. 3 represents a regular hexagonal mesh of an electron tube grid according to the prior art. This mesh has nodes 36. From each node 36, only three bars 35 leave.

Now each mesh 34 is hexagonal it is defined by six bars 35 connected by their ends.

The surface of the nodes 36 has been reduced compared to the meshes conventionally used. The electrons intercepted by a grid of this type will be less numerous and an electronic power tube having a grid of this type will have a reduced grid current compared to the grid current of a conventional power tube.

The mesh shown in Figure 3 is regular, each mesh 34 is made up of equal bars 35 and two successive bars 35 make an angle of 120 °. All the meshes are substantially identical.

One could envisage that the meshes are not all identical and that the hexagons are irregular. Such a mesh is shown in FIG. 4. Large meshes 41 have been shown in line and smaller meshes 42 also aligned. Each mesh 41 or 42 is an irregular hexagon. The angles between two successive bars can be larger or smaller than 120 °.

Figure 5 is a view of a mesh grid of electron tube according to the prior art. The grid has 50 regular hexagonal meshes. It has a honeycomb structure. It comprises a meshed part 51 of cylindrical shape. The two ends 52 of the cylinder are each held on a support 53.

In Figures 3,4,5, the hexagonal meshes shown have all the same orientation. This is only an example, their orientation can be arbitrary. The meshes could, in particular, have been turned 90 °.

Preferably, the grid will be produced from a sheet of refractory material, for example pyrolitic graphite, molybdenum, in the form of a cylinder. We cut holes. in this sheet by any known means, machining, sandblasting, EDM, for example. The orifices are distributed regularly over the entire sheet. They are given the shape of hexagons. We obtain a hexagonal mesh. Each end of the cylinder is fixed on a support.

With such a grid, one gains in mechanical stability and in resistance to deformation compared to grids with mesh in the shape of a quadrilateral.

The interception surface has been reduced, at the level of the nodes, if we compare it to that of the grids with triangular meshes and meshes in the form of a quadrilateral.

The regularity of the hexagons and their orientation are chosen according to the mechanical and electrical parameters that the grid must have.

The geometry of the bars, that is to say their length and their cross section, as well as the angle of intersection between two bars are chosen so as to ensure transparency to the electrons and a control of the potential around the cathode, corresponding to the characteristics that the tube must have.

For a given section of bars and a given grid transparency, a regular hexagonal mesh allows smaller meshes than those usually used. This results in better control of the potentials between the bars and near the cathode (if the grid is a control grid), an improvement in the tube blocking voltage or "cutt off" as well as a better distribution of the electron trajectories.

For a given section of bars and the same control of the potentials between bars and near the cathode, (if it is a control grid), a regular hexagonal mesh allows larger meshes than those usually used. This results in greater transparency of the grid and a reduction in the grid current, in particular in high power operation.

Another advantage of regular hexagonal mesh grids appears when using a cage cathode. You can align the cathode and the grid. In multigrid tubes, the cathode will be aligned with the control grid and also with the other grids.

FIG. 6 represents a mesh 60 of a cage cathode covered with a mesh 70 of the control grid of an electronic tube according to the invention. The cathode mesh 60 consists of two groups of wires 61, 62 which are substantially parallel, the two groups being crossed. We made 63 diamond shaped meshes.

The wires 61,62 of the cathode emit electrons when they are heated. An intersection 64 between two wires 61, 62 has a large surface which emits a high density of electrons.

The grid mesh 70 comprises hexagonal and regular meshes 65 made up of bars 66.

We can arrange to align the intersection 64 between two cathode wires 61, 62, with the central part of a grid mesh 65. This arrangement increases the amount of electrons that pass through the grid.

In the case of multigrid tubes, all the grids will be aligned with one another and will be identical, so that the intersection 64 between two cathode wires 61, 62 is placed in the central part of all the grid meshes.

We can also try to minimize the areas of cathode wires 61, 62 covered by a grid bar 66. Arrangements are made so that the grid bars 66 which cover a cathode wire 61,62 are perpendicular to this cathode wire 61,62. Compared to conventional structures, for the same control of the potentials between bars and near the cathode, the transparency of the grid has been improved.

The invention applies both to control grids and to other grids (screen grid, stop grid ...)

This type of hexagonal mesh is particularly suitable for tubes in which the interelectrode distance is small because the mesh offers very good mechanical stability and very good resistance to deformation.

The hexagonal mesh makes it possible to minimize the grid current and to properly control the potential between the bars.

A grid with hexagonal meshes can advantageously be integrated into a tube of high gain and low attack power.

Claims (8)

1. Thermionic valve with concentric cylindrical electrodes, among them a central cathode and at least one mesh-type grid, a mesh (34) being defined by several bars (35) in contact via their extremities, characterized in that the cathode includes two groups of substantially parallel wires (61, 62), the two groups being crossed over, and in that the grid meshes (65) are hexagonal, the intersection between two cathode wires (61, 62) being aligned with the central part of a grid mesh (65).
2. Thermionic valve according to Claim 1, characterized in that the meshes (34) are substantially identical.
3. Thermionic valve according to one of Claims 1 and 2, characterized in that the hexagons are substantially regular.
4. Thermionic valve according to one of Claims 1 to 3, characterized in that the cathode wires (61, 62) form lozenges.
5. Thermionic valve according to Claim 4, characterized in that, when a grid bar (66) overlaps a cathode wire (61, 62), the grid bar (66) and the cathode wire (61, 62) are perpendicular so as to minimize the overlap surface area.
6. Thermionic valve according to one of Claims 1 to 5, characterized in that the meshes (34) are produced by apertures pierced in a cylinder-shaped sheet of a refractory material.
7. Thermionic valve according to Claim 6, characterized in that the material is pyrolytic graphite.
8. Thermionic valve according to Claim 6, characterized in that the material is molybdenum.

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