EP0481052B1 - Grid amplification tube with varying width bars - Google Patents

Abstract

The invention relates to electron power tubes with grid. For high powers and high energies, very high, localized heating of the grid takes place due to the high currents. It is the case for example for the screen grid (grid G2) of meshed grid tetrodes with vertical or oblique bars (20, 22) of pyrolytic graphite. In order to improve the operational reliability and the life of the tube, the invention proposes to use grid bars having a larger width at the bottom of the grid, where the high frequency currents are at their highest.

Description

The invention relates to power amplification tubes such as for example tetrodes.

The more the tubes are capable of supplying a high power, the more the energy losses in the tube must be taken into account and eliminated so as not to risk causing the deterioration or destruction of the tube by abnormal heating.

In tubes operating at high frequency, there are in particular losses due to the circulation of high frequency currents in the grids located between the cathode and the anode of the tube. In particular, in the case of a tetrode, the screen grid, most often called grid G2, is traversed by high frequency currents flowing vertically between the top and the bottom of the grid. These currents come from the fact that the grid is placed in a resonant circuit output from the tube, and that in all high frequency resonant circuits steady wave regimes are established with nodes and bellies of current and voltage. The highest microwave currents are of course produced at the current bellies.

The grid is then subjected to a very significant heating. We do not know precisely how to measure this heating (inside a closed vacuum tube), but we have noticed the appearance of reverse grid currents during very high power and high frequency operations. In other words, while the normal gate current is a current consumption in one direction, it is found that an increase in the operating power of the tube leads to the reversal of the direction of current flow in the connection of wire rack. It has been observed, for example, that when starting a tetrode, the gate current very quickly passes from a positive normal value of a few amperes to a negative value of a few amperes (in a few seconds).

This inversion of the grid current suggests that the grid begins to emit electrons in large quantities (when it should not). This emission of electrons is probably caused by the increase in temperature of the grid. Indeed, the material used for the grid is most often pyrolytic graphite which has a relatively low emissivity at the normal operating temperature of the tube. It is therefore likely that it is a very significant abnormal heating of the grid which gives it a high thermoemissive power. The amount of current that can be measured suggests that the grid reaches temperatures of the order of 2000 ° C. Only such temperatures can indeed explain the appearance of such a high reverse grid current.

This high temperature of the grid can be the cause of tube malfunctions: the grid radiates a very large amount of heat towards the colder parts of the tube and causes abnormal degassing thereof. The ions released in the tube are then sources of electrical breakdowns, disjunctions, etc. Grid insulation ceramics can deteriorate (cracks) under the action of heat. In any case, this results in a reduction in the reliability and the service life of the tubes.

The object of the invention is to reduce the risks of malfunction which appear to be due to an abnormal increase in the temperature of the grid, in tubes whose grid is placed in a high frequency resonant circuit and is traversed by high frequency currents generated. by this resonance.

According to the invention, it is proposed to use a grid having bars whose width is variable and is larger at the places where the high frequency currents which flow under the effect of the resonance are the highest. Preferably, the width varies along a bar between one side where the bar is subjected to weaker currents and another side where the bar is subjected to higher currents. In particular, in a certain number of cases, it will be provided that the width of the bars is greater at the bottom of the grid (that is to say on the side of the connection towards the outside of the tube) than at the top .

In practice, it is in the case of cylindrical grid tubes that the invention turns out to be particularly advantageous. The top of the grid is most often placed with a current node and a high frequency voltage belly, but the bottom is much closer to a current belly.

In one embodiment, the bars have a regularly increasing width as one approaches the bottom of the grid. Growth can be continuous or discontinuous.

The invention is applicable to grids with vertical bars or grids with oblique bars. Grids with oblique bars are often built to improve the mechanical strength of these grids.

In practice, the application of the invention is intended especially for grids produced by machining or cutting, such as grids of pyrolytic graphite machined by sandblasting or molybdenum grids cut by laser by electroerosion or by stamping.

• la figure 1 représente un exemple de grille classique de tétrode de puissance;
• la figure 2 représente un autre exemple de grille classique, à barreaux obliques;
• la figure 3 représente une réalisation de grille selon l'invention à barreaux verticaux;
• la figure 4 représente une autre réalisation pour une grille à barreaux obliques.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the appended drawings in which:

• FIG. 1 represents an example of a conventional power tetrode grid;
• FIG. 2 represents another example of a conventional grid, with oblique bars;
• FIG. 3 represents an embodiment of a grid according to the invention with vertical bars;
• FIG. 4 shows another embodiment for a grid with oblique bars.

The invention will be described in detail with regard to a triode grid or high power tetrode having a cylindrical cathode, a grid (triode) or two cylindrical grids (tetrode) of mesh structure, surrounding the cathode, and an anode surrounding the grids. But of course, the invention is applicable to other tube structures where the same problems are encountered (anode surrounded by the grids and the cathode for example).

The grid, whether it is a modulation grid (G1) or a screen grid (G2), is very often made up (and it is this case which interests us especially here) of a sheet of refractory material in the shape of a machined cylinder. in a mesh structure.

The function of these grids is to establish a determined potential distribution in the vicinity of the cathode while letting pass the major part of the flow of electrons emitted by the cathode towards the anode. The bars of the mesh structure are close enough to each other to allow the establishment of potentials as well distributed as possible, and yet they are sufficiently separated from each other by the free space of the meshes to let pass such a large proportion. as possible electrons.

In conventional grid structures, this compromise is reached with networks of bars forming regular meshes. The bars are either vertical bars (to allow an optimal evacuation of the high frequency currents because these propagate from top to bottom taking into account the distribution of the high frequency potentials along the height of the cylindrical grid), or bars crossed obliques (to improve the mechanical strength of the structure). The bars are very thin compared to the intervals between bars. The vertical direction conventionally chosen here is the axis of the cylinder constituting the grid.

FIG. 1 represents a conventional grid of high power amplification tube. This is a pyrolytic graphite grid, but it could also be a metal. The grid 10 essentially consists of a network of vertical bars 12 extending between the top 14 of the grid and the bottom 16. The grid is electrically connected to the outside of the tube by a contact made at the bottom of the grid and not shown.

In the following, when we talk about the bottom of the grid we will refer to the side of the connection.

From place to place, horizontal circular bars 18 make it possible to mechanically connect the vertical bars to one another in order to increase the rigidity of the structure.

The horizontal bars do not or hardly participate in the evacuation of the currents in the grid. Very few high frequency currents develop in the horizontal bars. On the contrary, the vertical bars are the seat of high frequency currents which, in this type of structure, are all the higher the closer you get to the bottom of the grid.

The reason is that the grid is most often placed in a high frequency resonant circuit in which the top of the grid is at a belly of current and a node of tension, while the bottom of the grid approaches a current belly.

FIG. 2 represents another conventional grid structure in pyrolytic graphite. In this structure, the bars are oblique and there are two networks of crossed oblique bars 20 and 22. The whole forms a diamond mesh network. Although it has not been shown in FIG. 2, there could also be horizontal circular bars to increase the rigidity; for example, there may be horizontal bars connecting the crossing points of the two oblique networks, so as to transform the diamond mesh into a triangular mesh (each diamond divided into two triangles). These horizontal bars would again be provided to increase the rigidity of the structure.

Both in the case of Figure 1 and in the case of Figure 2, the bars have constant widths from top to bottom of the grid.

According to the invention, it is proposed to give the vertical or oblique bars variable widths according to the distribution of high frequency current densities in the grid for the desired operation of the tube (that is to say in particular for a frequency and a desired power). Where the current density tends to be higher, wider bars will be used.

The increase in the width of the bar makes it possible to increase the cross section through which the currents flow, therefore to reduce the power dissipated by the Joule effect.

In addition, this increase in width makes it possible to increase the radiating surface of the bar. For the same power dissipated by the Joule effect and evacuated mainly by radiation (very low evacuation by thermal conduction in these cylindrical grid structures and even lower by convection since it is in a vacuum), the temperature of the bar will be reduced.

The currents at different points on the grid can be calculated from Maxwell's equations; currents and potentials indeed follow well-known physical and mathematical laws; we can therefore determine which are the places (for a given operation) where the current density will be the highest, and we give the bars a wider section in these places.

In practice, the current density in the bars of the grid is often very high at the bottom of the grid, on the connection side, for a cylindrical grid conventionally having a connection on only one side of the cylinder.

In a particular preferred embodiment, the width of the bars increases from the top of the grid downwards, at least in the lower part of the grid.

We then obtain bars of non-uniform width as we extend their length, the bars being wider where the currents which are likely to the most important, and where the temperature of the grill is therefore most likely to rise too much.

In practice, in the cases which will be the most conventional, we will therefore choose to give the vertical bars an increasing width at the bottom of the grid. The vertical bars are indeed the most concerned by the problems of circulation of high frequency currents and it is at the bottom of the grid that the risks of abnormal heating are the highest.

For example, the vertical bars have a continuously variable width from top to bottom of the grid. But the variation can also be in stages.

Or again, the vertical bars have a constant width over part of the height of the grid, then, downwards, the width increases regularly or in stages.

For oblique bars, the solutions are the same: continuous growth or in stages, from the top of the grid or only in the lower part of the grid.

The horizontal bars themselves may be wider at the bottom of the grid than at the top, if only for convenience of manufacture.

FIG. 3 illustrates an example of the constitution of a grid, for vertical bars: the width (L1, L2) of the bars goes downwards. This figure represents a detail of the grid; the proportions are not respected, for reasons of convenience of representation, so that the increase in width of the bars can be clearly seen; in practice, in fact, the bars can be very thin compared to the interval between consecutive bars; on the other hand the interval between horizontal bars can be much wider than the interval between vertical bars.

• d'une part, la largeur des barreaux peut rester faible devant l'ouverture même là où les barreaux sont les plus larges;
• d'autre part, le plus souvent on aura des barreaux plus larges justement dans les régions (bas de la grille) où la densité de courant électronique issue de la cathode et récupérée par la grille G2 est la plus faible.

The width of the openings between bars can be constant or not: the simplest is to use a mesh of constant pitch, which implies that the openings are reduced as the bars widen. The transparency of the electron grid therefore decreases where the bars are wider, but this is acceptable for two reasons:

• on the one hand, the width of the bars can remain small in front of the opening even where the bars are the widest;
• on the other hand, most often there will be wider bars precisely in the regions (bottom of the grid) where the density of electronic current coming from the cathode and recovered by the grid G2 is the lowest.

The invention is applicable in the same way to grids whose bars are not vertical but oblique, such as for example a grid such as that of FIG. 2 comprising a series of oblique bars all parallel crossed with another series of oblique bars all parallel.

An exemplary embodiment of the invention with a grid of oblique bars reinforced by horizontal bars (triangular mesh) is shown in Figure 4. We see that the two networks of oblique bars 20 and 22 have widths increasing from top to low. The horizontal bars 24 also have increasing widths from top to bottom, but only for reasons of manufacturing convenience; they could all have the same width because the heating due to the flow of current in these horizontal bars is low.

The grids can be made of pyrolytic graphite; they are then generally cut by sandblasting by means of sand projection nozzles.

It is also possible to provide that the grids are made of metal (preferably molybdenum). They are then produced by laser cutting or by mechanical cutting or by electroerosion.

Claims (8)

1. High-frequency electronic amplifier valve, comprising a grid placed in a resonant output circuit of the valve, characterised in that the grid has rods whose width is variable and greater at the places where the high-frequency currents set up by the resonance tend to be highest.
2. High-frequency electronic amplifier valve, according to Claim 1, characterised in that the grid has elongated rods whose width varies along the rod.
3. Electronic valve according to one of Claims 1 and 2, characterised in that the grid is in the shape of a vertical cylinder and the rods whose width is variable are rods which are vertical or oblique with respect to the vertical.
4. Electronic valve according to Claim 3, characterised in that the rods have a greater width towards the bottom of the grid, on the side of an electrical grid connection, than towards the top.
5. Electronic valve according to Claim 4, characterised in that the rods have a regular growth in width as they approach the bottom of the grid.
6. Electronic valve according to Claim 4, characterised in that the valves have a continuous growth in width.
7. Electronic valve according to Claim 4, characterised in that the rods have a constant width over one part of the height of the grid and a variable width in the bottom of the grid.
8. Electronic valve according to one of the preceding claims, characterised in that the grid is of pyrolytic graphite or of metal.

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