US2143916A - Electron discharge device - Google Patents

Description

Jan, 17, 1939. J. L. H. JONKER ET AL 2,143,916

ELECTRON DISCHARGE DEVICE Filed April 26, 1958 INVENTORS JOHAN L.H. JONKER g 1' g ADRIANUS J.W.M. VAN OVERBEEK W k ATTORNEY constant potential, usually positive.

Patented Jan. 17, 1939 UNITED STATES PATENT OFFICE ELECTRON DISCHARGE DEVICE Application April 26, 1938, Serial No. 204,298 In Germany May 13, 1937 '7 Claims.

This invention relates to electron discharge tubes and more particularly to tubes comprising an electrode system which is closed at the ends by end shields or screening plates which are conductive, at least at their surface.

In screen grid tubes and pentodes the conductive end shields or screening plates are usually connected to the cathode, the screen grid, or another grid-like electrode maintained at a It has been found that this arrangement has certain disadvantages due to undesirable deformation of the electric field set up in the electrode system, as the electric lines of force extending, for instance, from one grid to the anode, are distorted or deflected in the vicinity of the charged end shields or screen plates. As a result, the end or marginal electrostatic field is distorted and the field in the electrode assembly is non-homogeneous, as the lines of force are curved at the regions of distortion, so that the distribution of the electron stream is irregular. This irregular distri bution causes the anode to be struck by a nonhomogeneous electron stream and there is a corresponding irregular distribution of the degree of amplification along the anode with corresponding undesirable variations in tube characteristics. This non-homogeneous field and irregularly distributed electron stream with resulting variations in the characteristics is undesirable in the usual thermionic cathode amplifying tubes such as triodes, tetrodes, pentodes, hexodes, pentagrid tubes, and octodes in tubes operating with secondary electron emission, and particularly in beam tubes wherein the electron stream is either split up or is concentrated into bundles or rays, as the non-homogeneous field produces a very disturbing deflection or diffusion of the beam. The phenomena due to a distorted marginal field may also be caused by local surface charges on end shields or closure plates of insulation, such as mica, hence metal plates are generally used for closing and shielding the electrode assembly or system in beam or cathode ray tubes.

The principal object of the invention is to provide an electron discharge tube which is free from variations of the tube characteristics, and in which the intensity of the field and the degree of amplification is substantially the same through out the length of the electrode assembly.

According to the invention, the electrode system, as viewed in a direction normal to the main direction of the electron paths, is closed at the ends by shields or closure members which are conductive, at least at the surface, and consist of two or more parts which are at different potentials during operation of the tube. The two parts of each end shield are preferably metal sheets or plates, but may be made in other forms if desired. To render the distribution of the shield may, for instance, be made as a metal sheet having recesses in its edge or having holes or slits through which may pass the field of a parallel metal sheet which forms another part of the end shield. The cooperating parts of the end shield may also have interleaved strips, or lips. During operation of the tube at least two parts of the end shield or closure member should be at different potentials, one of which is to advantage constant, and the other variable. For example, the part of variable potential may be connected to a control grid, to the anode, or the like, and another part at constant potential may be connected to the cathode, to a space charge grid, or to the screen grid. Instead of connections to electrodes, the different parts of the end shields may obviously have connections taken out through the wall of the tube by lead-in conductors connected, for instance, to contacts in the base of the tube.

The invention will be more clearly understood by reference to the accompanying drawing, in which Figure 1 is a diagram illustrating the lines of force and the electron paths in a conventional tube, such as a pentode; Figure 2 a similar diagram of a tetrode embodying my invention; Figure 3 a longitudinal section of a tetrode embodying one form of my invention; Figure 4 an end view, partly broken away, of the tube shown in Figure 3; and Figure 5 a plan view of a modified form of a part of the tube of Figures 3 and 4.

Figure 1 shows diagrammatically a conventional pentode having a cathode l, in the present case an indirectly heated cathode, a control grid 2, a screen grid 3, and a grounded suppressor grid 4, all surrounded by a tubular anode 5. The electrode assembly is closed at the top and bottom by mica spacers, such as sheets 6 and l, which space and center the electrodes, and also by metal end shields 8 resting on the outer surfaces of the mica spacers and connected, for instance, to the suppressor grid 4, which is connected inside the tube to the cathode by a conductor 9. In the diagram the full lines l9 extending from the screen grid 3 to the-anode 5 represent the electrostatic lines of force which, as appears from the drawing are straight at the middle of the electrode assembly, going straight from the screen grid through the suppressor grid to the anode, but are distorted and bent near the metal end shields 8 at the ends of the assembly, so that the field in the electrode assembly is non-homogeneous, the marginal fields at the ends of the electrode assembly being distorted as indicated. As a result, the electron paths near the ends of the assembly are curved, as indicated in the diagram by the dotted line arrows II, the electrons being repelled by the end shields 8 and pushed toward the middle of the'electrode assembly, hence all the electron streams are not perpendicular to the grid i, and there is an irregular distribution of the degree of amplification across the grid i with consequent undesirable variations of the tube characteristic. The tube is shown connected in the usual way to a battery l2 through a load circuit l3 and an input circuit M.

Figure 2 is a similar diagram of a similarly constructed screen grid or tetrode tube which embodies one form of the invention and in which the electrostatic field in the electrode assembly is homogeneous and all the electron paths are straight. In this particular embodiment, each end shield consists of an outer part l5, which is connected to and varies in potential with the anode, and an inner part I6 parallel to and separated from the outer part by the mica spacers and maintained at a constant potential different from that of the outer part l5 by a connection to the screen grid 3. The outer part 55 is of imperforate sheet metal, while the inner part i8 is a sheet or plate mounted on the screen grid and having slots or perforations i i. In this construction the field of the outer part l5 extends through the inner part it, so that the resultant field produced by the two parts 55 and It in the space between the cathode and anode is such that the field between the cathode and the anode is completely homogeneous throughout the length. of the electrode assembly.

Figure 3 is a longitudinal section of a screen grid tube or tetrode constructed as shown diagrammatically in Figure 2, and in which the end shields consist of outer parts !5 in the form of annular discs or washers of sheet metal, and the inner parts iii are annular sheet metal discs with semi-circular concentric slots or perforations ii, as best shown in Figure 4. The annular inner part or member i6 is mounted so as to be near and parallel to the outer member IS, with its inner edge on the screen grid 3 and its outer edge near the anode 5. The width of the slots l 'i preferably increases with distance from the cathode, and these slots are so proportioned, and are of such width and spacing, that the intensity of the resultant field produced by the combined fields of the outer part l5 and inner part it; gradually changes or shades from substantially the field of the inner part it at the screen grid to substantially the field of the outer part 55 at the anode. This field shaping or shading efiect of the two parts l5 and ii; at different potentials eliminates the distortion of the marginal field produced by a one part end shield, and the result is a homogeneous field inside the electrode assembly and along the anode and the screen grid.

Figure 5 is a plan view of a modification of the inner part or member of the end shield in which the inner part is in the form of a stellate disc l8, having in its edge recesses [9 by which the gradual changes in intensity of the resultant field of the outer part and the inner part of the end shield is obtained. The number and depth of the notches or recesses E9 in the disc i8 is obviously dependent on the difference in potential between the outer and inner parts of the end shield, and on the dimensions and proportions of the tube.

It has been found that in tubes embodying the invention the field is completely homogeneous even at a slight distance from the end shields or closing plates, and that the invention is of value not only for electron discharge tubes equipped with grids, but also for current converting tubes, gas discharge tubes and the like, and in general for tubes in which a homogeneous field distribution is desirable.

We claim:

1. An electron discharge device comprising a cathode an anode having parallel surfaces and an end shield between corresponding ends of said cathode and anode comprising two conductive members of difierent configuration positoned to cooperate when at different potentials to produce in the space between said cathode and anode a resultant electrostatic field having lines of force perpendicular to the parallel surfaces of said cathode and anode.

2. An electron discharge device comprising a cathode, an anode parallel to said cathode, a fiat end shield with one edge adjacent one end of said cathode and the other edge adjacent the corresponding end of said anode and comprising two parallel conductive sheet members insulated from each other and having overlapping portions which differ in shape to cause the resulting electrostatic field produced by said two members to vary gradually in intensity from one edge of said end shield to the other edge, and connections for maintaining said members at diiierent average potentials with reference to said cathode.

3. An electron discharge device comprising a cathode, an anode parallel to said cathode, a fiat end shield with one edge adjacent one end of said cathode and the other edge adjacent the corresponding end of said anode and comprising two parallel overlapping sheet metal members insulated from each other, one of said members having in the portion overlapping said other member perforations spaced to cause the electrostatic field of said end shield to vary gradually in intensity from one edge of said end shield to the other edge, and connections for maintaining said members at different average potentials with reference to said cathode.

4. An electron discharge device including an electrode assembly comprising a cylindrical thermionic cathode, a screen grid, an anode surrounding and coaxial with said cathode, and end shields at both ends of said electrode assembly, each comprising an outer substantially annular metal disc electrically connected to said anode and an inner substantially annular metal disc connected to said screen grid and underlying and insulated from said outer member, said inner member having a plurality of openings of different sizes and at different distances from said screen grid with the larger openings near the outer edge of said member to permit the electrostatic field of said outer member to extend through said openings into the interelectrode space between said screen grid and said anode.

5, An electron discharge, device including an electrode assembly comprising an elongated thermionic cathode, two cold electrodes surrounding and coaxial with said cathode, and end shields at both ends of said electrode assembly each comprisng an outer conductive member electrically connected to one of said cold electrodes and an inner conductive member connected to the other of said cold electrodes and insulated from said outer member and having a configuration which permits the electrostatic field of said outer member to extend through said inner member into the interelectrode space between said cold electrodes when said cold electrodes are at difierent potentials.

6. An electron discharge device including an electrode assembly comprising a cylindrical thermionic cathode, a screen grid, an anode surrounding and coaxial with said cathode, and end shields at both ends of said electrode assembly, each comprising an outer substantially annular metal disc electrically connected to said anode and an inner substantially annular metal disc connected to said screen grid and underlying and insulated from said outer member, said inner member having a plurality of semi-circular openings at different distances from and concentric with said screen grid.

7. An electron discharge device including an electrode assembly comprising a cylindrical thermionic cathode, a screen grid, an anode surrounding and coaxial with said cathode, and end shields at both ends of said electrode assembly, each comprising an outer substantially annular metal disc electrically connected to said anode and an inner substantially annular metal disc connected to said screen grid and underlying and insulated from said outer member, said inner member being stellate and having in the edge recesses of a depth greater than one-half the width of the annular part of said inner member.

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