US2932758A - Electron beam tube - Google Patents

Description

April 12, 1960 o. H. SCHADE, JR

ELECTRON BEAM TUBE Filed March 4, 1957 IN VEN TOR.

Dun I-I. S:1-1amE,.Tn Y 742 mar/9n United States Patent ELECTRON BEAM TUBE Otto H. Schade, Jr., North Caldwell, N.J., assignor to Radio Corporation of America, a corporation of Delaware Application March 4, 1957, Serial No. 643,572

6 Claims. (Cl. 313-313) This invention relates to electron beam tubes of the kind containing an auxiliary electrode or grid for suppressing secondary electrons released by the impact of primary electrons upon the anode of the tube.

Tubes of the subject variety find useful application as amplifiers in the vertical and horizontal deflection circuits of black and white and color television receivers. In such receivers the vertical and horizontal deflection circuits require tubes capable of handling a relatively large amount of current. In such tubes, especially those used to energize the horizontal deflection circuit and provide the second-anode voltage for the kinescope (e.g. 25,000 volts), the voltage difference between the suppressor electrode and the adjacent anode is often of the order of 7,000 volts. (This voltage diiference may be higher than 7,000 volts during the warm up time interval which occurs immediately after the television receiver is turned on.) For best tube operation, the portions of the suppressor electrode which define the window through which the electron-beam passes are usually positioned closely adjacent to the anode; as a consequence, there is a tendency for arc-over to occur between the suppressor electrode and the anode. The arc-over occurs as a result of a relatively high voltage gradient existing at the more closely adjacent parts of the suppressor electrode and anode. If an attempt is made to prevent arc-over by employing such well-known expedients as (a) narrowing the window in the suppressor electrode (to appreciably reduce the voltage gradient thereat), or (b) to increase the distance between the suppressor electrode and anode, then the effectiveness of the suppression of secondary emission from the anode is adversely afiected.

Accordingly, the principal object of the present invention is to provide an improved electron beam tube of the kind having an anode and a closely adjacent secondaryemission suppressor electrode, and one capable of handling high voltages and high beam current densities without arc-over between said adjacent electrodes.

The foregoing and related objects are achieved in accordance with the principles of the invention by the provision of an electrode assembly comprising a suppressor electrode of a novel construction (later described) cal culated (a) to establish a relatively low and uniform voltage-gradient along the surface areas thereof which face the anode, and (b) a desirable space charge configuration adjacent to the window for the beam.

The invention is described in greater detail in connection with the accompanying single sheet of drawings.

Fig. l is a partially cut-away view in perspective of an electron beam tube embodying the invention;

Fig. 2 is a transverse sectional view taken on line 2--2 of Fig. l;

Fig. 3 is a view in perspective of a suppressor electrode constructed in accordance with the principle of the invention; and

Fig. 4 is a view, on an enlarged scale, of a part of an 2,932,758 Patented Apr. 12, 1960 electrode assembly similar to the one shown in Fig. 2; the drawing being marked with equipotential lines which will be referred to in explaining the principle of the invention.

In Figs. 1 and 2, 10 designates an electron beam tube of the kind having a secondary emission suppressor electrode 12 and useful in large screen (e.g. 21 to 30 inches) telesvision receivers. The tube 10 contains a cathode 14, a control grid 16 around the cathode, a screen grid 18 around the control grid, the secondary emission suppressor electrode 12 around the screen grid, and an anode 20 around the suppressor electrode. The control grid 16 and the screen grid 18 are made of wire while the suppressor electrode 12 is made of sheet metal. These electrodes 12, 16, and 18 are suitably spaced from one another by means of mica spacers 22 and 24.

The tube 10 as thus far described is of more or less standard construction and, when used as an amplifier in the horizontal deflection circuit of a television receiver, has two operating conditions. In the first condition, which will be referred to as the conductive state, the tube provides horizontal scansion energy. In the conductive state the anode 20 is ordinarily operated at about 50 to volts positive (with respect to the cathode) and draws about A ampere direct current; the screen grid is operated at about 200 volts positive and the control grid is operated at from about minus 50 to about zero volts. During the conductive state the suppressor electrode 12 is operated at about cathode potential. The suppressor electrode contributes to the production of the desired electron beam by suppressing the passage of secondary electrons from the anode toward the screen grid. The secondary emission from the anode is suppressed by space charge in the region between the screen grid and the anode, the space charge being produced by the slowing up of the electrons traveling from the screen grid to the lower voltage anode. This space charge causes a reduction in space potential suflicient to repel secondary electrons emitted from the anode and cause them to return to the anode.

Now, in order to provide the desired space charge as above described (and hence the desired beam tube operation during the conductive state) the suppressor electrode 12 (Fig. 2) and anode 20 geometries must be so chosen that the electron beam is of substantially uniform density in the region between the screen grid 16 and the anode, in planes normal to the long axis of the tube. In general, in order to provide the desired configuration of the space charge the portion of the suppressor electrode adjacent to the electron window 21 therethrough should be located at least about half-way between the anode 20 and the screen grid 18, preferably closer to the anode. However, as Will be explained, a greater spacing, between the suppressor electrode 12 and anode 20, is desirable during a second state of operation in order to avoid arcover problems.

When the tube is in its second condition of operation, hereinafter referred to as its nonconductive state, the horizontal deflection coil, to which the tube is coupled, subjects the anode of the tube to a high voltage pulse at the end of each scansion. The magnitude of this pulse is ordinarily of the order of about 7,000 volts (even higher than this when a television receiver is warming up) with respect to the cathode and suppressor electrode. in this nonconductive state the control grid 14 is biased at about minus 200 volts to maintain the tube nonconductive. In tubes constructed in accordance with the prior art the high voltage pulse often gives rise to arc-over between the suppressor electrode and the anode. Attempts to prevent arc-over by increasing the distance between the suppressor electrode and the anode result in a deterioration of the space chargecharacteristics of the tube when it is in its conductive state. It would thus appear that if a beam tube is to be used as a horizontal deflection amplifier it should'have two'apparently mutually exclusive requirements, (a) relatively large spacing between the suppressor electrode and the anode, to prevent arc-over during the nonconductive state of the tube, and-(b) a relatively small spacing between said electrodes to preserve the tube from undesirable space charge characteristics during the conductive state.

The foregoing problem is solved, in accordance with the invention, by the provision of a suppressor electrode 12 (Figs. 2 and 3) having an elongated window 21 defined by a pair of elongated, oppositely located edgeportions 26 parallel to the long axis of the tube, the edge portions having a curvature in a' direction away from the anode 20 for minimizing the tendency to arc-over between this electrode and the anode. Too small a curvature of the edge portions would not obviate arcover; too great a curvature would adversely affect the secondary emission suppression characteristics. There fore, the degree of inward curvature of the edge portions 26 is critical. In accordance with this invention, these inwardly curved edge portions 26 each have a curvature in a direction away from the anode 20 such that the distance A, as measured along the beam path, between the edge and the portion of the suppressor electrode 12 closest to the anode is equal, substantially, to one third of the minimum distance B between the anode and said suppressor electrode. This optimum curvature minimizes the tendency to arc-over between the anode and the suppressor electrode during the nonconductive state of normal operation when the tube is used in large screen television receivers, without adverse effect on the space charge configuration within the tube during its.conductive state.

The sheet metal suppressor electrode 12 (Figs; 2 and 3) is made up of two elongated, spaced apart sections 28 and 30 connected by two sheet metal straps 32. Each strap 32 is welded at each end to an inside surface of one of the electrode sections 28 and 30 and provides an outer electrode-surface free from sharp discontinuities. The electrode sections 23 and 30 are, generally, C-shaped in cross section and are disposed with the open side of each C facing the open side of the other andwith the lateral edges 26 of each C curled inwardly away from the anode 20. Each of these lateral edges 26 forms a 90 segment of a cylindrical surface having a radius of curvature equal to about one-third of the shortest distance between the suppressor electrode 12 and the anode 20. Thus, as previously mentioned, each of the lateral edges 26 of the suppressor electrode 12 has a curvature in a direction away from the anode 20, and the distance A along the beam path is substantially equal to one-third of the minimum distance B between the anode and the suppressor electrode.

Fig. 4 shows the equipotential lines 34 measured in an electrode array similar to the electrode array in the tube of Figs. 1 and 2. The space potential distribution here shown involves a 10,000 volt potential difference between adjacent electrodes and corresponds to the potential distribution in a tube of the kind shown in Fig. 1 in its nonconductive state." Corresponding electrodes in Figs.

1 and 4 are designated by the same numerals with the exception that the electrodes in Fig. 4 are designated with primes added to the numerals. In plotting the equipotential lines shown in Fig. 4 the grid wires of the control and screen grids were omitted (to facilitate the electrical measurements) but side rods were left in place (as indicated at 16' and 13). It will be noted that the equipotential lines 34 in the region of the lateral edge 25 of the suppressor electrode 12' have substantially the same contour as the outside surface of the electrode 12. Thus thereis no high field-concentration at any place along the suppressor electrode12'. Consequently,'any

tendency toward, arc-over between the suppressor elec trode 12' and the anode 20 is minimized. Also, the space charge distribution present during the conductive state is not disturbed because the spacing between the suppressor electrode 12 (Fig. 2) and the anode 20 is approximately of the same order as the spacing between the screen grid and the suppressor electrode.

From the foregoing description it should now be apparent that the present invention provides an improved electron beam tube of the kind having an anode and a closely adjacent secondary-emission suppressor electrode, and one capable of handling high voltages and high beam current densities without arc-over between said adjacent electrodes.

What is claimed is:

1. An electron beam tube comprising an anode, a cathode spaced from said anode and adapted to supply electrons for acceleration in a beam having a. path to said anode, and an electrode disposed'between said anode and said cathode, said electrode including a planar portion relatively close to said anode and an edge portion positioned adjacent to a portion of said path, said edge portion having a curvature in a direction away from said anode and such that the distance, as measured along said beam path, between the free edge of said edge portion and the planar portion of said electrode closest to said anode is substantially equal to one-third of the minimum distance between said anode and said electrode to reduce arc-over tendencies between said anode andsaid electrode while preserving desirable space charge characteristics within said tube during the normal operation thereof.

2. The electron beam tube claimed in claim 1, wherein said curvature is cylindrical.

3. An electron beam tube comprising, a cathode, an anode spaced from said cathode, and an electrode disposed between said cathode and said anode, said electrode including edge portions onopposite' sides of said beam and planar portions adjacent to said edge portions, said edge portions having a cylindrical curvature, in a direction away from the planes of said planar portions and from said anode, with a radius of curvature equal to about one-third of the minimum distance between said planar portions of said electrode and said anode, the curved edge portions each forming a segment of a cylindrical surface.

4. An electron beam tube comprising a cathode, an anode spaced from said cathode, a control grid disposed intermediate. said cathode and anode, a screen grid disposed intermediate said control grid and said anode, and.

a sheet metal suppressor electrode disposed intermediate said screen grid and said anode, said anode surrounding said electrode and said electrode surrounding said cathode and said grids, said electrode having two elongated sections each having a generally C-shapedcross-section and arranged with the open side of each of said sections facing the open side of the other and with both lateral edges of: the G of each section curled inwardly away from said anode the inwardly curled lateral edges extending toward the screen grid a distance equal to approximately /3 the minimum spacing between the suppressor electrode and said anode.

5. An electron beam tube comprising a cathode; an anode disposed around, said cathode; a control grid disposed around said cathode and between said cathode and said anode; a screen grid disposed around said control grid and between said control grid and said anode; and a secondary. emission suppression electrode disposed around said screen grid and between said screen grid and said anode; said electrode including two spaced-apart, substantially parallel, elongated, sheet metal portions each having a generally C-shaped cross section and arranged with the open side of each of said portions facing the open side of the other and with the edges of said portions forming the ends of each C oppositely located and inwardly concave and forming a 90 segment of a cylindncai surface having a radius of curvature equal to about one-third of the minimum distance between said electrode and said anode, and a pair metallic straps joining said portions to each other, each strap having each end fixed to an inside surface of one of said portions; whereby said tube is adapted to be operated at relatively high potential differences between said electrode and said anode while preserving the region of said tube adjacent to said grids substantially free of secondary emission from said anode.

6. An electron beam tube comprising a cathode; an anode disposed around said cathode; a control grid disposed around said cathode and between said cathode and said anode; a screen grid disposed around said control grid and between said control grid and said anode; and a secondary emission suppression electrode disposed around said screen grid and between said screen grid and said anode; said electrode including two spaced-apart, substantially parallel, elongated, sheet metal portions each having a generally C-shaped cross section and arranged with the open side of each of said portions facing the open side of the other and with the edges of said portions forming the ends of each C oppositely located and inwardly concave and forming a 90 segment of a cylindrical surface having a radius of curvature equal to about one-third of the minimum distance between said electrode and said anode, whereby said tube is adapted to be operated at relatively high potential differences between said electrode and said anode while preserving the region of said tube adjacent to said grids substantially free of secondary emission from said anode.

References Cited in the file of this patent UNITED STATES PATENTS 2,036,069 Morrison Mar. 31, 1936 2,132,175 Machlett Oct. 4, 1938 2,254,096 Thompson Aug. 26, 1941 2,487,592 Rishell Nov. 8, 1949 2,562,242 Pohle July 31, 1951 2,568,552 Magnusson Sept. 18, 1951 2,573,397 Burnside Oct. 30, 1951 2,840,750 White June 24, 1958

Images