US2877369A

US2877369A - Electron beam tube

Info

Publication number: US2877369A
Application number: US512757A
Authority: US
Inventors: Frederick H Nicoll
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1955-06-02
Filing date: 1955-06-02
Publication date: 1959-03-10
Anticipated expiration: 1976-03-10

Description

March -10, 1 959 F. H. NICOLL ELECTRON BEAM TUBE 2 Sheets-Sheet 1 Filed June 2. 1955' March 1959 F- H. QNICOLL 25,877,369

ELECTRON BEAM TUBE Filed June 2. 1955 2 Sheets-Sheet 2' wwwi Application June 2, 1955,. SerialNo. 512,757 10 Claims. (Cl. 313-82l The present invention relates generally to an electron beam tube and, though not limited thereto, is herein described as embodied in a cathode ray tube having a multi-apertured electrode for modulating the intensity of the electron beam therein.

The tube of the invention is characterized by a relatively high transconductance, a relatively lowmodulation voltage requirement, and a relatively good focus at high beam currents and consequently proves advantageous in television receivers, especially in such receivers using-1 transistors in the video amplifiers thereof. 3

Previous cathode ray tubes have ge'nerallyused a control'electrode having a singleaperture through which. the beam passed and which required a relatively large negative signal voltage for the whole range of beam modulation. Thus, where such tubes areused for image viewing purposes, e. g. for television, additional amplifier stages have been required to provide the required relatively large negative control 'or'signal voltages.

.It is therefore an object of the invention to provide an electron beam tube having an improved electronoptical system for relatively high beam currents.

It is a more particular aim to prbvide an improved cathode ray tube having a multi-apertured modulating electrode and which exhibits a relatively high transconductance, low cut-off beam modulating voltage, and good beam definition and which is adapted to operate at relatively high beam currents.

The foregoing and related objects are achieved in accordance with the invention by providing an improved-- electron beam tube havingv a cathode and, grid spaced along the tube axis. The grid has"-atleast one aperture therethrough and has an axial thickness'of between about 5 and about 60 percent of the minimum distance across at least one aperture. A' multi-apertured diaphragm or mesh is fixed to the side of the grid adjacent to the cathode. Consequently, eachof f the apertures of the mesh acts as a separate beam control element for individually focusing a portion of the beam current at a common point within the tube or at a plurality of-points having a predetermined spacing, therebet-ween, the focus usually being at a target electrode of the tube.

The invention is described in greater detailin connection with the accompanying two sheets of drawings wherein:

Figure l is a side view, partly in section, of a cathode ray tube embodying the inventionj I Figure 2 is an enlarged sectional'view of a portion of the tube of Figure 1 and. illustrating'a space potential distribution between some of the electrodes of the tube;

Figure 3 is a chart illustrating the'control'grid voltage required to bias one conventional cathode ray tube under various conditions in comparison with. the control grid voltage required to bias one tube of the invention under similar conditions;

Figures 4 through 8 illustrate diflerent types of control electrodes which may be used in the tube of Figure 1;

a United States Patent 0 v 53 of the grid 30 adjacent to the cathode pins 18: for providing 1C6 Patented Mar. 10, 1.959

Figures 9 through 11 illustrate portions of diflf'erent types of electron gun structures useful inthe tube of Figure 1;

Figure 12 illustrates a portion of an electron gun for producing two electron beams within a cathode ray tube of the invention; and

Figure 13 shows a control electrode of a type useful in the electron gun of Figure 12.

Reference is made to Figure l where there is shown a side elevational view, partly in section, of a cathode ray tube 10 according to the invention and comprising an; evacuated envelope 12 made of an insulating material, such as glass. portion 14 terminated by a base 16 having a plurality ofelectrical access to some of theelements within the tube 10; the other part of the envelope includes a flared bulb portion 20 closed by a trans-1 parent end-wall 22. The. transparent end-wall carries on the inside surface thereof a target element in the form of a fluorescent screen 24, the screen becoming lumines-- cent onbeing bombarded by electrons.

An electron gun 26 is disposed within the neck portion 14 to provide a beam of electrons directed toward the. gun 26 includes a cathode 28 v having a heater therein (not shown),

of tube, are fixed; by being joined to insulating over the inner surface of the envelope bulb portion 20- to a point adjacent to the fluorescent screen 24. This. coating 40 prevents the operation, which would and deflection.

Means are provided for focusing and scanning the electron beam over the screen 24 to form a raster. means may include a focusing coil 42 These tion 14 adjacent to the flared envelope yoke 44 comprises a plurality of coils deflecting fields in the beam path for providing scansion of the electron beam over the screen 24.

In Figure 2 there is shown the approximate shape of an electron beam (shown in the drawing bounded by lines 46) and an electrostatic field, illustrated by equipotential lines 48, in relation to the cathode 28, control grid 30, and first accelerating electrode 32 of Figure 1. The control grid 30 has a single, large aperture 51 there'- through in a direction along the tube axis. A multiaperturcd diaphragm or mesh element 50 is disposed across the single aperture 51 and is fixed to the end-wall 28. The mesh element may be in any multi-apertured planar shape and is fixed to the control grid by any of the known techniques for welding a mesh element to a support. For example, the welding may be effected by a hydrogen firing operation while the mesh element is maintained in contact with the solid portion of the grid by means of a jig. The equipotential field lines 48 adjacent to the aperture 51 are given a curvature at the aperture due to the penetration of the field into the end-Wall 53 a substantial thickness. tical field which penetrates the small apertures 49 of the mesh element 50 accelerates electrons at the cathode 28 into a plurality of substantially parallel beams directed from the surface of the cathode through the small 'aper-' tures 49. Then, while the electrons are still traveling at portion 20. The

One part of the envelope-includes a neck electrodes, which The second accelerating electrode" glass portions of the envelope. 20 from becoming indiscriminately charged during tube adversely affect beam focusing and a deflection yoke 44 disposed around the part of the tube neck porforming magnetic:

which has The portion'of the electron-opresolution at relatively a relatively low velocity, that within and just beyond the large aperture 51, the portion of the electron-optical field between the grid 30 and the first accelerating electrode 32 converges the electrons into a restrained region, D, where the beam exhibits a minimum cross-sectional area. The focusing coil 42 (Figure l) is then used to image the beam from this region (D) at the screen 24.

The curvature of the electron-optical field in the large aperture 51, which produces the minimum cross-sectional area at point D, is determined by the spacing between the cathode 28 and the end-wall 53 and by the axial thickness (dimension B) of the end-wall. It has also been found that the optimum aforedescribed converging action within the large aperture 51, when a mesh is used across this aperture, is obtained when the thickness of the endwall portion 53, dimension B, is from about 5 percent to about 60 percent of the distance, dimension A, across this aperture.

' In order to obtain a relatively large penetration of the electron-optical field (Figure 2) through the control grid mesh 50 and adjacent to the active surface of the cathode 28, a relatively close cathode-to-mesh spacing is used. The cathode-to-mesh spacing is preferably smaller than the distance across an aperture in the multi-apertured mesh 50. For example, one tube made according to the invention had a distance of about .003 inch across each of the small apertures in the multi-apertured mesh and a. cathode-to-mesh spacing of about .0015 inch during normal tube operation.

It has been found that for a given grid thickness B, a good focus at the screen or target of the tube can be obtained only for a certain range of control grid to first accelerating electrode spacings (dimension C in Figure 2). A ratio of the distance represented by the thickness B of the control grid wall to the distance C between the control grid and the first accelerating electrode equal to about 1:3 to about 1:10 has been found to give satisfactoryresolution at the target. A ratio of from about 1:3 to about 1:5, i. e. of the order of 1:4, gives an optimum high beam currents such as, for example, 300 microamperes. It has also been found that the distance between the control grid and the first ac-" celerating electrode should be from about .2 to about one and a half times the distance A across the control grid aperture for the best resolution with the aforementioned: ratios.

For example, cathode ray tubes incorporating electron guns having the following electrode dimensions and spacings have yielded the results indicated below. In each case the resolution at the target electrode was measured when the beam current was of the order of 300 microamperes (the beam being cut-off when the grid volt-r age was lowered to -5 volts with respect to the cathode), the distance across the large aperture in the grid being .035 inch and the cathode-mesh spacing with the cathode at the temperature of normal tube operation) being about .0015 inch.

Control Grid Control Grid- Thickness, First Accnlcr- Resolution B ating Electrode at Target Spacing, C

.005 .010 Poor. .005 .020 Good. .007 032 D0. .009 032 Do.

' and a 60% transmissivity and with crossed wires at the center of the control grid aperture 51. Similarly, Figure 5 shows a wire-mesh 50b of the type depicted in Figure4- but with a mesh opening centered in the large aperture 51 in the control grid. Figure 6 illustrates yet another I mesh element 50c, this one having a spider-web-like mesh.

, minimum distance across the aperture.

element 502 therein (Figure 13).

As shown in Figures 7and 8 the single large aperture 51 in the control grid 30 may be square or rectangularly shaped, the dimension A in each case referring to the As illustrated in 50d may take the formcf an apertured plate instead of a wire mesh as in the afore; described mesh elements. Also, the apertures in the, control grid 30. may take the form of a plurality of apertures and with each of the apertures itself having a mesh In the last named ar rangement there is provided, in effect, a control element having a plurality of apertures wherein each of the aper; tures is itself sub-divided by a mesh element into a plurality of apertures. In each of the aforementioned ex amples the mesh element is of a multi-apertured, substantially planar shape and is preferably substantially symmetrical about the center of the large aperture 51.

Figures 9, 10, vand 11 illustrate portions of different. electron gun assemblies which may be used with a mesh element 50 fixed to a control grid. In these three figures.

Figure 8, the mesh element as well as in Figure 1 the mesh element 50 is indicated schematically; it will be appreciated that the mesh element is actually fixedto the grid 30 as shown in Figure 2. While the mesh element 50 may be a flat element (Figure 9) fixed around its periphery to a fiat control grid 30,

' beam being subject from each other at the control grid surface may instead be bowed either out-Q wardly (Figure 10) or inwardly (Figure 11) in order that the mesh follow the contour of the control grid surface to which the mesh is fixed. The bowing illustrated in Figures 10 and 11 (which is exaggerated in the drawing for purposes of illustrating the bowing) is used to' prevent the mesh from buckling when heated, during nor-iv mal tube operation, and short circuiting to the cathode- 28. The bowing gives a pro-stressed effect to the mesh forcing it to move, if at all, in the same direction as' that in which the control grid surface is bowed. Thus the distance of the mesh to the cathode is determined 'by', the control grid surface contour. The aforedescribedj bowing is not chosen to be of such a degree that it ap-I preciably affects the electron-optics (focusing in particu': lar) of the tube in which the gun is used. [1 Reference is made to Figure 12 where there is illus-" trated a portion of an electron gun of a type capable of producing two separate, focused electron beams, each to control by a grid-cut-off bias of about --5 volts. The electron gun of Figure 12 has a; control grid 30 having two-large apertures 51 and a wire mesh 50 across each of the large apertures. While the use of appropriate second accelerating electrode to first acf celerating electrode voltage ratios brings the two beams to focus at a single point on the screen of the tube, when the second accelerating electrode to first acceleration elec-'' trode voltage ratio is greater than about 50 to 1; the two beams come to two foci at the screen, and separated by about .075 inchithereat, when the second accelerating electrode to.first accelerating electrode voltage ratio is between about 10 to l to about 30 to 1. The use of two beams having individual foci at a predetermined distance the screen is useful in eliminating the appearance of a line structure between adjacent scan sions on the screensof a television viewing tube.

What is claimed is:

1. An electron beam tube having a tube axis, a cathode and a control grid spaced along said axis, said grid having a portion lying substantially in a predetermined surface, said portion having at least one aperture therethrough and having an axial thickness of between about 5 and about 60 percent of the minimum distance across said at least one aperture, and a multi-apertured mesh fixed to the side of said grid portion adjacent to said cathode and across said at least one aperture and lying substantially parallel to said surface.

2. The electron beam tube described in claim 1 and wherein said control grid has two apertures.

3. The electron beam tube described in claim 1 and wherein said at least one aperture is of substantially circular cross-sections.

4. An electron beam tube comprising a cathode for providing a source of electrons, a target, an accelerating electrode intermediate said cathode and said target for accelerating electrons from said cathode toward said target, and a control electrode intermediate said cathode and said accelerating electrode for controlling the flow of electrons toward said target, said control electrode having a portion thereof defining an aperture therein and a multiapertured mesh element across said aperture and fixed to the side of said electrode adjacent to said cathode, the thickness of said control electrode portion being between about 5% and about 60% of the minimum distance across said aperture, the ratio between the distance represented by the thickness of said control electrode portion and the distance between adjacent portions of said electrode portion and said accelerating electrode being from about 1:3 to about 1:10.

5. An electron beam tube comprising a cathode for providing a source of electrons, a target, an accelerating electrode intermediate said cathode and said target for accelerating electrons from said cathode toward said target, and a control electrode intermediate said cathode and said accelerating electrode for controlling the fiow of electrons from said cathode toward said target, said control electrode including an apertured element and a multi-apertured diaphragm across the aperture in said element and fixed to the side thereof adjacent to said cathode, the ratio between the distance represented by the thickness of said apertured element and the distance between said element and said accelerating electrode being from about 1:3 to about 1:10.

6. An electron beam tube comprising a cathode for providing a source of electrons, a target, an accelerating electrode intermediate said cathode and said target for accelerating electrons from said cathode toward said target, and a control electrode intermediate said cathode and said accelerating electrode for controlling the flow of electrons from said cathode toward said target, said control electrode including an apertured element and a multi-apertured diaphragm across the aperture in said element and fixed to the side thereof adjacent to said cathode, adjacent sides of said diaphragm and said apertured element having substantially the same contour, the ratio between the distance represented by the thickness of said apertured element and the distance between said element and said accelerating electrode being from about 1:3 to about 1:5.

7. An electron beam tube comprising, spaced along a tube axis, a cathode for providing a source of electrons, a target, an accelerating electrode intermediate said cathode and said target for accelerating electrons from said cathode toward said target, and a control electrode intermediate said cathode and said accelerating electrode for controlling the flow of electrons from said cathode to said target, said control electrode including an element having a single circular aperture axially therethrough and a multi-apertured diaphragm across said single aperture and fixed to the side thereof adjacent to said cathode, the smallest cross sectional distance across each of the apertures in said diaphragm being greater than the distance between adjacent surfaces of said diaphragm and cathode during normal tube operation, the ratio between the distance represented by the thickness of said apertured element and the distance between said element and said accelerating electrode being of the order of 1:4.

8. A cathode-ray tube comprising a cathode for providing a source of electrons, a target, an accelerating electrode intermediate said cathode and said target for accelerating electrons from said cathode toward said target, and a control electrode intermediate said cathode and said accelerating electrode for controlling the flow of electrons from said cathode toward said target, said control electrode including an apertured portion lying substantially in a predetermined surface and a multiapertured diaphragm across the aperture in said portion and lying substantially in said surface and fixed to the side thereof adjacent to said cathode, the thickness of said portion being from about .05 to about .6 of the average distance across the aperture in said element.

9. A cathode-ray tube comprising a cathode for pro viding a source of electrons, a target, an accelerating electrode intermediate said cathode and said target for accelerating electrons from said cathode toward said target, and a control electrode intermediate said cathode and said accelerating electrode for controlling the flow of electrons from said cathode to said target, said control electrode including an element having a circular aperture therein and a multi-apertured diaphragm across the aperture in said element and fixed to the side thereof adjacent to said cathode, the distance between said element and said accelerating electrode being from about .2 to about one and a half times the diameter across the aperture in said element.

10. An electron beam tube comprising a cathode for providing a source of electrons, a target, an apertured accelerating electrode intermediate said cathode and said target for accelerating electrons from said cathode through said accelerating electrode and toward said target, and a control electrode intermediate said cathode and said accelerating electrode for controlling the flow of electrons from said cathode toward said target, said control electrode including an element having a circular aperture therein and a multi-apertured wire mesh across the aperture in said element and fixed to the side thereof adjacent to said cathode, the smallest cross sectional distance across each of the apertures in said mesh being greater than the distance between adjacent surfaces of said mesh and cathode during normal tube operation, the ratio between the distance represented by the thickness of said apertured element and the distance between said element and said accelerating electrode being of the order of 1:4, the thickness of said disc being from about .05 to about .6 of the diameter of the aperture in said element, and the distance between said element and said accelerating electrode being from about .2 to about one and a half times the diameter of said aperture in said element.

References Cited in the file of this patent UNITED STATES PATENTS 2,153,223 Young Apr. 4, 1939 2,299,047 Winans Oct. 13, 1942 2,644,906 Bondley July 7, 1953 2,726,347 Benway Dec. 6, 1955