ELECTRON GUN FOR COLOUR PICTURE TUBE

Abstract: In an electron gun for a color picture tube having three cathodes (1A, 1B, 1C) which are arrayed orthogonally to the axial direction of the tube and at equal intervals on a straight line, and a first grid (10) and a second grid (11) which are successively arrayed and each of which has apertures aligned with electron beam paths corresponding to the three cathodes (1A, 1B, 1C); the improvement comprising the fact that the surface of at least one of the first grid (10) and the second grid (11) opposing to the other includes aperture-vicinity flats (12a, 12b, 12c) which correspond to the respective apertures, and annular margins (13a, 13b, 13c) which are protruded toward the other opposing grid around the aperture-­ vicinity flats (12a, 12b, 12c), and that the annular margin (13a, 13b, 13c) correspond­ ing to the central aperture is retracted from a plane which forms the two annular margins (13a, 13c) correspond­ ing to the outer apertures, while the aperture-­ vicinity flats (12a, 12b, 12c) corresponding to the three apertures are so formed as to be substantially coplanar.

Description: BACKGROUND OF THE INVENTION: The present invention relates to an in-line type electron gun for a color picture tube, and more particularly to the structure of a first grid and a second grid which constitute the electron gun. A prior-art electron gun for a color picture tube has a structure which forms the basis of the prior art portion of claim 1 and which is shown in Figs. 1 and 2 by way of example. As illustrated in Fig. 1, the electron gun includes three cathodes 1A, 1B and 1C which are arrayed orthogonally to the axis of the tube and at equal intervals on a straight line, and a first grid 2, a second grid 3, a focusing electrode 4 and an anode 5 which are disposed at predetermined intervals in this order from the side of the cathodes 1A - 1C toward a screen not shown and each of which has apertures aligned with beam paths corresponding to three electron beams emitted from the cathodes 1A - 1C. The cathodes 1A, 1B and 1C, the first grid 2 and the second grid 3 construct a so-called "triode portion." Usually, variable voltages of 0 - 200 V are applied to the cathodes 1A - 1C, a voltage of 0 V is applied to the first grid 2, and a voltage of about 600 V is applied to the second grid 3, whereby the electron beams 6A, 6B and 6C are formed. Further, the focusing electrode 4 is supplied with a voltage with which the electron beams 6A - 6C are focused to the optimum on the screen though not depicted in the figure, and the anode 5 is supplied with a high voltage equal to that of the screen. In order to maintain the orthogonalities of the electrodes to the beam paths, the parallelism among the electrodes, and the coaxialities between the respectively corresponding apertures of the electrodes, the electron gun for the color picture tube constructed as stated above is assembled in such a way that three mandrels arranged on straight lines and held parallel to one another are respectively passed through the three apertures of the electrodes, and that spacers each having surfaces parallel to each other are inserted in the interspaces between the respectively adjacent electrodes. In case of such assemblage, the first grid 2 and the second grid 3 have heretofore been set up in order to secure the mutual parallelism thereof as disclosed in, for example, the document JP-Y2-60-15242. More specifically, as illustrated in Fig. 2, regarding the first grid 2, the peripheral parts 7a and 7c of respective outer apertures 2a and 2c opposing to the second grid 3 are protruded to the side of the second grid 3 more than the peripheral part 7b of a central aperture 2b, while regarding the second grid 3, the peripheral parts 8a and 8c of respective outer aperatures 3a and 3c opposing to the first grid 2 are protruded to the side of the first grid 2 more than the peripheral part 8b of a central aperture 3b. Thus, only the outer peripheral parts 7a and 8a, and 7c and 8c of the electrodes 2 and 3 come into contact with the spacers (not shown) which are used for setting the mutual interval between the first grid 2 and the second grid 3. Therefore, the mutual parallelism between the first grid 2 and the second grid 3 can be enhanced. With the prior art, the intervals ℓa and ℓc between the outer apertures of the first grid 2 and the second grid 3 become, in effect, smaller than the interval ℓb between the central apertures thereof. In general, in a color picture tube, cathode cutoff voltages (namely, cathode voltages with which cathode currents become "0") E kco need to be equalized for three electron beams to the end of equalizing the cathode drive characteristics of the electron beams corresponding to red, green and blue. It is known that the relationship of the following equation holds between the cathode cutoff voltage E kco and the dimensions of the triode portion: where A denotes a constant, D the diameter of each aperture of the first grid 2, S the spacing between each cathode and the corresponding aperture of the first grid 2, T₁ the thickness of the vicinity (for example, 22a in Fig. 2) of the aperture of the first grid 2, ℓ the interval between the corresponding apertures of the first grid 2 and the second grid 3, and E c2 the voltage of the second grid 3. In the case of the prior art, since the intervals ℓa and ℓc are smaller than the interval ℓb as shown in Fig. 2, the spacings Sa and Sc need to be made greater than the spacing Sb in accordance with the relationship of Eq. (1). In the triode portion in which the individual dimensions ℓ and S are unequal, however, differences develop in lens characteristics which are formed in the triode portion, and differences also develop in the divergent angles of the electron beams which are emitted from the triode portion. As a result, the angles of incidence of the electron beams on a main focusing lens become unequal, and the focusing conditions of the electron beams become different. That is, the optimum focusing voltages V f of the electron beams become unequal. Moreover, this tendency intensifies as beam currents I b increase. It has been experimentally and calculatively revealed that, in a case where the interval ℓ is small and where the spacing S is great, the divergent angle enlarges relative to a case where the interval ℓ is great and where the spacing S is small, so the optimum focusing voltage V f of the electron beam rises. In the prior art, accordingly, the voltage V f of each of the outer beams becomes higher than that of the central beam. In actuality, when the beam currents I b are changed as shown in Fig. 3, the optimum focusing voltage V f of the central electron beam 6B shown in Fig. 1 becomes a characteristic 20 indicated by a solid line, and that of each outer electron beam 6A or 6C becomes characteristic 21 indicated by a broken line. In this manner, with the prior art electron gun, when the beam currents I b are changed, the central electron beam 6B and the outer electron beam 6A or 6C exhibit the different variations of the optimum focusing voltages V f . The prior art has therefore involved the problem that, when either electron beam is set at the optimum focusing condition, the other electron beam deviates therefrom, so a vivid picture is not produced on the phosphor screen. Summary of the Invention An object of the present invention is to provide an electron gun for a colour picture tube in which the focusing voltages of a central electron beam and outer electron beams are equalised to attain a good picture quality. The above object is accomplished in the electron gun of the invention as defined in claim 1. Brief Description of the Drawings Figure 1 is a sectional view of essential portions showing the construction of a prior art example of an electron gun for a colour picture tube; Figure 2 is an enlarged sectional view of a triode portion in Figure 1; Figure 3 is a characteristic diagram showing the relationships between the beam current and the optimum focusing voltage of an electron gun for a colour picture tube; Figure 4 is a sectional view showing the construction of an embodiment of a triode portion for use in an electron gun for a colour picture tube according to the present invention; Figure 5 is a sectional view showing the construction of another embodiment of the present invention; Figure 6 is a view showing the construction of still another embodiment of the present invention; and Fig. 7 is a characteristic curve diagram for explaining the operation of the present invention. DESCRIPTION OF THE PREFERED EMBODIMENTS: Owing to the construction of the present invention as stated in the section of SUMMARY, at least the outer margins of each grid come into contact with spacers (not shown) for setting the mutual interval between the first grid and the second grid. Therefore, the orthogonalities of and the mutual parallelism between the first grid and the second grid can be enhanced. Furthermore, the interval between the central apertures of the first and second grids can be substantially equalized to each of the intervals between the outer apertures thereof. Accordingly, the cutoff voltages of the central beam and the outer beams need to be set equal, so that the spacings between the cathodes and the apertures of the first grid can be substantially equalized for the central beam and the outer beams. Thus, the lens characteristics of the triode portion for the individual electron beams can be brought into agreement, and the optimum focusing voltages of the respective electron beams can be finally brought into agreement. Now, an embodiment of the present invention will be described with reference to Fig. 4. The surface of the first grid 10 opposing to the second grid 11 includes in correspondence with respective electron beam apertures 10a, 10b and 10c, flats 12a, 12b and 12c in the vicinities of the apertures or defining the apertures, and annular margins 13a, 13b and 13c protruded to the side of the second grid 11 around the aperture-vicinity flats 12a, 12b and 12c. In addition, the central aperture-vicinity flat 12b and the outer aperture-vicinity flats 12a and 12c are so formed as to be substantially coplanar, while the central annular margin 13b is so formed as to be retracted from the plane of the outer annular margins 13a and 13c. Likewise, the surface of the second grid 11 opposing to the first grid 10 includes in correspondence with respective electron beam apertures 11a, 11b and 11c, flats 14a, 14b and 14c in the vicinities of the apertures or defining the apertures, and annular margins 15a, 15b and 15c protruded to the side of the first grid 10 around the aperture-vicinity flats 14a, 14b and 14c. In addition, the central aperture-vicinity flat 14b and the outer aperture-vicinity flats 14a and 14c are so formed as to be substantially coplanar, while the central annular margin 15b is so formed as to be retracted from the plane of the outer annular margins 15a and 15c. By forming the first grid 10 and the second grid 11 in this manner, the interval ℓb between the central apertures of both the electrodes 10 and 11 becomes substantially equal to each of the intervals ℓa and ℓc between the outer apertures thereof. Fig. 5 shows another embodiment of the present invention. The first grid 10 has the same configuration as in Fig. 4, while the second grid 3 has the same configuration as that of the second grid of the prior art in Fig. 2. Further, annular margins formed around aperture-vicinity flats 8a, 8b, 8c and an identical plane containing them. In case of the present embodiment, the interval ℓb between the central apertures of both the electrodes 10 and 3 and each of the intervals ℓa and ℓc between the outer apertures thereof are not equal, but they have their difference made smaller than in the prior art and can be substantially equalized. Fig. 6 shows another embodiment of the present invention. The first grid 10 is the same as shown in Fig. 4, while the second grid 33 is such that flats 38a - 38c in the vicinities of a central aperture 33b and outer apertures 33a, 33c opposing to the first grid 10 are formed on an identical plane. Thus, the interval between the central apertures of both the grids can be equalized to each of the intervals between the outer apertures thereof. On this occasion, in a case where an error has developed between the flatness of the central aperture-vicinity flat 38b of the second grid 33 and that of the outer aperture-vicinity flat 38a or 38c thereof, the stabilities of spacers might become somewhat unsatisfactory. Since, however, the second grid has a higher voltage applied as compared with the first grid, the structural mechanical accuracy of the second grid is usually less influential on the behaviors of electron beams than that of the first grid. Accordingly, the required mechanical accuracy of the second grid is not so severe as that of the first grid, and the embodiment in Fig. 6 can be put into practical use. The difference between the intervals ℓb and ℓa (or ℓc) has heretofore been 20 - 50 µm. In contrast, it becomes 10 µm or less with the embodiment shown in Fig. 4 or Fig. 6, and it becomes 10 -25 µm with the embodiment shown in Fig. 5. Thus, any of the embodiments has the effect that the difference of the intervals can be reduced to a half or less with respect to the prior art. Moreover, the embodiment is so constructed that, when the spacers for setting the mutual interval between the first grid 10 and the second grid 11 or 3 are interposed between these grids 10 and 11 or 3, the outer beam aperture portions of at least the first grid 10 come into contact with the spacers. Therefore, the stabilities of the grids are good, and the orthogonalities of and the mutual parallelism between the first grid 10 and the second grid 11 or 3 are held favorable. In this way, the interval ℓb between the central apertures of the first grid 10 and the second grid 11 or 3 can be equalized or substantially equalized to each of the intervals ℓa and ℓc between the outer apertures thereof, and also the spacing Sb between the cathode 1B and the aperture 10b of the first grid 10 can be substantially equalized to each of the spacings Sa, Sc between the cathodes 1A, 1C and the outer apertures 10a, 10c of the first grid 10. Therefore, the electron optical characteristics of the central electron beam and the outer electron beams in the triode portion can be equalized, and the variations of the optimum focusing voltages to arise when the beam currents of the electron beams can be brought into agreement for both the sorts of electron beams. Fig. 7 shows the experimental result of the relationship between the difference of the interval ℓb of the central apertures and the interval ℓa or ℓc of the outer apertures of the first and second grids, and the difference of the optimum focusing voltage V fb of the central electron beam and the optimum focusing voltage V fa or V fc of the outer electron beam at beam currents I b = 4 mA. Experimental results indicate that the differences V fa - V fb and V fc - V fb should desirably be within 100 V for the purpose of attaining a good picture quality. It is accordingly understood from Fig. 7 that the differences ℓb - ℓa and ℓb - ℓc need to be held at, at most, 25 µm. The embodiments of the constructions in Figs. 4, 5 and 6 satisfy this requirement. As apparent from the above description, according to the present invention, the mutual parallelism between a first grid and a second grid can be maintained, and besides, the electron optical characteristics of a triode portion for a central electron beam and outer electron beams can be brought into substantial agreement. Therefore, the beam current - versus -optimum focusing voltage characteristics of the central electron beam and the outer electron beams can be brought into substantial agreement, and excellent picture qualities are attained over all beam currents. Although, in the above, only the case of plate-like electrodes has been described, it is needless to say that a similar effect is produced by the use of cup-like electrodes. Incidentally, the annular margins stated before are not necessarily formed into margins in the shape of continuous lines. Insofar as the intended purpose can be met in case of inserting the spacers and assembling the electrodes as already explained, the margin for each of the apertures of the grids may well be formed of dot parts or the likes.

Claims: An electron gun for a colour picture tube, said gun having three cathodes (1A,1B,1C) which are arrayed orthogonally to an axial direction of the tube and at equal intervals on a straight line, and a first grid (10) and a second grid (11,3,33) which are successively disposed in a direction towards a screen from the three cathodes (1A,1B,1C) each of the grids having apertures (10a,10b,10c; (11a,11b,11c) respectively aligned with electron beam paths corresponding to three electron beams emitted from the said cathodes (1A,1B,1C), a surface of said first grid (10) opposed to said second grid (11) comprising aperture-vicinity flats (12a,12b,12c) of the respective apertures (10a,10b,10c) aligned with the electron beam paths, and annular margins (13a,13b,13c) of the said apertures protruded toward said second grid (11) around said aperture-vicinity flats (12a,12b,12c); said electron gun being characterised in that the plane in which the annular margin (13b) of the aperture (10b) aligned with the central electron beam path is formed is retracted from a plane in which the two annular margins (13a,13c) of the apertures (10a,10c) aligned with the outer electron beam paths are formed, while said aperture-vicinity flats (12a,12b,12c) of the said apertures (10a,10b,10c) of said first grid (10) aligned with the three electron beam paths are all formed in an identical plane. An electron gun as defined in claim 1, characterised in that a surface of said second grid (11) opposed to said first grid (10) comprises aperture-vicinity flats (14a,14b,14c) of the respective apertures (11a,11b,11c) aligned with the electron beam paths, and annular margins (15a,15b,15c) of the said apertures (11a,11b,11c) protruded toward said first grid (10) around said aperture-vicinity flats (14a,14b,14c), the plane in which the annular margin (15b) of the aperture (11b) aligned with the central electron beam path is formed being retracted from a plane in which the two annular margins (15a,15c) of the apertures (11a,11c) aligned with the outer electron beam paths are formed, while said aperture-vicinity flats (14a,14b,14c) of the said apertures (11a,11b,11c) of said second grid (11) aligned with the three electron beam paths are all formed in an identical plane. An electron gun as defined in claim 1, characterised in that a surface of said second grid (33) opposed to said first grid (10) comprises aperture-vicinity flats (38a,38b,38c) of the respective apertures aligned with the electron beam paths, annular margins of the said apertures formed around said aperture-vicinity flats (38a,38b,38c) being formed in an identical plane, the aperture-vicinity flats (38a,38b,38c) of the said apertures of said second grid (33) aligned with the three electron beam paths all being formed in an identical plane. An electron gun as defined in claim 1, characterised in that a surface of said second grid (3) opposed to said first grid (10) comprises aperture-vicinity flats (8a,8b,8c) of the respective apertures aligned with the electron beam paths, annular margins of said apertures formed around said aperture-vicinity flats (8a,8b,8c) being formed in an identical plane, the plane in which the annular margin of the aperture aligned with the central electron beam path is formed being retracted from a plane in which the two annular margins of the apertures aligned with the outer electron beam paths are formed.

Inventors: MIYAZAKI, MASAHIRO
Applicants: HITACHI, LTD.
Application year: 1988
Main classification: H01J29
IPC codes: H01J29