In-line electron gun
FIELD OF THE INVENTION AND RELATED ART STATEMENT 
     1. Field of the Invention 
     The present invention relates to an in-line electron gun adapted to be incorporated into a color picture tube. 
     2. Description of the Related Art 
     Picture quality of a color picture reconstructed on a color picture tube depends on a resolution characteristic and a convergence characteristic of the color picture tube. In order to provide a good resolution characteristic, beam spots of small diameter and of nearly circular shape must be produced on not only the center part but also the circumference parts of a phosphor screen. 
     On the other hand, in driving a color picture tube having an in-line electron gun, a horizontal deflection field having a pincushion shaped distribution and a vertical deflection field having a barrel shaped distribution are generated by a deflection yoke, and a self-convergence effect is obtained as a result. Beam spots formed by electron beams passing through such distorted vertical and horizontal deflection fields are heavily distorted on the circumference parts of the phosphor screen. Namely, the beam spot formed on the center of the phosphor screen is circular, whereas the beam spots formed on the circumference parts of the phosphor screen are horizontally oblong ellipses 3 with vertically large hazes 3a, which are shown in FIG. 2. 
     In a color picture tube shown in Japanese published unexamined patent application Sho No. 58-192252, electron beams are impressed with a non-rotational symmetry dynamic additional focusing electrical field in order to reduce the above-mentioned distortion in beam spots. As shown in FIG. 1, the color picture tube in this device has cathodes 4, a control grid electrode 5, an accelerating electrode 6, a first focusing electrode 7, a second focusing electrode 8 and an anode 9. At least one group of through-holes through which the electron beams pass and which are disposed on facing surfaces of the first focusing electrode 7 and the second focusing electrode 8 are formed by non-rotational symmetry. A fixed focusing potential V f  is applied to one of the first and second focusing electrodes 7 and 8, and a dynamic potential which gradually increases or decreases from the potential V f  in proportion to the increase of the deflection angle of the electron beam, is applied to the other. 
     By constituting an inline electron gun as mentioned above, the aberration due to the deflection of the electron beam can be reduced and almost circular beam spots are formed even on the circumference parts of the phosphor screen however, the application of the dynamic potential may cause discordance in the convergence of the three electron beams. If the above-mentioned non-rotation symmetry dynamic additional focusing electric field is adapted to another color picture tube shown in Japanese published unexamined patent application Sho No. 59-51440, where side through-holes of a focusing electrode through which electron beams pass are deflected outward in comparison with those of a control electrode and of an accelerating electrode in order to produce static convergence, then the magnification of a main lens of the above-mentioned color picture tube varies, and also an equivalent electron source moves. As the result, further misconvergence occurs. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The purpose of the present invention is to prepare an in-line electron gun in which good resolution and convergence can be obtained in the entire region of a phosphor screen and which can be adapted for use with the static convergence constitution as shown in Japanese published unexamined patent application Sho No. 59-51440. 
     An in-line electron gun in accordance with the present invention comprises: 
     a group of horizontally in-lined cathodes, 
     a control grid electrode, 
     an accelerating electrode, 
     a first focusing electrode, 
     a second focusing electrode, and 
     an anode which are respectively in-lined along the axis of the electron gun, wherein: 
     the cathodes are in-lined with a first distance S 1  between each respective electron gun beam axis, 
     the control grid electrode and the accelerating electrode have plural through-holes respectively in-lined at the first distance S 1 , each respective electron gun beam axis being in-lined with respective through-holes on the control grid electrode and the accelerating electrode, 
     the first focusing electrode has, on a surface facing the accelerating electrode, plural through-holes in-lined at a second distance S 2  which is larger than the first distance S 1 , 
     one of the surfaces of the first focusing electrode faces one of the surfaces of the second focusing electrode, each surface having plural vertically oblong through-holes in-lined at a third distance S 3  which is smaller than the first distance S 1 , 
     the other facing surface of one of the first focusing electrode and the second focusing electrode has one or more horizontally oblong through-holes, centers thereof being coincident with those of the vertically oblong through-holes, 
     the second focusing electrode also has a surface facing the anode containing a plurality of through-holes horizontally in-lined at a fourth distance S 4  which is smaller than the third distance S 3 , 
     the anode has a surface facing the second focusing electrode containing a plurality of through-holes horizontally in-lined at the fourth distance S 4 , and 
     the first focusing electrode receives a fixed focusing potential and the second focusing electrode receives a dynamic potential which changes to a potential higher or lower than the fixed focusing potential in response to an increase of the electron beam deflection angle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is the cross-sectional side view showing the electron gun of the conventional color picture tube. 
     FIG. 2 is the schematical view showing the shape of distortions of the beam spots at various positions on the phosphor screen. 
     FIG. 3 is a cross-sectional side view showing an electron gun of a color picture tube in accordance with the present invention. 
     FIG. 4 and FIG. 5 are respective front views showing through-holes of focusing electrodes in accordance with the present invention through which electron beams pass. 
     FIG. 6 is a schematic view showing misconvergence occurring when the potential of the second focusing electrode is higher than the potential of the first focusing electrode. 
     FIG. 7 is a front view showing through-holes through which electron beams of main lenses pass. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of an in-line electron gun in accordance with the present invention is described with reference to FIG. 3. The in-line electron gun comprises three cathodes 10a, 10b and 10c which are disposed horizontally in a line, a control grid electrode 11, an accelerating grid 12, a first focusing electrode 13, a second focusing electrode 14 and an anode 15. These elements are respectively disposed along an axis Z of the electron gun. The control grid electrode 11 has a circular center through-hole 11a and two circular side through-holes 11b and 11c through which electron beams pass, and the accelerating electrode 12 also has a circular center through-hole 12a and two circular side through-holes 12b and 12c through which the electron beams pass. The center axes of the center through-holes 11a and 12a are coincident with the axis Z of the electron gun, and center axes of the side through-holes 11b, 11c, 12b and 12c are respectively apart by the distance S 1  from the axis Z of the electron gun. 
     The first focusing electrode 13 has a circular center through-hole 13a and two circular side through-holes 13b and 13c on a surface which faces the accelerating electrode 12, and it also has a vertically oblong center through-hole 13d and two vertically oblong side through-holes 13e and 13f on the other surface which faces the second focusing electrode 14. The vertically oblong through-holes 13d, 13e and 13f are shown in FIG. 4 in detail. Center axes of the center through-holes 13a and 13d are respectively coincident with the axis Z of the electron gun. Center axes of the circular side through-holes 13b and 13c are apart by a distance S 2  from the axis Z of the electron gun, whereas those of the vertically oblong side though-holes 13e and 13f are apart by a distance S 3  from the axis Z of the electron gun. The second focusing electrode 14, on the other hand, has a horizontally oblong center through-hole 14a and two horizontally oblong side through-holes 14b and 14c on a surface facing the first focusing electrode 13. Second focusing electrode 14 also has a circular center through-hole 14d and two circular side through-holes 14e and 14f on the other surface facing the anode. The horizontally oblong through-holes 14a, 14b and 14c are shown in FIG. 5 in detail. Center axes of the center through-holes 14a and 14d are respectively coincident with the axis Z of the electron gun, center axes of the horizontally oblong side through-holes 14b and 14c are apart by the distance S 3  from the axis Z, and center axes of the circular side through-holes 14e and 14f are apart by a distance S 4  from the axis Z. 
     The anode 15 also has a center circular through-hole 15a and two circular side through-holes 15b and 15c. The center axis of the center through-hole 15a is coincident with the axis Z, and the center axes of the side through-holes 15b and 15c are respectively apart by the distance S 4  from the axis Z. When the electron gun is active, three main lenses L M  are generated between the through-holes 14d, 14e and 14f of the second focusing electrode and the through-holes 15a, 15b and 15c of the anode 15. 
     Typical DC (direct current) potentials applied to the respective electrodes in an active state are as follows: 
     Cathode: 50-150 V 
     Control grid electrode: 0 V 
     Accelerating electrode: 300-500 V 
     First focusing electrode: 6-8 KV 
     Second focusing electrode: dynamic potential 
     Anode: about 25 KV, 
     and the dynamic potential is varied synchronously with the horizontal deflection of the electron beams. Namely, when the horizontal deflection angle of the electron beams is zero, the second focusing electrode 14 receives a potential of 6-8 KV which is nearly equal to the potential of the first focusing electrode 13, and the dynamic potential gradually increases in response to the increase of the horizontal deflection angle of the electron beams. When the horizontal deflection angle of the electron beams is a maximum, the potential of the second focusing electrode 14 is about 0.2-0.5 KV higher than the potential of the first focusing electrode 13. 
     The misconvergence which occurs when the potential of the second focusing electrode is higher than that of the first focusing electrode is described with reference to FIG. 6. The distance S 4  is set to be e·M (S 4  =e·M) when the magnification of the main lens L M  is M and the distance between the equivalent electron source P for a side electron beam and the axis of the main lens L M  for the side electron beam is e. When an additional lens L S  is not formed by the first focusing electrode 13 and the second focusing electrode 14, namely, when the horizontal deflection angle of the electron beam is zero, the convergence of the three electron beams is kept. On the other hand, when the horizontal deflection angle of the electron beams increases and the additional lens L S  is formed between the main lens L M  and the equivalent electron source P, another equivalent electron source P&#39; is formed by the additional lens Ls, and at the same time, the magnification of the main lens L M  changes from M to M-Δ M. As a result, the arrival point of the side electron beam moves from point Q to point Q&#39;, which is at the distance ΔX from Q. Thus, the misconvergence occurs on the circumference parts of the phosphor screen. 
     The distance X between the center axis of the main lens L M  for the side electron beam and point Q&#39; is given as: ##EQU1## Then the distance ΔX, namely, the amount of misconvergence is approximated as 
     
         ΔX=Δe·M-e·ΔM           (2), 
    
     where Δe, ΔM, e and M are respectively positive values. In case of ΔX&gt;0, the side electron beam is overconverged with the center electron beam, and in case of ΔX&lt;0, the side electron beam is under-converged. 
     In order to avoid such misconvergence, the absolute value of ΔX needs to be as small as possible, which is realized by setting the distances as a relation, S 4  &lt;S 3  &lt;S 1 . The validity of this relation can be confirmed in the following fashion. When the distance S 3  between the center axis of the additional lens L S  for the side electron beam and the axis Z is set nearly equal to the distance between the equivalent electron source P and the axis Z, the distance Δe is nearly equal to zero. As the equivalent electron source P is generated nearly on the common axis of the through-holes of the control electrode 11 and the accelerating electrode 12, the distance between the equivalent electron source P and the axis Z is nearly equal to the distance S 1 , which is the distance between the centers of the side through-holes 11b, 11c of the control grid electrode 11 or 12b, 12c of the accelerating electrode 12. When Δe=0, the distance ΔX is given as ΔX=-e·ΔM from the expression (2), so the misconvergence occurs. The absolute value of the misconvergence is about 0.5 mm. Therefore, the distance S 3  should be smaller than distance S 1  (S 3  &lt;S 1 ). 
     Further, when the distance of the additional lens L s  from the axis Z is set nearly equal to the distance S 4  of the main lens L M  for the side electron beam from the axis Z, the distance Δe becomes excessively large. The large distance between the axis of the additional lens L S  and the equivalent electron source P is magnified by the lens L S , thereby causing an excessive shift of the equivalent electron source P&#39;. As a result, a misconvergence in which the absolute value of ΔX is about 0.5 mm occurs. Therefore, the distance S 3  should be larger than the distance S 4  (S 3  &gt;S 4 ) 
     According to the present invention, the occurrence of the misconvergence is prevented by setting the relation of the distances as S 4  &lt;S 3  &lt;S 1  so that ΔX is set nearly equal to zero. Also, in the present invention, in order to incline the side electron beams for making static convergence, the side through-holes 13b and 13c on the surface of the first focusing electrode 13 facing to the accelerating electrode 12 are disposed outward from the axis Z of the electron gun, in comparison with the side through-holes 11b, 11c, 12b and 12c on the control grid electrode 11 and the accelerating electrode 12. Namely the relation of the distances S 1  and S 2  is set as S 1  &lt;S 2 . 
     The most preferrable concrete values for use when the present invention is embodied in a 20 inch size (diagonal of the phosphor screen) and 90 degree deflection type color picture tube, are shown as follows. The diameter of each circular through-hole of the control grid electrode is 0.4 mm; the diameter of each circular through-hole of the accelerating electrode is 0.4 mm; the distance S 1  is 6.1 mm; the length of the first focusing electrode in the axial direction of the electron gun is 7.5 mm; the diameter of each circular through-hole of the first focusing electrode on the surface opposing the accelerating electrode is 1.5 mm; the distance S 2  is 6.2 mm; the distance between the accelerating electrode and the first focusing electrode in the axial direction of the electron gun is 1.0 mm; each vertically oblong through-hole of the first focusing electrode on the surface facing the second focusing electrode has a width of 2.2 mm and a length of 4.95 mm; the distance S 3  is 5.9  mm; the length of the second focusing electrode in the axial direction of the electron gun is 16.7 mm; each horizontally oblong through-hole of the second focusing electrode on the surface facing the first focusing electrode has a width of 4.95 mm and a length of 2.2 mm; the diameter of each circular through-hole of the second focusing electrode on the surface facing the anode is 4.5 mm; the distance S 4  is 5.5 mm; the diameter of each circular through-hole of the anode is 4.5 mm; the distance between the second focusing electrode and the anode is 1.0 mm; the dynamic potential has a minimum value of 8.0 KV and a maximum value of 8.4 KV (or when the minimum value is 6.0 KV the maximum value is 6.3 KV). 
     In the above-mentioned embodiment, the horizontal oblong through-holes of the second focusing electrode 14 on the surface facing the first focusing electrode 13 are provided as a set of three through-holes. However, one wide horizontal oblong through-hole without vertical partitions is usable. Also the through-holes 14d, 14e and 14f of the second focusing electrode 14 and the through-holes 15a, 15b and 15c of the anode 15, which face the respective counterparts for generating the main lens, can be made continuous into one through-hole 17 as shown in FIG. 7. In this case, however, as shown in Japanese patent Sho No. 58-20093, an electrode 18 for compensating the electric field must be disposed in order to form the three main lenses. Hereupon, the distance S 4  should be the distance between the center of the equivalent main lens for a side electron beam and the center axis of the electron gun.