Abstract:
An electron gun that can provide a desired electron beam modulation effect without preventing a modulation magnetic field from passing from an exterior of the vacuum portion is provided. A part of a tubular G3 electrode in an electron gun is formed into a coiled portion to allow the modulation magnetic field to pass through the clearances between parts of a wire composing the coiled portion.

Description:
FIELD OF THE INVENTION 
     This invention relates to an electron gun for a cathode ray tube. More specifically, this invention relates to a technique to improve high frequency magnetic field transmission property of an electron gun. 
     BACKGROUND OF THE INVENTION 
     FIG. 5 shows a structure of a conventional electron gun for a projection-type monochrome cathode ray tube disclosed in Unexamined Published Japanese Patent Application (Tokkai-Hei) 10-74465. In FIG. 5,  14  denotes a neck tube having an electron gun disposed inside the tube. The electron gun is formed by sequentially arranging a cup-shaped G 1  electrode (control electrode)  5  housing a cathode  6 , a cup-shaped G 2  electrode (acceleration electrode)  7 , a tubular G 3  electrode (pre-anodic electrode)  8 , a G 4  electrode (focusing electrode)  9 , and a G 5  electrode (anodic electrode)  10  enveloping the top end part of the G 4  electrode  9 . A main electron lens is formed between the G 3  electrode  8  and the G 4  electrode  9 . Another electron lens is formed inside the G 5  electrode  10  at a position between the G 4  electrode  9  and the G 5  electrode  10 . Outside of the neck tube  14 , a velocity modulation coil  18 , a convergence yoke  15 , and a deflection yoke  16  are disposed. 
     As shown in FIG. 5, the state-of-the-art for improving focusing performance is subjecting the electron gun disposed inside the neck tube  14  to magnetic field modulation caused by the velocity modulation coil  18  from outside of the neck tube  14  in order to carry out velocity modulation of an electron beam. Namely, a track of an electron beam outgoing from a cathode  6  is modulated by an alternating magnetic field generated by the deflection yoke  16 , the convergence yoke  15 , the velocity modulation coil  18  and the like, before the electron beam reaches a phosphorous screen surface. The deflection yoke  16 , which is attached to a funnel cone portion of the cathode ray tube, generates an alternating magnetic field  17  to deflect an electron beam track, so that the electron beam scans the phosphorous screen surface of the cathode ray tube. The convergence yoke  15 , attached to the outside of the neck tube  14  of the cathode ray tube, corrects raster distortion and color displacement by generating an alternating magnetic field  20  to modulate the electron beam track. The velocity modulation coil  18  is attached to the outside of the neck tube  14  of the cathode ray tube and generates alternating magnetic field  19  to modulate the scanning speed of the electron beam in order to prevent a high-intensity part on the phosphorous screen from extending to a low-intensity part and to sharpen images. 
     The frequency of an alternating magnetic field for modulating an electron beam reaches a mega-Hertz order equivalent to a frequency for images. Therefore, when an electron gun includes metal portions formed by deep-drawing metal materials such as stainless steel, the alternating magnetic field is damped and a desired electrode beam modulation cannot be obtained. 
     As shown in FIG. 5, most of the alternating magnetic field  20  generated by the convergence yoke  15  passes the G 5  electrode  10 . The deflection yoke  16  is attached to the funnel cone portion. A portion of the alternating magnetic field  17  generated by the deflection yoke  16  passes the G 5  electrode  10 . The velocity modulation coil  18  is disposed between the G 3  electrode  8  and the G 4  electrode  9 . Most of the alternating magnetic field  19  generated by the velocity modulation coil  18  passes the G 3  electrode  8  and the G 4  electrode  9 . 
     When an alternating magnetic field is applied through these metal electrodes, eddy current is generated at the metal electrode. The eddy current loss is increased as the frequency of the alternating magnetic field becomes high. Thus, modulation effect of the electron beam track due to the magnetic field in the high frequency modulation band is reduced. For example, eddy current is generated at the G 5  electrode  10  due to the alternating magnetic field  20  generated by the convergence yoke  15 . This decreases the electron beam track modulation effect provided by the convergence yoke  15 . 
     Furthermore, this eddy current loss can heat the electrodes and break the neck tube. If the source of the alternating magnetic field and the metal electrodes of the electron gun are positioned fully apart in order to prevent the loss of the alternating magnetic field or the electrode heat, the electron beam focusing lens is arranged inevitably separated from the phosphorous screen surface. As a result, the electron beam magnification becomes high and the resolution is lowered. Especially for image display apparatuses having high deflection frequencies and wide signal bands such as high definition television, the loss in the alternating magnetic field is increased. This increased loss causes problems in use. 
     Tokkai-Hei 8-115684 discloses the improvement of transmission property of the magnetic field by dividing the deep-drawn metal portions into several sections and providing clearances between the respective sections. However, this method causes problems such as deterioration in assembly accuracy or increased cost. Moreover, in order to maintain the mechanical strength, the divided sections cannot be made too small and thus, the magnetic field transmission property cannot be improved remarkably. 
     SUMMARY OF THE INVENTION 
     It is an object of one or more embodiments of this invention to solve these problems and provide a cathode ray tube having an electron gun that can provide a desired electron beam modulation effect without interrupting transmission of the modulation magnetic field from the exterior of the vacuum portion. 
     An electron gun in accordance with an embodiment of the present invention includes a tubular electrode for an electron beam to pass through the inside and at least one part of the tubular portion of the electrode is formed into a coiled portion. Accordingly, a modulation magnetic field passes through the clearances between parts of the coiled portion and thus, eddy current loss can be decreased. 
     Preferably in one or more embodiments of the electron gun, at least one part of the pre-anodic electrode (G 3  electrode) is formed into a coiled portion, so that an equipotential space can be formed inside the pre-anodic electrode. 
     Preferably, in one or more embodiments, the coiled portion is composed of a nonmetal material, so that the transmission effect of the modulation magnetic field is further improved. 
     In an embodiment the electron gun, the coiled portion may be a coiled wire. 
     Preferably, in one or more embodiments, the coiled portion is formed so that clearances between the parts adjacent to each other in the axial direction are not more than 2.5 mm. Accordingly, influences from the outer electric field can be reduced. Furthermore, the coiled portion may be formed so that the parts adjacent in the axial direction are contacted with each other. Accordingly, the strength of the electrode members can be improved while maintaining the effect of transmission of the modulation magnetic field. 
     A manufacturing method in accordance with an embodiment of the present invention is applied to provide an electron gun having a tubular electrode for passing an electron beam inside the electrode, in which at least one part of the tubular portion of the electrode is formed into a coiled portion. In the method, a coiled portion is formed by cutting spirally a tubular electrode member and then stretching the electrode member in the axial direction. According to the method, the coiled portion can easily be manufactured. 
     A cathode ray tube device in accordance with an embodiment of the present invention includes a cathode ray tube having an electron gun inside the neck portion, and a velocity modulation coil outside the cathode ray tube. At least one part of a pre-anodic electrode of the electron gun is formed into a coiled portion, and the velocity modulation coil is provided around the coiled portion of the pre-anodic electrode. Accordingly, velocity modulation effect can be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially broken side view to show an electron gun in accordance with an embodiment of the present invention. 
     FIG. 2 is a perspective view of a cathode ray tube in accordance with an embodiment of the present invention. 
     FIG. 3 is a graph to indicate the magnetic field modulation effect of an embodiment of the present invention. 
     FIG. 4 is partially broken side view to show an electron gun in accordance with an embodiment of the present invention. 
     FIG. 5 is an enlarged cross-sectional view to show an electron gun of a conventional cathode ray tube. 
     FIG. 6 is an enlarged cross-sectional view showing an electron gun in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following is a description of the preferred embodiments of the present invention in which an electron gun is used for a monochrome cathode ray tube, with reference to the accompanying drawings. 
     FIG. 1 is a side view of an electron gun in accordance with an embodiment of the present invention. An electron gun  4  is formed by sequentially arranging a cup-shaped Gi electrode (control electrode)  5  housing a cathode  6 , a cup-shaped G 2  electrode (acceleration electrode)  7  facing backward to the bottom of the G 1  electrode  5 , a tubular G 3  electrode (pre-anodic electrode)  8  disposed at a predetermined spacing to the opening of the G 2  electrode  7 , a G 4  electrode (focusing electrode)  9  for forming a main electron lens  21  in the space to the G 3  electrode  8 , and a G 5  electrode (anodic electrode)  10  enveloping the top end portion of the G 4  electrode  9 . Another electron lens will be formed in the internal of the G 5  electrode  10  at a position between the G 5  electrode  10  and the G 4  electrode  9 . 
     The G 3  electrode  8  has an equipotential space inside thereof, and a coiled portion  11  is provided to one part of the G 3  electrode  8 . A plate electrode  13  is provided to the coiled portion  11  at the end portion facing the G 4  electrode  9  in order to form an electron lens, while the other end portion is connected with the end portion  12  facing the G 2  electrode  7 . The coiled portion  11  is preferably located at the position where the velocity modulation coil is attached in view of penetrating the velocity modulation magnetic field. Therefore, the G 4  electrode  9  can be partially coiled in an alternative method. However, it is further preferable to form a coiled portion  11  at the G 3  electrode  8  rather than the G 4  electrode  9 , since the G 3  electrode  8  is effective in velocity modulation because the velocity of the electron beams is low in the G 3  electrode  8 . 
     A wire made of an electrode material is formed into a coiled portion  11  and welded to the end portion  12  and to the plate electrode  13 . Alternatively, the G 3  electrode  8  is formed integrally by a deep-drawing, partially cut in a spiral shape, and stretched in the longitudinal direction (axial direction) to form integrally the end portion  12 , the coiled portion  11 , and the plate electrode  13 . This allows the coiled portion  11  to be formed easily. 
     As shown in FIG. 2, the electron gun  4  is integrated in the neck portion of an envelope including a face plate  2  and a funnel  3  to compose a cathode ray tube  1 . 
     In a preferable embodiment described below, the present invention is applied to a monochrome cathode ray tube for a projection-type tube that is sized to be 16cm (7 inches), and the neck tube diameter φ is 29.1 mm. The coiled portion  11  is made of a stainless wire 0.8 mm in diameter. The length is 8.6 mm, the inner diameter is 10.4 mm, and the pitch is 1.6 mm. 
     The spacing between the adjacent wire parts of the coiled portion  11  is preferably 0 to 2.5 mm. Even if the adjacent wire parts are contacted with each other when the spacing is 0 mm, sufficient effects in transmitting modulation magnetic field can be obtained when compared to a case in which there is no joint, e.g., a simple deep-drawn plate. However, slight clearance is preferably provided between the adjacent wire parts to obtain a better modulation effect. When a spacing between adjacent wire parts exceeds 2.5 mm, influence of the exterior electric field is increased. 
     The coiled portion  11  can be made from a nonmetal material, such as a conductive ceramic or a sintered material of a mixture of carbon graphite and a binder. 
     An applicable conductive ceramic includes, for example, TiC or TiN as a main component to which a metal such as Co, Ni or Mo is mixed, or including an element such as Cu, Sr and ReO 3 . When such a conductive ceramic is used for the coiled portion  11 , raw material is shaped to be a pipe before being cut to be a coil or the raw material is directly coiled, and then sintered. 
     FIG. 3 is a graph showing an effect of the present invention, indicating the relationship between the frequency of the magnetic field modulation and the effect of the magnetic field modulation. The measurement was carried out in case picture signals of rectangular shape for displaying vertical stripes on the phosphorous screen are supplied to the picture tube. The “effect of magnetic field modulation” indicates how much the width of the vertical lines on the phosphorous screen varies (arbitrary unit) between conditions with and without the velocity modulation. Higher value indicates the better effect for the magnetic field modulation. In FIG. 3, the curve (a) indicates a conventional electron gun without a coiled portion, the curve (b) indicates an electron gun of the present invention having a coiled portion of a metal material, and curve (c) indicates an electron gun of the present invention having a coiled portion of a conductive ceramic. As shown in FIG. 3, electron guns of the present invention can provide a greater magnetic field modulation effect than the conventional gun over a wide range of frequencies. It is also known from FIG. 3 that a coiled portion made of a conductive ceramic can provide a better magnetic field modulation effect than a coiled portion made of a metal material. 
     FIG. 4 is an embodiment where the present invention is applied to a G 3  electrode  8  of an electron gun for a monochrome cathode ray tube, as in the case of FIG.  2 . In this embodiment, no plate electrode is provided to the end portion of the coiled portion  11  facing the G 4  electrode  9 . The end face of the coiled portion  11  forms an electrode. Also, a main electron lens  21  is formed between the end face of the coiled portion  11  and the G 4  electrode  9 . 
     Though the present invention is applied to a monochrome cathode ray tube in the above-mentioned embodiments, it can also be used for a color cathode ray tube. In such a case, for example, a coiled portion is provided to a G 3  electrode enveloping three electron beams. 
     The present invention can provide a cathode ray tube having an electron gun to obtain a desired electron beam modulation effect without preventing modulation magnetic field from passing from exterior of the cathode ray tube. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.