Electron gun assembly for cathode ray tube

Abstract: The electron gun assembly for cathode ray tube, has the structure in which the first, second and third grids (G3, G5 and G6) are arranged in the traveling direction of an electron beam (6R 6G, 6B) emitted from cathodes (K), and so is the fourth grid (G7) to which a anode voltage (Eb) is applied, a suppressing ring (25) is provided to be adjacent to the second and third grids (G5 and G6), a dynamic focus voltage (Vd) which is varied in synchronism with the deflection of the electron beam (6R 6G, 6B) is applied to the first and third grids (G3 and G6), and the first and second grids (G3 and G5) are connected to each other by means of the resistor (24) provided adjacent thereto.

Description: The present invention relates to an electron gun assembly for a cathode ray tube, used in a color image receiving tube (television picture tube), and more specifically, to an improved structure of an electron gun assembly for a cathode ray tube. In general, a color image receiving tube has, as shown in FIG. 1, an outer enclosure consisting of a panel 1 and a funnel 2. On the inner surface of the panel 1, a phosphorous screen made of three-color phosphorous layers which emit blue, green and red light rays, namely, a target 3 is formed. A shadow mask 4 is provided on a further inner side of the panel, so as to face the phosphorous screen 3. Also, an electron gun assembly 7 for emitting three types of electron beams 6B, 6G and 6R is provided in a neck 5 of the funnel 2. In the tube, as three electron beams 6B, 6G and 6R emitted from the electron gun are deflected by vertical and horizontal deflecting magnetic fields which are generated by a deflecting device 8 mounted on an outer side of the funnel 2, the phosphorous screen 3 is scanned vertically and horizontally by these three electron beams 6B, 6G and 6R via a shadow mask 4, thus displaying a color image. In connection with the color image receiving tube described above, the self-convergence-inline type color image receiving tube is widely used in practice. The color image receiving tube of this type has the following structure. That is, an inline type electron gun assembly is employed, and which emits three electron beams 6B, 6G and 6R arranged in a line, which are specifically, a center beam 6G and a pair of side beams 6B and 6R running on the same horizontal plane. These three electron beams 6B, 6G and 6R arranged in the line are deflected by a pin-cushion shaped horizontal deflection magnetic field and a barrel shaped vertical deflection magnetic field, which are generated by a deflection device 8 to which a deflection signals is supplied from a deflection unit (not shown). Therefore, the three electron beams 6B, 6G and 6R can be converged without providing a special converging device. However, in connection with the self-convergence inline type color image receiving tube, the vertical and horizontal deflection magnetic fields generated by the deflection device 8 have pin-cushion shape and barrel shape, respectively, which are not uniformed within themselves. As a result, the three electron beams 6B, 6G and 6R are affected by these non-uniform magnetic fields to create astigmatism, and therefore at the peripheral portion of the screen, they are strongly converged in the vertical direction. Consequently, at the peripheral portion of the screen, the beam spot is made to have an elliptical shape elongated in the horizontal direction, including a core portion having a high luminance and a halo portion having a low luminance located in the vertical direction to the core portion. Such a phenomenon deteriorates the resolution at the peripheral portion of the screen. As a solution to the deterioration in the resolution at the peripheral portion of the screen, an electron gun assembly of the dynamic focus type, has been proposed, which has a structure in which quadruple-pole lens is formed in the main electron lens unit for focusing an electron beam. In this electron gun assembly, an electron beam generating unit, a quadruple-pole electron lens and an ultimate focusing lens are formed in the order in the traveling direction of the electron beams, that is, along the direction from the cathodes towards the phosphorous screen. With this structure, a dynamic focus voltage which varies in synchronism with the deflection of the electron beam, is supplied to an opposite electrode which constitute the quadruple-pole lens. Thus, the electron beams traveling towards at the peripheral portion of the screen are diverged in the vertical direction, and converged in the horizontal direction. Further, the horizontal- and vertical-directional focusing effect of the ultimate focusing lens is weakened, thus suppressing the deterioration in the resolution at the peripheral portion of the screen. The electron gun assembly of this type entails the problem of a withstanding voltage of the voltage supply unit since two types of medium voltages must be supplied to the opposite electrode from an external device in the quadruple-pole lens. Jnp. Pat. Appln. KOKAI Publication No. 1-232643 and U.S. Patent No. 4,945,284 and corresponding EP 0 332 469 A each provide a solution to the above-stated problem, and they disclose an electron gun assembly having the following structure such as shown in FIG. 2. That is, the electron gun assembly has cathodes K, and first to seventh grids G1 to G7 which are arranged in the order from the cathodes K towards the phosphorous screen. Further, a resistor 10 is provided adjacent to the fifth and sixth grids G5 and G6 which constitute a quadruple-pole lens QL, so as to connect the fifth and sixth grids G5 and G6 via the resistor 10, thus supplying a voltage to the fifth grid G5 via the resistor 10. However, the cathode ray tube having the above-described structure entails the following problem. That is, in the color image receiving tube shown in FIGS. 1 and 2, a anode voltage of 23 to 35 kV is supplied to the seventh grid G7 via a conductive film 12 applied on over an area from an inner surface of a portion of the funnel 2, having a larger diameter, to an inner surface of the adjacent portion to the neck 5, a valve spacer 13 brought into elastically contact with the conductive film 12 and a shield cup C. A focusing potential, which is about 20 to 35% of the anode voltage is applied to the sixth grid G6 via a stem pin 14 which air-tightly pierces through the end portion of the neck 5. In some cases, the inner surface of the neck 5 is charged under the influence of the conductive film 12, and an electric field emission phenomenon may occur from a grid which constitutes the electron gun assembly. If an electron emission occurs in the sixth grid G6 due to the electric field emission phenomenon, the focusing potential is varied, resulting in that the electron beam is not focused in an optimal fashion. Consequently, the resolution is lowered, and the image quality is deteriorated. Further, a discharge may occur due to the electron emission, and the discharge current or noise thus created may damage the electrical circuit of the cathode ray tube apparatus, or may cause a malfunction of the computer connected thereto. In order to avoid such a problem, the following measures have proposed. That is, a suppressor ring made of a metal wire is set so as to surround an insulation support rod 16 for fixing the electrodes of the electron gun assembly into an integral body, and then the suppressor ring is evaporated by heat to form a metal deposition film on the inner surface of the neck 5 in a manufacturing step of the color image receiving tube, thereby stabilizing the potential of the inner surface of the neck 5. However, as can be seen in FIG. 2, with regard to the electron assembly gun having the structure in which a resistor 10 is placed adjacent to the fifth and sixth grids G5 and G6, it is sometimes very difficult to dispose the suppressor ring at an effective position. More specifically, in the case where the suppressor ring is placed between the terminal 17a of the resistor 18 and the shield cup C, for example the sixth grid G6, the metal deposition film is formed adjacent to the conductive film 12, and therefore the potential of the inner surface of the neck 5 cannot be decreased. In the case where the suppressor ring is disposed between terminal 17a and terminal 17b of the resistor 10, parts of the metal deposition film may be attached to the resistor 10 located near-by, which creates a short-circuit between both terminals 17a and 17b of the resistor 10. For this reason, the suppressor ring cannot be provided at the above positions. On the other hand, in the case where the suppressor ring is provided close to the cathodes K rather than to the resistor 10, the metal deposition film is attached partially onto the inner surface of the neck 5 at a low potential, and therefore the potential of the inner surface of the neck 5 can be stabilized. However, the portion of the inner surface of the neck 5, which is close to the sixth grid G6, is located near the conductive film 12, and therefore the potential cannot be sufficiently lowered. The effect of suppressing the electric field emission from the sixth grid G6, the terminal 17a of the resistor 10 connected to the sixth grid G6, or the like, may not become satisfactory. EP-A-0 367 250 discloses an electron gun assembly for a cathode ray tube wherein an S-ring is provided in such a manner as to surround a resistor. This document is directed to the arrangement of a grommet between a S-ring and the resistor, that is the coupling portion between the resistor and an electrode. EP-A-0 332 469 relates to an electron gun assembly using quadropole dynamic focus system. Two focus electrodes are connected to a resistor so that the electron gun assembly may be suitable for use in a high resolution color cathode ray tube. This document does not specifically describe an S-ring. JP-A-04-322039 discloses an electron gun assembly comprising two S-rings provided in such a manner as to surround a resistor. This document is directed to the location to arrange the S-rings and how to determine the diameters of the neck. The object of the present invention is to provide an electron gun assembly which can prevent the deterioration of the image quality and the lowering of the reliability, which may be caused by the electric field emission from a grid or a resistor, and which is capable of constituting a high-quality and high-reliability cathode ray tube. According to the present invention there is provided an electron gun assembly according to claim 1. The dependent claims are related to further advantageous aspects of the present invention. Further, the suppressor ring is connected to the fifth grid at a position closer to the sixth grid rather than to the resistor. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a diagram schematically showing the structure of a color image receiving tube; FIG. 2 is a diagram showing the structure of a conventional electron gun assembly for a color image receiving tube; FIG. 3 is a diagram showing the structure of an electron gun assembly for a color image receiving tube, according to an embodiment of the present invention; FIG. 4A is a diagram showing the shape of electron beam passage holes of a surface of the fifth grid of the electron gun shown in FIG. 3, which is opposite to the sixth grid; and FIG. 4B is a diagram showing the shape of electron beam passage holes of a surface of the sixth grid shown in FIG. 3, which is opposite to the fifth grid. An electron gun assembly according to an embodiment of the present invention will now be described in detail with reference to drawings. FIG. 1 shows an electron gun assembly for color image receiving tube, according to the embodiment. The electron gun assembly includes three cathodes K arranged in a line in a horizontal direction, three heaters H for heating these cathodes K respectively and individually, first to seventh grids G1 to G7 each arranged such as to be a certain distance away from the respective cathode K, in the direction of the phosphorous screen, that is, the target, and a shield cup C mounted to the end of the seventh grid G7, which is on the phosphorous screen side, and the heaters H, cathodes K and the first to seventh grids G1 to G7 are fixedly integrated into one body by means of a pair of insulation support rods 20. The first and second grids G1 and G2 are made of plate electrodes having an integrated structure, and the third to seventh grids G3 to G7 are made of cylindrical electrodes having an integrated structure. In each of the first and second grids G1 and G2, three, relatively small, and substantially circular electron beam passing holes, which respectively correspond to three cathodes K, are made in one row. In a surface of the third grid G3, opposite to the second grid G2, three, larger than those of the second grid G2, and substantially circular electron beam passing holes, which respectively correspond to three cathodes K, are made in one row. Further, in a surface of the third grid G3, opposite to the fourth grid G4, three, larger than those of the second grid G2, and substantially circular electron beam passing holes, which respectively correspond to three cathodes K, are made in one row. In a surface of the fourth grid G4, opposite to the third and fifth grids G3 and G5, three, same-sized as those of the surface of the third grid G3, opposite to the fourth grid G4, and substantially circular electron beam passing holes, which respectively correspond to three cathodes K, are made in one row. In a surface of the fifth grid G5, opposite to the fourth grid G4, three, substantially the same-sized as those of the surface of the fourth grid G4, and substantially circular electron beam passing holes, which respectively correspond to three cathodes K, are made in one row. In a surface of the fifth grid G5, opposite to the sixth grid G6, three electron beam passing holes 21a, 21b and 21c, substantially elongated in a vertical direction (V-axis direction), a diameter of which is taken in that direction, and respectively correspond to three cathodes K, are made in one row, as shown in FIG. 4A. In a surface of the sixth grid G6, opposite to the fifth grid G5, three electron beam passing holes 22a, 22b and 22b, substantially elongated in a horizontal direction (H-axis direction), a diameter of which is taken in that direction, and respectively correspond to three cathodes K, are made in one row, as shown in FIG. 4B. In a surface of the sixth grid G6, opposite to the seventh grid G7, three substantially circular electron beam passing holes having substantially the same size as those of the surface of the fifth grid G5, opposite to the fourth grid G4 are made in one row. In a surface of the seventh grid G7, opposite to the sixth grid G6 and the shield cup C, three electron beam passing holes having substantially the same size as those of the surface of the sixth grid G6, opposite to the seventh grid G7, and respectively correspond to three cathodes K, are made in one row. Further, in a bottom portion of the shield cup C, three substantially circular electron beam passing holes having substantially the same size as those of the seventh grid G7 are formed in one row. In the electron gun assembly, a resistor 24 is provided near the third and fifth grids G3 and G5, and the third and fifth grids G3 and G5 are connected to each other by means of the resistor 24. Further, a suppressor ring 25 is mounted for the fifth grid G5 to be closer to the phosphorous screen, rather than to the resistor 24. In the electron gun assembly, a voltage obtained by superimposing a video signal on a DC voltage of 100 to 200V is applied to the cathodes K, and the first grid G1 is grounded. The second grid G2 and the fourth grid G4 are connected to each other within the tube, and a dynamic focus voltage Vd obtained by superimposing a voltage Vf which varies in a parabolic manner, synchronously with the deflection of the electron beam to a DC voltage Vd, which is 20 to 35% of a anode voltage Eb applied to the seventh grid G7, is applied to the third and sixth grids G3 and G6. To the fifth grid G5, at least a DC voltage component of the dynamic focus voltage Vd applied to the third grid G3 is applied via the resistor 24, and due to the static capacitance of the fifth grid G5 and the sixth grid G6, about 50% of the dynamic focus voltage is superimposed. Further, a anode voltage Eb of 25 to 35kV is applied to the seventh grid G7. By the application of the above-described voltages, in the electron gun assembly, an electron beam generating unit GE for forming an electron beam by controlling an electron emission from the cathodes K, and acceleration-focusing emitted electrons, is made of the cathodes K and the first to third grids G1 to G3, and a main electron lens unit ML for acceleration-focusing an electron beam generated from the electron beam generating unit GE, onto the phosphorous screen, is made of the third, fourth, fifth, sixth and seventh grids G3, G4, G5, G6 and G7. Particularly, in the main electron lens unit ML, a quadruple-pole lens QL for correcting a deflection aberration, by changing its lens intensity in synchronism with the deflection of the electron beam, is made of the fifth and sixth grids G5 and G6, and an ultimate focusing lens EL for focusing the electron beam ultimately on the phosphorous screen is made of the sixth and seventh grids G6 and G7. The electron gun assembly having the above-described structure exhibits the following effects. (a) The third grid G3 and the fifth grid G5 are connected to each other by means of the resistor 24, which is located at a position distant from the conductive film 27 to which a anode voltage Eb is supplied, and therefore the electric field emission from the terminals of the resistor 24 can be reduced. (b) Since the resistor 24 is connected to both of the third and fifth grids G3 and G5, a suppressor ring 25 can be mounted for the fifth grid G5 which is an electrode, to which a middle voltage is applied, located at a side of the seventh grid G7 to which a higher voltage than that of the resistor 24 is applied. With the suppressor ring 25 provided at the above-mentioned position, a metal deposition film can be formed on the region of the inner surface of the neck 28, which is closer to the seventh grid G7, as well as the region distant from the seventh grid G7, in the case where a metal deposition film is formed on the inner surface of the neck by applying a high-frequency magnetic field on the suppressor ring 25. Consequently, the resistor 24 can be provided at a position where the voltage on the inner surface of the neck is stable, and the electric field emission from the connection portion of the resistor 24 can be further efficiently reduced. Further, the metal deposition film can be formed at a position distant from the region where the anode voltage is applied, and therefore the leak current can be reduced. As shown in FIG. 3, in particular, when the suppressor ring 25 is mounted for the fifth grid G5 to which the resistor 24 is connected, the suppressor ring 25 and the resistor 24 can be maintained at the same potential, and thus the leak current between the suppressor ring 25 and the resistor 24 can be prevented. The above-described embodiment was described in connection with the case of an electron gun assembly of a bipotential type in which the ultimate focusing lens for focusing an electron beam ultimately on the phosphorous screen is symmetrical with respect to its axis; however, the present invention can be applied also to electron gun assembly including other types of symmetrical or asymmetrical lens, or a combination of these lenses. Further, the quadruple-pole lens is not limited to the type illustrated in FIG. 4. The present invention can be applied also to electron gun assemblies for cathode ray tube, other than the assembly for the color image receiving tube. In the case where the main electron lens portion of an electron gun assembly for cathode ray tube is constructed to have a structure as claimed in claim 1, a dynamic focus voltage which varies in synchronism with the deflection of the electron beam, is applied to the third and sixth grids, and the third and fifth grids are connected to each other by means of a resistor provided close to these grids, the following advantage can be obtained. That is, since the resistor is located at a position distant from the seventh grid G7 to which a anode voltage Eb is applied, and the conductive film on the inner surface of the neck, the electric field emission from the terminals of the resistor can be reduced. Further, with the structure in which the resistor is connected to the third and fifth grids, and the suppressor ring is connected closer to the seventh grid rather than to the resistor of the fifth grid, a metal deposition film can be formed on the region of the inner surface of the neck, which is closer to the seventh grid than to the resistor. Consequently, the resistor can be provided at a position where the voltage on the inner surface of the neck is stable, and the electric field emission from the connection portion of the resistor can be further efficiently reduced. In particular, when a suppressor ring is mounted for the fifth grid to which the resistor is connected, the suppressor ring and the resistor can be maintained at the same potential, and thus, for example, the leak current between the suppressor ring and the resistor can be prevented.

Claims: An electron gun assembly for a cathode ray tube, which emits an electron beam (6R, 6G, 6B) subjected to deflection, and having one end and an other end, comprising: cathodes (K) provided for one end, for emitting the electron beam (6R, 6G, 6B); directing means (G1, G2) consisting of a plurality of grids (G1, G2) provided adjacent to the cathodes (K) for directing the electron beam (6R, 6G, 6B) toward the other end, the plurality of grids of the directing means includes a first grid (G1) and a second grid (G2); a main electron lens portion for focusing the electron beam (6R, 6G, 6B) from the directing means (G1, G2) on a target (3), including third, fourth, fifth and sixth grids (G3, G4, G5, G6) arranged in order along a travelling direction of the electron beam (6R, 6G, 6B) and seventh grid (G7) to which an anode voltage (Eb) is applied, the fifth and sixth grids (G5, G6) being arranged to be adjacent to each other, and a same voltage being applied to the third and sixth grids (G3, G6); at least two insulation support rods (20) fixedly supporting the plurality of grids (G1-G7) of the directing means and the main electron lens portion; a resistor (24) provided above the insulation support rods and located near the third and fifth grids (G3, G5) and connecting the third and fifth grids (G3, G5) to each other; and a metal suppressor ring (25) separated from the resistor (24), provided between the resistor (24) and the other end of the electron gun assembly, surrounding said insulation support rods (20) and connected to the fifth grid (G5). An electron gun assembly according to claim 1, characterized by further comprising means (Vf) for applying a dynamic voltage to the third and sixth grids (G3, G6). An electron gun assembly according to claim 1, characterized in that the suppressor ring (25) is provided on either one or both of the fifth and sixth grids (G5, G6).

Inventors: HASEGAWA TAKAHIRO, KIMIYA JUNICHI, SUGAWARA SHIGERU, SUZUKI FUMIHITO, HASEGAWA, TAKAHIRO, KIMIYA, JUNICHI, SUGAWARA, SHIGERU, SUZUKI, FUMIHITO
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1997
Main classification: H01J29
IPC codes: H01J29