Electron gun for color cathode-ray tube

Abstract: An electron gun for a color cathode-ray tube for enhancing the convergence characteristics and removing the flare of beam spot formed at the boundary of screen, comprising first and second (9) grid electrodes and a first accelerating and focusing electrode (10) each having first, second and third electron beam passing holes (91-93) for allowing first, second and third electron beams emitted from cathodes to pass therethrough so as to be accelerated and focused; first and third slots formed around said first and third electron beam passing holes and having asymmetrical depth for allowing equipotential intervals at inner side to be larger than equipotential intervals at outer side, the first and third electron beam passing holes being symmetrical to each other with respect to the second electron beam passing hole; and a second slot formed around the second electron beam passing hole and provided with a symmetrical depth so as to have same equipotential interval at its inner and outer side with respect to the center of the second electron beam passing hole.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron gun for a color cathode-ray tube for enhancing the convergence by efficiently focusing electron beams emitted from three cathodes of in-line alignment on a fluorescent screen and removing the flare of beam spot which is produced around the fluorescent screen of color cathode-ray tube in terms of the deflection magnetic field for self-convergence. 2. Description of the Prior Arts In general, a color cathode-ay tube is structured, as shown in Fig. 1, such that three electron beams Bs, Bc and Bs are emitted from an electron gun 2 contained in a neck portion 1 in backward of a glass bulb and focused on a point of a shadow mask 3, and then combined with R.G.B. colors so as to reproduce desired images on a fluorescent screen 5 which is doped on the internal surface of a panel 4. The electron gun is of an in-line type for emitting three electron beams in parallel with the axis (A-A) of the color cathode-ray tube, and must have an electron beam focusing structure in order to focus the three parallel beams on one point of the fluorescent screen. Figs. 2 and 3 illustrate an electron gun which is generally applied to a color cathode-ray tube. As shown in Figs. 2 and 3, the electron gun comprises three cathodes 7 each having a heater 6 therein, first and second grid electrodes 8 and 9, first accelerating and focusing electrode 10 each of which has three electron beam passing holes 81, 82, 83, 91, 92, 93, 101, 102 and 103 being spaced with each other as much as a predetermined distance S and aligned in the same axial line, and second accelerating and focusing electrode 11 of which a central electron beam passing hole 112 is aligned in the same axial line as the electron beam passing holes 82, 92 and 102 of the first and second grid electrodes and first accelerating and focusing electrode 10 and side electron passing holes 111 and 113 are aligned eccentrically to the electron beam passing holes 81, 83, 91, 93, 101 and 103 of the first and second grid electrodes and first accelarating and focusing electrode 8, 9 and 10 as much as a predetermined distance ΔS toward the outer side. In the above structure, the amount of eccentricity ΔS is determined by establishing such that the diameters of the electron beam passing holes 111 and 113 of the second accelerating and focusing electrode 11 are larger than or the same as the diameters of the electron beam passing holes 101 and 103 of the first accelerating and focusing electrode 10, and the distance S' between the electron beam passing holes of the second accelerating and focusing electrode 11 is larger than the distance S between the electron beam passing holes of the first accelerating and focusing electrode 10. Referring to Fig. 4 which shows a convergence structure in which the electron beam passing holes 101, 103 and 111, 113 of the first accelerating and focusing electrode 10 and the second accelerating and focusing electrode 11 are formed in eccentric as much as the amount of eccentricity ΔS, when a voltage is applied from the outside of the electron gun 2, equipotential lines V1, V2 ..., which are called as a main electron lens, for focusing the electron beams Bs, Bc and Bs are formed at the space between the first and second accelerating and focusing electrodes 10 and 11 so that a plurality of electron beams which are emitted from the cathodes 7 can be focused on the fluorescent screen as a beam spot. At this moment, the equipotential lines at the second accelerating and focusing electrode 11 are formed In asymmetrical with respect to the electron beam path between the electron beam passing holes 101, 103, 111, and 113, by the eccentricity ΔS. Accordingly, the electron beam Bs which passes through the above path advances refractively toward the central beam Bc as much as predetermined angle ' by an equation of refraction VYQ=V'Y'Q', and then focused on a point on the fluorescent screen 5. Meanwhile, the main electron lens formed between the first accelerating and focusing lens 10 and the second accelerating and focusing lens 11 has to focus respective electron beams and converge the side beams Bs. However, in practice since the refranctive index of the main electron lens is varied when the focusing voltage is adjusted to enhance the focusing characteristics, and shape of the equipotential lines between the electron beam passing holes 101, 103, 111 and 113 becomes also varied. As a result, the focusing characteristics are varied so that the two requirements as above can not be satisfied. In addition, since the convergence rate must be varied depending upon the size of the color cathod-ray tube, there occurs a problem in that the eccentricity ΔS must be adjusted properly in correspondence with the size of the color cathode-ray tube, and also a further problem occurs in that the number of parts of the second accelerating and focusing electrode 11 Is large so that the workability for assembling the electron gun becomes lower. Furthermore, in the color cathode ray tube which adopts a circular symmetrical lens system, although a thin and round beam spot can be obtained at the center of the fluorescent screen by a strong quadrupole magnetic field within a color cathode-ray tube having a deflection yoke of non-uniform magnetic field for self-convergence, a flare that electronic density is low is formed at the circumferentical portion of the beam spot so that the focusing characteristics are deteriorated and thus the resolution of the color cathode-ray tube becomes lower. The self-convergence is a method for directing three electron beams to focus on a point by a deflection of electron beams even at the circumferential portion of the screen of a color cathode-ray tube. That is, the magnetic forces applied to three electron beams form non-uniform magnetic fields differently by means of the deflection yoke positioned just before the electron gun 2, as shown in Fig. 1. By such an arrangement, although the self-convergence characteristics may be obtained, but it is inevitable that the focusing characteristics of electron beams become deteriorated. Considering the problems mentioned above, an electron gun with a convergence structure as shown in Figs. 5A and 5B has been proposed. In such a type of electron gun, the second grid electrode 9 has longitudinal slots 94, 95 and 96 each of which has the same width as that of electron beam passing holes 91, 92 and 93. The slot 95 is positioned symmetrically with respect to the central electron beam passing hole 92 while other two slots 94 and 96 are in eccentric with respect to the center of the side passing holes 91 and 93. In Fig. 5A, the electron beam passing holes 101, 102 and 103 of the first accelerating and focusing electrode 10 and the electron beam passing holes 91, 92 and 93 of the second grid electrode 9 are disposed in the same axial line, and the dimension of the slots 94, 96 and 96 in lengthwise is determined by the equation of ℓ1 + ℓ2 = 2ℓ3 and ℓ2 > ℓ1. According to this type of electron beam convergence structure, the equipotential lines V1, V2 ... are formed asymetrically on the slots 94 and 95 of the second grid electrode 9 which are disposed asymetrically around the electron beam passing holes 91 and 93. That is, at the outer position ℓ1 where the length of the slot Is short with respect to the center of the electron beam passing hole, the gradient of the equipotential lines is abrupt, while at the inner position ℓ2 where the length of the slot is large the gradient thereof Is gentle. So, the electron beams Bs which have been passed through the side electron beam passing holes 91 and 93 pass through the second grid electrode 9 and then converged into the central beam by refracting toward inner side at a predetermined angle . Such an electron gun having a convergence structure at the second grid electrode 9 gets good convergence characteristics because the convergence structure between the first accelerating and focusing electrode 10 and the second accelerating and focusing electrode 11 compensates for the convergence deterioration caused by a variation of a convergence voltage. And, also since the slots 94, 95 and 96 strengthen the focusing operation in the breadthwise direction and deteriorates the longitudinal direction, the electron beams Bs, Bc and Bs passing through the passing holes 91, 92 and 93 are strongly focused in the breadthwise direction so that longitudinally extended electron beam is formed and then neutralized with an inverse quadrupole while passing through the main electron lens and the asymmetric magnetic field for self-convengence, thereby forming a beam spot of low density and low flare on the fluorescent screen, resulting in the increase in the resolution of the color cathode-ray tube. Such an electron gun is known of US-A-4 523 123. Another electron gun is revealed in JP-A- 2 012 740, wherein the influence to the static focus property of electron beams is reduced at an accelerating and focusing third grid electrode by forming inclines at both outer electron beam passing holes of that focusing electrode. The alignment of these inclines to the outer electron beam passing holes is asymmetrical. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide an electron gun for a color cathode-ray tube having a second grid electrode which is capable of being easily manufactured and applicable to various types irrespective of size of the cathode-ray tube. Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. The electron gun for a color cathode-ray tube of this invention comprises first and second grid electrodes and a first accelerating and focusing electrode each having first, second and third electron beam passing holes for allowing first, second and third electron beams emitted from cathodes to pass therethrough so as to be accelerated and focussed, wherein said second electron beam passing holes being centered on a center axis of said color cathode-ray tube; first and third slots formed around said first and third electron beam passing holes of the second grid electrode (9); and symetrically disposed with respect to the corresponding electron beam passing hole axis, said slots having an asymmetrical depth for producing equipotential intervals being on a side closer to the central axis of a color cathode-ray tube greater than on a side further from central axis of said cathode-ray tube, said first and third electron beam passing holes being symmetrical to each other with respect to the second electron beam passing hole of the second grid electrode (9); and a second slot formed around said second electron beam passing hole of the second grid electrode (9) having symmetrical depth to produce an uniform equipotential intervall with respect to the center axis of said color cathode-ray tube. According to a prefered embodiment of the invention said first and third slots are formed such that a depth of the first and the third slots on the side further from the center axis of said color cathode-ray tube is less than a depth of the first and third slots on the side closer to the central axis of said color cathode-ray tube and the depth of the first and third slots on the side closer to the central axis of said color cathode-ray tube and the depth of the first and third slots on the side closer to the central axis of said color cathode-ray tube is less than half of a thickness of the second grid electrode. In another prefered embodiment said second grid electrode including slots around said first and third electron beam passing holes on a side facing said electron gun and symmetrical slots around the first, second and third electron beam passing holes on a side opposite the side facing said electron gun. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: Fig. 1 is a longitudinal sectional view of a conventional color cathode-ray tube; Fig. 2 is a longitudinal sectional view of an electron gun of Fig. 1; Fig. 3 is a schematic sectional view of Fig. 2; Fig. 4 is a longitudinal sectional view of the conventional electron gun in partial, showing the electron beam convergence structure; Fig. 5A is a longitudinal sectional view of another type conventional electron gun, showing the electron beam convergence structure; Fig. 5B is a plane view of a second grid electrode of Fig. 5A; Fig. 6 is a longitudinal sectional view of an electron gun in partial, showing the electron beam convergence structure according to an embodiment of the present invention; and Fig. 7 is a view the same as Fig. 6, but showing another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in detail to the drawings for the purpose of illustrating preferred embodiments of the present invention, the electron gun of the present invention is similar in structure to that in Figs. 1 to 3, but the structure of the second grid electrode 9 is changed as shown in Figs. 6 and 7. Accordingly, the present invention will now be described in connection with the second grid electrode 9 with reference to the first accelerating and focusing electrode 10. As shown in Fig. 6, longitudinally extended slots 94, 95 and 96 are formed around electron beam passing holes 91, 92 and 93 of the second grid electrode 9. The width of the slots 94, 95 and 96 is nearly the same as that of the passing holes 91, 92 and 93 and the length thereof is in symmetric with respect to the center of each of the passing holes 91 92 and 93 and larger than the diameter of each of the passing holes 91, 92 and 93. Furthermore, the depth of the central slot 95 is formed such that the depths(t) at both sides on the basis of the passing hole 92 are the same and has the relationship with the thickness(T) of the second grid electrode 9 of t ≤ T/2. And, the depth(t') of each of the slots 94 and 96 is the same as that of the central slot(t) in its inner side, but that in outer side is smaller than the depth(t) of the central slot 95 as t' < t. The second grid electrode 9 is disposed at a certain space from the first accelerating and focusing electrode 10 and the electron beam passing holes 91, 92, 93 and 101, 102, 103 of the second grid electrode 9 and the first accelerating and focusing electrode 10 are aligned in the same axial line. Referring to Fig. 7, longitudinally extended slots 94a and 96b are formed only at the inner side of the electron beam passing holes 91 and 93 of the second grid electrode 9 toward the first accelerating and focusing electrode 10, and the depth(t 0 ) of each of the slots 94a and 96a has the relationship with the total thickness(T) of the second grid electrode 9 as t 0 < T/2. In addition, on the opposite side of the slots 94a and 96a of the second grid electrode 9, longitudinally extended slots 94b, 96b and 96b are formed around the electron beam passing holes 91, 92 and 93 such that the width thereof is the same as the diameter of the passing holes 91, 92 and 93, and the length thereof is larger than and symmetrical with respect to the center of each of the passing holes 91, 92 and 93. And also the depth(to') of the slots 94b, 95b and 96b has the relationship with the total tickness(T) of the second grid electrode 9 as to' ≤ T/4. According to the present invention, equipotential lines V1, V2 ... having an abrupt gradient at their outer side and gentle gradient at their inner side are formed around the electron beam passing holes 91 and 93 at both sides, as shown in Fig. 6, and the electron beams Bs which have been passed through the passing holes 91 and 93 of the second grid electrode 2 are refracted toward inner side at an angle  by the refraction of the asymmetrical equipotential lines V1, V2 ... and thus converged toward the central beam Bc. Moreover, since the slots 94, 95 and 96 are formed such that the width thereof is the same as the diameter of the electron beam passing holes 91, 92 and 93 and the length thereof in the longitudinal direction is larger than the diameter of the passing holes 91, 92 and 93, the equipotential lines in the breadthwise are abrupt in their gradients so that their converging operation is strong while gentle in the longitudinal direction so that their converging operation is somewhat weak, thereby forming the electron beams Bs and Bc in the longitudinally extended shape. The electron beams Bs and Bc which have been focused in the longitudinally extended shape pass through the main electron lens to compensate for the magnetic quadrupole operation of the deflection yoke so that the flare of beam spot around the cathode-ray tube is suppressed. According to another embodiment of the present invention, as shown in Fig, 7, the longitudinally extended slots 94a and 96a formed around the passing holes 91 and 93 of the second grid electrode 9 function to converge the electron beams and the longitudinally extended slots 94b, 95b and 96b formed around the passing holes 91, 92 and 93 function to suppress a flare at the circumferential portion of a screen of the color cathode-ray tube. As described above in detail, the present invention provides the effect that it is possible to increase the convergence characteristics by converging efficiently the three electron beams on a point of the fluorescent screen and to remove the flare which may be produced at the circumferential portion of the screen in terms of the deflection magnetic field for self-convergence. Also, there is provided the effect that the manufacturing of the electrode is made simple by aligning the electron beam passing holes of the second grid electrode and the first accelerating and focusing electrode in the same axial line as well as forming the slots of the the second grid electrode in symmetrical, thereby being applicable to various types of cathode-ray tubes irrespective of the size thereof. The invention being thus described, it will be obvious that the same may be varied in many ways. Such modifications as would be obvious to one skilled in the art are intended to be included in the scope of the following claims.

Claims: An electron gun for a color cathode-ray tube, comprising: first and second grid electrodes (8,9) and a first accelerating and focusing electrode (10) each having first, second and third electron beam passing holes for allowing first, second and third electron beams emitted from cathodes to pass therethrough so as to be accelerated and focussed, wherein said second electron beam passing holes being centered on a center axis of said color cathode-ray tube; first and third slots (94,96,94b,96b) formed around said first and third electron beam passing holes (91,93) of the second grid electrode (9) and symmetrically disposed with respect to the corresponding electron beam passing hole axis, said slots having an asymmetrical depth for producing equipotential intervals being on a side closer to the central axis of a color cathode-ray tube greater than on a side further from central axis of said cathode-ray tube, said first and third electron beam passing holes (91,93) being symmetrical to each other with respect to the second electron beam passing hole (92) of the second grid electrode (9); and a second slot (95,95b) formed around said second electron beam passing hole (92) of the second grid electrode (9) having symmetrical depth to produce an uniform equipotential interval with respect to the center axis of said color cathode-ray tube. The electron gun of grid claim 1, wherein said first and third slots (94,96) are formed such that a depth of the first and the third slots (94,96) on the side further from the center axis of said color cathode-ray tube is less than a depth of the first and third slots (94,96) on the side closer to the central axis of said color cathode-ray tube and the depth of the first and third slots (94,96) on the side closer to the central axis of said color cathode-ray tube and the depth of the first and third slots (94,96) on the side closer to the central axis of said color cathode-ray tube is less than half of a thickness of the second grid electrode (9). The electron gun of claim 1, said second grid electrode (9) including slots (94a, 96a,) around said first and third electron beam passing holes (91, 93) on a side facing said electron gun and symmetrical slots (94b,95b,96b) around the first, second and third electron beam passing holes (91,92,93) on a side opposite the side facing said electron gun.

Inventors: KOH NAM JE, KOH, NAM JE
Applicants: GOLD STAR CO, GOLDSTAR CO. LTD.
Application year: 1991
Main classification: H01J29
IPC codes: H01J29