In-line type electron gun for color cathode ray tube

An in-line type electron gun for a cathode ray tube includes a cathode, a control grid, a screen grid, and a main lens including a final focus electrode and an anode forming a major lens, at least one of the focus electrode and anode including a first member with three independent beam passing holes disposed at a beam passing plane thereof and a second member with a common beam passing hole disposed at a beam passing plane thereof, the beam passing planes being separated from each other by a fixed distance. According to the present invention, electron beam control errors originating from manufacturing errors can be reduced to a great degree. The electron gun of the present invention is simple and convenient to manufacture with a low product defect rate.

FIELD OF THE INVENTION 
The present invention relates to an electron gun for color cathode ray tube 
and, particularly, to an improved electron gun for a color cathode ray 
tube in which the electrodes of the main lens are improved, and 
consequently, the beam characteristics are improved to produce good 
quality images. 
BACKGROUND OF THE INVENTION 
In an in-line type electron gun is constituted such that three electron 
beams advance along paths in substantially the same plane. The electrons 
are emitted from three cathodes and are focused by means of a plurality of 
electrodes, each having three beam passing holes, ultimately focusing the 
beams on a screen. A dynamic deflecting magnetic field is established 
between the electron gun and the screen for determining the landing 
positions of the beams. The magnetic field is formed by a deflecting yoke 
to which horizontal and vertical deflecting signals are supplied and which 
sequentially deflects the beams onto the whole surface of the screen. 
When electron beams advance toward the screen after having departed from 
the electron gun, and are deflected by the deflecting magnetic fields, the 
electron beams are distorted by non-uniform vertical and horizontal 
magnetic fields. The distortions are particularly significant when the 
beams scan the peripheral portions of the screen. Due to such distortions 
of the electron beams, laterally elongated beam spots may be formed on the 
screen. 
The deflection aberrations are improved by the electron gun which is 
disclosed in U.S. Pat. No. 4,558,253, and which, as illustrated in FIG. 1, 
includes a cathode K, a control grid G1, a screen grid G2, and electrodes 
G3,G4,G5,G6. Particularly, the beam passing planes of the electrodes G5r, 
G6, which constitute a major lens, are respectively provided with recesses 
5R,6R at the centers thereof and three beam passing holes 5H,6H have 
projecting peripheral rims 5B,6B. 
Accordingly, a complex electric field is formed between the electrodes G5r, 
and G6, and consequently, three major lenses having large diameters and 
reduced spherical aberrations are formed. The peripheral major lenses are 
formed in an asymmetrical manner, thereby reducing the deflection 
aberrations of the peripheral electron beams. 
However, in the major lens formed by the electrodes G5r, G6, a focusing 
difference occurs between the vertical and horizontal directions with the 
result that the focus quality characteristics are reduced increasing the 
likelihood of spherical aberrations. 
Further, the electrodes G5r, G6 are complicated in their structures, and, 
therefore, the manufacturing process is difficult. Particularly, the 
portions requiring a high processing precision, i.e., the beam passing 
holes and the rims around them undergo plastic deformations during forming 
in a press with the result that high stresses are concentrated on the 
portions where the deformations have occurred, thereby reducing precision. 
Further, depending on the circumstances, partial disruptions can occur, 
thereby bringing a failure to the manufacturing of the electrodes. 
SUMMARY OF THE INVENTION 
Therefore, it is the object of the present invention to provide an in-line 
type electron gun for a color cathode ray tube in which the structure is 
improved such that the manufacturing is convenient and the focusing 
characteristics are improved, thereby upgrading the resolution of the 
screen. 
In achieving the above object, the in-line type electron gun for a cathode 
ray tube according to the present invention comprises a triode comprising 
a cathode, a control grid and a screen grid, and a main lens means 
including a final focus electrode and an anode, at least one of which 
includes: 
a first member having three independent beam passing holes disposed in the 
beam passing plane of the first member; and 
a second member having a common beam passing hole disposed in the beam 
passing plane of the second member, said second member being separated by 
a fixed distance from the beam passing plane of said first member. 
The electrode is very convenient to manufacture. Particularly, because the 
beam passing holes are formed in vertically elongate and laterally 
elongate shapes, the electron beams passing through these holes become 
vertically elongate to form normally circular beam spots upon arrival at 
the screen, thereby improving the resolution of the screen to a great 
degree.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 illustrates an electron gun provided with electrodes according to 
the present invention. As in the case of the usual electron gun, this 
electron gun includes a cathode K, control grid G1, a screen grid G2, and 
a plurality of electrodes G3,G4,G5,G6. The electrode G5 is a focusing 
electrode assembly to which a high potential focus voltage is applied. 
This electrode assembly includes a prefocus electrode G5f forming a 
prefocus lens in cooperation with the electrode G4 and a major focus 
electrode G5r forming a major focus lens in cooperation with the electrode 
G6. 
As shown in FIG. 3A, the major focus electrode (to be called hereinafter 
focus electrode G5r) includes a cup shaped first member F1 having three 
vertically elongate rectangular beam passing holes VH5 therein and 
provided with skirt F1S and a cup shaped second member F2 with a single 
laterally elongate beam passing hole HH5 formed therein and provided with 
a skirt F2S. The edge of the second member around the beam passing hole 
HH5 is provided with a rim 5B with the end of the rim contacting the first 
member at its beam passing plane. Meanwhile, the skirt F1S of the first 
member F1 and the skirt F2S of the second member F2 closely contact each 
other. 
The electrode G6 on which the highest anode voltage is applied includes a 
first member A1 and a second member A2 which are cup-shaped and similar in 
their shapes to the first and second members F1,F2 of said electrode G5r. 
Here, the skirt of A1S of the first member F1 and the skirt A2S of the 
second member A2 also closely contact each other. The beam passing plane 
of the first member A1 is provided with three laterally elongate beam 
passing holes HH6 which form asymmetrical quadrupole lenses in cooperation 
with the three vertically elongate rectangular beam passing holes VH5. The 
second member A2 is provided with a rim 6B along the circumference of the 
beam passing holes HH7 contacting the first member A1 at its beam passing 
plane. 
The electron gun of the present invention as described above corrects for 
the deflection aberrations which originate from the large diameter 
asymmetrical lens formed between the electrodes G5r and G6. Further, 
improved focus characteristics can be expected from the asymmetrical 
quadrupole lenses which are formed between the beam passing holes VH5,HH6 
of the first members of the electrodes G5r and G6. 
The control of the electron beams realized through the electrodes G5r,G6 in 
the vertical direction is as shown in FIG. 4A. When the electron beams 
pass through the first member F1 of the electrode G5r, the electron beams 
receive weak focusing forces in the vertical direction, while, when they 
pass through the first member A1 of the electrode G6, they receive strong 
diverging forces in the vertical direction. 
The control of the electron beams in the horizontal direction is as shown 
in FIG. 4B. When the electron beams pass through the first member F1 of 
the electrode G5r, they receive strong focusing forces in the horizontal 
direction, while, when they pass through the first member A1 of the 
electrode G6, they receive weak diverging forces in the horizontal 
direction. 
Thus, the electron beams are controlled in different ways in the vertical 
and horizontal directions so that the electron beams are vertically 
elongated during passing through the major lens so that they form 
substantially normal circular beam spots upon landing on the screen owing 
to the influence of the deflection aberrations caused by the deflecting 
magnetic fields. 
As described above, the electron gun according to the present invention is 
provided with major lens-forming electrodes which are divided respectively 
into the first and second members which are simple and convenient to 
manufacture, and, therefore, the control errors for the electron beams 
originating from the manufacturing (processing) errors can be reduced to a 
great degree. 
The first members F1,A1 and the second members F2,A2 of the respective 
electrodes are shaped like a cup, and the skirts of them closely contact 
each other with the result that the relative positional errors between the 
beam passing holes VH5,HH6 of the first members F1,A1 and the beam passing 
holes HH5,HH7 of the second members F2,A2 are minimized. Further, the gaps 
between the pairs of the beam passing holes are controlled by the 
extensions of the rims 5B,6B, and, therefore, the separation between the 
beam passing holes can be adjusted. 
The electron gun of the present invention as described above is easy to 
manufacture and easily precisely assembled. In addition, it improves the 
focus characteristics of the electron beams through the major lens, 
including a large diameter lens and a quadrupole lens. Therefore, the 
product defect rate is lowered, and the quality of the cathode ray tube is 
improved.