Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
2000-03-09
2002-11-05
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
C313S414000, C313S412000
Reexamination Certificate
active
06476544
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a color cathode ray tube and, more particularly, to a color cathode ray tube having an electron gun in which a cathode structure arrayed in line within a cup-shaped first grid electrode is fixedly housed.
Color cathode ray tubes having a plurality of cathodes arrayed in line are generally used as image display devices for television receivers or monitors of data processing terminals.
This kind of cathode ray tube (CRT) has an evacuated envelope comprising a panel portion having a phosphor screen formed on its inner surface, a neck portion which houses an electron gun structure, and a funnel portion which connects the panel portion and the neck portion. A widely used type of electron gun structure is an inline type electron gun structure constructed to emit three electron beams toward the phosphor screen in a horizontal plane.
FIG. 7
is a view illustrating an example of a typical electrode gun for use in a cathode ray tube, which electron gun has a construction in which a cathode support and a first grid electrode are fixed. In
FIG. 7
, the electron gun has a cathode support
15
provided with a cathode inside, a cup-shaped first grid electrode
16
, welding spots
17
at which the first grid electrode
16
and the cathode support
15
are welded to each other, electron beam passing holes
18
provided in the first grid electrode
16
(reference numerals denote
18
S side electron beam passing holes, reference numeral
18
C denotes a center electron beam passing hole), and a bead portion
19
to be buried into a bead glass to fix the first grid electrode
16
.
The cathode support
15
is inserted inside of the cup-shaped first grid electrode
16
and is welded at the welding spots
17
in its open end portion. The welding spots
17
are also present in a back portion which is not shown in
FIG. 7
, so that first grid electrode
16
and the cathode support
15
are fixed to each other at four spots.
FIG. 8
is a cross-sectional view, taken in an inline direction, of a triode portion of the electron gun. Symbols K denote cathodes, and reference numerals
20
denote cathode structures each provided with a cathode K for emitting an electron beam toward the first grid electrode
16
(reference numeral
20
S denotes side cathode structures, reference numeral
20
C denotes a center cathode structure). The structure includes sleeves
21
to which the respective cathode structures are fixed, and a hermetic glass (insulating substrate)
22
. The first grid electrode
16
is a cup-shaped electrode in which the cathode support
15
is housed. Each of the cathode structures
20
is fixed in an electrically insulated state by the hermetic glass
22
, and is fixed to the cathode support
15
.
During the start-up period of the cathode ray tube, each of the cathode structures
20
is heated by a heater which is not shown. Each of the cathode structures
20
is thermally expanded by this heating and the distance between the cathodes K and the electron beam passing holes of the first grid electrode
16
becomes smaller, so that a larger amount of cathode current flows. Then, the first grid electrode
16
is thermally expanded and the distance between the cathodes K and the electron beam passing holes of the first grid electrode
16
becomes longer, so that the cathode current becomes gradually less. After that, the thermal expansion of the cathode structures
20
and that of the first grid electrode
16
comes to an end and the distance between the cathodes K and the first grid electrode
16
stabilizes at a constant value, so that the brightness on the screen becomes constant.
The first grid electrode
16
and the cathode support
15
used in the illustrated electron gun differ from each other in coefficient of linear thermal expansion (hereinafter referred to as the coefficient of thermal expansion). During the operation of the CRT, the electron gun is heated at a high temperature. In the electron gun constructed in this manner, since the first grid electrode
16
and the cathode support
15
are fixedly welded to each other, the first grid electrode
16
and the cathode support
15
are deformed by the difference between their coefficients of thermal expansion.
In the electron gun structure which constitutes the above-described electron gun, the amounts of thermal expansion assume the relationship of the first grid electrode
16
>the cathode support
15
. In this case, when the first grid electrode
16
is expanded, the cup-shaped first grid electrode
16
pulls the cathode support
15
in the directions indicated by arrows in FIG.
7
. Since the first grid electrode
16
is fixed to the bead glass by the bead portion
19
, the cathode support is deformed in the direction in which the central portion of the cathode support
15
approaches the first grid electrode
16
compared to the edge portion of the same. Accordingly, the cathode surfaces of the cathode structures
20
fixed to the cathode support
15
approach the first grid electrode
16
. Specifically, the distance between the cathode surface of the center cathode structure
20
C and the first grid electrode
16
becomes shorter than the distance between the cathode surface of the side cathode structures
20
S and the first grid electrode
16
.
FIG. 9
is a graph which shows a variation in cathode current with time, wherein the vertical axis represents cathode current and the horizontal axis represents time.
Reference numeral
23
denotes a variation in the cathode current of the center cathode, and reference numeral
24
denotes a variation in the cathode current of each side cathode.
For example,
FIG. 9
shows variations in cathode currents with time in a cathode ray tube of &phgr;29 neck in which the cathode support
15
is made of 42% Ni—Fe (coefficient of thermal expansion: 46×10
−7
/° C.) and the first grid electrode
16
is made of 50% NI—Fe (coefficient of thermal expansion: 100×10
−7
/° C.). As shown in
FIG. 9
, when about 10 minutes passes after power is turned on, the gap between the cathode surfaces and the first grid electrode becomes stable and the cathode currents become constant. For this reason, the best cathode current value (set value) is set to the value of each of the cathode currents obtained when about 10 minutes passes after power is turned on. As shown in
FIG. 9
, according to the welding positions in the above-described structure, when about 1 minute passes after power is turned on, the cathode current at the center cathode reaches 115% of the set value and the cathode current at each of the side cathodes reaches 150% of the set value, and the difference in cathode current between the center cathode and each of the side cathodes is about 35%. Normally, in the electron gun of &phgr;29, the difference in cathode current between the center and side cathodes reaches its maximum in about 1 minute after power is turned on, but in the case of the welding positions in the above-described structure, the difference in cathode current between the center and side cathodes reaches a maximum of 35% until the cathode currents become stable after power is turned on. The related art cathode ray tube has the problem that at the starting time of its operation, the difference between the cathode current of the center cathode and the cathode current of each of the side cathodes is so large that no desired colors can be displayed on the screen. In other words, in the related art cathode ray tube, the manner of variation of the distance between the cathode surface at the center portion and the first grid electrode differs from the manner of variation of the distance between the cathode surface at each side portion and the first grid electrode, so that it is difficult to stably supply electron beams to the phosphor screen.
It has recently been proposed to provide a cathode ray tube in which the sensitivity of its deflection yokes to electron beams is increased to reduce the power consumed for deflecting the electron beams. Such a cathode ray
Ishii Sakae
Miyamoto Satoru
Shiraishi Yasuhisa
Antonelli Terry Stout & Kraus LLP
Berck Kenneth
Hitachi , Ltd.
Patel Vip
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