Indirectly heated cathode assembly and its associated electron gun structure

An indirectly heated type cathode assembly comprises a cathode sleeve having a heater within itself and having an emitter-impregnated type cathode disc fitted at one end, a plurality of straps connected at one end to a lower end portion of the cathode sleeve, and a cylinder holder whose upper end is connected to the other end of each strap, the holder being located outside the cathode sleeve such that it is spaced a predetermined distance apart from the cathode sleeve. A heat reflecting cylinder is located between the cathode sleeve and the holder of the indirectly heated type cathode assembly such that it is coaxial with the cathode sleeve and holder. The heat reflecting cylinder is supported by the holder and each strap extends such that it is not in contact with the heat reflecting cylinder. The strap is made of a Ta-W alloy or a Ta-W-Hf alloy. An electron gun structure comprises the indirectly heated type cathode assembly, a first grid placed in front of the indirectly heated type cathode assembly and an insulation support into which the first grid and the holder of the indirectly heated type cathode assembly are embedded partially and directly through a securing piece, respectively. The cathode disc is hidden, by a heat reflecting cylinder, from view at least that portion of the insulating support which is defined between an embedded spot of the first grid and that of the securing piece.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electric-power saving type 
high-performance, indirectly-heated cathode assembly for use, for example, 
in a color CRT (cathode ray tube) and its associated electron gun tube 
structure. 
2. Description of the Related Art 
Recently, there is a growing demand for a color CRT of an improved 
resolution with added scanning lines, an ultrahigh frequency-responsive 
picture tube and so on. A demand is also made for improved brightness, for 
example, in a projection CRT. In order to meet these demands, the density 
of emission electron from the cathode need to be increased to a greater 
extent. 
An emitter-impregnated type cathode can obtain a greater current density 
than an oxide cathode. For this reason, the emitter-impregnated type 
cathode has been employed for a pickup tube, travelling-wave tube, 
Klystron and so on. In the field of color CRTs, however, the 
emitter-impregnated type cathode finds only a limited application. 
The emitter-impregnated cathode of indirectly heated cathode assembly is 
constructed, such a type as shown in FIG. 1. In the structure shown in 
FIG. 1, a heater 1 is located within a cathode sleeve 2. A cap 4 is fitted 
into one end of the cathode sleeve 2 and has an emitter-impregnated 
cathode disc 3. A cylindrical holder 6 is disposed outside the cathode 
sleeve 2 such that it is situated coaxial with the cathode sleeve 2. The 
cathode sleeve 2 is fixedly supported by three straps 5 made of tantalum. 
The operation temperature of the aforementioned indirectly heated cathode 
assembly is higher than that of the oxide cathode type by about 
200.degree. C. Thus the indirectly heated cathode assembly requires more 
heater's electric power, presenting a bar to its practical application. 
For economy in the electric power of the indirectly heated cathode 
assembly, it is necessary that it be made compact. In order to obtain a 
compact unit, it will be proved effective to reduce the cross-sectional 
area of the strap and the heat conduction loss. 
However, the straps are so employed as to support the cathode and, if made 
too small, will be deformed at the operation of the cathode due to a 
fatigue resulting from heat. As a result, the characteristics of the color 
CRT become defective, such as degraded brightness or color drift. 
Japanese Utility Model Publication (KOKOKU) 59-33146 discloses a heat 
reflective means which is provided outside straps. In the structure of 
KOKOKU, the means is placed outside of straps and thermally contacted with 
straps, failing to achieve a saving in electric power and a compactness. 
The Japanese Utility Model Publication (KOKOKU) 57-26514 also discloses a 
heat reflecting cylinder which is located between a sleeve and straps and 
fixed to the sleeve. Since, however, the heat reflecting cylinder is 
placed in direct contact with the sleeve, heat is dissipated through the 
sleeve during operation, failing to achieve a saving in electric power. 
SUMMARY OF THE INVENTION 
It is accordingly the object of the present invention to provide an 
indirectly heated cathode assembly of better thermal efficiency and its 
associated electron gun structure which can suppress a heater's electric 
power. 
The indirectly heated cathode assembly of the present invention is of such 
a type that a heat reflecting cylinder is located between a cathode sleeve 
and a holder and fixed to the holder. Furthermore, straps have both ends 
attached to the corresponding lower end portion of the cathode sleeve and 
corresponding upper end portion of the holder and are thermally insulated 
from the heat reflecting cylinder. 
Furthermore, the indirectly heated type cathode assembly according to the 
present invention is of such a type that the straps are made of a Ta-W 
alloy or a Ta-W-Hf alloy. 
An electron gun structure according to the present invention is such that a 
first grid is located in front of the indirectly heated type cathode 
assembly. The first grid and the holder of the indirectly heated type 
cathode assembly are embedded partially and directly through a securing 
piece, respectively. The cathode disc is hidden, by the heat reflecting 
cylinder, from view at least that portion of an insulation support which 
is defined between an embedded spot of the first grid and that of the 
securing piece.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An indirectly heated type cathode assembly and its associated electron gun 
structure according to one embodiment of the present invention are shown 
in FIG. 2. 
In FIG. 2, reference numeral 7 shows a cathode sleeve made of tantalum. A 
heater 8 is provided within the cathode sleeve 7 and is of a coiled-coil 
type. A primary coil at an area A in FIG. 2 is wound at a finer pitch on 
the heater portion than the rest of the heater. In this embodiment, the 
area A is wound at a rate of the pitch about 1/3 that of the rest of the 
heater. 
A cup 9 which is made of tantalum is fitted into an open upper end of the 
cathode sleeve 7. An emitter-impregnated type cathode disc 10 is fitted 
into the cup 9 and obtained by impregnating a porous tungsten (W) 
substrate of about 20% in porosity with an electron emissive material. An 
iridium (Ir)-tungsten (W) alloy layer is formed on the surface of the 
cathode disc 10. 
The insulation degradation of the heater 8 occurs due to the scattering of 
vapor-phase deposits of the emitter material, such as Ba, from the cathode 
disc 10 toward the heater 8. In order to prevent such scattering, the cup 
9 is fitted into the open upper end of the cathode sleeve 7. 
Outside the cathode sleeve 7, a cylindrical holder 11 is provided coaxial 
with the cathode sleeve 7 such that it is spaced a predetermined distance 
apart from the cathode sleeve 7. The cathode sleeve 7 is supported by the 
holder 11 through a plurality of strip-like straps 12, for example, three 
straps. In this case, the strap 12 is connected at one end to the lower 
end portion of the cathode sleeve 7 and at the other end to the upper end 
of the holder 11. 
From the result of tests it has been found that the strap 12, if being made 
of, for example, a Ta-10%W alloy, Ta-3%W alloy, Ta-8%W-2%Hf alloy or 
Ta-10%W-2.5%Hf, reveals a high heat resistance and low-heat conduction. 
The other characteristics as obtained as the result of the tests are as 
shown in Table 1 below: 
TABLE 1 
______________________________________ 
Chemical Cutoff 
Composition Voltage 
(Wt %) Variation 
Samples 
Ta W Hf (V) Workability 
______________________________________ 
Conven- 
100 -- -- 3.0 good 
tional 
Assembly 
Sample 1 
Bal 2.5 -- 1.5 " 
Sample 2 
Bal 7.5 -- 0.3 " 
Sample 3 
Bal 10.0 -- 0.6 " 
Sample 4 
Bal 12.5 -- 0.7 possible 
Sample 5 
Bal 15.0 -- -- difficult 
Sample 6 
Bal 8 2 0.6 good 
Sample 7 
Bal 10 2.5 0.6 good 
Sample 8 
Bal 5 5 0.5 possible 
Sample 9 
Bal 3 7 -- difficult 
______________________________________ 
As seen from the Table 1, 2.5 to 12.5% of W in Ta or 2 to 5% of Hf in Ta in 
the chemical compositions of the samples are preferable, all of which are 
percent by weight. Between the cathode sleeve 7 and the holder 11, a heat 
reflecting cylinder 13 is located coaxial with the cathode sleeve 7 and 
holder 11 and supported relative to the upper end of the holder 11 by a 
plurality of support members such as support pieces 14. The support pieces 
14 are L-shaped in cross-section. 
As the support member, use may be made of not only the support pieces 14 
but also an annular support member. Or it may be possible to strike a 
portion of the heat reflecting cylinder, as a struck-out portion, out of 
itself or upset the heat reflecting cylinder by a press to provide a 
flange portion. 
As seen from FIG. 2, the strap 12 for supporting the cathode sleeve 7 is 
located such that it is not in contact with the heat reflecting cylinder 
13. That is, the strap 12 extends below the heat reflecting cylinder 13 
with a major portion parallel to the axis of the cylinder 13, and is 
welded to the upper end of the holder 11. 
A first grid 15 is located in front of the indirectly-heated cathode 
assembly thus configured, so that it is spaced a predetermined distance 
apart from the cathode assembly. The peripheral portion of the first grid 
15 is embedded in an insulation support 16 made of glass. One end of the 
fixing or securing piece 17 is mounted on the outer peripheral portion of 
the holder 11. The other end of the securing piece 17 is embedded into the 
insulation support 16. 
In this case, the cathode disc 10 is hidden, by the heat reflecting 
cylinder 13, from view at at least that portion (a portion indicated by B 
in FIG. 2) of the insulation support which is defined between the embedded 
spot of the first grid 15 and that of the securing piece 17. 
As a result, the heat reflecting cylinder 13 is provided between the 
cathode sleeve 7 and the holder 11 to shield vapor deposits of the emitter 
material coming from the cathode disc 10. By so doing, it is possible to 
prevent vapor deposition of the emitter material on the insulation support 
and stem section of electron guns. This improves the withstand voltage 
characteristic and stray emission characteristic of a color CRT. 
The indirectly-heated type cathode assembly according to this embodiment 
has the heat reflecting cylinder 13 and employs a low heat conduction 
material for the strap 12. Furthermore, the heater 8 is of a variable 
pitch type and hence provides an electric power-saving structure. 
By so doing, the dissipation power has only to be about one-third that of 
the conventional assembly shown in FIG. 1, noting that the power 
dissipation of the invention assembly is 0.7 W and the power dissipation 
of the conventional assembly is 2 W. Therefore, the indirectly heated 
cathode assembly of the present invention can be mounted on an oxide 
cathode-incorporated CRT without the need of altering an associated 
circuit. 
The result of an electric power saving leads to a lowering in heater 
temperature and an improved heater-to-sleeve withstand voltage 
characteristic, noting that, under an artificially harsh test, 
conventional assembly could perform up to 600 V but the present invention 
could perform up to 1200 V. 
According to the present invention, the cathode degradation resulting from 
ion impact can be prevented during the manufacture of a color CRT. That 
is, at the exhaust and high-voltage aging steps of the color CRT, 
discharge occurs across the first grid 15 and the cathode disc 10. Due to 
such discharge, the cathode is subject to ion impact, causing defective 
emission. 
In this embodiment, however, owing to the presence of the heat reflecting 
cylinder, discharge is produced across the forward end of the heat 
reflecting cylinder 13 and the first grid 15, causing no loss in the 
cathode disc 10. 
According to the present invention, since the material for the strap allows 
an improved heat resistance, it is possible to improve, for example, the 
degraded brightness and color drift of the color CRT. 
That is, a change in the dimension of Ggl-K (a gap between the first grid 
and the cathode surface), if being caused for some reason or other, 
results in a change in the cutoff voltage and hence a change in the anode 
current. 
For the color CRT, the cutoff voltage of the red, green and blue electron 
guns is so controlled as to develop predetermined color. 
However, the prolonged use of the color CRT causes the deformation of the 
associated component parts resulting from their fatigue by heat, thus 
giving rise to the dimensional change of Ggl-K. Since the dimensional 
change is not constant for the red, green and blue electron guns, anode 
current which is incident to the phosphor screen varies, thus producing a 
color drift and degraded brightness. 
In order to evaluate a possible dimensional change for a different strap 
material, tests were conducted to allow the indirectly-heated type cathode 
assembly of FIG. 2 to cool after being heated. The tests were repetitively 
conducted at a cathode temperature of 1150.degree. C. with the cathode 
assembly ON for five minutes and OFF for 10 minutes. The dimensional 
change between the cathode and the first grid is proportional to a change 
in the cutoff voltage and, therefore, the deformation of the strap can 
relatively precisely be measured by measuring the change in the cutoff 
voltage. In this way, measurement was made of the change in the cutoff 
voltage. 
Since a slow change occurred under the normal operation temperature 
condition, the cathode was caused to be heated at 1150.degree. C. and, 
after a stable condition was reached, allowed to cool. Such operations 
were repeated to examine a change in the cutoff voltage. FIG. 3 shows a 
change in the cutoff voltage for the case of a conventional tantalum strap 
and an alloy strap of the present invention, noting that the numerals in 
FIG. 3 correspond to those in Table 1. 
As seen from FIG. 3, a change in the cutoff voltage emerges, after 1000 
times ON-OFF tests, for the case of the conventional tantalum strap and 
almost no change in the cutoff voltage emerges over a very long period of 
time, for the case of the alloy strap of the present invention, in which 
the ON-OFF tests were conducted under the same condition. 
Furthermore, the cathode was caused to be heated up to 1250.degree. C., but 
a very small change in the cutoff voltage occurred. Hence the strap of the 
present invention revealed a very small change over a very long period 
even after many ON-OFF tests. 
According to the present invention, the strap reveals an improved heat 
resistance and allows its smaller cross-section. It is thus possible to 
prevent deformation of the strap by heat. 
That is, the conventional strap was 0.025 mm.sup.2 in cross-section and the 
strap of the present invention can reduce its cross-section to 0.01 
mm.sup.2 in terms of using a heat resistant alloy, ensuring a power saving 
of 0.2 W (30% of full power). 
As already set forth above, the indirectly-heated type cathode assembly of 
the present invention has the heat reflecting cylinder which is not in 
contact with the cathode sleeve, heat radiation near the cathode disc is 
suppressed, ensuring an enhanced cathode heat efficiency. 
Furthermore, the heat reflecting cylinder shields a vapor-phase deposition 
of the emitter material from the cathode disc onto the insulation support 
and stem section of the electron guns, thus improving the withstand 
voltage characteristic and stray emission characteristic of the color CRT. 
Since the strap is made of a Ta-W alloy or Ta-W-Hf alloy, it is possible to 
prevent heat deformation and to obtain an enhanced heat resistant unit. As 
a result, if the indirectly heated type cathode assembly is used for a 
color CRT, it is possible to prominently improve degraded brightness, 
color drift and the other characteristics of the color CRT. According to 
the present invention, it is possible to enhance the heat resistance of 
the strap and to obtain a compact strap and hence contribute to power 
economy. 
The cathode disc is not restricted to the emitter-impregnated type. The 
heat reflecting cylinder, cathode sleeve and cylindrical holder may not 
necessarily be made coaxial with each other.