Electric lamp and discharge devices – Cathode ray tube – Envelope
Reexamination Certificate
2001-04-19
2003-08-05
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Envelope
C313S417000
Reexamination Certificate
active
06603253
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-119808, filed Apr. 20, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a color cathode ray tube, particularly, to a color cathode ray tube capable of suppressing the fluctuation in the cathode current brought about by the thermal expansion taking place in the members constituting the electron gun assembly.
In general, a color cathode ray tube comprises an envelope including a panel having a phosphor screen formed therein, and a funnel integrally bonded to the panel and including a neck. An electron gun assembly is received in the neck of the funnel.
A so-called “in-line-type” electron gun assembly, in which three electron guns are arranged in a line, is mainly used nowadays as the electron gun assembly of a color cathode ray tube. The in-line-type electron gun assembly comprises in general a beam generating section called triode, which includes a cathode electrode, a first electrode, and a second electrode, and a main lens section for focusing the three electron beams on a phosphor screen. The electron gun assembly is received in the neck of cylindrical structure having a diameter of about 20 to 40 mm, and a stem section in the shape of a circular glass is welded to the neck. Stem pins made of a conductive metal are buried in the stem section such that the electron gun assembly within the tube is connected to the circuit outside the tube via the stem pins, and the electron gun assembly within the tube is held by the stem pins in the stem section so as to be fixed within the tube.
In the triode section referred to above, voltage of hundred and scores of V is applied from outside the tube to the cathode electrode through the stem pins. Also, 0 V and hundreds of V are applied to the first and second electrodes, respectively, from outside the tube through the stem pins. The cathode electrode is provided with a heater for generating electrons from the cathode electrode. By heating the cathode electrode by the heater, an electron beam is emitted from the cathode electrode. The electron beam emitted from the cathode electrode passes through the beam-passing apertures of the first and second electrodes so as to be guided to the main lens section and, then, finally focused by the main lens section on the phosphor screen.
The main lens section is formed of at least two electrodes including a final accelerating electrode connected to an anode to which is applied a high voltage of about 25 to 30 kV through an inner conductive film coated on the inner surface of the neck section, and a focus electrode to which is applied a voltage about 20 to 40% of the anode high voltage through the stem pins.
In general, the final accelerating electrode and the focus electrode are arranged to face each other such that the beam-passing holes of these two electrodes are positioned apart from each other by about 1 mm. Applying a potential difference between these mutually facing two electrodes forms the main lens section, and the electron beam is focused on the phosphor screen by the main lens thus formed.
Each of these electrodes is fixed to and supported by an insulating supporting bar made of, for example, a glass. To be more specific, the strap mounted to each electrode is buried in the insulating supporting bar so as to have each electrode fixed and supported.
As described above, a heater mounted inside the cathode electrode heats the cathode electrode. Originally, the heater is intended to heat the cathode electrode alone. However, each of the electrodes of the electrode gun assembly including the cathode electrode and the heater is fixed to and supported by an insulating supporting bar made of, for example, a glass. As a result, the heat generated from the heater is transmitted to not only the cathode electrode but also to the other electrodes of the electron gun assembly by the heat conduction via the insulating supporting bar. These electrodes are also heated by the heat radiated directly from the cathode electrode itself so as to lead to the temperature elevation.
The temperature elevation caused by the heat of the heater is most prominent in the first electrode positioned closest to the cathode electrode. Then, the temperature elevation is gradually lowered in the second electrode, the third electrode, et seq. as the distance from the cathode electrode is increased.
What is serious is the temperature elevation of the first and second electrodes serving to control the generation limit (cutoff) of the electron beam from the cathode electrode. If the temperature of these electrodes is elevated, these electrodes are thermally expanded so as to change the distance between these electrodes and, thus, to change the cutoff. As a result, the cathode electrode current is changed with increase in the temperature elevation of the electrodes.
It should be noted that the first electrode and the second electrode are formed of relatively thin plates in many cases. Therefore, if the first electrode and the second electrode are thermally expanded and deformed, the distance between these first and second electrodes is changed so as to change the cutoff. As a result, the cathode electrode current is changed in accordance with elevation of the electrode temperature.
It is possible for the problem described above to take place also in the third electrode the electric field of which somewhat affects the cathode electrode. However, the effect of the electric field given from the third electrode to the cathode electrode is markedly smaller than that given from any of the first and second electrodes to the cathode electrode. In addition, the temperature elevation of the third electrode is small because the third electrode is positioned remote from the cathode electrode. It follows that the particular problem is substantially negligible when it comes to the third electrode.
It may be possible to overcome the above-noted problem by using materials low in thermal expansion coefficient for forming the first electrode, the second electrode, etc. However, it is impossible to suppress the thermal expansion to zero and, thus, it is necessary to design the first electrode and the second electrode based on a subtle combination of the thermal expansion coefficients.
It should also be noted that a wire made of a conductive material is welded to each electrode for the electrical connection to the circuit outside the tube. When it comes to the thin plate-like electrodes such as the first electrode and the second electrode, the electrode is provided with a welding margin to which one end of the wire for the electrical connection to the circuit outside the tube is welded, and the other end of the wire is welded to the stem pin.
Naturally, the wire welded to the electrode is also heated by the heat of the heater so as to be thermally expanded. What should be noted is that, since the wire is welded to a part of the electrode, the thermally expanded wire pushes the welded part of the electrode, giving rise to a problem that the distance between the adjacent electrodes is changed in the vicinity of only that portion of the electrode to which the wire was welded. It follows that, even if the materials of the first electrode and the second electrode are selected exquisitely, the first electrode or the second electrode is locally deformed.
The local deformation brings about a change in the cathode electrode current in only one of the three cathode electrodes arranged in-line so as to disturb the current balance of the three electron beams in accordance with temperature elevation of the wire. As a result, the color of the image displayed on the phosphor screen is prominently changed.
In order to overcome the problem, it is necessary to improve the supporting strength of the electrodes of each electron gun assembly, to improve the shape of the strap of each electrode buried in the insulating substrate, and
Hasegawa Takahiro
Oda Hiroyuki
Takeshima Masashi
Day Michael H.
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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