GB1596597A

GB1596597A - Cathode ray tubes

Info

Publication number: GB1596597A
Application number: GB1715278A
Authority: GB
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1978-04-29
Filing date: 1978-04-29
Publication date: 1981-08-26

Description

(54) IMPROVEMENTS RELATING TO CATHODE RAY TUBES (71) We, FERRANTI LIMITED a Company registered under the Laws of Great Britain of Hollinwood in the County of Lancaster do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed. to be particularly described in and by the following statement: This invention relates to cathode ray tubes, and in particular to cathode ray tubes each including at least one resistor to protect other, external electrical components connected in circuit with the cathode ray tube from damage by inadvertent current surges which may be generated by changes in the state of charge of capacitance associated with the final anode of the cathode ray tube as herein defined.

The final anode of the cathode ray tube as herein defined is in the form of a conductive coating on the glass envelope of the cathode ray tube, the conductive coating extending on substantially all the flared part of the envelope, and possibly also on the face plate of the cathode ray tube. The capacitance associated with the final anode is due, at least partially, to the extensive area of the coating, and may be due partially to external electrical components connected in circuit with the cathode ray tube.

It is known to provide at least one resistor to protect external electrical components connected in circuit with the cathode ray tube from such inadvertent current surges, for example, from the present applicant's British Patent Specification No. 1,448,223, in which there is described and claimed a current limiting resistor connected in series with the final anode of a cathode ray tube, the resistor being in the form of a pattern of resistive material printed onto an insulating substrate secured within the envelope of the cathode ray tube. Conveniently, the current limiting resistor is mounted on the electron gun assembly of the cathode ray tube.

The present invention relates in particular to the type of cathode ray tube each having a tetrode section electron gun, the electron beam produced by the gun having a high density. With such a type of cathode ray tube it is conventional to provide an apertured stop defining the cross-sectional shape of the electron beam at the stop. If this apertured stop is provided between the cathode and said at least one current limiting resistor, and if a part of the electron beam is intercepted by the stop, there is an inadvertent current flow through the stop and the current limiting resistor. This inadvertent current flow produces a potential difference across the current limiting resistor, which tends to change inadvertently the focus conditions for the electron beam.

It is an object of the present invention to provide a cathode ray tube of the type referred to in the preceding paragraph, and incuding at least one resistor to protect external components from inadvertent current surges, the construction of the cathode ray tube being such that inadvertent significant changes in the focus conditions for the electron beam, due to inadvertent potential differences across the current limiting resistor, are obviated.

According to the present invention a cathode ray tube having a tetrode section electron gun providing a high density electron beam, and having at least one resistor within the cathode ray tube to protect external electrical components connected in circuit with the cathode ray tube from inadvertent current surges generated by changes in the state of charge of capacitance associated with the final anode of the cathode ray tube as herein defined, and having an apertured stop defining the crosssectional shape of the electron beam at the stop, has the apertured stop connected in series with said at least one resistor. and between the final anode and said at least one resistor.

Thus, any inadvertent current flow through the apertured stop, due to part of the electron beam being intercepted by the stop, does not flow through said at least one resistor, and so does not change signficantly the focus conditions for the electron beam.

It is required that the arrangement is such that, under normally-encountered operating conditions for the cathode ray tube the electron beam does not impinge on said at least one resistor.

Conveniently said at least one resistor is mounted on the electron gun assembly of the cathode ray tube. When said at least one current limiting resistor is mounted on the electron gun assembly the arrangement is suitable for a cathode ray tube which has a narrow neck of less than 4 centimeters diameter.

Conveniently, the apertured stop is mounted onsaid at least one resistor, irrespective of whether said at least one resistor is mounted on the electron gun assembly, or not.

Each provided current limiting resistor may be in the form of a pattern of resistive material printed onto an insulating substrate.

The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure I is a section of one embodiment of a cathode ray tube according to the present invention, the tube including a resistor to protect external components connected in circuit with the cathode ray tube from damage by inadvertent current surges associated with the tube, Figures 2 is a side elevation of the resistor of Figure 1, indicating how it is mounted on the electron gun assembly of the cathode ray tube.

Figure 3 is a side elevation of an alternative mounting arrangement for the resistor, Figure 4 is a section on the line IV - IV of Figure 3, Figure 5 is a side elevation of another mounting arrangement, for three identical resistors, and Figure 6 is a section on the line VI - VI of Figure 5.

The conventional part of the construction of the cathode ray tube 10, illustrated in Figure 1, comprises a glass envelope 11 with a narrow neck 12 of less than 4 centimetres in diameter. A screen 13 is provided on a plane inner face 14 of the glass envelope 11.

An electron gun assembly of the cathode ray tube comprises a tetrode section electron gun including a thermionic cathode 15 and a grid 16. An accelerating electrode is shown at 17. Pins 18 for the electron gun assembly are provided in a conventional valve base 19 sealed into the end of the neck 12 of the glass envelope.

A high density electron beam is provided, and an apertured stop 20 defines the crosssectional shape of the electron beam at the stop 20. The axis of the electron gun assembly is indicated at 22.

On the flared part of the inner surface of the glass envelope 11, and on the screen 13, there is deposited a film 23 of electrically conductive material, such as graphite or aluminium, and forming a final anode coating. A contact 24 is hermetically sealed into a portion of the wall of the glass envelope and comprises an external connection for the final anode coating 23. The outer surface of this portion of the glass envelope is covered with a film 25 of electrically conductive material, such as graphite, except for a region surrounding the contact 24.

Whether, the outer film 25 is provided, or not, a significant capacitance is associated with the final anode coating 23. In addition, the capacitance associated with the final anode may be due partially to external electrical components connected in circuit with the cathode ray tube.

It is required to provide a surge limiting resistor to prevent other, external components connected in circuit with the cathode ray tube 10 from damage by inadvertent current surges, which may be associated with the cathode ray tube during its operation, due to changes in the state of charge of the capacitance associated with the final anode coating 23. Thus, it is required that this surge limiting resistor is connected to the final anode coating 23.

However, if the surge limiting resistor 30 is connected between the stop 20 and the final anode coating 23, any inadvertent current flow through the stop 20, produced by part of the electron beam being intercepted by the stop, produces a potential difference across the current limiting resistor, which tends to change inadvertently the focus conditions for the electron beam.

According to the present invention, any such inadvertent current flow through the stop 20 does not cause corresponding significant changes in the focus conditions for the electron beam, the stop 20 being provided between the surge limiting resistor 30 and the final anode coating 23, as is illustrated.

Thus, any inadvertent current flow through the stop 20 does not flow through the current limiting resistor 30.

The stop 20 is connected to the final anode coating 23 via blades of a contact spring 31 mounted on the stop 20.

As shown, the resistor 30 is mounted on the electron gun assembly by being secured to the accelerating electrode 17. The stop is mounted on the resistor 30.

The surge limiting resistor 30 may have any convenient form. The resistor shown in Figures 1 and 2 comprises a pattern of resistive material 33 printed onto the outer side wall of a hollow cylindrical substrate 34 of insulating material. The resistor 30 is mounted so that the axis 22 of the electron gun assembly passes through the bore 35 of the substrate 34. Terminal caps 36 are provided at each end of the substrate 34, and connect the resistive pattern 33 to the accelerating electrode 27 and to the stop 20.

The inside wall of the hollow substrate 34 may be coated with a layer of material providing protection against electrostatic charging of the substrate during the operation of the cathode ray tube, the material possibly comprising chromic oxide.

The diameter of the aperture 38 in the stop 20 is less than the diameters of the aperture 39 through the accelerating electrode 17, and of the bore 35 of the hollow substrate 34 of the current limiting resistor.

The arrangement is such that, under normally-encountered operating conditions for the cathode ray tube the electron beam does not impinge on the resistor.

The substrate is required to be securely mounted within the electron gun assembly so that it is incapable of vibrating under normally encountered operating conditions for the cathode ray tube.

A surge limiting resistor of the required magnitude easily may be provided for a cathode ray tube in the manner described above.

Figure 3 is a side elevation, and Figure 4 is a section, of an alternative mounting arrangement for the resistor 30, parts identical with or closely resembling corresponding parts shown in Figure 2 being identified by the same reference numerals in Figures 3 and 4 as those employed in Figure 2. The mounting arrangement is such that the axis of the electron gun assembly 22 does not pass through the resistor 30, although the resistor 30 is secured between the accelerating electrode 17 and the stop 20. Diammetrically opposite to the resistor 30 in relation to the axis 22 of the electron gun assembly is provided a ceramic insulating support rod 40, also secured between the accelerating electrode 27 and the stop 20, both the resistor 30 and the support rod 40 extending generally parallel to the axis 22 of the electrode gun assembly.Hence, there is provided a rigid mounting for the resistor 30 between the accelerating electrode 27 and the stop 20. In this arrangement the substrate 34 comprises a solid cylinder or rod, but otherwise is as shown in Figures 1 and 2.

A hollow cylinder 41 of conducting material encircles the part of the path of the electron beam between the accelerating electrode 17 and the stop 20, to shield the electron beam from the electric field associated with the resistor 30. The cylinder 41 is connected electrically only to the accelerating electrode 17 so as not to short-circuit the resistor 30.

The diameter of the aperture 38 in the stop 20 is less than the diameters of the aperture 39 through the accelerating electrode 17, and of the bore 42 through the hollow cylinder 41. The arrangement is such that, under normally-encountered operating conditions for the cathode ray tube the electron beam only, possibly, impinges on the apertured stop 20.

Figure 5 is a side elevation, and Figure 6 is a section, of another arrangement, in which three identical resistors 30 are secured between the accelerating electrode 17 and the stop 20. The three resistors 30 are equidistant from the axis 22 of the electron gun assembly. The resistors 30 extend generally parallel to the axis 22. The resistors each have the construction shown in Figures 1 and 2, or in Figures 3 and 4, but because they are connected in parallel with each other between the accelerating electrode 17 and the stop 20, preferably, they each have a resistance value three times that of the single resistor of Figures 1 and 2, or of Figures 3 and 4.

The supporting rod 40, and the hollow cylinder 41, of the embodiment of Figures 3 and 4 are not incuded within the embodiment of Figures 5 and 6.

The diameter of the aperture 38 in the stop 20 is less than the diameter of the aperture 39 through the accelerating electrode 17. The arrangement is such that, under normally-encountered operating conditions for the cathode ray tube the electron beam only, possibly, impinges on the apertured stop 20.

The mounting of each current limiting resistor on the electron gum assembly is especially advantageous if the cathode ray tube is required to have a narrow neck, for example, of a diameter less than 4 centimetres, and may be less than 2 centimetres, because the resistor is inserted with the electron gun assembly into the glass envelope of the cathode ray tube. Otherwise, each current limiting resistor may not be mounted on the electron gun assembly.

Whether said at least one current limiting resistor is mounted on the electron gun assembly, or not, it is convenient, but not essential, that the apertured stop is mounted on said at least one resistor.

Said at least one current limiting resistor may have any convenient form.

The cathode ray tube may have any convenient construction.

The tube may have more than one tetrode section gun.

An electrostatic lens may be provided within the cathode ray tube after the apertured stop.

Claims

WHAT WE CLAIM IS:

1. A cathode ray tube having a tetrode section electron gun providing a high density electron beam, and having at least one resistor within the cathode ray tube to protect external electrical components connected in circuit with the cathode ray tube from inadvertent current surges generated by changes in the state of charge of capacitance associated with the final anode of the cathode ray tube as herein defined, and having an apertured stop defining the crosssectional shape of the electron beam at the stop, and having the apertured stop connected in series with said at least resistor, and between the final anode and said at least one resistor.

2. A cathode ray tube as claimed in claim 1 in which said at least one resistor is mounted on the electron gun assembly of the cathode ray tube

3. A cathode ray tube as claimed in claim 2 which has a narrow neck of less than 4 centimetres diameter.

4. A cathode ray tube as claimed in claim 1 or claim 2 or claim 3 in which the apertured stop is mounted on said at least one resistor.

5. A cathode ray tube as claimed in any one of the preceding claims in which each provided current limiting resistor is in the form of a pattern of resistive material printed onto an insulating substrate.

6. A cathode ray tube as claimed in claim 5 in which only one resistor is provided, and the resistor is in the form of a hollow cylinder mounted to be coaxial with the electron gun assembly, so that the electron beam passes through the cylinder of the resistor.

7. A cathode ray tube as claimed in any one of claims 1 to 5 in which the arrangement of said at least one resistor is such that each resistor are spaced from the axis of the electron gun assembly and the electron beam does not pass through any constituent resistor, and a cylinder of conducting material is provided and encircles part of the electron beam path, to shield the electron beam from the electric field of said at least one resistor.

8. A cathode ray tube as claimed in any one of claims 1 to 5 or claim 7 and in which the arrangement of said at least one resistor is such that each resistor is spaced from the axis of the electron gun assembly, and the electron beam does not pass through any constituent resistor, and said at least one resistor is part of a rigid structure including a support of insulating material.

9. A cathode ray tube as claimed in claim 7 or claim 8 in which a plurality of resistors are provided, the resistors, and any provided insulating support, extending generally parallel to each other, and to the part of the electron beam path through the arrangement of the resistors.

10. A cathode ray tube substantially as described herein with reference to Figures 1 and 2, or the modifications thereof shown in Figures 3 and 4, or in Figures 5 and 6, of the accompanying drawings.