Electron Gun Assembly

This invention relates to an electron gun assembly, and more particularly to a quickly emissive type electron gun assembly.

In a television receiver, it is desirable that when its switch is turned on, a picture comes out as quickly as possible and a stable picture is quickly obtained. Conventionally, a stand-by system wherein a heater of the cathode ray tube is preheated has been adopted to meet that demand. In this stand-by system, however, even when the television receiver is out of use, a power necessary to such preheating is consumed. This is not preferable from the standpoint of energy saving. Under these circumstances, such an electron gun assembly as, when the switch has been turned on, permits the cathode electrode to be quickly heated without being preheated goes on being developed.

Such a quickly heatable or quickly emissive type cathode electrode can be constructed using, for example, a chrome alloy as the cathode sleeve, such that the sleeve surface is oxidized and blacked in a wet hydrogen furnace during a cathode electrode manufacturing process; the heat radiation rate is increased up to, for example, 0.8 as compared with 0.2 in the case of a nickel sleeve to increase the heat radiation during a high temperature (stationary operation temperature)-operation; and a heater power compensating that increased thermal loss, for example a heater power about four times as large as the conventional power, per cathode capacity is supplied to the cathode electrode, whereby to raise quickly the temperature of the cathode electrode.

However, what should be here taken into consideration is first to form a cathode body comprised of a base metal coated with an electron emitting material and a cathode sleeve so as to have a small heat capacity, second to reduce thermal loss due to the heat transmission upon heat-rising, said first and second problems being raised for the purpose of saving a heater-heating power as much as possible, and third to eliminate the difficulties in supporting and assembling the cathode body necessarily miniaturized as a result of solving the first and second problems. The same reference applies also to, for example, a pick-up tube for use in a TV-telephone set other than the cathode ray tube.

In addition to the above-mentioned problems, it is also a great problem to control the variation of the cathode electrode-to-grid electrode's interval due to the temperature variation. Particularly in the case of a color television, arrangement should be so made, in order to obtain a white picture, that three electron beams emitted from the electron gun assembly have the current density. To this end, the three electron gun units constituting the three electron gun assembly have to be so constructed that the assembly is in a state wherein the respective cathode-to-grid's intervals of the gun units are always kept equal to each other. Conventionally, however, the cathode electrode, the grid electrodes and the supporting members supporting them are dimensionally varied due to the temperature variation occurring from the start of operation to the achievement of full operation. For this reason, the cathode-to-grid's interval is varied, so that the respective cathode-to-grid's intervals of the three electron gun units become unequal, so that a purely while picture fails to be obtained within a prescribed length of time after the switch has been turned on.

In order to solve the above-mentioned problems, various improvements have been made. According to, for example, U.S. Pat. No. 3,354,340, filed Oct. 14, 1966, patented Nov. 21, 1967, a cathode electrode haivng a heater received therein is coaxially supported, using supporting strips, by a supporting cylinder, and this supporting cylinder is supported by an insulating ring, and a cathode assembly thus obtained is attached to a cap-shaped grid electrode to constitute an integral structure. Since, in this type of electron gun assembly, the cathode assembly and the grid electrode are combined into one integral structure, the heat of the cathode electrode is transmitted to the grid electrode and as a result a so-called "grid emission" occurs, that is, thermoionic emission from the grid electrode is effected. Further, in this type of electron gun assembly, the cathode assembly is fitted to the grid electrode with the insulating ring interposed therebetween. Due to the differences in thermal expansion coefficient between the insulating ring, cathode assembly and grid electrode, these are dimensionally varied during operation, so that the cathode-to-grid intervals become unequal. On the other hand, U.S. Pat. No. 3,265,920 discloses an electron gun assembly wherein the cathode electrode and the grid electrodes are individually separately fitted to the insulating support pillars. This type of gun assembly, however, is of the type wherein electron emission is slow, that is to say, is not of a quickly emissive type. Namely, in the disclosed type of gun assembly, the cathode structure is not so constructed that counter-measure is taken toward the variation of the cathode-to-grid's interval due to thermal expansion.

An object of the invention is to provide a quickly emissive type electron gun assembly wherein the cathode-to-grid's interval is difficult to vary due to the temperature variation.

Another object of the invention is to provide a quickly emissive type electron gun assembly having a constitution capable of removing excessive heat constituted by small parts can be easily assembled.

According to the present invention, the cathode assembly comprises an inner cathode cylinder having a metal base coated with an electron emitting material, an outer cathode cylinder coaxially supporting said inner cathode cylinder through a plurality of supporting strips with a space formed between the inner and outer cathode cylinders, and a cathode holder supporting said outer cathode cylinder at a first supporting point and supported by an electrode supporting piece at a second supporting point, respective portions of said outer cathode cylinder and cathode holder falling between said first and second supporting points being formed respectively of materials having substantially the same thermal expansion coefficient. According to such constitution, the respective dimensional variations due to expansion, of said outer cathode cylinder and cathode holder before and after the filament ignition are mutually cancelled, so that the cathode-to-grid's interval is always kept substantially constant. Further, excessive heat is removed through a space between the inner and the outer cathodes so that a constant operation temperature is obtained.

This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of an electron gun assembly according to an embodiment of the invention;

FIG. 2 is a side view, partly broken, of the cathode assembly;

FIG. 3 is a cross sectional view taken along the line 3--3 of FIG. 1;

FIG. 4 is an enlarged perspective view of a cathode assembly of the side electron gun unit;

FIG. 5 is an enlarged perspective view of a cathode assembly of the central electron gun unit;

FIG. 6 graphically shows the increasing percentage of anode current with respect to the time passage from the heater ignition;

FIG. 7 graphically shows the variation of cut-off potential coEc2 with respect to the variation of heater potential Ef;

FIG. 8 is a sectional view of the supporting structure of a cathode assembly according to another embodiment of the invention;

FIG. 9 is a sectional view taken along the line 9--9 of FIG. 8;

FIG. 10 is a perspective view of a strap of the cathode assembly of the side electron gun unit; and

FIG. 11 is a perspective view of a strap of the cathode assembly of the central electron gun unit.

In FIG. 1, a tri-electron gun assembly comprises three electron gun units 11a, 11b and 11c arranged in the same plane in prescribed convergence relationship and a converging cylinder 12 commonly provided on the tip ends of these three electron gun units. Each gun unit comprises a cathode electrode assembly 13, first grid electrode 14, second grid electrode 15, third grid electrode 16 and fourth grid electrode 17 which are coaxially arranged in sequential order, and a heater 18 provided in said cathode electrode assembly. To these electrodes 14 to 17 are fitted, respectively, electrode supporting pieces, i.e., straps 19, 20, 21, 22 and 23. To these straps are fitted three insulating support pillars, for example, three vitreous support pillars 24 axially of the tri-electron gun assembly in order to permit said three electron gun units 11a, 11b and 11c to be fixed into an integral assembly. The tri-electron gun assembly thus constructed is secured to a stem 26 by means of mount pieces 25 attached to ends of said insulating support pillars 24 on the cathode side, as shown in FIG. 2.

In FIG. 2 showing the peripheral portion of a cathode assembly 13, said stem 26 is fused and sealed to a neck portion of a funnel shaped envelope (not shown) of a cathode ray tube and is intended to exteriorly draw out the electrodes of said tri-electron gun assembly.

Said cathode assembly 13 has a cathode body 13e comprising an inner cathode cylinder 13b having a diameter of 1.9 mm and a length of 2.3 mm and having a base metal 13a coated with electron emitting material, and an outer cathode cylinder 13d having a diameter of 5 mm and coaxially supporting said inner cathode cylinder 13b on its tip end portion by three supporting strips 13c (only one supporting strip is shown) having a diameter of, for example, 5mm so that the tip portion of the inner cathode cylinder 13b is prosected from the tip end of the outer cathode cylinder. Since the inner cathode cylinder 13b is supported by the outer cathode cylinder 13d through said three supporting strips 13c obliquely fitted between the rear end of the inner cathode cylinder and the tip end of the outer cathode cylinder, transmission of the heat of the inner cathode cylinder 13b to the outer cathode cylinder is decreased and excessive heat is removed through a space between the inner and outer cathode cylinders. Further, since expansion of said supporting strips 13c obliquely provided so acts as to cancel an expanded portion of the inner cathode cylinder, the variation of the gap Ggl-k due to the expansion of the inner cathode cylinder does not take place. This cathode body 13e is fixed to a cathode holder 13f in a state wherein the outer cathode cylinder 13d is coaxially inserted into the cathode holder 13f secured to the strap 19. This cathode assembly is shown in detail in FIGS. 3, 4 and 5. Note that in FIGS. 4 and 5 the respective cathode assemblies of the side and central electron gun units are shown developed.

Upon constructing the above-mentioned cathode assembly, said cathode holder 13f is so constructed that the outer cathode cylinder 13d can be axially moved within the cathode holder 13f. For example, where the outer cathode cylinder 13d has an outer diameter of 5 mm, the cathode holder 13f preferably has an inner diameter 0.02 to 0.08 mm larger than the outer diameter of the outer cathode cylinder. The outer cathode cylinder 13d and the cathode holder 13f are separately manufactured so as to have such dimensional relationship with each other, and the outer cathode cylinder 13d is fitted with the inner cathode cylinder 13b thus to construct said cathode body 13e. When the cathode body 13e is fitted to the cathode holder 13f, the cathode body 13e is inserted into the cathode holder 13f so as to permit the electron emitting material layer of the metal base 13a to face the first grid electrode 14. At this time, said insertion is carried out while the gap Ggl-k between the first grid electrode 14 and the cathode electrode 13 is being measured using an air micrometer, and when said gap Gg1-k has a prescribed value, the outer cathode cylinder 13d is welded and fixed to the cathode holder 13f.

In fitting the heater 18, a filament 18a and a conductive frame 18b supporting said filament 18a and concurrently functioning as a lead are assembled in another step, and this filament assembly is disposed in the cathode assembly 13 in a state wherein the filament 18a is inserted into the inner cathode cylinder 13b. After the conductive frame 18b of the filament assembly has been welded to straps 27 fused respectively to the insulating support pillars 24, a portion 18c of said conductive frame 18b is notched.

When, in the electron gun assembly having the abovementioned construction, the gap Gg`1-k between the first grid electrode 14 and the cathode electrode 13 has the following relationship: ##EQU1## where Ggl-g2 represents the gap between the first and second grid electrodes 14, 15Tg1 the wall thickness of the first grid electrode 14, Dg1 the hole diameter of the first grid electrode, k the correcting coefficient, Ec1 the potential of the first grid electrode, and Ec2 the potential of the second grid electrode. The anode current is set at max. Ik of the equation: max. Ik-k' .times. coEc1.sup.3/2 established when the cathode electrode is operating within a region of space charge. This max. Ik is equal to the amount of anode current when the Ec1 is kept at a level of rated voltage and, by varying the Ec2 with the second grid taken as an anode electrode, the Ec2 is set to a beam cut-off voltage and thereafter the Ec1 is set to a bias voltage of zero.

As apparent from the equation (1), when the Gg1-k is varied due to the thermal expansion, etc. after ignition of the heater, the anode current is varied in proportion to the 3/2 power of the cut-off voltage (coEc1). In a stable condition after ignition of the heater, the Gg1-k (which is electrically expressed by a cut-off potential) can be set at a given value, but during a certain period of time beginning from heater ignition the Gg1-k is gradually reduced from a value before heater ignition as the temperature of the associated parts is increased, so that the anode current (in the case of a Braun tube, current flowing in the fluorescent screen) is gradually increased. This increasing amount of anode current is not only varied depending upon the temperature and thermal expansion coefficient of the associated parts but also does not always flow only in a fixed direction on account of, for example, the configuration of the structure, and particularly a strap of the configuration wherein its both ends are fixed is laterally displaced, inclined, etc., so that the variation in amount of the anode current does not always picture a fixed curve. To manufacture, in order to avoid this, a strap of the configuration wherein its both ends are not fixed is in fact difficult, and such configuration of strap has never been put to practical use. Particularly in the case of a color Braun tube comprised of three electron gun units, there is little possibility that respective kaleidoscopic dimensional variations of the three electron gun units occur in harmony with each other, and therefore these dimensional variations should be considered as occurring in an irregular manner. These dimensional variations appear as an anode current variation, accordingly as an unbalance between the current amounts of the three electron gun units. Although the color adjustment is performed by so adjusting the current of the three electron gun units as to permit a white picture screen to be obtained when this amount of current is maximum (this matter is hereinafter referred to as "white balance", which is expressed by W/B), occurrence of said dimensioned variations due to the reignition after the tube is switched off causes the color displacement, so that the picture does not become normal until the dimension of the three electron gun units becomes stable.

As one of the methods of removing the above-mentioned drawbacks it is contemplated that the cathode cylinder 13d and cathode holder 13f are formed of material having a small thermal expansion coefficient. By so doing, for example, such results as shown in FIG. 6 are obtained. FIG. 6 is a characteristic curve diagram wherein a curve I represents the anode current characteristic of a cathode electrode formed of a Fe--Ni--Co alloy (known under the trade name of (KOVAR) based material having a relatively low expansion coefficient and a curve II the anode current characteristic of a cathode electrode formed of a Fe--Cr--Ni alloy base d material having a relatively high expansion coefficient, with a time length after switch-on of the tube plotted on the abscissa, and anode current values plotted on the ordinate, said anode current values being those at respective points of times in the case where a length of time for permitting a complete stabilization of electron gun assemblies using said respective cathode electrodes is taken as about 200 seconds and this period of 200 seconds is taken as 100. As seen from FIG. 6, between both electron gun assemblies little difference exists in respect of a time length required from switch-on of the tube to the start of electron emission, an about 1-second difference exists in respect of a time length required from switch-on of the tube to the time when the picture is clearly seen (an anode current of about 50%), and an about 15-second difference exists in respect of a time length required from switch-on of the tube to the time when a substantially complete picture is obtained (an anode current of about 90%). These phenomena result from the comparison between the cathode electrodes wherein electron emission is normal, all occur within a space charge-limiting region, and originate in an occasional structural preveance as measured in terms of temperature. Therefore, the above-mentioned curve patterns can not be varied through varying the exhausting and aging conditions in the steps of manufacturing the electron gun assembly. Further, it is impossible to achieve the "white balance" even through varying the cathode-to-first grid's interval and the level of voltage applied. FIG. 7 shows the relation between the variation in the potential of the heater Ef (abscissa) and the variation in the cut-off potential coEc2 when the electron beam is cut off with the Ec1 set at -100V and the Ec2 varied (ordinate), in a sufficiently thermally stabilized condition. As seen from FIG. 7, one curve I' representing a cathode electrode formed of material having a low thermal expansion coefficient presents a smaller variation than the other curve II'. It will be understood that at the respective points of times of FIG. 6 as elapsed after ignition of the heater the variation of the gap between the first grid and cathode electrodes due to the variation in the temperature of the associated parts affects the amount of anode current.

Another one of the methods of eliminating the above-mentioned drawbacks is to reduce the effective length of the expansible members such as the outer cathode cylinder 13d. To this end, the cathode holding strap 19 and the cathode holder 13f are so constructed that their respective lengths extending toward the electron emission plane of the cathode from a fusing point at which 19 is fused to 13f may become as small as possible, and at least respective portions of the cathode holder 13f and cathode cylinder 13d falling between said fusing point and a securing point between 13f and 13d are made of the same quality of material, for example, KOVAR into a cylindrical shape, thereby permitting said respective portions to cancel their mutually opposite expanding actions, thus zeroing the effective length of said respective portions. For this reason, the effective length of the outer cathode cylinder 13d as resulting from expansion is determined only by the distance from said fusing point to the tip end of the outer cathode cylinder. From the above-mentioned point of view, the fusing point between the cathode holding strap 19 and the cathode holder 13f preferably is determined at a position as near to the tip end of the outer cathode cylinder as possible. It is sometimes synthetically preferable, with a view to avoiding strap distortions due to the fixation, of the holding strap 1a at its both ends, that said fusing point is set at a central portion of the strap width. In this case, the directions in which the respective members such as 19, 13f, 13d, 13b, etc. extend due to their thermal expansions are as follows. The strap 19 extends mainly in a lateral direction but little extends in a direction permitting the gap Gg1-k between the first grid and cathode electrodes to be varied. Further, in the case of comparison between the holder 13f and the strap 19, the holder 13f extends toward the cathode side of the electron gun assembly, whereas in the case of comparison between the holder 13f and the cathode cylinder 13d, the cylinder 13d extends toward the anode side of the electron gun assembly. Accordingly, the holder 13f and the cathode cylinder 13d cancel their mutually opposite expanding actions to permit a reduction of the variation with temperature of the gap between the first grid and cathode electrodes, thus preventing the W/B (white balance) from being lost. Further, the first grid electrode 14, cathode electrode assembly 13 and heater 18 are respectively independently attached to the insulating support pillars 24. Therefore, it little happens that the electrode 14, assembly 13 and heater 18 affect each other to vary the first grid-to-cathode's interval. Further, the amount of heat applied to the cathode electrode is rapidly decreased by the supporting strips 13c but the temperature gradient of the remaining portions 13d, 13f and 19 is gentle. Further, if the outer cathode cylinder 13d and the cathode holder 13f are formed of the same quality of material, both of them can be regarded as being heated at almost the same temperature (for example, at about 3500.degree. C). For the foregoing reasons, according to the structure of the present invention, it is possible to extend axially the cathode holder 13f and the outer cathode cylinder 13d without taking into consideration the effect upon the gap between the first grid and cathode electrodes. Further, according to the present invention, an overlapped portion of the cathode holder 13f upon the cathode cylinder 13d can be elongated, which offers many advantages--for example, the cathode cylinder 13d is prevented from being inclined due to a clearance between both 13d and 13f, the parallelism between the first grid and cathode electrodes is less deteriorated, the cut-off voltage variation is reduced, and so on.

In FIGs. 8 to 11, another embodiment of the cathode assembly-supporting structure is shown. In this embodiment, a cathode holder 30 supporting the cathode body 13e comprised of the inner cathode cylinder 13b and the outer cathode cylinder 13d has a flange portion 30a. The respective cathode holders 30 of side electron gun units 11a and 11c are secured to electrode supports or plate-like straps 31 in a manner that the flange portions 30a of the cathode holders 30 are welded respectively to the inner flanges 31b of cylindrical portions 31a of the plate-like straps 31 shown in FIG. 10. The cathode holder 30 of a central electron gun unit 11b is welded to a plate-like strap 32 shown in FIG. 11 in a manner that the flange 30a of the cathode holder 30 is welded to the inner flange 32b of a cylindrical portion 32a of said plate-like strap 32. These straps 31 and 32, as shown in FIG. 9, are attached to the insulating support pillars 24 through their attaching ends 31c and 32c, respectively.

Suppose now that in the above-mentioned construction the outer cathode cylinder 13d and cathode holder 30 are formed respectively of materials having substantially the same expansion coefficient. Then, in portions of the cathode assembly between a first welded point between the cathode holder 30 and the flange 31b (32b) of the strap 31 (32), and a second welded point, for example, at a lower part of the illustration between the outer cathode cylinder 13d and the cathode holder 30, the respective dimensional increases due to expansion, of the outer cathode cylinder 13d and cathode holder 30 are mutually cancelled, so that the gap Gg1-k between the cathode assembly 13 and the first grid 14 remains unvaried. Accordingly, the gap variation after and before the filament ignition little occurs, so that a good "white balance" is achieved. The preceding embodiments referred to the in-line type tri-electron gun assembly but it will be evident that this invention can be applied also to a single electron gun assembly and .DELTA.-type tri-electron gun assembly.