US3504225A - Electron gun - Google Patents

Description

March 31, 1970 SATOSHLSH'WDA E-rAL 3,504,225

ELECTRON GUN Filed April 2l, 1966 3 Sheets-Sheet 2 G1 G2 G3 v -Eco l I 5 i I I Eb (Kv) ECZ (y) :1 E SI2 f'co -80 Lrco -90 Ego -OO [NVEVTORS 4l .FE1 SAJ'OSH/ Sli/1MM Co 7420 BY AMO O//kos/y/ Sfm/ #7n/AOU? A WEP/V575 March 31, 1910 STOSHL SHMADA mL 3,504,225

" ELECTRON GUN Filed April 21, 1966 3 Sheets-Sheet 5 v Eb: 4.. um 34) ECO 33 Eb: foo- 90' 32) 5b: 15Kv 80- 70 Eb: OKV

INVENTORS Snroa/ Sli/1MM Y Amo OHKOSH/ United States Patent O 3,504,225 ELECTRON GUN Satoshi Shimada, Ako Ohkoshi, and Senri Miyaoka, Tokyo, Japan, assignors to Sony Corporation, Tokyo, Japan, a corporation of Japan Filed Apr. 21, 1966, Ser. No. 544,268 Claims priority, application Japan, Apr. 26, 1965, 40/ 24,937 Int. Cl. H011' 29/56 U.S. Cl. 315-31 5 Claims ABSTRACT F THE DISCLOSURE In an electron gun for generating and focusing an electron beam having at least a cathode and at least iirst, second, third and fourth electrodes successively axially arranged in the advance direction of the electron beam emitted from the cathode, the second electrode has an aperture therein with a diameter at least twice the diameter of an aperture in the first elecrode, and the second electrode is maintained at ground potential and the fourth electrode is maintained substantially at ground potential.

This invention relates generally to improvements in electron guns of the type employed in the reception of television signals, and more particularly to a cathode ray tube particularly suited for use in chromatron type television receivers.

In conventional cathode ray tubes the lens system gener-ally consists of four grid electrodes. The diameter of the aperture in the second grid is substantially the same as the diameter of the aperture in the iirst grid in order to prevent the cathode from being subjected to variations in the anode voltage. In this type of configuration the cathode is shielded by the second grid from variations in the anode voltage or from the high voltage present on the third grid. In addition, since the path of the electron beam is encircled by the tubular portion of the second grid the electron beam path is shielded from asymmetrical fields which are produced by the supporting pins on the electrodes. The disadvantages with this type of configuration, however, are that the modulation characteristic is poor and the assembling process is tedious and inefficient due to the necessity of aligning the apertures in the grids.

In another type of prior art cathode ray tube the second or accelerating grid is eliminated which avoids the disadvantages set forth above. In this type of arrangement, however, there are other disadvantages which render this type of construction unsuitable for a chromatron type of cathode ray tube.

In view of the foregoing it is the primary object of the present invention to provide an electron gun which has the advantages but not the disadvantages of prior art electron guns.

Another object of the present invention is to provide a cathode ray tube which eliminates any momentary discharge between adjacent electrodes, particularly between the anode and the first grid of the electron gun. In this manner, breakdown of electrical parts associated with the cathode ray tube such as transistors, diodes and other semi-conductor devices is avoided.

These and further objects, features and advantages of the present invention will appear from the following detailed description of one embodiment of the invention which is to be read in conjunction with the accompanying drawings wherein like components in the several views are identified by the same reference numeral.

In the drawings:

FIGURE l is a schematic diagram of a conventional electron gun having a lens system consisting of four grid 3,504,225 Patented Mar. 3l, 1970 ice electrodes, and illustrating the relative arrangement of the electrodes;

FIGURE 2 is a schematic diagram of a conventional electron gun having a lens system consisting of three grid electrodes, and illustrating the relative arrangement of the electrodes;

FIGUR-E 3 is a circuit diagram illustrating one example of a cathode ray tube produced in accordance with the present invention;

FIGURE 4 diagrammatically shows one example of an electron gun as used in the cathode ray tube illustrated in FIGURE 3;

FIGURE 5 is a graph illustrating the relationship between anode voltage and cut-off voltage in the device of this invention and a conventional device;

FIGURE 6 is a graph indicating the relationship between the second grid voltage and the cut-off voltage in the device of this invention and a prior art device;

FIGURE 7 is a graph representing the relationship hetween the second grid voltage and the cut-olf voltage with the anode voltage as a parameter in the device of the present invention;

FIGURE 8 is a graph illustrating the relationship of the aperture diameter of the second grid to the distance between the iirst and second grids in the device of the present invention, the cut-off voltage being a parameter;

lFIGUR-E 9 is a graph showing the relationship between the distance between the first and second grids and the lens effect in the device of the present invention, the aperture diameter of the second grid being the parameter;

FIGURE 10 is a diagram schematically illustrating the iield distribution of the device of the present invention; and

FIGURE l1 schematically illustrates another embodiment of this invention.

In order to facilitate a better understanding of the present invention, a description will first be given of a conventional electron gun such, for example, as an unipotential focus-type electron gun having a lens system consisting of four and three grid electrodes respectively, indicated generally as DA and DB in FIGURES l and 2. As illustrated in FIGURE 1, the electron gun DA comprises cup-shaped iirst, second, third, fourth and iifth grids designated as Gm, GA2, GA3, GAA and GA5. These grids are sequentially arranged in the direction X of travel of an electron beam emitted from a cathode KA, the first grid GAI constituting a first lens, the third grid GA3 being of an unipotential electrode structure and constituting a second lens or the main lens, and the iifth grid GA5 being held at the same potential as the third grid `GA3 and s'upplied with a high voltage. In this case, the diameter of an aperture 1A2 of the second grid GA2 is selected to be substantially the same as that of the aperture lz A1 in the first grid GAI.

The electron gun DB (FIGURE 2) is of the type in which the second or accelerating grid GA2 of the electron gun DA illustrated in FIGURE l has been eliminated. With this exception, the electron gun DB is exactly the same in structure as the electron gun DA. That is, the electron gun DB comprises a cathode KB, irst, second, third and fourth grids Gis1, GBz, GB3, GBA which are sequentially arranged in the same manner `as in FIGURE 1.

These electron guns DA and DB will now be compared from the viewpoint of their advantages and disadvantages.

In the electron gun DA which has a lens system consisting of four grid electrodes and which is illustrated in FIGURE l, the diameter of the aperture hA2 of the second grid GAZ is substantially the same as that of the aperture h A1 of the rst grid GAI. In this manner the cathode KA is shielded by the grid GAZ from the inuence of the anode voltage or a high voltage present on the third grid GA3. In other words, the cathode KA is not subjected to variations in the anode voltage. Further, since the path of the electron beam is encircled by the tubular portion of the second grid GA2, the electron beam path can be shielded from asymmetrical fields produced by the supporting pins of the respective electrodes. On the other hand, however, the second grid GA2 is placed relatively close to the first grid GA1 and the diameter of its aperture hA2 is small, with the result that the modulation characteristic is poor. In addition to this, since the diameter of the apertures zA1 and 11212 of the first and the second grids GA1 and GA2 are selected as small as 0.6 mm or so, the centering of the apertures /1A1 and hA2 requires great precision, which causes remarkedly reduced efficiency in the assembling process.

The other prior art electron gun DB illustrated in FIG- URE 2 is not provided with an accelerating electrode such as that corresponding to the second grid GA2 of the electron gun DA. Accordingly the electron gun DB does not have the disadvantages enumerated above for the electron gun DA. That is, the centering of the grid electrodes does not require great precision such as in the electron gun DA, so that assembly is relatively easy and the modulations characteristic gm. is excellent. On the other hand, however, the electron gun DB does not possess the advantages which emanate from the provision of the second grid GA2 in the electron gun DA, and accordingly this type of electron gun is not suitable for use in the Chromatron-type cathode ray tube.

The present invention will not be described in detail with reference to FIGURES 3 to 10 inclusive. In FIG- URE 3 the reference character T designates in general a device produced in accordance with the present invention. As illustrated in the figure, there is provided in the neck portion 41a of the cathode ray tube 41 an electron gun D comprising first, second, third, fourth and fifth grids G1, G2, G3, G4 and G5 sequentially arranged in the direction X of travel of an electron beam emitted from a cathode K. The third grid G3 is an anode which constitutes a unipotential-type focus lens and is supplied with a high voltage, the fourth grid G4 is the so-called focus electrode and the fifth grid G5 is an anode which is supplied with the same high voltage as that applied to the third grid G3.

The interior of the funnel portion 41b of the cathode ray tube 41 is coated with an inner conductive film 42 such as in conventional cathode ray tubes, and high voltage of from to 25 kv. is applied from a power source E to the inner conductive film 42 and the third and fifth grid electrodes G3 and G5 of the electron gun D through an anode terminal 43 or the so-called anode button provided on the funnel portion 41b. The outer circumferential surface of the funnel portion 4111 of the cathode ray tube 41 is coated with an outer conductive film 44 and the conductive film 44 is grounded to, for example, the chassis of the cathode ray tube 41.

In the device of the present invention both or either one of the second and fourth grids G2 and G4 of the electron gun D is connected to, for example, the outer conductive film 44 of the cathode ray tube 41, so as to be operated at chassis potential or ground potential which might be said to be the common potential of the cathode ray tube.

As illustrated in FIGURE 4, the first grid G1 of the electron gun D can be formed cup-shaped which comprises a tubular portion 1a and a closed end face 1b having bored therein an aperture h1. The aperture h1 has a diameter d1 which is a minimum value for the production of a necessary beam current. It is usually selected in the range of from 0.3 to 1.5 mm., and preferably is 0.9 mm.

The second grid G2 can also be formed cup-shaped and comprises a tubular portion 2a having a diameter substantially equal to that of the tubular 1a of the first grid `G1 and a closed end face 2b. The second grid G2 has formed in the closed end face 2b thereof an aperture h2 which is substantially coaxial with the aperture h1 of the first grid G1 in opposing relation thereto. In this case, the diameter d2 of the aperture h2 is selected to be more than two times the diameter d1 for example, with the relationship d2=4d1. The second grid G2 is not always required to be cup-shaped and the closed end face 2b may if desired be eliminated.

The distance S12 between the first and second grids G1 and G2 is selected to be in the range of from 0.08 to 0.5 mm., for example, 0.335 mm. Each of the third, fourth and fifth grids G3, G4 and G5 constituting a unipotential focus lens can be formed in such a shape as to correspond to each grid electrode of a conventional unipo tential-type electron gun. In accordance with the present invention, the diameters d3 and d3 of apertures h3 and h3 of the third grid G3 are selected to be about two times the diameter d1 of the aperture h1 of the first grid G1 and the diameter d5 of an aperture h5 of the fifth grid G5 is selected to be about three times the diameter d1 of the aperture h1 of the first grid G1. An anode voltage of, for example, about 18 kv. is applied to the third and fifth grids G3 and G5.

In order to ground the fourth grid G4, the third and fifth grids G3 and G5 are spaced apart a distance such that the unipotential-type main lens system constituted by the third, fourth and fifth grids G3, G4 and G5 is in an optimum focusing condition when the fourth grid G4 is at the ground potential. That is, the focal length of the main lens formed by the third, fourth and fifth grids G3 G4 and G5 decreases with an increase in the distance between the third and fifth grids G3 and G5 and the focal length also decreases with a decrease in the potential difference between the fourth grid G4 and the third and fifth grids G3 and G5. Therefore, the distance S35 between the third and fifth grids G3 and G5 is selected in such a manner that a predetermined main lens is constituted when the fourth G4 is at the ground potential. Heretofore, the fourth grid G4 has usually been operated at a higher potential than the ground potential. In this invention the fourth grid G4 is held at approximately the ground potential and the distance S35 between the third and fifth grids G3 and G5 is selected to be longer than that in the prior art, for example, 11 mm. in order that the potential difference between the fourth grid G4 and the third and fifth grids G3 and G5 may be great.

The reason Why the second grid G2 is held at ground potential can be seen by ascertaining the relationship between the potential E22 of the second grid G2 and the cut-off voltage. In FIGURE 7 curves 31, 32, 33 and 34 respectively illustrate the relationship between the second grid voltage E32 and the cut-off voltage Eco obtained when the anode voltage E1, was selected to be 10 kv., l5 kv., 16 kv. and 18 kv. respectively in the embodiment above described (FIGURE 4). As is apparent from the graph of FIGURE 7 the cut-off voltage is not substantially dependent upon the voltage of the second grid G2, and hence even if the second grid G2 is held at ground potential, the cut-off voltage is not appreciably affected. According to this invention, the electron gun D is provided with a second grid G2 and its tubular portion 2a serves to shield the electron beam path from the asymmetrical fields as mentioned previously.

In the graphs of FIGURES 5 and 6 the full-line curve 11 represents the relationship between the cut-off voltage E33 in the electron gun of the aforementioned structure (FIGURE 4) and the voltage Eb applied to the third and fifth grids G3 and G5 while the other full-line curve 12 indicates the relationship between the cut-off voltage and the voltage E32 of the second grid G2. In FIGURE 5 the broken line curve 13 indicates the relationship between the impressed voltage on the third and fifth grids GA3 and GA5 and the cut-off voltage Eco in the electron gun DA, while the other broken lineV curve 14 represents the relationship between the impressed voltage on the second and fourth grids G32 and G31 and the cut-off voltage 'Eco in the electron gun DB. In FIGURE 6 the dotted-line curve 15 is a characteristic curve of the impressed voltage on the second grid G22 with respect to the cut-off voltage Eco in the electron gun DA.

As is apparent from these curves, and particularly curve 12, in the present invention the cut-ofi voltage Eco is practically independent of variations in the second grid voltage E32. That is, the cut-off characteristics are not affected by the second grid G2. Further, the plot of cut-off voltage Eco vs. anode voltage E1, for the device of the present invention (curve 11) is similar to that for the electron gun DB (curve 14) having a lens consisting of three grid electrodes. Accordingly, the modulation characteristic gm of the electron gun D of the present invention is improved to the same extent as that for the electron gun DB.

In the electron gun D produced in accordance with the present invention the diameter d2 of the departure h2 of the second grid G2 is selected to be greater than one and one half times the diameter d1 of the aperture h1 of the first grid G1, so that the precision required in the centering of the apertures h11 and h2 is not as great as in the prior art electron gun DA. This facilitates the assembling of the electron gun device of this invention.

As has been described in the foregoing, the electron gun D has almost all the merits of conventional electron guns but in the electron gun D the cathode K is not protected from the anode voltage, since the diameter d2 of the aperture h2 of the second grid G2 is selected to be about two times the diameter d1 of the aperture h1 of the first grid G1. As indicated in FIGURE 5 by the curve 11, the cut-off voltage Eco varies in proportion to the anode voltage Eb. The variations of the anode voltage, however, can be suppressed satisfactorily by various circuit arrangements, so that it does not offset the operation of the device of the present invention.

In the electron gun D the diameter d2 of the aperture h2 of the second grid G2 is large and a high voltage of l5 to 20 kv. is applied to the third grid G3, so that the distance S12 lbetween the first and second grids G1 and G2 can be selected to be long. In addition to this, since the diameter, d2 of the aperture h2 of the second grid G2 is large, the 0pposing areas between the first and second grids G1 and G2 is small. As a result, the electrostatic capacity produced between the first and second grids can be decreased, which effectively prevents a discharge current from fiowing in a pulsating manner from the second grid G2 to the first grid G1 and the cathode K, as compared with the prior art electron gun DA having a lens consisting of four grid electrodes.

In prior art electron guns having a lens consisting of three grid electrodes, it is difficult to effect prefocusing and an electric field formed between the first grid G1 and the anode performs a concave lens effect, so that the structure of the prior art electron guns of this type have the disadvantage of spherical aberration.y In the present invention, however, a weak convex lens is produced by suitably selecting the diameter d2 of the aperture h2 of the second grid G2 and the distance S12 between the first and second grids G1 and G2, by which prefocusing can be effected. 1

In FIGURE 8 curves 16, 17, 18 and 19 respectively indicate the relationship between the diameter d2 of the aperture h2 of the second grid G2 and the distance between the first and second grids G1 and G2 in the cases where the cut-off voltage is -80 v., -90 v., 100 v. and 120 v. when the diameter d1 of the aperture h1 of the first grid G1 is constant. In FIGURE 9 curves 20, 21 and 22 respectively represent the relationship Ibetween the distance S12 between the first and second grids G1 and G2 and the lens effect when the diameter d1 of the aperture h1 of the first grid G1 is constant but the diameter d'2 of the aperture h2 of the second grid G2 is a variable, the abscissa representing the distance S12 between the first and second grids G1 and G2 and the ordinate representing the lens effect. In this graph the convex lens effect is shown above the zero point and the concave lens effect is shown under the zero point. In this case the inflection points of the curves shift to the right when the depth of the cupshaped grid G2 and the projection of the grid G3 are small, and the inflection points move to the left when the depth of the grid G2 and the projection of the grid G3 are great.

As can be seen from the curves in FIGURES 8 and 9, with suitable selection of the cut-off voltage, the diameter d2 of the aperture h2 of the second grid G2 and the distance S12 between the first and second grids G1 and G2, a pre-focusing lens having a convex lens effect can be produced by the first and second grids G1 and G2. That is, in a system having a cathode K, a first grid G1, a second grid G2 and a third grid G3, an electric field distribution can be produced which forms a pre-focusing lens L1 having a convex lens effect, as illustarted in FIGURE l0. In this case a concave lens L2 is for-med in the vicinity of the aperture h2 of the third grid G3, but it is sufficient that the composite lens formed by the lenses L1 and L2 has only a convex lens effect. In this figure reference character P indicates a crossover point.

According to this invention the second grid G2 and/or the fourth grid G1 are grounded, and hence the second grid G2 and/or the fourth grid AG1 serves as a lighteningconductor. As a result of this, it is possible to prevent discharges which might otherwise be caused between the third and fifth grids G3 and G5 when these grids are supplied with a high voltage as well as another electrode, for example, the first grid G1. At the same time, when discharge is actually caused between the third and fifth grids G3 and G5 and the second grid G2 and/or the fourth grid G1, this discharge current can be prevented from flowing to circuit elements, thereby avoiding breakdown thereof.

One embodiment of this invention as applied to a l2 inch gun is as follows:

L all 6.7

While the present invention has been described in connection -with a unipotential-type electron gun it is to be understood that in a tripotential-type one such as shown in FIGURE ll a similar effect of discharge prevention can be obtained where the diameter of the second grid G2 is selected to be two times that of the first grid G1 and then grounded.

It will be apparent to those skilled in the art that many modifications and variations may be effected without departing from the scope of the novel concepts of this invention.

What is claimed is:

1. An electron gun for generating and focusing an electron beam, said gun comprising a cathode, first, second, third and fourth control electrodes and an anode, said control electrodes and anode being axially arranged in the advance direction of said electron beam, said electrodes having axial apertures therein, the aperture in said second electrodes having a diameter at least twice as large as the diameter of the aperture of said first electrode, means maintaining said second electrode at ground potential, and means maintaining said fourth electrode substantially at ground potential.

2. A lens system for use in an electron gun for focusing an electron beam emitted from a cathode, said lens system comprising first, second, third, fourth and fifth electrodes, said electrodes being axially arranged in the advance direction of said electron beam, said electrodes having axial apertures therein, the aperture in said second electrode having a diameter at least twice as large as the diameter of the aperture of said rst electrode, means maintaining said second electrode at ground potential, and means maintaining said fourth electrode substantially at said ground potential.

3. A device in accordance with claim 2 wherein said second and fourth control electrodes are interconnected.

4. A device in accordance with claim 3 wherein said third and fifth electrodes are interconnected and have applied thereto a high voltage.

5. A device in accordance with claim 4 wherein the spacing between the rst and second electrodes is approximately 0.08 to 0.5 mm.

References Cited UNITED STATES PATENTS RODNEY D. BENNETT, JR., Primary Examiner C. E. WANDS, Assistant Examiner U.S. C1. X.R. 315-16

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