US2721995A

US2721995A - Cathode ray tube shield structures

Info

Publication number: US2721995A
Application number: US361627A
Authority: US
Inventors: Friend Albert Wiley
Original assignee: Magnetic Metals Corp
Current assignee: Magnetic Metals Corp
Priority date: 1953-06-15
Filing date: 1953-06-15
Publication date: 1955-10-25
Anticipated expiration: 1972-10-25

Description

Oct. 25, 1955 A. w. FRIEND CATHODE RAY TUBE SHIELD STRUCTURES 2 Sheets-Sheet 1 Filed June 15, 1955 Y W. L B A E M R E p LOW B l w F INVENTOR ALBERT W. FRI END ATTORNEYS Oct. 25, 1955 A. w. FRIEND CATHODE RAY TUBE SHIELD STRUCTURES 2 Sheets-Sheet Filed June 15, 1953 A 4 W F a llillilllll ALBERT W. FRIEND ATTORNEYS United States Patent CATHODE RAY TUBE sinnrn STRUCTURES Albert Wiley Friend, Bala Cynwyd, Pa., assignor to Magnetic Metals Company, Camden, N. J., a corporation of New Jersey Application June 15, 1953, Serial No. 361,627

Claims. (Cl. 34t 367) The present invention relates to cathode ray tube constructions and is more particularly concerned with a new and improved magnetic shield structure to be utilized in conjunction with such tubes.

As is well known, it is necessary to shield cathode ray and television tubes, particularly those of color television applications, from undesired magnetic fields inasmuch as failure to provide such proper and sufiicient shielding may effect pattern distortion, reduced sharpness of focus, and, in color kinescopes, may cause color fringing and ofi-color effects. It has accordingly been suggested that, in one form of shielding, the foregoing undesirable effects may be minimized by application to the tube of an essentially conical shield of magnetic metal, such as, for instance (4-79) molybdenum Permalloy (or mumetal) which possess high initial permeabilities and very low coervice forces. Such shields may be applied to the cathode ray tube either internally, i. e. within a metal or glass vacuum envelope, or externally. While both such applications tend to produce the desired shielding, internal application of the shield has the added advantage of the shielding effect from the metal envelope obtained in the electron beam space. The basic disadvantage of such molybdenum Permalloy (or mumetal) shielding resides in the very substantial cost of the material employed, and such cost factors become a major consideration when mass production techniques are utilized in an attempt to produce a competitive, but technically sound, cathode ray tube.

In order to meet the foregoing cost consideration, it has been the practice to fabricate metal cones from chromium-iron alloy materials. Such materials may contain considerable residual magnetism, however, and the fields induced by this remanent magnetism, as well as those which may be caused by external sources of magnetism, often have a most adverse effect upon the tube beam configuration. This adverse effect, further, is most active upon the electron beams in the region within the smaller end of the truncated cone comprising the metal envelope of the tube, for, at this point, the magnetic gap across the cone is the least. As a result, beam operation may be far from satisfactory when such structure is utilized, first because stronger fields may prevail in this neck region and, secondly, because a relatively small change in the beam direction or focus in this restricted zone produces a disproportionate effect upon the electron impact pattern, and angles, at the tube screen.

It is accordingly an object of the instant invention to provide a composite shield structure, of low cost, having shielding effects comparable with those of shields fabricated entirely of very high initial permeability, and low coercive force, materials.

Still another object of this invention resides in the provision of a new shield structure for cathode ray tubes having predetermined portions of greater shielding effect than the other portions thereof. In accordance with one g The high permeability small section embodiment of this invention these portions of higher shielding effect are located within that region of the tube wherein stray magnetic fields will have the most adverse effect upon the tube beam. In accordance with still another embodiment of the instant invention, the higher shielding portions are selectively distributed along the shield cone to provide an improved distribution of shielding within the entire tube envelope.

A still further object of the present invention is the provision of a low cost, multiple layer shield structure which may be used as an impedance component in electronic circuitry associated with a cathode ray tube, as well as providing magnetic shielding for the tube itself.

Another object of this invention is to provide a cathode ray tube shield which may be applied either internally or externally to the tube envelope, and which provides a section of greater initial permeability and lower coercive forces adjacent the tube neck than is present in the major portion of the remainder of the shield. An improved form of this structure also provides an additional section of greater permeability and low coercive force adjacent the tube screen to help reduce undesired magnetic fields adjacent the wide end of the envelope as well as near the neck portion thereof.

In keeping with the foregoing objects and discussion the instant invention provides an essentially full size shield cone, of either single or multiple layer structure, having at least two basic portions. The larger of these portions acts as the primary shield formed as a truncated cone, and is fabricated of a lowcost nickel-free material such as a silicon-iron alloy. The second and smaller of these portions is also formed as a further truncated cone, or as a cylinder, located adjacent the smaller end of the larger or primary shield portion. This second portion is in turn fabricated of a material possessing high initial permeability, such as (479) molybdenum Permalloy (Hy Mn an alloy, typically comprising by weight 4% molybdenum, 79% nickel, and the balance iron; (mumetal), an alloy typically comprising by weight 77.2% nickel, 4.8% copper, 1.5% chromium, and the balance iron; possibly even a fifty percent nickel-iron alloy; or similar metals. Because of the high initial permeability and low coercive force of this smaller shield portion, and due to its physical disposition with respect to the remainder of the shield, the smaller of the sections acts as a highly effective magnetic shield for the electron beam space within and near it and, at the same time, functions as a magnetic shunt for the lower permeability larger shield which covers the greater volume of the conical space within the cathode ray tube envelope.

thus directs magnetic fields, which might otherwise pass mostly through the internal space, through its own high permeability structure. The composite dual-permeability structure thus has much the same shielding effect as if the entire structure had been formed only of the high initial permeability material, but with a considerable economy in the use of high permeability material. It has also been found that even better shielding than that of my basic shield may be obtained by providing, in addition to the high permeability neck portion, a further high permeability portion adjacent the wide end of the main shielding cone. By utilizing the structure of this latter embodiment one may reduce the field near the wide end of the main cone, and thereby effect an improved distribution of shielding. The precise disposition of the two, or three, basic portions of my composite shield is subject to certain possible variations, as will be subsequently described. Further, in still another embodiment of my invention, the main shield may be formed in multiple layers of magnetic material, with a dielectric or electrically insulating layer interposed"therebetween. This structure can-therefore be utilized as'a capacitor; for-instance; as

well as a magnetic shield.

Theforegoing objects. and precise structure, as well as other objects and advantages of "my invention will'be' more readily seen from the following description'and' accompanying drawings wherein:

Figures 1A and 1B show, respectively, a lateral section and front view of'one' form of my invention,

Figures 2A andZB'shbw, respectively; a lateral section andfr'ont View of'a further'embodimentof my invention,

Figures 3Aiand '3'B'sl1ow, respectively, a lateral section and front viewof still another embodiment 'of my invention,

Figures 4A and 4B show, respectively, a lateral section and' front view of a still further embodiment of my invention,

Figure 5 is a lateral section, analogous to Figures 1A'-4A', illustrating stillanother structure incorporating myinvention, and

Figure 6 is a lateral section of a multiple layer shield, in accordance'withmy invention, as utilized in a cathode ray'tube.

Referring now to Figures 1 to 4 inclusive, it will'be seen that a shield constructed in accordance with my invention comprises a main, relatively large, shield portion 10 taking the form of a truncated "cone. The precise materials of which the shield portion ltl'may-be fabricated have already been indicated'in the previous-discussion, and it will suflice for purposes of "the following description'to merely indicate that shield'portion 10 is of magnetic material having a relatively low permeability when compared to the shield portion 20. Shield portion is of substantially higher initial permeability, as has also been discussed, and is disposed adjacent the smaller or neck portion of cone portion 10. This shield portion 20 may be formed as a smaller truncated cone, as is shown in Figures 1, 2, and 3, or may be a-modified cylinder; as shown in Figure 4, or a true cylinder.

As has been pointed out previously, the provision of a high permeability small shield portion adjacent the neckof the larger, but lower permeability shield, results in the smallershield acting as both a magnetic shield for the area it encompasses as well as a magnetic shunt for the remainder of the shield. This is true whether the smaller shield is inside or outside the larger cone shield, or even if it acts merely as an extension of that larger. shield, so long as the two shield portions are tightly fastened together. The maximum shielding effect is obtained, however, when the high permeability cone or cylinder is inside the smaller end of the larger, lower permeability cone, or when it acts as an extension of that larger cone. These particular embodiments of my-invention are illustrated in Figures 1, 3, and 4. Thus, referring to Figures 1A and IE, it will be seen that the shield portion 20, formed as a truncated cone, may be inserted inside the larger cone 110 and'attached to the inner surface thereof adjacent the smaller or neck portion of the entire shield. This structure may in turn be readily compared with the embodiment shown in Figures 2A and 28' where, as before, the high permeability shield 20 again takes the shape of-a truncated cone, but is attached to lower permeability shield -10, adjacent the neck, on the outer surface thereof.

In Figures 3A and 3B I have shown still another embodiment of my invention wherein smaller shieldportion 20 acts as an extension of larger shield portion 10. The respective shield diameters are so chosenthat, when the two shield portions are fastened together, the higher permeability neck shield extends beyond and away from the main shield 10. The embodiment of Figure 3 is analogous to that of Figure 1 in that. shield 20 is within shield '10 at the actual points of juncture. It should be noted, however, that it is withinthe contemplationof the l present invention to have the extended shield 20 of Figure-Sfastenedtothe outer surface of main shield lll, much as is the case in the embodiment of Figure 2. This same consideration applies to the embodiment of Figures 4A and 4B, now to be described.

In Figures 4A and 4B I have illustrated another form of the extended high permeability shield concept. In this particular case, however, the smaller shield 20 in turn comprises two portions, namely, a smaller truncated cone section 21-.which is fastened to main cone 10, and a cylindrical extension 22 projecting to the rear of and away from main shield 10. The particular configuration of shield portion 20 shown in Figure 4 has been chosen to facilitate the joining of shields lll'and '20 to one another. As may be readily seen, however, and provided the appropriate fastening techniques are employed, shield portion 26 may take the shape of a true hollow cylinder fastened to truncated cone 10.

Figure 5 illustrates an improved form of "my"inven tion wherein the composite shield structure'is soconstructed that additional magnetic shielding is providedadjacent the cathode ray tube-screen; as wellas'at the tube neck; The structure'of Figure 5 actually gives 'results favorably comparable with a unitary-shield formed completely of highinitial permeabilityand low coercive force material. In accordance withthis particular embodiment of my invention, the shield again comprises a main portion 19 and a-higher permeability portion 20 attached adjacent the neck end. Whilethe-particular form-of shield 20, shown in Figure 5, is analogous to that of- Figure 1, it must beunderstood-that the neckportion of-the shield'may actually take any of the configurations shown in Figures 1 to 4, inclusive. In addi tion to the foregoing structure, .I'alsoprovide in this improvement-a further ring or truncated cone portion-25' from the improved shielding effects, and ring ZS-has been so shown in Figure 5. Again, high permeabilityring-ZS hasbeen shown attached to theinner surface andwithin the. outer boundary of main surface 10, but it may as readily be attached to the outer surface of shield 10 and/or may extend beyond the boundarylimits thereof.

Each of the foregoing embodiments of my invention has been primarily concerned with a single layer shield.

Such shields may be placed, with good results, either inside or outside the metal-or glass vacuum envelope of the cathode ray tube or television picture tube. The effectiveness of the shield. is in fact less when the shield is on the outside of the tube because'the residual magnetism of a metal envelope may then be adjacent'to the electron beam space, but even in that event the externally placed structure of ferromagnetic material acts as a mag netic shunt to minimize any internal fields. If the mag netic shield is actually external to a metal vacuum envelope, it should be placed in as close proximity thereto as is possible.

Figure 6 illustrates theuse of a shield, in accordance with my invention, external to the cathode ray tube being shielded. While the shield structure-of Figure 6 incorporates the basic concepts illustrated in Figures 1 t05, it contains certain additional structure allowing for uses of-the complete device for purposes other than shielding. The cathode ray tube 5t comprises a neck portion SIspaced from a screen 52 by a metal envelope 53, in accordance with well known constructions. In close proximity to metal envelope-53 is placed a main shield Edofrelatively low permeability, nickel-free, and lower cost construction, analogous to shield it ofthe prior discussion. Snugly placed around the smallerend of main shield 541s a further shield 55 of high permeability material. Smaller shield 55 extends to the rear of main shield 54, and covers substantially the balance of metal envelope 53; the combined shields 5455 present a structure closely analogous to that discussed in relation to Figure 3. In addition to this basic structure, a further cone 56 of electrically insulating material (e. g. plastic) covers the magnetic shield structure 5455, and a final outer magnetic shield cone 57, formed of a low-cost and nickel-free composition such as silicon-iron alloy, is placed over the electrically insulating cone 56. The outer magnetic cone 57 may be utilized as a mechanical support member, for instance in conjunction with a support rim 58 attached thereto, and may in addition act as one plate of an electrostatic capacitor in opposition to the inner shield cone 54, in addition to its magnetic shielding function, the insulating cone 56 then acting as the dielectric of the capacitor combination. When such a capacitor function is in fact desired of the magnetic shield, it may be effected by electrically connecting the metal tube envelope 53 to

metal shield portions

54 and 55, one lead being then taken from any of

metal portions

53, 54, or 55, while the other capacitor lead is taken from outer shield 57 or mechanical support rim 58. The capacitor so derived may be used, for instance, as a portion of the filter circuit and energy storage system of the high-voltage beam-accelerating power supply.

Having thus described my invention, I claim to have invented:

1. In a cathode ray tube apparatus a cathode ray tube, a shield for said tube comprising a main magnetic shield of relatively low permeability formed as a truncated cone, and a further shield portion of substantially greater initial permeability than that of said main shield, said further shield being attached to the said main shield adjacent the smaller end of said truncated cone structure.

2. In a cathode ray tube structure, a cathode ray tube, a magnetic shield for said tube comprising a first portion having relatively low magnetic permeability, and a second shield portion of substantially higher magnetic permeability and low coercive force attached to said first portion adjacent the neck of said cathode ray tube, said first and second portions being formed as overlapping truncated cones.

3. The structure of claim 2, wherein the said second shield portion is attached to said first portion at the inner surface of said first portion.

4. The structure of claim 3 wherein the said second shield portion extends beyond the boundary limits of said first portion.

5. In a cathode ray tube apparatus, a cathode ray tube, a magnetic shield for said tube comprising a first portion having relatively low magnetic permeability, a second shield portion of substantially higher magnetic permeability and low coercive force attached to said first portion adjacent the neck of said cathode ray tube, said first portion comprising a truncated cone extending adjacent the envelope Walls of said cathode ray tube substantially to the picture face thereof, and a third shield portion, of substantially higher magnetic permeability than that of said first portion, in close proximity to said first portion adjacent said picture face.

6. In a cathode ray tube apparatus, a cathode ray tube, a magnetic shield for said tube comprising a first portion having relatively low magnetic permeability, a second shield portion of substantially higher magnetic permeability and low coercive force attached to said first portion adjacent the neck of said cathode ray tube, said first portion comprising a truncated cone, and said second portion being substantially cylindrical in configuration.

7. In a cathode ray tube apparatus, a cathode ray tube, a magnetic shield for said tube comprising a first portion having relatively low magnetic permeability, a second shield portion of substantially higher magnetic permeability and low coercive force attached to said first portion adjacent the neck of said cathode ray tube, each of said first and second portions being formed as overlapping truncated cones, a third cone of electrical insulating material covering each of said first and second portions, and a fourth cone of relatively low permeability magnetic material covering said third cone.

8. In a cathode ray tube structure, a cathode ray tube, a magnetic shield for said tube comprising a main shield portion fabricated of nickel-free magnetic material, said main shield portion being formed in a substantially truncated conical configuration and being disposed closely adjacent the envelope Walls of said cathode ray tube, and a second shield portion of substantially higher initial permeability than that of said main shield portion and attached to a limited area of said main shield portion.

9. In a cathode ray tube structure, a cathode ray tube, a magnetic shield for said tube comprising a main shield portion formed in a substantially truncated conical configuration and disposed adjacent the envelope walls of said cathode ray tube, said main shield portion being fabricated of a magnetic material having relatively low magnetic permeability, and a second shield portion contiguous with a limited area of said main shield portion, said second shield portion being fabricated of a magnetic material having substantially higher magnetic permeability than said main shield portion whereby said second shield portion acts as a magnetic shunt for the said main shield portion.

10. The structure of claim 9 wherein said second shield portion is fabricated of a magnetic material exhibiting lower coercive forces than said main shield portion, said second shield portion defining a closed magnetic configuration contiguous With said main shield portion adjacent the neck of said cathode ray tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,185,562 Nielsen Jan. 2, 1940 2,440,260 Gall Apr. 27, 1948 2,497,078 Gall Feb. 14, 1950 2,505,736 Herscher et al Apr. 25, 1950 2,563,525 Foster Aug. 7, 1951