US3119946A - Cathode - Google Patents

Description

Jan. 28, 1964 D. R. KERSTETTER ETAL CATHODE Filed May 5, 1961 INVENTOR5 .Dana/a R Kerseher Samue/JManfgamery ATTORNEY United States Patent 3,119,946 CATHODE Donald R. Kerstetter and Samuel .1. Montgomery, Em-

porium, Pa., assignors to Sylvania Electric Products Inc., a corporation of Delaware Filed May 5, 1961, Ser. No. 108,118 4- Claims. (CL 313337) This invention relates to indirectly heated cathodes employed in electron tubes.

In such cathodes there is often used a metallic sleeve exteriorly coated with an electron emissive material and, upon occasion, interiorly coated with an insulating medium such as aluminum oxide. Within this coated sleeve there is positioned an insulatingly coated heater.

Very often, due to the thinness or absence of insulating coating on the interior of the sleeve and the poor condition of the heater insulation, the heater wire is improperly electrically isolated from the metal sleeve and leakage between the heater and metal sleeve occurs. This is particularly evident and undesirable when the electron tube is used in pulsed applications, in which use, in some circuits, high potentials may exist between the heater and cathode.

In an attempt to overcome this condition, thicker insulation between a coated heater coil and sleeve has been proposed and even a second insulated wire coil has been Wound about the insulated heater coil and bonded thereto for insertion into the sleeve. Both of these constructions are expensive to fabricate and they do not lend themselves to consistent reproducibility from tube to tube, of the desired heat transfer, microphonism and heather-cathode leakage characteristics.

Accordingly, it is an object of this invention to inexpensively achieve excellent insulation and reproduceable heat transfer characteristics between the heater wire and the cathode sleeve in a heater-cathode assembly.

In carrying out the invention, a length of wire, either bare or insulated, subsequently to become an insulating structure, is coiled about a mandrel after which the resilient spacer coil is loosened by allowing it to partially uncoil and expand to enable the mandrel to be removed. An insulated, folded, or coiled heater is subsequently inserted within the void left by removal of the mandrel. The assembly of inner heater and outer spacer coil is then inserted into a cathode sleeve. After this insertion, outer portions of the turns of the spiral come in contact essentially at points only with the inner wall of the sleeve, while inner portions of the turns engage and hold the heater in place. The resulting structure is a heatercathode assembly with predictable point area contact of the insulating coil and the cathode sleeve and predictable bundling and spacing of the heater from the sleeve. These features provide consistent relative positioning of the heater and cathode assembly components and minimization of leakage between the heater and cathode sleeve.

For a better understanding of the invention, attention is directed to the following detailed specification when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a mandrel about which is wound a length of spacer coil wire;

FIG. 2 is an exploded view of the cathode heater assembly showing the spacer coil, a heater ready for insertion into the coil and a cylindrical cathode sleeve in position to receive these elements;

FIG. 3 is an end view of the complete cathode-heater assembly; and

FIG. 4 is a perspective view of the cathode-heater assembly.

Referring to the drawings with greater particularity,

FIG. 1 shows a mandrel 10 having a square cross-section although other polygonal shaped mandrels having relatively sharp angular edges may be employed. As will be hereinafter pointed out, mandrels having an elliptical cross section or a round mandrel having a series of diameters of repetitive variance may also be used. About the mandrel 10 in FIG. 1 is wound an insulated wire 12 fabricated from refractory metals such as tungsten, molybdenum or alloys thereof. The insulating coil is here shown as using a resilient wire 14 coated with a suitable insulating coating 16 such as aluminum oxide.

The coated wire 12 is helically wound about the angular mandrel 10 with tension not exceeding the elastic limit of the wire since a degree of resiliency is desired. To facilitate winding, the free end of the wire may be temporarily secured to the mandrel 10 by suitable clips 18. The tight winding of insulated wire 12 about the mandrel forms relatively sharp corners on the helix at the corners of the mandrel 10, which corners are desirable in the final cathode assembly for positioning the heater 21 relative to the cathode sleeve 22. The length of wire wound may encompass the requirements of several individual coils 12, the finished coil being cut to given lengths approximating the length of the cathode cylinder 22. Upon removal from the mandrel 10, the spiral will relax and form a structure with angular bends or peripheral portions 13 interconnected by substantially straight-sided inner portions 15. This outside dimension defined by the bends preferably exceeds the inside diameter of the cathode sleeve 22.

If so desired, Wire 14 may be wound as bare wire, and upon removal from the mandrel It may then be coated with the insulating material 16 by any conventional process such as dipping or electrophoresis.

During assembly, insulated heater 21, illustrated as being of the spade wound type, is bunched and inserted into the central opening or void in the spiral of the insulating coil 12. If desired, a coating of alundum or other insulating material may be applied to these assembled elements by dip or electrophoretic processes to form an integral sub-assembly.

The sub-assembly of integrated components is then inserted into the cathode sleeve 22 by bundling or carefully depressing the spiral 12, and threading, preferably by circular motion compatible with the helix direction of the heater spacer coil, into the cathode sleeve 22. Since the relaxed diameter of the spiral 12 exceeds the inside diameter of the sleeve 22, the spiral, after insertion, will press against the interior sleeve surface and frictionally hold the heater in appropriate spaced relationship in the central area of the coil, as can be seen in FIGS. 3 and 4.

To achieve a greater degree of insulation, the interior surface of the cathode sleeve may be covered with an insulating coating of aluminum oxide 24 or the like. Thus, the cathode assembly formed, as shown in FIGS. 3 and 4, provides a frictionally mounted resilient spacer between the heater 21 and the cathode sleeve 22 wherein the angular spacer coil has only point random contact with the sleeve 22 or its insulating coating 24 at the bends or peripheral portions 13 while the straight inner turn portions 15 of the coil hold the heater centrally of the sleeve substantially along its longitudinal axis, and removed from the wall of the sleeve.

If desired an elliptically wound coil could be formed so that when relaxed the smaller are portions would make random peripheral contact with the inner cathode wall while the larger arc portions of the ellipse would contact and support the heater centrally of the sleeve. Also, a modification of the coil spacer could be used which comprises a convoluted insulating structure of repetitive hour-glass contour. Such a coil could be produced on the above described round mandrel which has a series of diameters of repetitive variance. The coil would, upon partial relaxation, aiford spacing and support within the cathode. The larger diameter outer spirals would bear against the inner cathode wall While the smaller internal spirals would centrally support the heater.

The exterior of the cathode sleeve 22 may be coated in any desired fashion with an electron emissive coating material 26.

The cathode assembly described herein has a firmly bundled heater 21 spaced substantially centrally of sleeve 22. Such a structure provides predictable uniform heat transfer characteristics, and minimizes microphonism and heater-cathode leakage. The structure is relatively inexpensive and lends itself to simple fabrication techniques.

Although several embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

What is claimed is:

1. A cathode assembly comprising a substantially cylindrical cathode sleeve provided with an exterior electron emissive coating, a heater mounted within said sleeve, and a resilient spacer coil positioned between the heater and said sleeve, the coil comprising an insulated wound Wire spiral having randomly positioned exterior portions defining a peripheral surface and randomly positioned internal portions defining an interior surface, said peripheral portions bearing against the inner wall of the cathode sleeve and said internal portions engaging the heater to provide confinement thereof and spacing from the sleeve.

2. A cathode assembly comprising a cylindrical cathode sleeve coated on its exterior with an electron emissive material, a heater positioned within the sleeve, and a resilient spacer coil lying between the heater and sleeve, said coil comprising an insulated wound wire with angular bends interconnected by substantially straight-sided portions, the angular bends bearing against the inner wall of the sleeve at random points therearound and the heater lying and held within said straight-sided portions of the coil at a position spaced from the sleeve and extending substantially along the longitudinal axis thereof.

3. A cathode assembly comprising a cylindrical metallic cathode sleeve coated with electron emissive material on its exterior wall and with insulation material on its interior wall, a heater, a resilient insulater heater spacer coil lying inside the sleeve, said coil having straight sides interconnected by sharp bends, said bends engaging the insulated inner wall of the sleeve at random points and the straight walls of said coil engaging and holding the heater between them at a position spaced from the sleeve and extending substantially along the longitudinal axis thereof.

4. A cathode assembly comprising a hollow metallic sleeve with an inner wall devoid of sharp bends, a bunched heater positioned substantially centrally within the sleeve and extending substantially along the longitudinal axis of the sleeve, and a resilient spacer frictionally mounted between the sleeve and heater comprising a metallic resilient wire enclosed in an electrical insulating material, said spacer being generally a spiral with straight-sided portions joined by sharp bends, said bends and straight portions being randomly located about the longitudinal axis of the sleeve, the sharp bends of the spiral lying in contact with the inner wall of the sleeve, and the heater being engaged and held by said straight-sided portions of the spacer.

References Cited in the file of this patent UNITED STATES PATENTS 2,831,140 Blickwedel et al Apr. 18, 1958

Claims (1)

1. A CATHODE ASSEMBLY COMPRISING A SUBSTANTIALLY CYLINDRICAL CATHODE SLEEVE PROVIDED WITH AN EXTERIOR ELECTRON EMISSIVE COATING, A HEATER MOUNTED WITHIN SAID SLEEVE, AND A RESILIENT SPACER COIL POSITIONED BETWEEN THE HEATER AND SAID SLEEVE, THE COIL COMPRISING AN INSULATED WOUND WIRE SPIRAL HAVING RANDOMLY POSITIONED EXTERIOR PORTIONS DEFINING A PERIPHERAL SURFACE AND RANDOMLY POSITIONED INTERNAL PORTIONS DEFINING AN INTERIOR SURFACE, SAID PERIPHERAL PORTIONS BEARING AGAINST THE INNER WALL OF THE CATHODE SLEEVE AND SAID INTERNAL PORTIONS ENGAGING THE HEATER TO PROVIDE CONFINEMENT THEREOF AND SPACING FROM THE SLEEVE.

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