US2884554A

US2884554A - Electron tube

Info

Publication number: US2884554A
Application number: US594538A
Authority: US
Inventors: William E Lynar; William J Helwig
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1956-06-28
Filing date: 1956-06-28
Publication date: 1959-04-28
Anticipated expiration: 1976-04-28

Description

April 28, 1959 'W. E. LYNAR ET AL ELECTRON TUBE Filed June 28, 1956 wm/MM WillzamEZ nu)" ELECTRON TUBE Application June 28, 1956, Serial No. 594,538 6 Claims. (Cl. 313-470) This invention relates to an electron tube mount, and

particularly concerns a novel support for a cathode of the indirectly heated type. I One type of electron tube useful at relatively high frequencies, known as a pencil tube, is made up of concentric tubular electrode elements having relatively small inter-electrode spacings. tubular cathode, closely spaced within a tubular grid, is mounted at one end on a tubular support with the other end free. It is a practice to have an end portion of the cathode telescoped within an end portion of the tubular support. This support has a relatively low heat conductivity at the cathode operating temperature for thermally insulating the cathode from the portions of the tube to which it is electrically connected.

Two problems are presented by the use of a support of the type described. One of these problems arises because of the difference in coefficients of expansion between the material of the cathode and that of the support. The support is usually made of a material known commercially as Kovar which has an appreciably lower coefficient of expansion than the cathode, which is usually made of nickel. Therefore, when the end of the tubular cathode, within the support expands due to a heating ofthe cathode during tube operation the expansion is constrained. Thus, when the tubular supports are of seamless construction the difference between expansions of the cathode and the support causes a mechanically restrictive force to be exerted on the cathode by its support. When such a composite structure is alternately heated and cooled, such as when a tube is turned on and olf during normal operation, the region where the cathode and support are joined tends to bulge outwardly. During normal tube operation the bulge increases in size with each on and olf cycle until the region of the bulge contacts the grid, short-circuiting thereto, or the bulge becomes sulficiently asymmetrical at the region of the joint to cause the cathode to tilt from its normal'position and contact the grid. Also, when the seamless support is made relatively thin the stresses caused by the differences in expansion often causes the support material to fissure; this weakens the structure. Then, too, seamless tubes of the dimensions useful for such supports are relatively costly and are relatively difficult to handle.

The other problem arises in connection with tubular supports of an overlapping seam construction. While this seamed construction is generally desirable in that the seam appears to otter mechanical relief to the support, by allowing the support to more readily accommodate expansion of the cathode sleeve, asymmetrical relief only is afforded. This asymmetrical relief arises since the seam includes a double thickness'of support material in the region where the cathode sleeve is joined to the support. The double thickness seam, in the region of the sleeve-support joint, resists movement with the sleeve (when the sleeve expands during tube operation) by a greater amount than the resistance to movement exhibited by the single thickness of support material along the other portions of the support joined to the cathode sleeve. As an end result asymmetrical forces are produced which cause the cathode to tilt off its axis and to contact a portion of the grid so that the cathode is short-circuited to the grid.

Accordingly, it is an object of the invention to provide an improved support for a tubular cathode wherein the cathode is supported at one end portion thereof only.

Another object is to fix a cathode to an electricallyconductive, thermal isolation support in such a manner as to preserve the cathode and the support from contact with an adjacent electrode.

A further object of the invention is to provide an improved indirectly heated cathode mount wherein the cathode is supported at one end only and is free from a tendency to tilt away from a predetermined axis during normal tube operation.

In one kind of pencil tube a v The foregoing and related objects are realized in an indirectly heated cathode mount tion and which includes a cathode sleeve and a thermally insulating, tubular support therefor. In one embodiment of the invention the support is in telescoped relation with the sleeve. The support has a longitudinally extending double ply seam. The seam is longitudinally spaced from the telescoped region. Therefore, the structure of this mount provides a cathode sleeve secured at one end por tion thereof to a thermally insulating support havingv a construction for aifording substantially symmetrical me-' chanical relief for forces having components transversely of the mount structure in the region of the telescoped joint.

Fig. 2 is an enlarged elevational view of a portion of 8.

wherein like numerals refer cathode mount of the tube in Fig. 1; and

Fig. 3 is a sectional view taken along line 3-3 of Fig. 2. v v

The pencil type electron tube shown in Fig. 1 includes opposite end sections 10 and 12 between which is mounted a disc 14. The disc 14 supports a grid 16 and is electri: cally insulated from the end sections 10 and 12referred to by means of

glass tubes

18 and 20. Within the first end section 10 there is mounted a cylindrical anode 22, the first end section 10 being closed at its free endby an exhaust tubulation 24. p

The second end section 12 includes an outer tubular, member 26 which serves as a cathode terminal. Within the tubular member 26 there is fixed a cathode mount comprising tubular member 28 for supporting a cathode mount portion 29. The mount portion 29 is comprised of a cathode sleeve 30, which is coated with an electron emissive material 31 (Fig. 2), and a cathode-sleeve support 33 to be described. A heater 32, having legs 34 and 36, is connected to lead-in wires 38 and 4tl which in turn are adapted to be connected to a suitable power source (not shown) for raising the cathode sleeve to operating temperature. The lead-in wires 38 and 40 are held in spaced relation by an insulating spacer disc 42. Also mounted within the second end section 12 is a flashable getter 44 mounted across lead-in

wires

46 and 48, the getter being activated by a resistance heating thereof. The lead-in

wires

38, 40, 46, and 48 are'sealed through a glass stern (not shown) which serves to close the free end of the second end section 12 of the tube. j

Reference is now made to Figs. 2 and 3 in describing the cathode mount portion 29 according to the invention. The cathode mount portion 29, as has been indicated before, is comprised of a cathode sleeve 30 and a support 33 therefor. The support 33 is substantially concentric with the sleeve and has one end portion 52 telescoped according to the invenaround an end portion 54 of the cathode sleeve 30. The support 33 and the sleeve 30 are fixed at the telescoped region as by a number of spot welds 56. The support 33 is comprised of a tubular sheet metal structure having a longitudinally extending two-ply seam 57. The region Of the support adjacent to the sleeve, however, is free of overlap as indicated in Fig. 2 by the gap 58. The overlapped portions or" the support are also fixed to each other as by a number of spot welds 6! The discontinuous transverse extent of the support 33 provides mechanical relief in the telescoped region thereby allowing the end portion 54 of the sleeve to expand or contract substantially symmetrically about an axis common to the sup port, cathode sleeve, and an adjacent electrode such as the grid 16.

The tubular support 33 (Figs. 1 and 2), between the cathodesleeve 30 and the tubular member 28, provides reduced thermal conductivity at normal tube operating temperatures. This reduced thermal conductivity, which enables the cathode sleeve 30 to be maintained at operating temperatures with a relatively small amount of heater current, is provided by two means. Firstly, the tubular support is made of a material having a relatively low heat conductivity at normal tube operating temperatures. By way of example, the tubular support 33 may be made of an alloy known commercially as Kovar, which includes nickel, iron, and cobalt. Secondly, the support 33 is pro vided with a wall thickness which is as thin as is compatible with sufficient rigidity to enable the support to withstand the usual shocks encountered in handling the tube. It has been found that a Wall thickness of the order of .5 mil provides the required rigidity and at the same time provides a relatively small cross sectional area. This reduced area provides a relatively high thermal resistivity at normal tube operating temperatures. Moreover, it has been found that the thinnest sheet metal material which is readily, commercially available is material having a thicknessof the order of .5 mil.

The usual material used in the cathode sleeve 30 is nickel. Nickel has a higher coeflicient of expansion than the alloys which are desirable for the support 33. Therefore, the end portion 54- of the sleeve 30 within the support 33 will tend to expand to a greater extent than the end portion 52 of the support. Also, the wall thickness of the sleeve 30 is usually of the order of 2 mils for manufacturing convenience. The sleeve wall is thus four times as thick as the support wall. Consequently, the sleeve end portion 54 within the support will, when the mount portion 29 is heated, exert a relatively large radially outward force on the support end portion 52 around it. This expansion tendency of the cathode sleeve end portion 54 is partially restrained by the support, the support end portion 52 yielding symmetrically to a small extent. The discontmuous construction of the support end portion 52 at the gap 58 allows this yielding to be symmetrical so that no bulges are formed in the region of the sleevesupport joint. The symmetrical yielding preserves the sleeve 3i from tilting away from the common axis of the sleeve, support, and grid so that the sleeve is preserved from contacting the grid 16 and producing a short circuit therewith.

It is apparent from the foregoing that the invention provides a rugged electron tube having an improved heat isolating cathode mount portion, closely spaced from and within a grid, and which is preserved fromshort circuitin g contacts with the grid.

'What is claimedis:

1. An electron tube .;subassembly comprisinga tubular cathode sleeve having fixed to one end thereof and sub- 'stantially coaxial therewith a tubular support having a longitudinally extending overlapped seam, said support being free of overlap in the region thereof adjacent to said sleeve, said support being of a material having an appreciably different coefiicient .ofexpansion characteristicffrom .thatof thematerial of.said sleev,e.

assaults 2. An electron tube sub-assembly comprising a tubular cathode sleeve having one thermal coefficient of expansion characteristic and having fixed around one end thereof and substantially coaxial therewith a tubular support having a lower thermal coefficient of expansion characteristic than that of said sleeve, said support having a longitudinally extending overlapped seam and being free of overlap in the region thereof adjacent to said sleeve.

3. An electron tube having an envelope, and a plurality of concentric tubular electrodes within said envelope and including a grid and a cathode sleeve supported closely spaced from and within said grid, said sleeve being joined to a tubular support having a discontinuous transverse extent in the region of the sleeve-support joint for providing said joint with mechanical relief under conditions of thermal stress, said support having a continuous transverse extent in regions remote from said joint for contributing to the rigidity and ruggedness of the sleeve-support assembly, whereby said sleeve is characterized by a freedom from a tendency to short circuit to said grid within said envelope.

4. An electron tube having an envelope, and a plurality of concentric tubular electrodes within said envelope and including a grid, a cathode sleeve supported closely spaced from and within said grid, and a tubular support for said sleeve fixed with respect to said grid, said sleeve being joined within said grid to said support, said support having a less wall thickness than that of said sleeve and having a discontinuous transverse extent in the region of the sleeve-support joint for providing said joint with mechanical relief under conditions of thermal stress, said support having a continuous transverse extent in regions remote from said joint for contributing to the rigidity and ruggedness of the sleeve-support assembly, whereby said sleeve is characterized by reduced heat loss and by a free.- dom from a tendency to tilt from its axis and short circuit to said grid within said envelope.

5. An electron tube having an envelope, and a plurality of concentric tubular electrodes within said envelope and including a grid and a cathode sleeve supported closely spaced from and within said grid, said sleeve having one thermal coefficient of expansion characteristic andbeing joined within said grid to a tubular support having a lower thermal coeificient of expansion characteristic than that of said sleeve and being substantially concentric with said sleeve, said support having a portion adjacent to said sleeve fixed around said sleeve and having a discontinuous transverse extent in the region of the sleeve-support joint for providing mechanical relief characteristics under conditions of thermal stress, said support having a continuous transverse extent in regions remote from said joint for contributing to the rigidity and ruggedness of the sleeve-support assembly, whereby said sleeve is characterized by a jreedorn from a tendency to tilt from its axis and short circuit to said grid within said envelope.

6. An electron tube having a plurality of concentric electrodes including a tubular grid and a tubular thermionic cathode within said grid, a support for said grid, a support for said cathode spaced from said grid support, said cathode support comprising an elongated tubular structure having a larger transverse extent than said cathode, one end of said tubular structure being fixed with respect to said grid support, the other end of said structure receiving an end portion of said cathode with said cathode extending to a region within said structure adjacent to said other end thereof, means fix ing said cathode endportion to said tubular structure in a joint, said cathode having a larger coeflicient of expansion than said tubular structure, a heater within said cathode for periodically heating said cathode to electron emitting temperature, whereby said joint is periodically heated and cooled, said tubular structure having a discontinuous perimeterin a longitudinal portion thereo .extendins.troa said other end t sa n 5 6 whereby distortion of said joint in response to said 1,937,849 Slack Dec. 5, 1933 periods of heating and cooling is reduced for maintaining 2,056,157 Buttolph Oct. 6, 1936 said cathode in concentric relation with respect to said 2,220,909 Kershaw Nov. 12, 1940 gn'd. 2,244,356 Bucklin June 3, 1941 5 2,708,249 Pryslak May 10, 1955 References Cited in the file of this patent UNITED STATES PATENTS 1,712,402 Robinson May 7, 1929