US2726346A

US2726346A - Indirectly heated cathode of increased efficiency

Info

Publication number: US2726346A
Application number: US268322A
Authority: US
Inventors: Busby John Wesley; Morris Clifford Mandville
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1952-01-25
Filing date: 1952-01-25
Publication date: 1955-12-06
Anticipated expiration: 1972-12-06

Description

Dem 1955 J. w. BUSBY ET AL 2,726,346

INDIRECTLY HEATED CATHODE OF INCREASED EFFICIENCY Filed Jan. 25, 1952 v [0 29 M 5 y /Za 1/ 5 in j/W 1a f /Z\ 1 1/ jj l if; L S, 'f V If 4/ ZS atent 2,726,346 Patented -Dec. 6, 19.55

1 N DIRECTLY HEATED CATHODE 1 OF INCREASED EFFICIENCY ApplicationJanuary 25, @1952, Serial No..268,322 '6 Claims. (Cl. 31337) This invention relates to a cathode assembly, including a cathodeof the indirectly heated type, of increased efiiciency, and more particularly. to a rugged cathode assembly. characterized by reduced. heat losses.

In sometypes of electrontubes, such asthe pencil type, the cathode is spaced appreciably from the header or stem through which heater leads extend. This appreciable spacing has rendered it necessary to use heater legs integral with the heater of considerable length for connecting the heater to the heater leads referred to. The heater structure employed in the pencil type tube aforementioned includes a double coil-which is received within thesleeve, the legs of the. heater extending outside of the sleeve. :-Since.the-heaterlegs are integral with the double coil and have the same diameter raszthe .coil wire, they .become heated tosubstantially the same .degreeas thecoil and dissipate anv appreciable amount of heat to the region .surroundingthem. The heat dissipated is not only the heat .directly generated in .the heater legs ;but also heat xtransferredzby conduction by the heater .proper:to TIh legs. Such transfer is likely to-occur-because-of,the higher temperature of the, heater proper, resulting from the greater heater concentration in the-heateraproper than ;in ithe'glegs, and the shielding efiect provided ;-by the cathode sleeve, which is absent, from the region-of theheater legs.

.This heat dissipation by the cathode sleeve and heater consumesv an appreciable part of-the power intended to-energize theheater. Because of this, either the-heat transferfrom theheaterto thesleeve isinadequate' for a desired emission, or added, power to-theheater is necessary to makeup for the-heat dissipation referred to.

.In additionto undesirable heat dissipation, the relatively. long heater legs heretofore employed in the fpencil type tube have contributed to. lossof ruggedness ofthe cathode assembly. The heater coil received by the cathode sleeve relies in parton a snug fit in the sleeve and in part onthe heater legs for support. Where the heater legs are relatively long and of relativelyrsmall cross-section, as is the'case when theyare integral with the heater coil, theyprovide a relatively Weak, support for the heater coil. This. supportmay-be inadequate, and impacts incidental -to normal use of a tube in which theassembly is used,-may--cause the heatercoil tobecome displaced from the sleeve, thereby impairingfnrther utilityof the tube.

The, lack of ruggedness of a support providedby relatively long heater legs integral with the heater, is alsoa disadvantage when mounting the heater in the :cathode sleeve. -Some force is usually involved in inserting a heater in. a, sleeve, especiallywhen the transverse dimensions .of a. heater structure are. such as,to providea relatively snug fitof theheater in the'sleeve. Sincethis'force is transmitted through the heater legs,.collapse or: buckling of theheaterflegs sometimes occurs during amounting operation.

Accordingly, itis an object of the inventiontoprovide an improved cathode assembly of the indirectly heated yP ,A further object is to provide an indirectly heated cathode of increased efficiency.

Another object is to provide an indirectly heated cathodeassembly. wherein undesired heat losses by the cathode sleeve and heater, is reduced.

A furtherobject is to provide an indirectly heated cathode assembly including an elongated heat isolating ,sup- .port forthe cathode sleeve-of such assembly resultingrin increased spacing of the sleeve;from lead-ins serving the heater of the assembly, and leads of novel construction between the heater .and said lead-ins for reducing -heat losses-therefrom.

Another object is to providea tubular heatisolating cathode sleeve. support having a falling characteristieof heat conductivity atincreasing temperatures thereofand leads within said support'having relatively highheatradiating properties forraising the temperature of thesupport to thereby reduce its heat conductivity, whereby heat losscsjfroin said sleeve are reduced.

A further object is to provide an indirectly heated cathode assembly of increased ruggedness.

Briefiy considered, the indirectly heated cathode assembly or" the invention includes a tubular support for the cathode sleeve, having reduced heat conductivity and characterized by appreciably less heat conductivity in relation-to applied heat, at relatively high temperatures than atrelativelylow temperatures. In combination with this tubular-support the invention provides for a heater disposed within the cathode sleeve andhaving relatively shortintegral legs, and tubular leads having relatively large area surfaces connecting the short heater legs..to heater lead-ins. The tubular leads extend with-in -the tubular'support and are substantially coextensive 'longitudinally therewith.

This combination is of appreciable advantage. *The tubular structure of the leads provides-relatively large heat radiating surfaces. Therefore, whatever heat is -generated in the leads is readilyradiated from their relatively large area surfaces, to the inner walls of'the tubular support. The temperature of the tubular cathode support is'therefore. increased as a result of such radiation,-which serves to decrease its characteristic of heat conductivity. -While one advantage of the tubularleads resides-in theirgreater heat radiation, as aforementioned, further advantages 'of the tubular structure of the leads comprise increased ruggedness of support provided thereby, increased electrical conductivity thereby reducing heat generation therein as compared to conventional'heatcr legs,and reduced heat conductivity as compared to conventional solid lead-ins. 'A tubular structure has a favorable characteristic of stiifness over a solid structure ofequal diameter. -Thisstiffness enables the leads to support firmly in a cathode sleeve a heater to which theyare connected and facilitates insertion of the heater into a cathode sleeve. The combination of the-cathode sleeve tubular support ahd-the-tubular leads for the heater, therefore, substantially reduces-heat dissipation both from thecathode sleeve as Well-as from the heater leads, thus permitting reduced heater-power input without sacrifice of emission. Furthermore, the-tubular leads referred to provide the desired support of the heater, particularly when such support is called uponto traverse an appreciable distance as is involved in a pencil type tubestructure utilizing'atubular support for thermal isolation of the sleeve, as aforementioned. Further objects and advantages of the invention .will become apparent'as the present description proceeds.

Referring to-the drawing, in connection witha more detailed-description of the invention,

Figure lshows a sectional elevationof a conventional -penc iltypeelectron'tube; Figure 2 is a view in sectional elevation of the cathode assembly of a pencil type tube wherein the cathode sleeve issupporte'd on a sleeve made of a material of low heat-conductivity for reducing heat dissipation from the oVer the relatively long sleeve by conduction and wherein the structure requires relatively long heater legs from lead-ins to the heater proper; and

Figure 3 is a longitudinal sectional view of a cathode assembly according to the invention useful in a pencil type electron tube and wherein tubular leads are employed for connecting the heater lead-ins to the heater distance separating the heater and the lead-in as a consequence of the use of a low heat conducting support for the cathode sleeve.

The pencil type of electron tube, as shown in Figure 1, includes opposite end sections 1 0 11 between which is mounted a disc 12. The disc 12 supports a grid 12a and is electrically insulated from the end sections referred to by means of glass tubes 13, 14. In section 1 0 is mounted a. cylindrical anode 15 and the section is closed at its free end by exhaust tubulation 16.

Section 11 comprises a cathode assembly, and includes an outer tubular member 17 serving as a cathode lead-in. Within member 17 is fixed a tubular support 18 for the coated cathode sleeve 19. A heater 20 having

legs

21, 22 is connected to lead-

ins

23, 24 which in turn are adapted to be connected to a suitable power source, not shown, for raising the cathode sleeve to an electron emitting temperature. The lead-

ins

23, 24 are held in spaced relation by an insulating spacer disc 25. Also included in the cathode assembly is a getter 26 mounted across

leadins

27, 28, the getter being flashed by resistance heating thereof. The lead-

ins

23, 24, 27, 28, are sealed through glass stem 29 which serves to close the free end of section 1] of the tube.

The foregoing tube structure is subject to several disadvantages. The sleeve support 18 is usually made of a metal having relatively high heat conductivity. Therefore, an appreciable amount of heat is conducted by it away from the cathode sleeve 19. Furthermore, the

heater legs

21, 22, being integral with the heater 20 and of the same diameter as the heater wire, become heated during energization of the heater. The heat so generated in the legs referred to, serves no useful purpose and on the contrary, since it is dissipated by radiation, constitutes an undesired heat drain. To compensate for the heat lost by the sleeve 18 and

legs

21, 22, as aforementioned, it is necessary to provide an increased power input to the heater 20. In addition, the structure, as shown in Figure 1, provides a relatively weak support for the heater. In this connection, it may be noted that the heater 20 and the

legs

21, 22 thereof are usually made of wire having a diameter of only 4 mils. The legs therefore, are readily yieldable, not only during the operation of inserting the heater into the cathode sleeve during manufacture of the tube, but also in response to stresses tending to displace the heater from the sleeve during operation.

In attempting to remove the aforementioned difficulties, applicants interposed a tubular support 30, shown in Figure 2, between the cathode sleeve 19, and support 18, the tubular support having reduced conductivity. By way of example, the tubular support 30 may be made of an alloy known commercially as Kovar, and including nickel, iron and cobalt. This alloy is characterized by a relatively low order of heat conductivity. To further lessen heat conductivity by the support 30, its walls may be reduced to a foil thickness of 0.0005 inch.

However, several objections were found to characterize the improved structure shown in Figure 2. The interposition of the tubular or sleeve support 30 between the cathode sleeve and support 18 involved an appreciable increase in the spacing between the heater 20 and the

leadins

23, 24. To traverse this increased spacing, the

heater legs

21, 22 were made substantially longer than in the structure shown in Figure l. This increased length of the heater legs increased the electrical resistance and consequently the temperature thereof to an appreciable degree resulting in severe losses in heater power. While the hot legs radiated some heat to the sleeve support, the

effect of such heat in reducing the heat conductivity of the support, was negligible in respect of the efiiciency of the cathode assembly.

To fully solve the problem of undesired heat loss from the cathode assembly, applicants found it necessary to resort to a novel combination of the thermal isolation sleeve 30 with a novel lead structure connecting the legs of the heater to their associated lead-ins. As shown in Figure 3, a cathode assembly is provided according to the invention in which the heat insulating sleeve 30, which may be similar to die sleeve 30 shown in Figure 2, is interposed between cathode sleeve 19 and support 18. The

heater legs

21, 22 are appreciably shorter than in Figures 1 and 2 and are connected to lead-

ins

23, 24 by means of tubular leads 31, 32. The tubular leads may have an outer diameter of 10 mils and walls 2 mils thick. In one example, the tubular leads 31, 32 were 11 mm. long. One or both ends of the tubular leads may be flattened for welding or brazing to the

heater legs

21, 22 and to the lead-

ins

23, 24. Preferably the tubular leads referred to may have a coating of insulation thereon to prevent shorts.

The tubular leads 31, 32 may be made of any suitable material. Preferably they are made of a refractory metal such as tungsten, molybdenum or tantalum having relatively-high melting point temperatures. This is because relatively high temperatures are usually used in processing the cathode after mounting in a tube. If relatively low temperatures should be feasible during such processing, other materials, such as nickel, may be employed for the tubular lead-ins referred to.

The tubular leads 31, 32 coact with the thermal isolation sleeve support 30 to increase the efiiciency of a tube in which they are used, by decreasing appreciably undesired heat dissipation by the cathode assembly, thereby rendering a reduced power input to the heater 20 adequate for good operation of the tube. The increased cross-sectional area of the tubular leads over that of the heater legs serves to decrease their electrical resistance to thereby conserve the heater power. Whatever heat is generated in the leads is effectively radiated by their relatively large area surfaces to the support 30. As a consequence, sleeve support 30 is heated, and since its heat conductivity characteristic becomes less at higher temperatures, its utility as a thermal isolation means for the cathode sleeve 19 is increased. Therefore, the tubular leads 31, 32 not only contribute to increased efliciency of a tube in which they are used, by virtue of their increased conductivity, but also because they increase the effectiveness of the thermal isolation sleeve 30 in reducing heat conduction from the cathode sleeve.

Apart from their aforementioned coaction with the heat isolation shield 30 to improve the efficiency of a tube, the tubular leads 31, 32, have individual advantages. One of these advantages resides in a contribution of ruggedness to the cathode assembly. This is important where the heater is spaced appreciably from its lead-ins. Thus, the tubular leads provide a relatively rugged support for the heater 20 as a consequence of which it is restrained from displacement from the sleeve 19. Furthermore, the tubular supports provide a relatively stifi heater assembly which is of advantage when threading the heater into the cathode sleeve. In addition, the tubular construction of'the leads, renders a flattening thereof at one or both ends, relatively easy, for welding or brazing to the heater legs and leads 23, 24. In one embodiment of the invention, only one end of the tubular leads is flattened for welding to the

leads

23, 24, the other ends being open for receiving the free ends of the heater legs to facilitate fixing the heater legs to the tubular leads referred to. It will be noted from the dimensions previously given herein of the heater legs and tubular leads, that the heater legs are readily extendable into the end openings in the tubular leads. When the heater legs are once extended into the end openings in the tubular leads, they are effectively held in position without special holding means, while a welding or brazing operation takes place. In view of the foregoing advantage, this embodiment is preferred in practicing the invention.

It will be noted from the foregoing, that one advantage of the tubular leads 31, 32, is their relatively large area cross-section, as a result of which their electrical conductivity is increased. However, an increase in the crosssectional area of the heater leads also increases their heat conductivity. Therefore, it is desirable to keep the crosssectional area of the heater leads within prescribed bounds, even if this should involve appreciable heat generation therein, if the advantage of increased electrical conduc tivity is not to be negatived by the disadvantage of excessive heat conductivity. Applicants have found that the tubular structure of the heater leads 31, 32 according to the invention, contributes appreciably in realizing the aforementioned advantage while reducing to tolerable limits the disadvantage of heat conductivity.

Thus, the relatively large radiating surfaces provided by the tubular structure effectively dissipate by radiation to the sleeve support 30 any heat generated therein, as a consequence of a reduced cross-section thereof desired for reduced heat conduction from the heater.

However, the cross-section of tubular leads 31, 32 according to the invention is such that most of the heat radiated by the tubular leads 31, 32 is heat conducted to them from the heater, the cross-section being such that a relatively high electrical conductivity results and reduced resistance losses take place.

Applicants have found that a heater assembly having a heat isolation support 30 of a wall thickness of about 0.05 mil, and tubular heater leads having an outer diameter of about mils and a wall thickness of about 2 mils, is characterized by substantial increase in efficiency over structures having either relatively long heater legs or con ventional lead-ins of increased length for reaching relatively short heater legs. In this connection, it may be mentioned that heater lead-ins usually employed in the type of electron tube aforementioned have a diameter of 16 mils and are solid in cross-section. A lead having this structure and connected directly to a heater would conduct a prohibitive amount of heat from the heater and in view of its smaller radiating surface in relation to its diameter, would contribute to reduced eificiency of the cathode assembly.

It will be apparent from the foregoing that the invention provides an electron tube of increased efficiency. To this end the cathode assembly of the tube is provided with a thermal isolation support for the cathode sleeve and the heater is provided with tubular leads coacting with the support referred to, to reduce heat losses from the assembly. In addition, the tubular structure of the leads contributes to ruggedness of the cathode assembly and facilitates manufacture of the assembly. These features of the invention contribute to an improved electron tube.

What is claimed is:

1. A cathode assembly for an electron tube comprising a cathode sleeve, a heater within said sleeve and having relatively short legs extending from one end thereof, a support fixed to said one end of said sleeve, a lead-in for said cathode sleeve connected to said support, lead-ins for said heater spaced from said heater legs and said support, and leads connecting said legs to said lead-ins, at least a portion of said leads being adjacent said support, said support being made of a material having a coefiicient of thermal conductivity that becomes smaller with increases in temperature of said support, said leads being tubular for good heat radiation, whereby appreciable heat transfer from said leads to said support takes place for reducing the heat conductivity of said support.

2. A cathode assembly comprising a cathode sleeve, a metallic support for said cathode sleeve made of an alloy of nickel, iron and cobalt having reduced heat conductivity at elevated temperatures, a wire heater in said cathode sleeve, lead-ins for said heater, said support being disposed between said heater and said lead-ins, and tubular leads connected to said lead-ins, said leads having a smaller cross-section than said lead-ins and being connected to said heater, whereby said leads are heated by resistance losses and by conduction from said heater, said leads being relatively close to said support, whereby said support is heated by said leads for reducing its heat conductivity and improving the efiiciency of said assembly.-

3. A cathode assembly of increased efiiciency comprising a cathode sleeve, afirst elongated metallic support having one end engaging an end of said cathode sleeve, a second elongated metallic support having one end engaging said first support adjacent to the opposite end of said first support, a heater wire within said cathode sleeve, said heater wire having-legs extending adjacent to said first support, and tubular metallic leads of larger crosssectional area than said legs and extending adjacent to said second support, whereby said first'support is heated by said legs and said second support is free from heat from said tubular leads, said first support being made of a metal having a relatively low coetficient of heat conductivity at relatively high temperatures, said second support being made of a metal having a relatively high coefficient of heat conductivity at relatively high temperature.

4. A cathode assembly for an electron tube comprising a cathode sleeve, a metallic support for said sleeve, said support having a relatively low coetficient of heat conductivity at relatively high temperatures and engaging one end of said sleeve, a heater within said sleeve, said heater having two legs extending from said one end of the sleeve, two lead-ins spaced from said legs and adapted to be connected to a power supply, and two tubular leads having relatively large radiating surfaces connecting said legs to said lead-ins and being substantially co-extensive with and relatively close to said support, whereby heat generated in and conducted to said tubular leads is effectively transferred to said support, for improved efliciency of said assembly.

5. A cathode assembly for an electron tube comprising a cathode sleeve, a metallic support having relatively low coeificient of heat conductivity fixed to one end of said sleeve, whereby heat conductivity from said sleeve is reduced, a heater within said sleeve having legs extending from said one end thereof, lead-ins for said legs spaced from the legs, and cylindrical leads having a larger radius than said legs connecting said legs to said lead-ins, said metallic support being made of a material having a relatively low coefiicient of heat conductivity at relatively high temperatures, said leads being substantially coextensive with said metallic support and adjacent thereto for effective heat transfer by radiation to said support, whereby said support is heated by said leads for reduced heat conduction from said sleeve.

6. In combination: a cathode sleeve; a tubular metallic support having a relatively low coefiiicient of heat conductivity at relatively high temperatures engaging one end of said cathode sleeve; a heater within said sleeve having legs extending from said one end thereof; solid electrical conductors spaced from said legs; and tubular leads hav- References Cited in the file of this patent UNITED STATES PATENTS 1,984,897 Rothe Dec. 18, 1934 2,200,954 Glassberg May 14, 1940 2,432,513 Depew Dec. 16, 1947 2,441,224 Hector et a1 May 11, 1948 2,693,546 Sorg et a1. Nov. 2, 1954