US2841736A

US2841736A - Electron tube and filamentary cathode

Info

Publication number: US2841736A
Application number: US359770A
Authority: US
Inventors: Merrald B Shrader
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1953-06-05
Filing date: 1953-06-05
Publication date: 1958-07-01
Anticipated expiration: 1975-07-01

Description

July l, 1958 A M. B. SHRADER 2,841,736

ELECTRON TUBE AND FILAMENTARY CATHODE INVENTOR.

MERRELL: B. SHRHDER ORNEY July l, 1958 l M. B. sHRADER 2,841,736

ELECTRON TUBE AND FILAMENTARY CATHODE Filed June 5, 1953 2 Sheets-Sheet 2 lijf.

INVEN TOR.

MERRHLD B. SHRHDER United States Patent O ELECTRON TUBE AND FILAMENTARY CATHODE Merrald B. Shrader, Mount Joy, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application June 5, 1953, Serial No. 359,770

13 Claims. (Cl. 313273) This invention relates to electron tubes and more particularly to such tubes having a filamentary type cathode.

Filamentary cathodes are characterized by dependable electron emission over an exceedingly long useful life. Furthermore, such filaments have successfully been utilized in tubes having a wide range of power output capabilities even though relatively complex arrangements are required. While iilamentary cathodes have come tobe more widely used in power tubes than indirectly heated or unipotential cathodes, this is not the case with respect to high frequency tubes. The flamentary cathode structures heretofore utilized, in addition to having certain mechanical disadvantages, have also been unsuitable for use at the higher frequencies.

One of the disadvantages of the structures heretofore utilized may be best pointed out in connection with the so-called self-supporting lamentary structure. In such a structure lamentary strands are usually arranged around a circle with adjacent strands extending parallelV to one another. The adjacent strands are connected at one end to different heating current lead-ins while at their opposite ends the strands are bound together to form the self-supporting structure. The heating current which heats the filament strands to electron emission temperature travels up alternate strands to the bound ends and then down the remaining strands. One of the filament heater lead-ins serves also as the radio frequency current lead-in thereby resulting in the radio frequency current traveling up alternate strands and then down the remaining strands. Obviously such an arrangement is limited to relatively low frequencies since otherwise the electrical phase difference' between adjacent strands would be found objectionable. Furthermore, the low frequency heating" current e. g., 60 cycles, usually utilized adversely affectsA the operation of such tubes and its harmful effects must be overcome. Often this is extremely diflicult and costly to carry out.

Some of the foregoing short comings are ameliorated by utilizing a central support connected to the bound ends or electrical center of the filamentary strands. The center tap thus provided may be utilized as theradio frequency' lead-in or connection to the filament structure. However, such a structure also is limited to relatively low frequency. As is well known, with increasing frequency the elements of a tube become more and more a significant part of the high frequency circuit. Theinduct'ance of leads and filament strands becomes a major portion of the circuit and effectively limits the upper frequency of operation.

While the filamentary type of cathode structure is widely used, the structures utilized have often proven to be: fragile and subject to deformations which have limited their usefulness.

It is therefore a principal objectof my invention to providean improved electron tube having a` substantially unipotential filamentary cathode.

Another important object is to provide an electron tube having a substantially unipotential lamentary cathode and capable of operation at high frequencies.

Still another object is to provide an electron tube having a substantially unipotential lamentary cathode capable of operation at high frequencies and high power. Another object is the provision of an electron tube in which the filament heating current leads are cornpletely shielded from the radio frequency current leads and do not themselves carry any radio frequency current. Yet another object is the provision of an electron tube having a substantially unipotential lamentary cathode and having reduced overall dimensions.

Stated generally, I provide a pair of filament heating current lead-ins which extend into the electron tube f envelope and to each of which are connected filament support rods. Alternate support rods are connected to one lead-in while the remaining support rods are connected to the other lead-in. I utilize filament strands which are doubled over or joined together at one end so as to form interconnected pairs. The` free ends of the doubled over or paired filament strands are each connected to adjacent support rods while the strands themselves extend back along the supportrods. A tubular radio frequency 1eadin is sealed through the envelope so as to completely shield the heating current lead-'ins from the remaining lead-ins and elements of the tube which may include control electrode, screen electrode, and anode lead-ins and electrodes. The filamentary` cathode radio frequency lead-in extends into the envelope and` is conductively connected to each of the filament strands at their electrical center. In such an arrangement the radio frequency currents travel up all of the filamentary strands in phase and do not travel along the filament support rods at all. Thus the electrical length of the cathode structure with respect to radio frequency is that o'f the radio frequency lead-in and the filament strands themselves. The electrical length of the support rods is not included at all.

My invention as well as further objects and advantages thereof will be most clearly understood from the following description thereof and the accompanying drawings in which:

Figure 1 is a sectional view of an electron tube constructed in accordance with my invention;

Figure 2 is a fragmentary sectional view through the line 2 2 of Figure l;

Figure 3 is a fragmentary perspective view of a portion of the filament structure;

Figure 4 is a sectional view of yet another filament and filament support structure constructed in accordance with my invention; and

Figure 5 is a fragmentary perspective view of a filament structure similar to that shown in Figure 4.

Referring now to the drawings in detail and to Figures 1, 2 and 3 in particular, electron tube 10 comprises a filamentary cathode 11, anode 12, and may also include control electrode 13 and screen electrode 14. The control and screen electrodes 13, 14 themselves` may be of any suitable form as is well known in the art and need not be described in detail here. Anode 12 in the present instance forms part of the `envelope of tube 10 and may be provided with an array of cooling fins 15, as shown, to form a radiator through which a coolant such as air may be forced under pressure.

Extending into the recess formed by anode 12 are a plurality of

filament support rods

16, and 16a which may be molybdenum and are arranged alternately in an annular array ini the present instance. All of the support rods 16 are joined at one end as by welding to a support ringl 17 which may be of molybdenum or tung,- sten or the like. Support rods 16a extend'through `holes or recesses in support ring 17 and are joined to a similar support ring V18. A pair of nested coaxial tubular leadins'and supports which may be copper are sealed through the envelope of tube 10. Lead-in and support 19 is joinedat -its-inner end Vto support ring 17 whilelead-in andfsupport `20 isjoined to support ring 18. Insulating ,support means may be provided between the supports 19,120. Llor example, an insulating bushing 21 which may..` be of ceramic or other suitable material may be utilized as a. spacer between the inner ends of supports 19. and 20; Bushing or spacer 21 may be shouldered asl shown -so as to provide additional support for support ring 17. An'annular metallic member 22 which maybe .readilysealed tofglass is joined as by brazing or the like Vto support member 20vadjacent the outer end thereof.

Member 22 is sealed as by means of glass bushing 23 to another annular metallic member 24 which also may .and iron.

Support member 19 extends outwardly beyond member l20 and its outwardly extending portion or, as shown, a

terminal cap 25 serves as a Contact or terminal for one of the filament heater connections. As shown, the terminal member or cap 25 may have an outer diameter which is substantially equal to that of the outwardly extending portion of support 20 which serves as the terminal member for the other side of the heating current circuit. Intermediate the ends of support 20 and inwardly of the portion thereof which serves as a terminal, an annular member 26 is sealed thereto and serves to support an annular terminal member 27 which is somewhat U shaped in cross section. Members 26 and 27 form part of the envelope of tube 10 and are insulatingly joined together as shown at 28 by glass or other suitable material.. It should be noted that terminal vmember 27 forms the innermost portion of a re-entran part ofthe envelope of tube 10 as shown.

A tubular or cylindrical conductive member 29 is joined to and supported on terminal member 27 and extends inwardly and engirdles the end portions of support members 19 and 2l) as well as support rings 17 and 18 which extend inwardly of terminal member 27. Aiplurality of filaments 30, one of which is most clearly shown in Figure 3, are provided. Each filament 30 has a pairof parallel portions or

strands

31 and 32. The `free end of each of the strands 31 is connected to the end portion of one of the support rods 16 as by means lof molybdenum clips 33 which are welded thereto. The free end of each of the strands 32 is similarly connected to one of the support rods 16a. In the present instance filaments 30 are each unitary and are doubled back somewhat in theV form of a hairpin to form the parallel extending

portions

31, 32. Each of the filaments 30 is connected at its midpoint to conductive member 29 by means of thin flexible radio frequency connectors or lstraps V34 which may be tantalum or other suitable refractory or high melting .point metal. The flexible straps orconnectors while serving to rigidify and strengthen the filament lstructure also permit elongation and contraction of the filaments without causing deformation. As clearly shown

portions

31 and 32 of each of the filaments 30 extend back along

support rods

16, 16a. The array formed by the support rods'and the array formed by the filament strands are thus arranged in sideby-'side spaced relation. As will be more fully pointed out,..such a filament structure may be made so as to bodies several important advantages.

have such an extremely short electrical length as is desirable and necessary at the higher frequencies.

An annular terminal member 35 has one peripheral portion insulatingly sealed to a peripheral portion of terminal member 27. Terminal member 35 may serve as part of the electrical lead-in and support for control electrode 13 which may be supported therefrom as shown by conductivetubular member 36. rlhe other peripheral portion of terminal member 35 is insulatingly sealed to terminal member 37 which is also annular and serves to similarly support screen electrode 14 by means of conductive tubular member 38.

While the radiator formed by fins 15 may serve as the anode terminal, in the present instance terminal member 39 is preferably provided. Terminal member 39 has one end portion thereof sealed to anode 12 while the other end thereof is sealed to one end of tubular insulator 40 which may be glass. The other end of tubular insulator 40 is sealed to an end portion of terminal member 37.

The envelope and terminal structure of tube 10 em-

Terminals

27, 37 and 39 are all interposed in the envelope wall and are preferably of highly conductive material or plated with silver or other suitable highly conductive material. Such terminals provide low impedance paths and particularly provide low impedance paths to `high frequency currents. yAll of the terminals including also the filament heater terminals 20 and 25 are arranged to facilitate the axial mounting and dismounting of the tube in its circuit. A

Another important advantage of such a tube structure resides in the'relatively short overall axial length which is made Apossible by the re-entrant arrangement shown. More particularly the re-entrant structure is provided by the Vfact that all of the filament terminals including the high frequency terminal 27, are nested within other parts of the envelope structure. Tubular insulator 40 which necessarily must be relatively long to afford proper insulation between

terminals

39 and 37 does not add to the overall axial length of tube 10. This is made possible because of the arrangement of terminal 35 as'well as most of the filament support structure within tubular insulator 40. As shown terminal 35 is of substantially smaller diameter than the diameter of insulator 40 and is nested within insulator 40 and adjacent the portion thereof connected to terminal 39. The radio frequency filament terminal 27 is seen to be alsoV nested within insulator 40 and is substantially in alinement with anode terminal 39. Filament heater terminal 20 is seen to be located entirelyl within insulator 40 while `the yother heater terminal 25 is substantially alined with terminal 37. Thus, it is apparent that the structure combines lthe advantage of long glass or leakage paths with short overall tube or lead length.

A further advantage accrues from the construction of radio frequency terminal 27. Because of the well known skin effect at the high frequencies, losses which occur along a conductor increase not only with power but also Vwith frequency. In the case of radio frequency conductors which are sealed through insulators it is necessary to avoid excessive power loss. Such power loss is vmanifested by the generation of heat in the conductor, which, if excessive, results in damage to or destruction of the seal. In the present instance the radio frequency termi.- nals or leads which are sealed through orform part of the envelope wall of tube 16 are readily provided with sufficient surface area to avoid excessive losses. In particular, the filament radio frequency terminal 27 and the seals associated therewith may be made large enough in diameter in accordance with the present invention so as to prevent excessive sealvheating and circuit losses. A l Another important advantage, accrues fromthelfact thatsucha tube may be mounted in its circuitwith conmentary cathode and another electrode, a plurality of elongated conductive support members in an annular array, said filamentary cathode comprising a plurality of filament strands in an annular array, said array of filament strands being interposed between said other electrode and said array of support members, heating current lead-in and 'support means connected to said support members adjacent one end thereof for applying heating current to said filament strands, each of said filament strands being connected adjacent one end thereof to the other end portion of one of said support members, a leadin and support for said other electrode, and a lead-in for radio frequency current interposed in said envelope between said heating current lead-ins `and the lead-in for said other electrode and connected to the other end portions of each of said filament strands.

2. An electron tube comprising an envelope, a plurality of electrodes spaced in said envelope and including a plurality of cathode filament strands in an annular array with the remaining electrodes all on one side of said array, an annular cathode terminal member interposed in said envelope, a tubular conductive member connected to said cathode terminal member `and to each of said filament strands adjacent one end thereof, heating current lead-ins on one side of Isaid cathode terminal member providing the sole heater current path, a plurality of conductive support members connected adjacent one end thereof to said heating current lead-ins and forming an annular array on the side of said cathode filament array away `from said remaining electrodes, said support members extending into said envelope beyond said tubular conductive member, and each of said filament strands being connected adjacent the other end thereof to the inner end portion of one of said support members.

3. An electron tube comprising an envelope, a plurality of coaxial, concentric electrodes spaced in said envelope and including a plurality of cathode filament strands in an annular array, an annular cathode terminal member for radio 'frequency current interposed in said envelope, a tubular conductor for radio frequency current connected to said cathode terminal member extending adjacent to one end of said filament strand array and connected thereto, heating current lead-ins for supplying heating current to said filament strands and extending into said envelope interiorly of said cathode terminal, a plurality of filament support members in an annular array coaxial and concentric with said filament strand array a-nd within the same, said support members extending through said tubular conductor with the inner ends thereof spaced axially inward of said tubular conductor, and the end portion of each of said filament strands remote `from said tubular conductor being connected to the innerend portion of one of said support members.

4. An electron tube as defined in claim 3 wherein said filament strands are substantially hairpin-shaped, and said tubular conductor is connected to substantially the electrical center of each of said strands.

5. An electron tube comprising an envelope, a plurality of cathode filament strands in an annular array, coaxial concentric control and anode electrodes spaced about said cathode array, annular cathode, control and anode terminal members interposed in said envelope in that sequence and insulated one from the other, the diameter of said cathode terminal member being the smallest, the `diameter of said anode terminal member being the largest, the diameter of said control electrode terminal member being intermediate that of said cathode and anode terminal members, concentric coaxial tubular conductive members connecting said cathode and control electrode terminal members respectively to one end of said filament strand array and to said control electrode, and heating current supply and support means forming with said filament strands the sole path for heating current, said means including a pair of heating current lead-ins extending through said envelope interiorly of said cathode terminal member, a plurality of conductive support members in an annular array and connected adjacent one end thereof to said lead-ins, said support members extending through the tubular conductive member connected to said filament strand array, the end portion of each of said lament strands remote from 4the tubular conductor connected thereto being connected to the inner end portion of one of said support members.

6. An electron tube comprising an envelope, a plurality of electrodes spaced in said envelope and including a filamentary cathode and another electrode, a plurality of elongated conductive support members in an annular array, said filamentary cathode comprising a plurality of filament strands in an annular array, said array of filament strands being interposed between the other electrode and said array of support members, a pair of coaxial concentric tubular heating current lead-ins sealed through said envelope, support means connected to said heating current lead-ins and to said support members adjacent one end thereof, each of said filament strands being connected adjacent one end thereof to the other end portion of one of said support members, a lead-in and support for said other electrode, and a lead-in for radio frequency current interposed in said envelope between said heating current lead-ins and the lead-in for said other electrode and connected to the end portions of each of said filament strands remote from said support members.

7. An electron tube comprising an envelope, an annular array of a plurality of cathode filament strands, an annular array of a plurality of conductive support members spaced radially inward of said filament strands with said filament strands extending downward along the same, the inner end portions of each of said filament strands being connected to the inner end portion of one of said support members, a pair of nested conductive tubular lead-ins sealed through said envelope in insulated relation, alternate ones of said support members being connected to one of said lead-ins, the remaining support members being connected to the other of said lead-ins, an annular cathode terminal member interposed in said envelope and insulatingly sealed to the outer one of said lead-ins, said cathode terminal member engirdling said lead-ins and being adjacent to the inner ends thereof, and a tubular conductive member joined adjacent one end to said cathode terminal member and adjacent the other end to the end portions of said filament strands remote from said support members.

8. An electron tube comprising an envelope containing a cathode, said cathode including a pair of cathode filaments, a tubular cathode terminal and support member connected to said filaments at one end thereof, and a pair of filament terminal members extending in insulating relation through the tubular opening of said `cathode terminal and support member and connected respectively to said cathode filaments at the other end thereof.

9. An electron tube comprising an envelope containing a cathode, said cathode including a pair of cathode filaments, a grid adjacent to said cathode, a tubular cathode terminal and support member forming part of said envelope connected to all of said filaments at one end thereof, a tubular grid terminal and support member surrounding said cathode terminal and support member, and a pair of filament terminal members extending in insulating relation through the tubular opening of said cathode terminal and support member and connected respectively to said cathode filaments at the other end thereof.

10. An electron tube comprising an envelope containing a cathode, a grid adjacent to said cathode, a tubular radio frequency cathode terminal connected to said cathode and forming a part of said envelope, a tubular radio frequency grid terminal of larger diameter than said cathode terminal connected to said grid and forming a part of said envelope, and means for conducting heating current to said cathode including terminal means surrounded by said cathode terminal, said cathode terminal forming the inner-most extension of a re-entrant portion of said envelope.

11. An electron tube comprising an envelope, a plurality of cathode ilament strands, grid and anode electrodes spaced from said cathode iilament strands, a pair of heating current lead-ins extending into said envelope at one end thereof and connected to said iilament strands, an annular cathode terminal member surrounding said pair of heating current lead-ins, an annular grid electrode terminal member surrounding said pair of heating current lead-ins and having a diameter larger than said cathode terminal member, said pair of heating current leadins, said cathode terminal member and said grid electrode terminal member being insulatingly sealed to each other and to said anode electrode, said cathode terminal member forming the inner-most extension of a re-entrant portion of said envelope.

12. An electron tube as dened in claim 5 wherein said heating current lead-ins, said cathode terminal member, said control electrode terminal member, and said e anode terminal member comprise a re-entrant portion of said envelope.

13. An electron tube comprising a iilamentary cathode, co-axial and concentric control, screen, and anode electrodes, support and heating current lead-in means comprising a first tubular conductor having a terminal portion at one end thereof, and a second tubular member co-aXial with and having a larger diameter than said first tubular member surrounding the other end portion of said first tubular member, an annular cathode terminal member surrounding said other end of said first tubular member and said second tubular member and insulatingly sealed to said second tubular member, an annular control electrode terminal member insulatingly sealed to said cathode terminal member and extending toward said terminal end of said rst tubular member, an annular screen electrode terminal member insulatingly sealed to said control electrode terminal member extending toward and surrounding said terminal end of said first tubular member; whereby said iirst tubular member, said second tubular member, said control electrode terminal member, and said screen electrode terminal member form a double reentrant portion of the envelope of said electron tube.

ieterenees Cited in the le of this patent UNITED STATES PATENTS 2,399,004 Crawford Apr. 23, 1946 2,451,987 Sloan Oct. 19, 1948 2,452,786 ONeil NOV. 2, 1948 2,489,872 Elder et al Nov. 29, 1949 2,510,171 Clark lune 6, 1950 2,534,548 Fay et al. Dec. 19, 1950 2,542,639 De Walt Feb. 20, 1951 2,617,960 Kohl Nov. 11, 1952 2,705,294 Schrader Mar. 29, 1955 2 726,349 Wing et al Dec. 6, 1955 UNITED STATES PATENT OFFICE CERTIFICATE OF vCORRECTIGN Patent No., 2,841,736 July l, 1958 Merrald Bo Shrader It is hereby certified that error appears in the-printed specification of the above 4'numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column l, line 33, after "a structureu insert the u; column 5,\ line 66, for "each" read each ne; column 8, lines 59 and 6D, strike out nforming part of said envelope; same column 8, line 60 strike out "all Of o Signed and sealed this 28th day of October 1958.,

(SEAL) Attest:

KARL H. AXLINE Attesting Officer ROBERT C. WATSON Commissioner of Patents