US2683831A - Electron tube structure - Google Patents

Description

July 13, 1954 w, sKEHAN 2,633,831

ELECTRON TUBE STRUCTURE Filed Nov. 22, 1950 2 Sheets-Sheet 1 FIG. I

INVENTOR JOSEPH w. 'SKEHAN BY 0 ATTORN July 13, 1954 J. w. SKEHAN 2 Sheets-Sheet 2 INVENTOR JOSEPH W. SKEHAN ATTORNEY 2 Patented July 13, 1954 UNITED STATES PATENT OFFICE ELECTRON TUBE STRUCTURE Joseph W. Skehan, Stamford, Conn., assignor to Machlctt Laboratories, Incorporated, Springdalc, Conn., a corporation of Connecticut Application November 22, 1950, Serial No. 196,983

4 Claims. (01. 31338) This invention relates to improvements in the construction of coaxial heater terminals, for use with high frequency tubes.

The small over-all size and the close spacing required for high frequency operation of electron tubes have led to rather common use of coaxial tube structures. In addition to the assembly advantagesafiorded by the symmetry of these constructions, a much stronger mechanical structure with only small amounts of glass is achieved by coaxial designs. Furthermore, since high frequency tubes find extensive use with cavities in order to obtain maximum efii-ciency in operation, it is a decided advantage to have the whole tube structure of such design that a single plugin operation is all that is required to make good contact between the tubes coaxial terminals and cavities.

Because they are not truly a part of the high frequency circuit, the leads to heaters for indirectly heated cathodes of most early coaxial designs were brought through their glass envelope walls as parallel wires, disregarding the resultant inconsistency in overall tube structure and the inconvenience in connecting the tube into its circuit. Demand for completely coaxial structures, however, soon caused designers to ailix one heater lead to the cathode and bring the other lead out as a rod on the tubes axis. This coaxial rod lead represented an improvement in the art, but, because it was used to plug into coaxial circuitry, it offered several problems. Chief among the diiiiculties encountered were the flimsy nature of the rod which made it subject to bending and the small surface area of the rod which prevented good plug-in contact. The bending, which was annoying in itself, often caused permanent damage to the tube because of breakage or cracking of the insulation between the heater terminals. The small surface area of the axial rod permitted only poor contact and thus presented substantial electrical resistance.

In order to correct these undesirable conditions, cups were finally tried as inner-terminal members. These cups had their bases aifixed to the axial lead from the heater and were separated from and held relative to the outer heater terminal by an insulator. Furthermore, the whole inner surface of the cup presented larger contactarea with a material reduction in effective resistance. This latter improvement was especially valuable for airborne equipment because it reduced the voltage necessary to obtain required heater current and thus reduced the size and weight requirements of power supplies,

In the first structures developed using cups as inner terminals, glass was cast between two terminals, as by melting powdered glass. To give added strength and better insulation a sufficient amount of glass was used to fill the space immediately below the cup surrounding the top of the axial lead. Although theoretically strong, the cast glass often proved structurally weak so that the vacuum envelope was broken by slight mechanical shock. In addition, the severe heating of the glass due to its proximity to the heater caused evolution of trapped gases thus poisoning the vacuum.

A somewhat later structure eliminated the cast glass and substituted therefore a metallic sleeve and glass bead seal. A typical structure of this type seals one end of the metallic sleeve to the axial rod below the cup by means of a glass bead seal and, thereafter, solders the other end of the sleeve to the outer tubular terminal. Although this glass seal is not difficult to make, it requires the attention of a skilled technician. This type of seal also requires the use of Kovar, or other glass sealing. metal, which tends to make a weak metal to metal joint at the terminal cup. Furthermore, because of its proximity to the heater, since it must be located below the terminal cup, the glass seal is subject to failure due to overheating, and its internal position makes it extremely difilcult to cool the seal from the outside of the tube.

Since the advent of successful ceramic seals, it has been contended that the substitution of ceramic would solve most of the problems arising because of the use of glass. But substitution of ceramic for glass poses many problems, particularly in light of the fact that size and shape of terminals have been standardized to fit cavity sockets already widely in use. The standardized terminal types frequently employ a cup which is only sufficiently smaller than the outer tubular terminal to permit the insertion of a thin tubular dielectric therebetween. This construction is common'because it permits the largest possible contact area at the inner terminal. It minimizes the distance between the terminals, however, so that no satisfactory ceramic annulus could be inserted directly between the terminals. Even if satisfactory quality ceramic annulus could be made sufficiently narrow to fit between the terminals, the rings of solder necessaryto braze them to the terminals have a thickness which would cause them to short the terminals either during assembly or during soldering due to flow across the ceramic surface. My invention makes possible the use of a larger ceramic annulus having sufficient width to prevent shorting of the terminals as described. To this end, my inner terminal differs from prior art constructions in that it consists of a deep drawn cup, the top portion of which is of standardized terminal diameter. The bottom portion is of somewhat reduced diameter and is joined to the top portion by an essentially radial shoulder. The reduced diameter of the bottom portion accommodates a ceramic washer which is soldered to it and to the outer tubular terminal using conventional ceramic to metal soldering techniques. Thus the ceramic simultaneously performs the functions of insulator, separator, and vacuum wall member.

Accordingly, one object of my invention is to produce a rugged coaxial terminal construction for cathode and heater elements without changing the size or shape of the terminal contact areas. The seals between the ceramic and the metal, like the ceramic itself, are inherently stronger than glass to metal seals, and the relatively large diameter, even at the inner terminal, makes possible an extremely strong joint. These seals are so strong that, the dielectric tube interposed between heater terminals in the past, largely for reinforcement to prevent any radial displacement of the cup towards the outer tubular terminal, may be completely omitted. Another feature which makes for a more rugged structure is use of a material for the axial lead which may be more easily affixed permanently to the bottom of the cup terminal. This replacement also means that better conductivity can be obtained in the axial lead than when Kovar was used.

Another object of my invention is production of a heater terminal construction capable of withstanding higher temperatures than those experienced in the prior art. The ceramic is placed even closer to the heater than was the glass of the prior art without softening or cracking and may be chosen of such quality that it will not give off gases when heated. But heating of the insulation of itself poses a serious problem only when no means for cooling is available. The reentrant shape of these heater terminal assemblies makes it very difiicult to cool the insulator within the reentrant portion, particularly since the interterminal space is practically closed in order to make the inner terminal as large as possible. The practice of inserting a tubular dielectric member between the terminals has completely closed off the seals from any circulation of air.

Within a cavity air circulation is never very satisfactory anyway so that to be at all efiective a large area must be exposed. My structure is strong enough to permit omission of the dielectric tube and its deep cup also permits air COOIiIlg on the other side of the metallic wall at and below the seal. But more important in the overall cooling of the ceramic is the short, direct, large area thermal path through each seal and out each terminal. This cooling effect plus the ceramics ability to withstand higher temperatures make much improvement over the prior art structures such as those which employ a wireglass seal. In such a structure the small area center conductor and the long path down the sleeve and up the outer terminal represent relatively high heat flow impedance.

Still another object of my invention is the simplification of tube construction. By my invention there are just three essential heater terminal parts, all of which are of simple preformed shape. When sealed together these heater terminals represent the ultimate in simplicity of arrangement. Such a terminal configuration was impossible in the prior art because of lack of knowledge about the use of ceramic in vacuum tubes and because such a construction necessitated placin the ceramic in a rather inaccessible position.

Another advantage stemming from the simplicity of my heater terminal assembly is its compatibility with a unique inter-electrode spacing device which I have employed with coaxial tubes in the past. This device consists of a tubular member which surrounds the tubular terminal, these tubular members being joined together toward the end of the tube above a radial fold or convolution in the outer member. Below this fold the outer tubular member is joined to glass or other envelope material insulating the cathode terminal from its adjacent terminal. By axially compressing this fold the outer member is caused to slide upon the inner member, thus reducing the spacing between the active elements which are ultimately connected to the respective tubular members.

A final object of my invention is to produce the seals in my novel terminal arrangement accurately and in a minimum of time. In order to accomplish this end I employ a novel method of assembling the parts necessary to my completed assembly before placin them in a brazing fur nace. This method as described below will be seen to be of advantage in that it eliminates the skilled techniques of glassing from this portion of the tube. Thus it may be seen that even an unskilled person may be taught the simple assembly procedure required for its manufacture. At the same time great uniformity in the size and position of the seals may be achieved.

For a better understanding of this invention reference is made to the following drawings:

Fig. 1 illustrates the preferred form of my heater terminal construction as used in a vacuum tube in partial axial section.

Fig. 2 illustrates the fixture used in soldering the ceramic insulator in place together with an exploded view of the stem members showing their assembly order.

Referrin to Fig. l the vacuum envelope generally designated I0 contains three active planar elements: an anode H, a control grid 12 and a cathode l3. Anode H is atop a copper block M which provides a path of high thermal conductivity to the outside of the tube for cooling purposes and through which is bored an evacuation channel terminated in seal-oil means l5. Reentrant shell member I 6, forming part of the vacuum envelope, is affixed at opposite ends to block l4 and anode terminal I! respectively. Anode terminal I! is separated from grid terminal l9 by insulator [8 which is sealed vacuum tight to each of these pieces. Tubular member 20 supports grid support member 2| to which is aflixed grid 12 and which has an extension Zla to prevent spewin of vaporized cathode emitter material onto the dielectric. Tubular member 20 is sealed between grid terminal I9 and metallic tubular envelope portion 22.

The cathode terminal is advantageously composed of two tubular portions formin two plies 24 and 25 sealed together at the end of the tube. The tubular envelope portion 22 is sealed to this outer ply 24 by insulating ring seal 23. In some portion of the tube envelope between the dielectrio member 23 and the inner ply of the cathode terminal 25, a small radial fold or convolution 24a is formed in the member 24. Attached to the internal end of inner ply is low heat conductivity foil tube 28 which is in turn attached to the cathode l3. By axially compressing the convolution 2 3a the tubular surfaces of 24 and 25 are caused to slide upon one another thereby ultimately moving the active surfaces of the grid and cathode closer together without opportunity for misalignment.

The heater coil 27 is placed adjacent the cathode |3 within the foil member 25 and surrounded by tubular heater shield 28 having a radial fiare 28a at one end which is attached to the cathode. One end of the heater is attached to the cathode terminal inner ply member 25 through conductor 29. lead 30 to the bottom of deep drawn inner heater terminal cup. 3|. Although a cup-shaped element is always employed as inner terminal member 3|, the bottom of the cup need not in every case be metal despite the obvious advantage of an all-metal piece. This cup member is shown in preferable form comprising a large diameter lip portion 31a separated from a small diameter base portion 3!?) adjacent the bottom by an essentially radial shoulder. The larger diameter portion 31a corresponds to the prior art inner terminal in size, shape, and location so that tubes employing my invention may be used in standardized cavity sockets. This cup might be of the same diameter as its base portion throughout its length when used in a tube without specified cavity requirements, or it may be of any convenient shape so long as it has some portion having a substantial diameter which is nevertheless small enough to permit the ceramic annulus 32 to be interposed between it and the inner ply member of the cathode terminal 25. Small bafile plate 33 is placed between the heater and the ceramic annulus 32 to prevent material from the heater and cathode from being sprayed onto the ceramic, thereby shorting the terminals. Tubular dielectric member 34 may be omitted for better cooling if desired because the terminals are held rigidly in place by the seals between them and the ceramic.

Fig. 2 illustrates a novel fixture used in the assembly of the heater terminal structure. It

also shows an exploded view of the essential parts cf the heater terminal construction and enables one to obtain a visual concept of how the various parts are assembled. The fixture itself is extremely simple and is composed of a planar base member and a tubular member M which may or may not be attached to the base. Initially I employed a structure which consisted of two opposing members attached perpendicular to the planar member thereby essentially forming a tube. This one piece fixture was advantageous in that it was extremely simple, but it suffered from the fact that it permitted misalignment of the parts.

The method of assembly illustrated in Fig. 2 permits ease of assembly and prevents twisting of the cup terminal, the cause of much misalignment in the past. It employs a modified fixture which consists of a planar member with a raised shoulder, normally circular in shape, which snug- 1y accommodates the mouth of the cup 3 I. Tubular member 34 is advantageously composed of dielectric material which may be retained be tween the assembled terminals as an insulator, if desired. It is or" such diameter that it may be easily slipped over the large diameter portion The other end is connected through axial of cup 3 la until it rests against the planar member 35. Thereafter ceramic annulus 32 is slipped over the smaller end of cup 31b and rests atop dielectric tube 34 which is made a correct predetermined height to locate the ceramic annulus 32 at the identical place each time the fixture is used. Solder ring 36 for making the seal between the ceramic and inner terminal is then slipped over cup end 31b until it comes to rest atop ceramic annulus 32. Larger solder ring 31 is of sufiicient diameter to fit snugly within terminal piece 25, and it is placed within the end of this terminal which will ultimately be remote from the cathode. This same end of terminal member 25 is then slipped over the whole assembly on the fixture and pmhed downward until it strikes member 35 during which process solder ring ill is pushed into soldering position by ceramic annulus 32. Thereafter the whole assembly is placed in a suitable brazing oven, the tempera-- ture of which is raised to a level sufiicient to cause flow of the solder.

t will be obvious to those skilled in the art that any unskilled worker can be taught to perform the above mentioned novel assembly procedure. Furthermore, much manufacturing time is saved because, despite the fact the brazing time is longer than glassing time, a large number of assemblies may be placed in a brazing oven at the same time.

Although the described construction is preferred, the description is not meant to limit the scope of the invention. For instance, the shape of terminal members, the active elements, and other tube parts may be varied. Since my invention is confined to heater terminal construction, it would seem obvious that the number of tube elements employed is immaterial. Modifications in the structure depicted which would occur to anyone skilled in the art or which are apparent from the claims are intended to be within the scope and spirit of my invention.

I claim:

1. In an electron tube having at least an anode, a cathode, and a heater within a vacuum envelope, a coaxial heater terminal construction comprising an outer terminal consisting of two tubular members joined together toward the tubes end above a radial fold in the surface of the outer member, the inner member being con nected to the cathode and one side of the heater, an inner cup-shaped terminal connected to the other side of the heater, and a ceramic annulus sealed between the two terminals.

2. In an electron tube having at least an anode, a cathode, and a heater within a vacuum envelope, a coaxial heater terminal construction comprising an outer terminal consisting of two tubular members joined together at the tubes end, the outer member having a radial told in its surface and the inner member being connected to the cathode and one side of the heater, an inner cup shaped terminal having smaller diameter side walls at its bottom than at its top,

' said walls being joined by an essentially radial shoulder, and a ceramic annulus sealed between the two terminals.

3. In an electron tube having planar electrodes, including at least an anode and an indirectly heated cathode, and coaxial terminals for said electrodes, a cathode stem construction comprising an outer tubular terminal of essentially uniform diameter, a generally tubular high heat flow impedance cathode support member terminating the tubular terminal internally of th vacuum envelope and efiectively extending its length, a planar cathode of the oxide emitter type aflixed to the cathode support, a cup-shaped terminal member lying within the tubular terminal member, a heater element connected between the outer terminal and the inner cup-shaped terminal, a ceramic annulus between the tubular outer terminal and the sidewalls of the cup-like inner terminal, metallic solder joints sealing the ceramic vacuum tight between the tubular and cup-like terminal members and a shield-like baffle between the heater and the ceramic annulus.

4. In an electron tube having planar electrodes, including at least an anode and an indirectly heated cathode, and coaxial terminals for said electrodes, a cathode stem construction comprising an outer tubular terminal of essentially uniform diameter, a generally tubular high heat flow impedance cathode support member terminating the tubular terminal internally of the vacuum envelope and efiectively extending its length, a planar cathode disk coated with oxide emitter material and afiixed to the cathode support, a cup-shaped terminal member having smaller diameter side walls at its bottom than at its top,

said walls being joined by an essentially radially extending shoulder and said cup-like terminal lying within the tubular terminal member, a helical heater element connected between the outer terminal and the inner cup-shaped terminal, a tubular heat shield surrounding the heater element and affixed to the cathode disk, a ceramic annulus between the tubular out r terminal and the smaller diameter side walls of the cup-like terminal, metallic solder joints sealing the ceramic vacuum tight to the tubular and the cup-like terminal members, respectively, and a shield-like baflle aflixed to the cup-like member and extending laterally between the heater and the ceramic annulus.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,871,371 Jackson Aug. 9, 1932 2,121,600 Knowles et a1 June 21, 1938 2,250,986 Dobke July 29, 1941 2,396,320 Gaudenzo Mar. 12, 1946 2,445,766 Derby July 27, 1948 2,446,271 Eitel Aug, 3, 1948

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