US3522467A - Electron tube filament support structure employing deformable loop portions of the filaments - Google Patents

Description

Aug. 4', 1970 J. A. BAKER 3,522,457

ELECTRON TUBE FILAMENT SUPPORT STRUCTURE EMPLOYING DEFORMABLE LOOP PORTIONS OF THE FILAMENTS Filed Oct. 26, 196' 2 Sheets-Sheet l 42 I so INVENTOR.

JEAN A. BAKER BY M1 249? ATTORNEYS Aug. 4, 1970 J. A. BAKER 3,522,457

ELECTRON TUBE FILAMENT SUPPORT STRUCTURE EMPLOYING DEFORMABLE LOOP PORTIONS OF THE FILAMENTS Filed Oct. 2'3, 1967 2 Sheets-Sheet 2 r4 7 a 70 (N (6) 2 v 64 64 INVENTOR. 1 f2 JEAN A. BAKER I BYW ATTORNEYS United States Patent U.S. Cl. 313278 16 Claims ABSTRACT OF THE DISCLOSURE An electron tube filament structure having a filament wire and a filament support. The filament wire has a bight looped over one member of the support and two rectilinear strands extending between the bight and another member of the support. The supporting members are spaced and biased from each other to hold the strands taut. The bight dampens deviations in strand tension through bowing action confined to a plane substantially parallel adjacent grid wires thereby maintaining a constant filament-to-grid spacing.

BACKGROUND OF THE INVENTION This invention relates to electron tube filament structures and particularly to a novel filament structure which provides improved stability of interelectrode spacing during tube operation.

In electron tubes having a directly heated cathode, the electrodes typically comprise a filamentary cathode, one or more grids, and an anode. The filamentary cathode conventionally consists of a number of parallel, thoriated tungsten filaments arranged in horizontally spaced, vertically extending orientation to form a cylindrical or caged pattern. The grid comprises a number of non-emissive wires similarly arranged into the shape of a cylinder and disposed coaxially with the filamentary cathode. A copper anode in the shape of a hollow cylinder is located coaxially about the grid. Though each electrode is radially spaced from each other, the cathode-to-grid spacing is quite small, frequently measuring only some ten to some two hundred thousandths of an inch.

Typical filamentary cathode and grip supports provide upper and lower members to which the ends of the filaments and grid wires are respectively fastened. Though both members of the grid support are fixed, only one member of the cathode support, usually the lower, is fixed while the other is biased therefrom by a spring. In this manner the filaments are maintained in a state of tension throughout the range of temperatures encountered during storage and tube operation.

Were the tension maintained on each filament the same as upon each other, the bias alone would be sufficient compensation for the elongations and contractions of the filaments caused by their extreme changes in temperature. This, however, is not actually the case. In practice it is impractical if not impossible to mount all the filaments in an identical manner. Furthermore, the physical geometry of each frequently varies ever so slightly. There are also latent strains and stresses in the filament supports which may be relieved at elevated temperatures causing nonuniform and unpredictable elongations and contractions. These variations, though tenuous, are frequently of enough significance as to cause one or more filaments to have insufficient tension at operative temperatures. In this situation, further expansion of such a filament will not be compensated longitudinally by means of the support bias. Expansion will thus cause the filament to move laterally, that is to bow. This bowing can take place in any direction, the vast majority of which "Ice have a component normal to the filament. In so bowing the interelectrode spacing between the filament and the adjacent grid wire is altered. As this spacing is critical to proper tube operation, any change adversely affects the performance of the electron tube. In the more extreme cases of bowing, the filament will actually touch an adjacent grid wire causing an electrical short circuit and probable tube destruction.

Heretofore, the above described problem has been partially abated through the use of filament supports having separate resilient members to which each individual filament is fastened. With this structure, divergent forces arising in each filament from thermal expansion are independently compensated. The prior art approaches to this problem has almost universally been directed towards various designs in these filamentary supports. The theme has been to impart general tensioning of the filaments as a whole through the use of a spring, and then to compensate for variations in individual filaments through the use of secondary biasing means in the filament support. Some supports, known in the art as spiders, have taken the form of a thin disk having radiating arms to which individual arms are held. These arms, which typically have significant widths, are made of molybdenum. The arms thus have more resiliency than the filaments which are made of thoriated tungsten. In some designs, a plurality of arms having varying degrees of resiliency have been used; in others, split arms have supported a filament simply looped over both sections of the arm. But in all of these designs of varying complexity, the central thought has prevailed: Impart resiliency into the filament support to provide secondary biasing means to damper adverse variations in the tensioning of individual filaments.

Accordingly, it is an object of the present invention to provide an electron tube filament structure which produces improved stability of interelectrode spacing under varying thermal conditions.

Another object of the invention is to provide a novel electron tube filament configuration which self-compensates for deviations from a predetermined norm of tension.

An associated object of the invention is to provide a simplified, rigid filament support which may be uniformly and inexpensively fabricated and mounted.

SUMMARY OF THE INVENTION Briefly described the present invention is an electron tube filament structure comprising a filament wire and a filament support. The filament wire has two rectilinear strands joined at one end by a unitary bight. Attaching means affix one end of each strand in the tube while the filament support abuts the filament bight. The filament support is biased from the attaching means to tension the two rectilinear strands to a predetermined degree. The portion of the bight which abuts the filament support does not exceed approximately half of the total length of the bight whereby deviations in tension of a rectilinear strand are dampened through change in the configuration of the bight.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevational view of an electron tube, portions of which are broken away revealing one embodirnent of the filament structure of the present invention.

FIG. 2 is a perspective view of one embodiment of a filament support made in accordance with the present invention.

FIG. 3a is an elevational View of a typical filament and filament support combination of the prior art. FIGS. 3b, 3c, and 3d are elevational views of three embodiments of the present improvement over the prior art structures.

FIG. 4a is an extended elevational view of the embodiment shown in FIG. 30 with an associated force diagram. FIG. 4b diagrammatically illustrates the result of adverse, filamentary tensioning. FIG. 40 is a partial plan view of filamentary and grid wires in association with a displacement diagram.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in more detail to FIG. 1 there is illustrated an electron tube of the 3CW20,000 general series, and in particular the EIMAC Triode 3CW20,000H3. The cut-away portion of the tube reveals in cross section an outer envelope housing a cup-shaped copper anode 12 having a tubular pinch-off 14 extending through the anode and envelope about the tube axis. The pinch-off is enclosed by a tubulation cap 16 which is afiixed to envelope 10. Housed within anode 12 are a molybdenum grid cap 18 which supports molybdenum grid wires 20. Though only two grid wires have been illustrated for the sake of clarity, it should be understood that grid Wires 20 are held all about the circumference of grid cap 18 to form a cylindrical cage. The lower ends of the grid wires are afiixed to a grid support 21.

The cathode consists of a plurality of thoriated tungsten filaments which are centrally bent to form bights, each of which integrally join two rectilinear strands 26. One free end of each rectilinear filament strand 26 is welded to a finger 30 which extends from an inner filament support 32. The other free end of each rectilinear strand is welded to an adjacent finger 31 extending from an outer filament support 22. Both the inner and outer supports and their fingers are made of nickel. Each filament bight 24 is looped over a cantilevered arm 34 of a centrally apertured spider. A hub with radially extending rods 34' assembly, as illustrated in FIG. 2, may be substituted for the spider and arms filament support. Such a hub and rods support is known as the Wagonwheel.

Near the free end of each arm 34 or rod 34' is a notch 38 into which the bight lays and is kept from sliding axially along the arm or rod. Once a filament has been looped over the notch and the ends welded to fingers 30 and 31, rectilinear strands 26 are aligned essentially parallel to each other.

Also shown in the cut-away portion of the tube are means for biasing the upper filament support away from lower filament supports 22 and 32. One end of a center molybdenum rod 40 is held in a recess 42 within copper housing 44. This housing also retains a spring 46 coaxially about center rod 40. Spring cups 48 are fitted about each end of the spring. Spacers 50 are inserted between the lower spring cup from the inner floor of the housing. A hollow, pusher rod 52 is placed coaxially about center rod 40 and affixed at one end to the upper spring cup. A hollow, cylindrical getter 41 is then placed about the pusher rod. Finally a dielectric collar 54 mechanically links but electrically insulates the upper end of pusher rod 52 from the spider and arms 34. Thus spring 46 exerts an upward force on the spider which tensions filament strands 26 aligning them in parallel. The degree of tension may be adjusted through the inclusion or exclusion of spacers 50. Approximately 3 pounds of tension is all that is typically required. The upper filament support is coaxially aligned with the grid by center rod 40 which extends through a central aperture in the spider.

The ease with which the described filamentary cathode and its support may be assembled should be appreciated. Once the free ends of rectilinear strands 26 have been welded to fingers 30 and 31, spring 46 may be manually depressed and filaments the bights easily slipped over the ends of arms 34 and into notches 38. Spring 46 may then be manually released thereby positioning all the filaments.

Reference to FIG. 2 will disclose the simplicity of the alternate upper filament support. This assemblage merely consists of an apertured hub to which a plurality of rods are radially attached. The rods need not be individually adjusted or tensioned, nor need they have resiliency.

Turning now to FIG. 3a there is illustrated in elevation the upper portion of a filamentary wire and that portion of its upper support by which it is directly supported. The filament is looped over and pressed against a mandrel having a rectangular cross-section causing it to be bent into two rectilinear strands 56 and a rectilinear bight 58 with right angled junctures 60 therebetween. The unitary filament is then placed over a filament support finger 62 having a cross-section essentially congruent with that of the mandrel. This filament structure is that in the prior art most closely resembling that of the present invention, It is here illustrated for comparative purposes in order that the significance of the present invention can be easily and fully appreciated. It is recognized that at first glance the structure of the present invention does not appear grossly distinctive over the prior art of FIG. 3a. A careful examination of the embodiments of the present invention shown in FIGS. 3b, 3c, and 3d will reveal, when read in context with the explanations presented in FIG. 4, the very significant physics resulting from the slight alterations in filament configuration which in turn result in greatly improved stability of inter-electrode spacing.

FIG. 31; illustrates a unitary filament having two rectilinear strands 64 and an interconnecting bight 66 looped over a filament support finger 62 of the same crosssection of that illustrated in FIG. 3a. In this embodiment bight 66 has a rectilinear midsection which abuts the upper surface of a notch in finger 62. At each end of the midsection the bight arcs for approximately at which point it integrates into parallel, rectilinear strands 64. Note that the breadth of finger 62 is slightly less than half the span between strands 64. Also note that no section of the bight nor of the rectilinear strands abuts the sides of the support finger.

Another embodiment of the present invention is shown in FIG. 30. Here the filament has two rectilinear strands 64 integrally connected by a bight. The center portion of the bight has an are 68 which is looped over a notch in rod-shaped finger 70. Each end of are 68 merges into a short rectilinear portion of the bight, the other ends of which integrally merge into other arcs 69 which in turn integrate with the ends of rectilinear strands 64.

Such a configuration conforms to the inner contour of an arch termed by architects a tudor arch. Alternatively are 68 may be crimped in which case the bight generally conforms to the inner contour of a structure known as an ogee arch. Note in both alternatives that only a small portion of the overall bight actually abuts support finger 70.

FIG. 3d illustrates yet another embodiment having two rectilinear filament strands 64 interconnected by a bight 71 looped over a notch in rod-shaped support finger 70. In this example, bight 71 assumes the form of a semicircle or round arch.

FIG. 4 illustrates the physics. experienced by the configurations of FIGS. 3b, 3c and 3d, due to thermal expansion during tube operation. In FIG. 4a there is again illustrated the filament structure of FIG. 30 together with fingers 30 and 31 to which the free ends of a filament are welded. The force of spring 46 transmitted to the rodshaped finger 70 is shown by vector S. This force is distributed to two reactant forces in the two rectilinear strands 64 illustrated by the force vectors T which bring the filament support into equilibrium. As force S is distributed among some 40 filaments, force T in any single filament is quite small, just sufiicient to align filaments 64 in parallel. It should be appreciated that force S must not become excessive so as to alter the configuration of the bight itself at nonoperative temperatures.

Now suppose for a moment that upon heating of the filament structure during tube operation an imbalance in forces T is created. This would result from the length of one rectilinear strand being different from that of the other strands once all have thermally expanded. This could be caused by several factors including the thermal release of latent stresses or strains in finger 30 or 31, nonconformative in the filamentary wire used, as in the slight slippage of a free end of one strand when it is welded to a finger. A very small deviation in the length of the filament and its associated tension can have very significant alteration in the interelectrode spacing between the filament and adjacent grid wires. For example, should the length of each strand 56 in the prior art filament structure of FIG. 3a be specified as 10.0 cms., and instead should one strand become 10.1 cms. through an error in mounting, then the erroneously mounted strand will bow laterally a distance of approximately 1 /2 cm. at its midpoint. This can be more easily visualized in FIG. 4b where A is the predetermined length 10.0 cm. and A is the actual length 10.1 cm. By the pythagorean theorem B, which represents the maximum degree of bowing, becomes 1.4 cms. Thus a longitudinal error of just 1 part in 100 produces a lateral error of approximately 14 parts which could easily be sufficient to completely traverse the spacing between the filament and an adjacent grid wire thereby causing an electrical short. But of course any significant deviation in this spacing is detrimental to the operation of the electron tube since the cathode-to-grid spacing is both quite close and critical. Variation from the designed spacing is one of the chief causes of power grid electron tubes failing to meet operative specifications.

In the case where a filament strand after expansion is shorter than the other strands, the filament of FIG. 3a does not provide any alternative configuration which the filament could assume. Rather support arm 62 has been designed with a degree of resiliency. Thus where one filament strand is under more tension than the others, arm 62 has bent downwardly. In doing so, however, the end of the arm to one which the filament is hung pivots slightly away from the adjacent grid wires. This of course alters the interelectrode spacing between the cathode and grid resulting in a change in tube perveance. Should the resiliency of arm 62 not ofiset the abnormal tension in the particular filament strand, the entire spider to which the arm integrates will be held too close to the other filament support. This is to say that but one arm 62 will be counterbalancing the biasing force of spring 46. In this situation the filaments held to other arms 62 will not be taut; they will then be at liberty to bow towards or away from the adjacent grid.

Though we have discussed the lateral displacement that may occur when a filament strand bows, we have yet to discuss the direction in which the bowing takes place. In FIG. 40 there is illustrated a plane view, a set of filaments 72 and a set of grid wires 74. About one of the filaments is drawn a dotted circle 72'. This circle, for the sake of illustration, is presumed to define the lateral bounds a bowed filament assumes at elevated temperatures. Should this bowing occur along the are 76 defined by the cylindrical cathode, the slight decrease in spacing between the bowed filament and one of the adjacent grid wires 74 is approximately offset by the decrease in its spacing with the other grid wire 74. This does not cause a significant alteration in the perveance of the tube. However, should such bowing occur in any direction other than along are 76, the spacing between filament 72' and grid wire 74 would change. The configuration illustrated in FIGS. 3b, 3c, and 3d serve to prevent this occurrence. In these figures each pair of rectilinear strands 64 define a plane which forms a segment of are 76. Should one of the strands 64 in FIG. 4a, for example, be mounted too low on fingers 30 and 31, the bight will take the shape illustrated by the dotted line x in FIG. 4a; should it be mounted too high on the finger it will take the shape of dotted line y. In this later case support rod 70 will move very slightly upward but this has not been shown for the sake of clarity.

These described deviations from the designed norm occur in the plane defined by the two strands 64, i.e., in the plane of this paper in FIG. 3. In this manner the excess tension is relieved through a change in bight configuration in a .predesignated plane. Likewise any slight bowing is confined to this same plane. This controlled direction in bight deviation occurs along are 76. In this manner the electrode spacing is maintained.

It should be understood that the above-described embodiments are merely illustrative of applications of the principles of the invention. Obviously many modifications may be made in the specific example without departing from the spirit and scope of the invention as set forth in the following claims.

What is claimed is:

1. An electron tube having a filament structure comprising a filament wire and a filament support, said filament wire having a bight and two rectilinear strands, means attaching an end of each strand in the tube, said attaching means aifording electrical connection to the respective ends, said filament support being relatively rigid and mounted in spaced relation to said attaching means and supporting said filament wire by abutting a portion of said bight, said filament support being biased away from said attaching means to tension the rectilinear strands to a predetermined degree, the abutting portion of said bight not exceeding approximately half the total length of the bight, and the remaining length portion of said bight being shaped and dimensioned relative to said support to form a deformable arch portion, whereby deviations in tension of a rectilinear strand from said predetermined degree are dampened through deformation of said deformable arch portion of said bight.

2. The election tube of claim 1 wherein said filament support is a rigid cantilever.

3. The electron tube of claim 1 wherein said filament support comprises a metal selected from the group consisting of molybdenum and tungsten, and wherein said filament wire comprises thoriated tungsten.

4. The electron tube of claim 1 wherein said filament support is cylindrical.

5. The electron tube of claim 4 wherein said rectilinear strands are substantially parallel and wherein the diameter of said cylindricalfilament support does not exceed half the spacing between the two strands.

6. The electron tube of claim 1 wherein said filament support is a parallelepipedon and wherein said bight abuts but one surface of said parallelepipedon.

7. The electron tube of claim 1 wherein said filament support is a parallelepipedon and wherein said bight abuts the paralllelepipedon only at two edges thereof.

.8. The electron tube structure of claim 1 wherein the filament support comprises a hub disposed perpendicular to the force of said bias and a plurality of spokes radially afiixed to said hub.

9. The electron tube of claim 8 having a plurality of said filament wires wherein said spokes are cantilevers each of which support a filament wire bight.

10. The electron tube of claim 9 wherein said spokes have a notch near their free end into which said filament wire bights are placed and held to the spoke.

11. The electron tube of claim '8 having a spring, a pusher having one end fixed to said spring and a dielectric spacer fixed to the other end of said pusher abutting said hub.

12. The electron tube of claim 11 wherein said hub and said dielectric spacer have aligned apertures and wherein said filament structure is in combination with a second filament support having an element coaxial with said pusher extending through the apertures in said hub and dielectric spacer.

13. The electron tube of claim 1 wherein the configuration of said bight is the inner contour of a round arch.

14. The combination of claim 1 wherein the configuration of said bight is the inner contour of a tudor arch.

7 8 a 15. The combination of claim 1 wherein the configura- 3,218,502 11/ 1965 Freggens 313-278 X tion of said bight in the inner contour of a round arch. 3,299,310 1/ 1967 Freggens 313-278 16. The combination of claim 1 wherein the coniignra- FOREIGN PATENTS t1on of sald bight consists of two arcuate sections oined together by a rectilinear section. 1331911 1/1946 Austraha- 5 73,735 2/ 1952 Denmark. uNlTg s zgfr l is gjfTENTs JOHN W. HUCKERT, Primary Examiner. 2 385 9/194 1 A. J. JAMES, Assistant Examiner ,435 5 W t 313-278 a a 10 US. 01. X.R.

2,602,907 7/1952 Shower 313-278 2,632,129 3/1953 Dailey 313278 X 313271, 275, 277

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