US3041495A - Grid mounts for electron tubes - Google Patents

Description

June 26, 1962 M. B. KNIGHT ErAL GRID MOUNTS Foa ELECTRON TUBES Filed Sept. 1, 1959 United States Patent O 3,041,495 GRE MUNIS FR ELECTRON TUBES Mark Berwyn Knight, West Caldwell, NJ., and John Joseph Thompson, Brooklyn, N.Y., assignors to Radio Corporation of America, a corporation of Delaware Filed Sept. 1, 1959, Ser. No. 837,449 12 Claims. (Cl. 313-350) This invention relates to grid mounts for electron tubes. In particular, this inventionrelates to 'grid mounts having grids that are of the multiple-siderod, cylindrical type and which are supported at one end only.

IThe grids used in certain types of tubes, such as some receiving tubes and some power tubes are of a cylindrical shape and comprise a helix of fine wire brazed to a plurality of supporting siderods of heavier wire. The siderods usually number eight or more while the helix usually is made with several hundred turns per inch. These grids are supported rmly at one end and usually are free at the other end for electrical purposes and for ease of manufacture.

The response of cantilever-supported electrode structures to mechanical vibration is entirely different from that of the electrodes in conventional receiving tubes. The conventional electrodes, which are supported between mica spacers, must be provided with a sliding lit in the spacers to allow for differential thermal expansion of the several electrodes. The tolerances associated with these fits permit more or less rattling in response to mechanical vibrations. The rattling modifies greatly the frequency characteristics of the mechanical excitation, and provides high-frequency excitation to the electrodes even though the tube envelope may not experience these frequencies. Also, the rattling tends to damp natural vibration frequencies of the whole electrode.

The cantilever structure, by contrast, is effectively one piece and allows virtually no relative displacements of electrodes except near the mechanical resonant frequencies of those electrodes. The resonances of the electrodes are not excited unless the applied mechanical motion contains components at or near the resonant frequencies. In further contrast to conventional structures, the vibrations in the cantilever structures are damped only by the losses involved in straining the materials. Still further, the resonant frequencies of cantilever elements are lower than those for similar elements supported at both ends.

Some mechanical resonances in this type of structure tend to produce relatively large, undesired electrical outputs (microphonism) when mechanical vibrations of that frequency reach the tubes. In order to reduce undesired microphonic'elfects, it is desirable to have the mechanical resonant frequency as high as possible, because the amplitude of vibration tends to be lower and because high mechanical frequencies are usually transmitted to the tube ineiciently. Furthermore, as the mechanical resonant frequency is increased, the vibrations tend to decay more quickly. In other words, high-frequency mechanical vibrations are less common, and are less harmful, than low-frequency vibrations.

The various vibrational modes of cantilever supported grids have been studied. One mode of vibration has been found to be especially serious in producing spurious electrical output signals from the tube. In this mode, the grid sways as a column, from its supported end. This sway could occur in any direction, -but it has been found that inadvertent dissymetrics in the construction of practical grids yield preferential directions of sway. Two resonances'of this mode have been observed, with the direction of motions of these two resonances at right angles to one another. In the type of grids under consideration,

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the frequencies of these two resonances typically differ by less than 5 percent.

It is therefore an object of this invention to provide a new and improved grid structure that is substantially free of microphonics.

It is another object of this invention to provide a novel tube including an improved grid structure of the cantilever supported type which minimizes mechanical vibrations of the grid while not substantially effecting the electrical parameters of the tube.

These and other objects are accomplished in accordance with this invention by providing a multiple-siderod, cylindrical grid of the cantilever-supported type with one or more coarse-pitch helices of line wire which may be considered as truss wires, mounted on the same grid supports with the conventional fine-pitch helix which is used for electron control. This small additional wire, with proper geometry, not only damps the vibrations more quickly, but stilfens the grid and thereby increases the resonant frequency of the grid substantially without significant change in the electrical characteristics of the tube.

The invention will -be more clearly understood when read in conjunction with the accompanying single sheet of drawings wherein:

IFIG. l is a top partially schematic view of a grid while under the harmful mode of mechanical vibration;

FIG. 2 is a side view of an improved grid in accordance with this invention;

FIG. 3 is a side view of anY embodiment of an improved grid in accordance with this invention;

FIG. 4 isa side View of an improved grid in accordance with my invention;

FIG. 5 is a side view of still another improved grid in accordance with my invention; and

FIG. 6 is a sectional view of a receiving type tube utilizing this invention.

One specic grid design of the type under consideration herein, and illustrated in FIG. l, uses as the control vgrid wire, a helix 12 of 320 turns per inch of 0.8 mil molybdenum wire or one of 400 turns per inch of 0.5 mil molybdenum wire. Mechanical support of the helix 12, as well as the electrical and the thermal conduction paths are provided by twelve siderods 14, arranged parallel to the axis of the cylinder, of 1.5 mil molybdenum wire brazed on the outside of the helix. This cylindrical grid is brazed to a flange (not shown) supported by three legs from a ceramic base wafer in one known tube type. The

' inside diameter of the cylinder is .066 inch and the free length above the ilange is .235 inch.

j When gridsV of the type brieily described above, and without benefit of the present invention, are subjected to a mechanical impulse, e.g., a sharp tap, a train of damped oscillations occurs. The wave-form of the oscillations produced is complex during the first one to five milliseconds after the impulse, indicating a mixture of several frequencies. Then, there is a relatively slowdecaying oscillation at about 3000 cycles for the 0.5 mil wire grid and about 5000 cycles for the 0.8 mil Wire grid. This damped oscillation characteristically appears to be amplitude modulated to 50 to 300 cycles. The amplitude decays exponentially to 37% of the original after 500 to 1000 cycles of oscillation.

When the tube is mechanically excited, e.g. with a loudspeaker and audio oscillator, it is found that the grid vibrations actually consist of two sharp resonant frequencies separated by only 50 to 300 cycles. The presence `of the two frequencies makes it obvious why the damped oscillations, described above, have the appearance of amplitude modulation, i.e., the amplitude changes are like beats between the :two frequencies. These two frequencies represent the two directions of sway mentioned previously. Other smaller resonances can often be found at higher frequencies but these smaller high frequency resonances do Vnot normally produce seriously objectionable microphonics, asv would -be expected from the results of the impulse test. f e

Several modesof vibration have been observed. With the'speciiic grid design described above, the sway mode vibrations, as shown in FIG. 1 (the displacement of the grid being indicated by the dotted lines), are at the lowest frequency and are of the largest amplitude among lthe modes observed. Also, the sway mode has been found to be the only mode which substantially aects the electrical characteristics of the tube. In the sway mode, two resonances Vhave been observed V(only one of which is shown) 4.which represents motion in two directions, 9'() degrees with respect to one another. The amount of frequency separation of the two sway mode resonances is an indication of the degrees of symmetry of the structure. In tubes using an 0.8 mil grid helix i12, the average of the two sway mode resonances is about 4800 cycles.

When the grid is under vibration in the sway mode, as shown in FIG. 1, the substantial rectangles normally formed -by the siderods 14 and helix wire 12 are distorted. Since the helix wire and siderods are brazed at the joints, the helix wire must bend, Therefore, Vthe strength of the individual spans of the helix wire is found to be the most important single factor in the mechanical strength of the grid described above. Y

It has been pointed out above that the siderods and helix Vwire intersect to form approximaterectangles. In

helix, or truss wire 10, of different pitch, which tends to form appproximate triangles 13 and which stiffens Ythe structure. The truss wire 10 in the embodiment shown in FIG. 2, is in the form of a coarse pitch helix in contact e with Va regulariine pitch helix 12 so thateach turn of the truss wire 10 crosses perhaps ll'turns of the helixv Wire 12. ySurprising improvements have been observed 'be cause of this structure, considering the small amount of wire that hasV been added and the negligible effect on electrical characteristics.

The truss wire I10 is preferably the same diameter as the helix wire '12, although it is shown as a smaller diameter wire for simplicity ofV illustration, so that there is little disturbance of dimensions at the cross-over points between theY truss and helix wires. The regular helix 12 has the proper inside diameter except in the immediate vicin- Vity of a cross-over point. Tests indicate that it is equally effective to place the truss wire 10 between the helix 12 and the siderods 14. Although it is less Veffective for the stiifening purposes to place the truss wire over the side-V rods, this is of special conveniencein manufacturing.

Manyv tests have shown that the truss wire 10 is most eifective if the turns-per-inch of the truss wire is such that the pitch (reciprocal of the turns per inch) is in the range of from one-third of to equal to the circumference malte the truss wire diameter larger than the helix wire.

' accordance with this invention and as shown in FIG. 2, y lthere is added to the conventional structures, another of the grid. The use of multiple truss wires 16 and d8 as f in a double or triple thread, is particularly advantageous. As shown in the following table, each additional truss Wire 16 and 118 gave the same percentage improvement as characterizedV in the previous one. The table shown below summarizes these -tests as well as the effect of varying the truss wire diameter. Other `tests have shown that the vibrations decay more rapidly in the grids having truss wires. These multiple truss wiresaltl, 16 and 18 (IFIG. 1)'permit the use of near optimum turns-per-inch of each wire and yet more stiffening effect may be achieved. e That is, a greater number of srtuctural triangles isvformed by truss wires crossing at a favorable anglerto make strong triangles. Accordingly, the several truss wires should be spaced quite uniformly although no severe tolerance or uniformity of spacing is involved.

One interesting result, thathas been found, 4and that is shown in the table, is that there is little advantage to When the truss wire is considerably smaller than the helix wire, however it is not strong enough for the relatively large mass of the grid.

Relative Resonant Stitiness Helix Wire Truss Wire Frequency, With Truss c.p.s. Compared To No Truss 400 t.p.i., 0.5 rnil none 3,200 1.0 Do 7 t.p.i., 0.5 ruil 4, 300 1. 8 l2 t.p.i., 0.5 mil. 4, 500 2.0 12 t.p.i., 0.8 mil 4. 600 2.1 2-7 t.p.i., 0.5 mil 6, 000 3. 5 2-14 t.p.i., 0 5 mil 6, 400 4. 0 3-6 t.p.i., 0 5 m 8, 1.00 6. 4 none 4, 800 1. 0 12 t.p.i., 0.8 mil 6,300 1. 7 3-6 t.p.i., 0.5 mil 7, 500 2. 4 D0 3-6 t.p.i., 0.8 mil 9, 800 4. 2

T his invention is also Iapplicable to a grid of the type shown in FIG. 3. In this embodiment, the grid helix 12 is on the inside of the side rodsv 14 while the truss Wire 1S is on the inside of the helix 12. The dimensions, geometry and materials used for this embodiment may be similar to those previously described. Also, multiple Vtruss wires may be used as has been explained.

In accordance with my invention, the grid helix wire and the truss wire may be on opposite sides of the grid side rods. As shown in FIG. 4, the side rods 1-4 have a grid helix `12. applied inside thereof while the truss wire 11S is applied around the outside of the said side rods 14. The said rods -14 are brazed to both the grid helix and the truss wire. Also, as shown in IFIG. 4, four truss wires are used in quadruple thread. The pitch of the grid helix is fine compared to the pitch of the truss wire 18 in a similar manner to the relative pitches disclosed in connection withy FIGS. 2 and 3 hereof.

Further, inv accordance with my invention, the truss wire :18 may be between the side rods 14 and the grid helix 12 and may be brazed to both thereof, as illustrated in FIG. 5. This ligure also shows four truss wires wound in multiple thread.

FIG. l6 is a partial sectional view of a tube embodying this invention. The tube includes a cathode member 20, a grid 22 and an anode 24 all of which are ruggedly supported at one end on flanges l26, 28 and 30 respectively. The flanges referred to have an appreciable lateral extent and each is supported adjacent its periphery by a tripod array of lead-in and support wires 32 firmly fixed to a wafer 34. It should be noticed that the upper end of the grid' 22 is free and would be susceptible to certain mechanical movements, as has been explained, which in turn would cause spurious electrical outputs, were it not forl the presence of the truss wires 10, 16 and 11S in accordance with this invention.

It should be understood that the tube shown in FIG. 6 is exemplary and that this invention is also applicable to other tubes, such as power tubes, having a grid of the cylindrical multiple siderod, cantilever supported type.

What is claimed is:

. 1. A cylindrical type grid structure comprising a plurality of siderods, a helix wire having a relatively large number of turns per inch joined to said siderods, and a ytruss wire having a relatively small number of turns per inch joined to at least one of said helix Wire and said plurality of siderods.

2, A grid structure as in claim 1, wherein said truss wire is between said helix wireyand said plurality of siderods.

3. A grid structure as in claim l, wherein said helix wire is within said plurality of siderods, and said truss wire is around said plurality of siderods.

V4. A cylindrical type grid structure of the cantilever supported type comprising a plurality of siderods, a helix grid wire having between and 500 turns per inch brazed to said plurality of siderods, and between 1 and 4 truss wires each in the form of a helix having 5 to 15 `turns per inch brazed to at least one of said helix grid wire and said plurality of siderods.

5. A grid structure as in claim 4, wherein said truss wire has a diameter at most equal to the diameter of said helix wire.

6. A grid mount comprising a support structure, a tubular grid mounted at one end thereof only on said support structure, said support structure having an annular iange of larger transverse extent than said grid, a wafer of insulating material, three parallel support rods iixed to said Wafer and iixedly engaging at one group of adjacent ends thereof regions of said iiange equidistantly spaced therearound, said grid comprising a plurality of parallel siderods yand a helix ixed to said rods, said helix having a relatively large number of turns per inch, and means for restraining vibrations of the other end of said grid, said means comprising a wire helix having relatively small number of turns per inch iixed to -said grid and being substantially coextensive axially therewith.

7. A grid mount comprising a cylindrical grid having a plurality of parallel siderods and a wire helix having a relatively large number of turns per inch and fixed `to said siderods, means for ruggedly supporting one end of said grid, said means comprising an annular flange member, said siderods being spaced equidistantly around said flange and disposed in a circle having a larger diameter than said helix, and additional means for extending the support of said one end of the grid to the other end thereof, said additional means comprising a wire helix having a relatively small number of turns per inch fixed to said grid and in coextensive relation therewith.

8. A cylindrical type grid structure comprising a plurality of siderods, a grid Wire helix having a relatively large number of turns per inch joined to said plurality of siderods and forming a plurality of approximately rectangular coniigurations with said siderods, a truss Wire helix joined lto one of said grid Wire helix and said siderods, said truss wire having a relatively small number of turns per inch and forming a plurality of triangular configurations with said grid W-ire helix and said siderods.

9. A cantilever supported, cylindrical type grid structure comprising a plurality of side rods, a grid Wire helix bonded to said siderods and having a relatively iine pitch, a truss wire helix secured to one of said grid Wire helix and said plurality of siderods, the pitch of said -truss wire being within the approximate range of one third of the circumference of said grid structure to equal the circumference of said grid structure.

10. A cylindrical type grid structure comprising a plurality of side rods, a helix Wire having a relatively large number of turns per inch and a plurality of truss Wires having a relatively small number of turns per inch helically wound in multiple lthread arrangement, said side rods, said helix wire `and said truss Wires being electrically and mechanically joined to each other at points along their lengths.

11. A grid structure such as in claimed in claim 10 in Which said plurality of truss Wires is four.

12. A cylindrical type grid structure comprising a plurality of side rods, a helix wire having a relatively large number of turns per inch, a truss Wire wound in a helix and having a relatively small number of turns present per inch, said side rods, said helix Wire and said truss wire being electrically and mechanically joined to each other at points along their lengths.

References Cited in the tile of this patent UNITED STATES PATENTS 1,780,033 Nolte Oct. 28, 1930 2,438,113 vDenmark Mar. 23, 1948 2,457,626 Atlee Dec. 28, 1948 2,489,873 Thorson v Nov. 29, 1949

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