US2462921A - Electron discharge tube - Google Patents

Description

March 1, 1949. w. G. TAYLOR ELECTRON DISCHARGE TUBE Filed May 5, 1946 I N V EN TOR, WE r1922 a- B Y Z. P WW ATTORNEY Patented Mar. 1, 1949 ELECTRON DISCHARGE TUBE Warren G. Taylor, Essex Fells, N. J., assignor to Federal Telephone and Radio Corporation, New

York, N. Y.,

a corporation of Delaware Application May 3, 1946, Serial No. 666,972

Claims.

This invention relates to vacuum tubes of the ultra-high frequency type, especially those known H as Lighthouse tubes, in which the electrodes are spaced a minute distance apart to provide an extremely short transit time of travel of the electrons between the electrodes. The invention relates more particularly to a mountin arrangement assuring accurate spacing between the oathode and grid electrodes of such tubes.

The principal object of the invention is to provide a mountin arrangement for the cathode and grid which will permit an extremely close spacing between these electrodes without incurring variations in spacing due to heating or other conditions.

Vacuum tubes capable of handling large amounts of power have been developed for use i at ultra-high frequencies, in which the electrodes are spaced extremely close together to create an extremely small time of transit of the electrons in their path of travel. Such tubes with parallel plane electrodes have been commonly called Lighthouse tubes. They ordinarily comprise a cathode, grid and anode, and have been used for example, as oscillators and amplifiers. The electrodes have usually been built into a resonant cavity system, there being, for example, a resonant cavity for the grid-cathode circuit, and another cavity for the grid-anode circuit.

Owing to the fact that the grid is placed at only a minute distance from the cathode in such tubes, for example, the order of .005", it has been an important problem in the construction and arrangement of these electrodes, to provide suitable means for providing and maintaining accurately the spacing inasmuch as a very slight change of spacing during operation due to heating or other conditions will produce an undesired change in the tube characteristics. A difficulty heretofore encountered in the mountings of such electrodes has been the uneven expansion of the material with heat, which undesirably changes the spacing between the cathode and grid.

According to my invention, I provide means for mounting the grid and cathode in relation to each other so that the spacing between them can be accurately established without variation. I do this by attaching the grid to a grid support, and providing a cathode by covering a surface of an insulating support facing the grid, with an electron emissive substance. For the purpose of providing the desired critical spacing between the grid and cathode, I provide on the cathode support, a raised rim or spacing member having a surface which engages the grid support. Preferably the cathode support has a cup-like shape with the rim of the cup held against the grid support and the cathode supporting surface is raised from a central location of the cathode support. By accurate dimensioning of the cathode support in relation to the grid and grid support, there will be provided the desired minute spacing between the cathode and grid.

I prefer to make the cathode support of a ceramic material or glass, capable of withstanding heat and with as low a temperature coeiiicient of expansion as possible. By making the ceramic structure or high silica glass of low temperature coefficient of expansion, the tendency for variation of distance between the cathode and grid is practically eliminated, and the electrodes will accurately maintain their spacing through variations in temperature. Moreover, such tendency as may exist for heat expansion to occur will be selfcompensating owing to the shape of the cathode support.

In order to provide for heating the cathode, the member on which the cathode material is placed is preferably provided with a recess at the side opposite the grid, and a cathode heating element inserted in the recess.

The foregoing and other features of my invention will be better understood from the following detailed description and the accompanying drawings of which:

Fig. 1 is a perspective cross-section view taken at line l! of Fig. 2, showing a grid-cathode mounting arrangement according to my invention;

Fig. 2 is a top view of the arrangement shown in Fig. 1;

Fig. 3 shows the grid used in the arrangement of Figs. 1 and 2;

Fig. 4 is a cross-section of the grid taken at line i-fi of Fig. 3;

Fig. 5 is a top view of a rin or washer used for holding the grid in the construction of Figs. 1

and 2;

Fig. 6 is a cross-section view taken at line 6-6 of Fig. 5; and

Fig. 7 shows a grid-cathode structure mounted accordin to my invention and included in a vacuum tube.

Referring to Figs. 1 and 2, there is provided a grid i which may be constructed as shown in Figs. 3 and 4 of the usual grid wires 2, fastened within a suitable holder 3 which may be annular in shape, For the purpose of holdin the grid there are provided a pair of annular mounting washers or rings 4 and 5, each of which is constructed as shown in Figs. 5 and 6.

Referring to Figs. 5 and 6, the washer comprises the peripheral portion 6 having a circular cutout portion 1 at its inner edge, providing a shoulder 8. In Fig. 1, two of these washers are placed together with their cutouts I placed against each other, providing the annular space between the washers into which the supporting holder 3 ofthe grid snugly fits so that it is held between surfaces 1 and abuts shoulders 8. The grid may, of course, be attached to one or the other of the washers in any other suitable manner, as may be desired.

There is provided a cathode supporting means in the form of a cup-likemember'il of ceramic material or high temperature glass having a flat base portion l and a circular peripheral rim ll upstanding from the base portion, and providing a fiat planar surface l2 at the top of the lip which engages the flat undersurface of the hollow washer 5.

There is centrally formed fromthe base portion within the cup, an upstanding cylindrical portion l3'having a fiat circular top l4. On this circular topl l there is deposited in a suitable manner a cathodic coating I5 which may for example, be a deposition of metal providing an active electronemissive surface such as the oxides of barium and strontium.. This cathode is located centrally with reference to the grid and is provided with leads The dimensions of the circular peripheral lip portion l l relative to the central cylindrical portion I3 and the-cathode layer on top of it are so related to each other that when the planar surface l2 of the circular lip is held directly against the washer 5, the spacing between the upper surface of cathode l5 will be the desired minute distance from the lower surface of grid I. This distance may, for example, be as close as .005.

In order to guard against appreciable variation between cathode and grid, I make the cup-like member 9 of an insulating material having av low temperature coefiicient of expansion, so that subto that of Figs. 1 and 2 mounted into a vacuum tube, of which only a part of the tube is shown. Sufficient of the tube is illustrated, however, to show the relative position of the cathode and grid to the anode 23, the anode being in the form of a solid cylindrical member having a diameter substantiallythe same as that of the cathode l5 and located c'oaxially with the cathode. The grid is situated between the cathode and anode, as .1 Wu. The lower surface 24 of the anode is a plane fiat surface parallel to the surface of the grid and of the cathode. The anode is very closely spaced to the grid, although it will not ordinarily be spaced quite as close as the cathode is to the grid. The spacing between the grid and anode may, for example, be in the order of .020.

The anode may be held in relation to the grid in a suitable manner, for example, by the metallic flange element 25, which is fastened to a shoulder of the anode and extends laterally. A glass section 21 is-sealed to flange 25 and also to an annular flange 28 which is bolted or otherwise suitably attached or sealed to grid washer 5, as shown.

If desired, the annular cavity l'l between the rim H and the central portion l3 of the cathode support, andalso the cavity 29 between the grid and the flange 25 may form one vacuum chamber; and this may be done by sealing the metal ring 22 to the metal washer 5i where they join. In such case, the chambers II and 29 may be evacuated in any suitable manner, for example, through a central bore extending through the anode member and providing an opening into cav ity 29 (not shown).

In order to support the electrode structure, there is shown a glass supporting bowl 3!] which is sealed around its upper rim to the ring 28 and has the cathode heater leads l9 and 20 sealed stantially no change in its shape'or dimension I occurs when the structure is heated. By a low temperature 'coefficient of expansion within the meanin of this specification and the appended claims, I mean a temperature coefficient less than 1x l0 Such a low temperature coefficient will givethe desired'freedom from excessive dimension changes. A suitable glass for the purpose is, for example, one having predominantly one of the following compositions:

1. 96% S102, 3% B203, 0.4% R2O3+RO2(A12O3) 2. Pure fused SiOz 3. A1203 (Alundum) is placed in recess l6 and terminal leads l9 and 20 are taken from the filament. In order to prevent'the filament from burning out, the recess It should be evacuated, and a glass seal 2| is provided for maintaining this'vaicuum.

The glass cup may be held against the grid washer in any suitable manner, for example, by,

screwing means or nuts and bolts, or other suitable means. In Figs. 1 and 2, spring clamps 22 are shown clamping the elements together.

Fig. '7 shows a mounting arrangement similar through it.

It will be understood that the tube elements shown associated with the grid and cathode mounting arrangements in Fig. '7 is only for purposes ofillustration, and should not be considered as limiting in any way. The grid-cathode mounting arrangement according to my invention may be incorporated in any desired form of tube or arrangement, as may be dictated by individual circumstances.

Suitable cavities may conveniently be formed byattaching a cylindrical member to the gridcontacting ring 28 and extending the cylinder in either direction from the ring so as to form a coaxial cavity with the anode which can readily beadjusted by a suitable plunger arrangement, and a similar cavity with a cylindrical member which may be attached to the cathode.

It will be recognized that the grid-cathode mounting arrangement according to my invention is'useful with many forms of tube and resonant cavity arrangements, and that the advantage attending the use of the mounting will be utilized in whatever circuit arrangement is in use.

Owing to the low temperature coefficient of the ceramic cup-like mounting device 9, there is substantially no change in its dimensions when heated, and accordingly, very little variation in spacing between the cathode and grid. Furthermore such tendency toward change of dimension as may 'exist will be mostly'compensated because of the fact that the upstanding central member i3 is substantially the same length as the upstanding rim; and accordingly any change of dimension would be practically the same in both the upstanding members. This would result in maintenance of the prescribed spacing between the grid and cathode.

I claim:

1. A grid and cathode assembly for use in vacuum tubes comprising a disk-shaped grid having a plane surface, an annular washer holding the grid within the Washer, a cup-shaped member of refractory insulating material havin a base portion and a circular rim with a peripheral surface in contact with the surface of the washer, and an upstanding central portion protruding from the base portion toward the grid, said upstanding central portion having a plane surface facin the grid, an electron-emissive cathode covering the plane surface, and means making electrical connection with said cathode covering.

2. A grid and cathode assembly for use in ultra-high frequency vacuum tubes comprising a disk-shaped grid having a plane surface, an annular washer holding the grid at its periphery, a

cup-shaped glass member having a base portion, a circular rim with a peripheral surface in con-- tact with a surface of the washer and an upstanding central portion protruding from the base portion toward the grid, said upstanding portion having a plane surface facing the grid, an electron-emissive cathode covering the plane surface and placed very close to the grid, said upstanding portion having a recess within it, and a cathode heating element within the recess.

3. A grid and cathode assembly for use in Vacuum tubes adapted. to operate at ultra-high frequencies, said assembly comprising a grid elec trode supported around its periphery by a holding element, a ceramic element comprising a base portion and a centrally located upstanding portion providing a surface facing the grid and conforming with the shape of the grid surface, an electron emissive cathode on said surface and spaced close to the grid, said ceramic member having an upstanding portion around the central upstanding portion and having aperipheral surface held against a surface of the grid-supporting member, the surface of the cathode being spaced at a minute distance from the surface of the grid, the annular space within the ceramic member between the peripheral portion and the central upstanding portion providing a cavity, and a recess in the upstanding portion, and a cathode heating element within said recess.

4. An assembly according to claim 3 in which the ceramic member is a glass having a low temperature coefficient of expansion.

5. A grid and cathode assembly for use in vacuum tubes adapted to operate at ultra-high frequency, said assembly comprising a thin fiat grid, a pair of washers holding the grid between them, a cup-shaped glass member having a base portion, a circular rim portion extending from the base portion and having a peripheral surface in contact with a surface of one of the washers, and an upstanding cylindrical central portion protruding from the base portion and providing an annular space between the central portion and the peripheral portion, said upstanding portion having a surface facing the grid and a cathode placed on said surface at a minute distance from the grid, and means holding the peripheral surface of the rim portion against the washer.

G. A grid and cathode assembly for use in a vacuum tube adapted to operate at ultra-high frequencies and providing relatively short transit time of electron travel between the cathode and grid, comprising a disk-shaped grid having a plane surface, an annular washer to which the grid is attached, a cup-shaped glass member having a low temperature 'coefiicient of expansion, said glass member having a base portion and an upstanding cylindrical portion providing a plane surface parallel to the surface of the grid and facing the grid, an electron-emissive cathode layer on said fiat surface, and a circular rim portion upstanding from the base portion and having a peripheral surface adapted to make contact with a surface of the washer, and means clamping said peripheral surface and washer surfaces together, whereby the surface of the cathode is held spaced at a minute distance from the surface of the grid and said minute spacing is accurately maintained.

7. A cathode assembly for use in vacuum tubes comprising supporting means of ceramic material of low temperature coefiic-ient of expansion including a base portion and an upstanding cylindrical portion having a flat top, said cylindrical portion being provided with a central bore having an opening at the side opposite said fiat top, a coating of emissive material on said flat top, said coating having leads electrically connected thereto, and a heating filament arranged in said central bore adjacent said fiat top and having terminal leads arranged through said opening.

8. A cathode assembly for use in vacuum tubes comprising supporting means of ceramic material of low temperature coeficient of expansion including a base portion and an upstanding cylindrical portion having a flat top, said cylindrical portion being provided with a central bore having an opening at the side opposite said fiat top, a coating of emissive material on said flat top, said coating having leads electrically connected thereto, a heating filament arranged in said central bore adjacent said flat top and having terminal leads arranged through said opening and a glass seal closing said opening, said central bore being evacuated.

9. A cathode assembly according to claim 8 in which said emissive coating is barium oxide.

10. A cathode assembly according to claim 8 in which said emissive coating is strontium oxide.

WARREN G. TAYLOR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES "PATENTS Number Name Date 1,913,978 Ewen June 13, 1933 2,414,137 Branson Jan. 14, 1947

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