US2873401A - Electron discharge devices - Google Patents

Description

Feb. 10, 1959 H. D. DOOLITTLE 2,373,401

, ELECTRON DISCHARGE DEVICES Filed Dec. 12, 1956 :s Sheets-Sheet 1 INVENTOR. HOWARD o DOOLITTLE AGENT Feb. 10, 1959 -H. D. DOQLITTLE ELECTRON DISCHARGE DEVICES Filed Dec. 12. 1956 5 Sheets-Sheet I5 FIG] INVENTOR.

HOWARD o. DOOLITTLE AGENT United States Patent ELECTRON DISCHARGE DEVICES Howard D. Doolittle, Stamford, Conn assignor to Machlett Laboratories, Incorporated, Springdale, Conn., a corporation of Connecticut Application December'll, 1956, Serial No. 627,834

6 Claims. (Cl. 313-498) tures which do little to provide or maintain a high plate current to grid current ratio or used complicated beaming structures which involved parts difficult to manufacture and assemble, and in some cases prior art tubes were not sufiiciently rugged to withstand transiental faults such as. flash arcs in the tube which are difiicult to eliminate completely. Such flash arcs are sometimes caused by transients in the power mains or byparasiticoscillations of the tubes circuits.

The present invention is directed primarily to the provision of an electron tube wherein a selected normal or high amplification factor, i. e. approximately from 10 to 100, is achieved by positioning the respective pairs of parallel grid and filament wires of substantially cylindrical electrodes in radial alignment with respect to the axis of the electrode structure so that each filament wire will be in the most effective portionof the field influenced by a respective grid wire.

An additional feature is the provision of means for reducing grid current in such a structure either by restricting cathode emission to those portions of the filament wires which lie farthest from the grid wires, or by providing ashield grid between the filament and control grid, which shield grid is maintained at or near cathode potential for beaming the electrons emitted by the cathode so that they will not impinge upon the control grid wires. Either of the methods will reduce grid current and obviate the necessity for cooling the control grid, and also at the same time will provide a large safety factor under transient faults.

A further control of the amplification factor so as to provide a tube with a relatively high mu is achieved, in accordance with this invention, by controlling the spacing of the respective wires of the grid structure and controlling their diameters relative to the diameters of the cathode wires so as to produce the desired mu on the useful electron emitting areas of the cathode.

' Known means andmethods of increasing the amplification factor of an electron tube are by moving the grid away from the anode or by decreasing the space between grid wires, or a combination of both. However,

I further modification of the invention.-

the results of such means and methods have not been entirely satisfactory, particularly in a structure wherein the grid wires were closely spaced, because the problems of grid heating and ratio of plate current to grid current became serious due to lack of means for preventing the grid from intercepting current. e

2,873,401 Patented Feb. 10, 1959 "ice It is known also that grid current in a tube can be reduced by providing a grid with relatively tine wires which, due to their small diameters, will not intercept as many electrons as relatively large diameter wires. This results in a lower screening fraction for a given grid wire pitch, which may be satisfactory for certain. applications. However, in most power tubes the use of fine wires is not desirable because of their fragility.

In order to minimize grid driving power, mu should be kept as high as possible and intercepted grid current should be kept as low as possible. This goal is achieved in certain known tube structures but such structures require a relatively large number ofvery accurately aligned parts, some of the parts having shapes requiring costly fabrication.

It is, therefore, a primary object of this invention to provide a simple, rugged, and relatively inexpensive electron tube having a selected normal or relatively high amplification factor with low driving power and capable of offering high power levels.

Another object is to provide an electron tube with a novel electrode structure whereby a normal or relatively high amplification factor is obtained without undesirable rise in grid current and resultant temperatures, thus eliminating any requirement for grid cooling.

A further object is the provision of an electrode structure within an electron tube wherein cathode emission is restricted to controlled areas thereof as determined by the relative spacings of grid wires, whereby the tube is pro- 'vided with a controlled normal or high amplification factor. A still further object is the provision in a tube of the above character of means at or near cathode potential for beaming the electron How to provide a still higher mu and substantially reducing electron bombardment of the control grid with resultant reduction in incidental grid current and heating.

Other objects and advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings, in which,

Fig. l is an axial sectional view of an electron tube embodying the invention;

Fig; 2 is a reduced sectional view taken on line 2-2 of Fig. 1;

Fig. 3 is a schematic diagram illustrating diagrammatically the electron emission of a filament having restricted emission areas; I

Fig. 4 is a view similar to Fig. 1 illustrating a modification of the invention; n

Fig. 5 is a sectional view taken on line 55 of Fig. 4;

Figs. 6 and'7 are diagrams illustrating certain principles of the invention;

Fig. 8 is a schematic diagram illustrating diagrammatically the beaming effect produced in the tube shown in Fig. 4; and

Fig. 9 is a sectional view similar to Fig.-

showing a k Referring to' the drawings, the electron tube shown in Fig. 1 includes an anode 10 formed as a deep cylindrical cup of highly conductive metal such as copper. Mounted around the anode 10 near the open end thereof is a ring 11 which provides the anode terminal. A short tubular member 12, preferably of Kovar, is sealed throughout one end to the terminal ring 11 and has its other end sealed to one end of a dielectric ring 13. The opposite end of ring 13 is likewise sealed to a second tubular Kovar member 14 which is carried by an annular grid terminal 15.

Grid terminal 15 also carries a tubular grid support 16 which extends inwardly of the structure toward the anode 10, the grid support 16 having a supporting ring 17 screwed .05 otherwise secured in coaxial relation to its inner end. The ends of a plurality of control grid wires 13 are secured to the inner surface of ring 17 and extend into the interior of the anode to form a cagelike structure of parallel wires, which structure, in cross-section, defines a circle. The ends of the control grid Wires 18 within the anode are secured to a disc 19 which maintains the wires in desired spaced relation.

A second dielectric ring 20, of smaller diameter than ring 13, is sealed at one end by a Kovar member 21 to the grid terminal 15, and at its other end by a Kovar member 22 to a cathode terminal ring 23. A first cupshaped cathode support 24 extends inwardly from terminal ring 23. One end of a plurality of filament supporting posts 25 are mounted in the end 24a. of the support 24 and extend into the control grid structure. A second group of posts 26 are arranged alternately with posts 25 so as to define therewith, in, cross-section, a circle of posts within the grid wires 18. Posts 26 extend. through clearance openings 27 in the end 24a of the cup-shaped support 24 and their ends are mounted in the end 28a of a second cup-shaped support 28 which is located within and. spaced from support 24. The, open end of support 28 is sealed to a second cathode terminal ring 29 which is sealed, by a pair of Kovar rings 30 and 31 having a vitreous ring 32 therebetween, to the terminal ring 23.

The inner ends of the filament support posts 25 and 26 extend slightly into the control grid structure and each carries a respective filament wire 33. The wires 33 are mounted on the. posts. 25-26 by means of clips or clamps 34 and extend substantially parallel with grid wires 18. In cross-section, wires 33 define a circle of wires. within and in predetermined spaced relation to the circle of control grid wires. The inner ends of wires 33 are joined at 35 to complete the cathode structure. An exhaust tubulation 36 is sealed in the open end of support 28.

In the manufacture of electron tubes of. the triode type, it is necessary to accurately position the grid wires with respect to both the anode and the cathode. Various spacings of the electrodes relative to one another may be used to provide tubes having diiferent amplification factors. A common arrangement is to dispose the filament wires of the cathode opposite the spaces between the grid wires so that electrons emitted by the cathode may pass through the spaces directly to the anode. In such cases, where the respective wires in the grid are spaced apart a distance which is substantially the same as the grid-cathode spacing, the mu across, thecathode is substantially constant. To increase the amplification of such a tube, common practice has been either to move the anode away from the grid or to decrease the spacings between the respective grid wires, or both. Likewise, it has been common procedure to decrease the amplification factor of a. tube either by decreasing the space between grid and. anode or by increasing the spacings between grid wires, or both. I

I have found, however, that the amplification factor of a tube can be increased by positioning respective pairs of grid and filament wires inradial alignment with respect to the axis of the electrode structure. The grid wires 18 thus are each located in the shortest path between a respective filament wire 33 and the nearest portion of the anode 10 as shown in Fig. 2. By thus being directly in the electron paths between the filament wires and anode, the grid wires have their greatest influence on the cathode emission.

In a structure as described, upon operation of the tube the filament wires 33 will emit a copious supply of elec! trons which are intended to flow to the anode 10. It is apparent, however, that a large number of the electrons will travel directly to the grid wires 18 due to the par ticular positions of the grid Wires directly in the electron paths. Such bombardment of the grid wires causes undesired grid current.

In accordance with another feature of this invention, bombardment of the grid wires is considerably reduced by providing the filament wires with limited emission areas. For example, the areas of the filament wires nearest the grid wires can be made substantially non-emissive while the remaining areas can be made to emit copiously. Since electrons theoretically leave the surfaces of the filament wires substantially normal to the tangent of the surface at the point of emission, such emission will thus pass by the grid wires. This is exaggeratedly illustrated in Fig. 3 wherein a filament wire. 40 is made to be emissive only at the surface areas indicated by the double line 41. The electrons therefrom will fiow in initially generally arcuate paths, indicated by dotted lines, through the spaces between the grid Wires and toward the anode.

- Thus, a. grid wire 52 will not be bombarded because of the fact that the filament wire 4t) will not emit, electrons from surface 43 which is nearest grid Wire. 42.

The filament wires can be provided with limited emis; sion areas by conventional methods which are slightly modified fromthe usual practice. If the filament wires 40 are formed of thoriated tungsten, for example, they can be provided with limited emission areas by masking the surfaces 43 during the carburizing process so that only the unmasked surfaces will be carburized and thereby made capable of copious electron emission. Another method which may be used is to coat only the surface to be carburized with aquadag; before the carburizing step. After carburizing, surface 43. will be, substantially nonemissive compared to the remaining surfaces of the Wires. If the filament wires are oxide-coated, wires, with the emission being provided by the oxide coating, the coating can be applied only to the surfaces of the wires which are to be emissive, or the entire surface of each, wire can be oxide coated, with the non-emissive areas being subsequently masked by a separate thin overlying coating of non-emissive material.

Other methods of making filament wires only partially emissive can be accomplished so that the wires can be used in connection with the present invention.

The amplification factor of a tube can be increased also by increasing the spacings between grid wires to an extent where the spacings begin to get larger than the grid-to-cathode distance. This causes the efiective mu tobecome variable across the face of a planar or cylindrical cathode. Under these circumstances the parts of the cathode situated centrally between the grid wires have a much lower mu than those parts immediately adjacent the grid wires.

This is illustrated in Figs. 6 and 7 wherein a cathode 44 is spaced a known distance a from grid wires 45 and the spacing between grid wires is indicated by b. It will be noted that b is larger than a, This produces a variable mu across the face of the cathode which is llustrated. by curve 0 in Fig, 7. The high peaks in curve 0 indicate the relatively high mu adjacent each grid wire 45,, nd the. dip in the curve indicates th t h m is lower between grid wires.

Each of; the. pea s in curve. C is r lativelylevel or constant throughout a restricted length as indicated at d. While the mu in areas d is not absolutely constant, the variation is relatively small and is not sufficient to cause distortion of the waveshape imposed on the tube. Therefore, in accordance with this invention, I have provided a tube as shown in Fig. 4 wherein the parts common to the tube. shown in Fig. 1 have been given like characters of reference with primes added. In this modification of the invention I have spaced the grid wires 18' apart a distance b which is larger than the grid cathode spacing a, which produces a variable mu across the cylinder defined by the cathode, and have located the filament wires 33 of the cathode at the locations where the mu is highest. Thus the filament wires 33 and respective grid wires 18 are disposed in pairs extending radially from the center of the cathode structure, as seen in Fig. 5. Since the filament wires extend parallel with the grid wires, there is no emissive portion of the cathode in the relatively low mu areas between the grid wires, all emission being restricted to the high mu areas.

It is apparent that the actual mu value can be ad justed as desired by appropriately spacing the grid wires from each other and/or by adjusting the space between the grid and anode, so as to produce a curve wherein the relatively level areas a! are located at the desired mu. In accordance with this invention, however, the cathode emission is restricted to those portions of the cathode which are within those areas.

In the example shown in Figs. 6 and 7, a mu of about 96 is obtainable. However, if a higher or lower mu is desired it is merely necessary, in accordance with this invention, to appropriately space the wires 45 so as to produce a curve wherein the relatively level areas d are located at the desired mu, and the cathode emission is restricted to those portions of the cathode which are within those areas.

Another method is shown in the structure of Figs. 4, and 8 for substantially reducing the cathode emission which impinges upon the grid wires. Such means is in the form of a shield grid similar to the control grid but of smaller diameter. The shield grid is located between the control grid and filament and is comprised of a plurality 'of wires 38, each of which wires extends parallel with and between respective control grid and filament wires 18 and 33, the shield grid wires 38 being secured at one end to a disc 39 which maintains them in desired spaced relation.

The opposite ends of shield grid wires 38 are, in accordance with this invention, connected to a source of potential which will repel electrons. For example, the wires 38 may be mounted on the filament support 24 if desired, as shown in Fig. 4, and in this case the shield grid will be at ground or cathode potential. If desired, and at an increase in the resultant cost of a tube, an additional terminal may be provided in the tube envelope, with the shield grid being suitably connected to the added terminal. In such a device the terminal and shield grid may be provided with a negative potential which is the same as or somewhat negative with respect to cathode potential.

By providing a shield grid of this type, the electron supply emanating from the filament wires 33 will be beamed by the shield grid wires 38, as indicated diagrammatically in Fig. 8, and will pass on to the anode with substantially no bombardment of the control grid wires 18'. This provides little or no grid current and consequently requires no cooling means for dissipating excess heat from the control grid.

This last-described method of preventing grid current is equally adaptable to the tube structure shown in Fig. 1.

It is also possible to apply the present invention to tubes wherein the cathode is cylindrical in shape. This is shown in Fig. 9 wherein a cylindrical cathode 46 is positioned coaxially within an anode 4'7 and within a control grid structure 48 and a shield grid structure 49.

In this structure the distance between the wires of the control grid 48 is considerably larger than the control grid to cathode spacing, which produces the variable mu across the face of the cathode 46 as desired in accordance with the invention.

The cathode 46 is provided on its outer surface with elongated strips 5t) of electron emitting material, such as an oxide coating. Strips 50 extend parallel with the grid wires 48 and 49 and are of controlled width so that no portions thereof extend transversely out of the high mu areas, the emission thus being restricted to the high mu areas.

In providing electron tubes of the high mu type constructed' as described, the structures employ the so-called beaming efiect to reduce currents intercepted by the control grid. This low interception not only reduces heating of the control grid wires, and thus aids in re ducing deformation of the control grid due to uneven heating, but also reduces distortion of the input waveshape.

From the foregoing description it will be apparent that I have provided an electron tube having improved characteristics in accordance with the objects of the invention. While the invention is primarily concerned with power transmitting tubes, it will be obvious to those skilled in the art that the structures and tube designs given herein are applicable to negative grid operation.

While the novel features of the invention have been shown and described and are pointed out in the annexed claims, it is to be understood that various omissions, substitutions and changes may be made by those skilled in the art without departing from the spirit of the invention. Therefore, it is to be understood that all matter shown or described is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. An electron tube having a high amplification factor comprising an envelope containing an anode, a cathode and a control grid, the control grid having a plurality of parallel wires positioned substantially parallel with and at predetermined distances from and between the anode and cathode and spaced apart a distance which is greater than the grid-to-cathode spacing whereby a variable mu is produced across the cathode, which mu varies from relatively high levels adjacent each grid wire to relatively low levels in areas adjacent the spaces between the grid wires, the cathode having a plurality of spaced parallel electron emitting portions positioned in the areas where the mu across the cathode is relatively high, and the grid wires each being located in'the shortest path between a respective cathode emitting portion and the nearest surface of the anode for controlling the flow of electrons from the cathode to the anode.

2. An electron tube having a high amplification factor comprising an envelope containing an anode, a cathode, and a control grid, the control grid having a plurality of parallel wires positioned substantially parallel with and at predetermined distances from and between the anode and cathode and spaced apart a distance which is greater than the grid-to-cathode spacing whereby a variable mu is produced across the cathode, which mu varies from relatively high levels adjacent each grid wire to relatively low levels in areas adjacent the spaces between the grid electron emitting portions, the cathode having a plurality of spaced parallel electron emitting portions positioned in the areas where the mu across the cathode is relatively high, and thegrid wires each being located in the shortest path between a respective cathode emitting portion and the nearest portion of the anode for controlling the flow of electrons from the cathode to the anode, and means for preventing the major portion of the electrons emitted by the cathode from impinging upon the grid wires, whereby grid current is reduced.

3. An electron tube having a high amplification factor comprising an envelope containing a hollow anode, a cathode embodying a plurality of spaced electron emitting wires defining a cylinder extending longitudinally of and parallel within the anode, and a control grid located between the anode and cathode and embodying a plurality of grid wires extending substantially parallel with the cathode and positioned with respect to one another so as to define acylinder encircling the cathode in predetermined spaced relation therewith, the grid wires being spaced apart a distance which is greater than the grid-to-cathode spacing whereby a variable mu is produced across the cylinder defined by the wires of the cathode, which variable mu varies from. relatively high levels adjacent each grid wire to relatively low levels in the area adjacent the spaces between thegridrwires, the electron emitting wires each being positioned in the areas where the mu is relatively high and the grid wires each being located in the shortest path between a respective electron emitting wire and the nearest portion of the anode.

4. An electron tube having a high amplification factor comprising an envelope containing, a hollow anode, a cathode embodying aplurality of spaced electron emitting wires defining a cylinder, extending longitudinally of and parallel within the anode and cathode and embodying a plurality of grid wires extending substantially parallel with the cathode and. positioned with respect to one another so as .to define a cylinder encircling the cathode in predetermined spaced relation therewith, the grid wires being spaced apart a distance which is greater than the grid-to-cathode spacing whereby a variable mu is produced across the cylinder defined by the wires of the cathode, which variable mu variesv from relatively high levels adjacent each grid wire to relatively low levels in the area adjacent the spaces between the grid wires, the electron emitting wires each being positioned in the areas where the mu is relatively high and the grid wires each being located in the shortest path between a respective electron emitting wire and the nearest portion of the anode, and means for preventing the major portion of the electrons emitted by the cathode from impinging upon the grid wires, whereby grid current is reduced.

5. An electron discharge device as set forth in claim 2 wherein the control grid wires and the electron emitting portions of the cathode all extend longitudinally of the cathode.

6, An electron tube having. a high amplification factor comprising an envelope containing an anode, a cathode, and a control electrode having a plurality of parallel effective portions positioned between and at predetermined distances from the anode and cathode, said efiective portions being spaced apart a distance which is greater than the cathode-to-control electrode spacing whereby a variable Inn is produced across the cathode, which mu varies from relatively high levels adjacent each effective portion of the control electrode to relatively low levels in areas adjacent the spaces between said effective portions, the cathode having a plurality of spaced parallel electron emitting portions positioned in the areas where the mu across the cathode is relatively high, and the elfective portions of the control electrode each being located in the shortest path between a respective cathode emitting portion and the nearest surface of the anode for controlling the flow of electrons from the cathode to the anode.

References Cited in the file of this patent UNITED STATES PATENTS 2,067,529 Heising Jan. 12, 1937 2,090,006 Knoll et al. Aug. 17, 1937 2,459,792 Chevigny Jan. 25, 1949 2,719,244 Dailey Sept. 27, 1955 2,727,177 Dailey Dec. 13, 1955

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