US2323364A - Filamentary cathode - Google Patents

Description

July s, ms

- A. K. WING, JR

FILAMENTARY CATHODES Filed March 18, 1942 Patented July 6, 1943 UNITED STATES PATENT OFFICE FILAMENTARY CATHODE Arthur K. Wing, .lr., Chatham, N. J., assignor to Radio Corporation of America, a corporation of Delaware 5 Claims.

My invention relates to cathodes for electron discharge devices, particularly to filamentary cathodes for power and transmitting types of such devices.

Self-supporting cathodes for power tubes have been proposed comprising straight parallel filaments arranged in a cylindrical surface with the filaments attached at one end to support and current supply conductors. Experience has shown that the long lengths of untensioned filament wire may either bow and buckle at operating temperature, or, if held straight, expand unequally and force the cathode off center. Electrode spacers of insulating material to brace the cathode are not feasible in many high powered tubes, because of the severe voltage and temperature conditions. It is undesirable to stiffen the filaments by using thicker wire because of the increased heating current, and because of the reduced effective cathode emitting area of the large diameter filaments, for a given cathode power.

An object of my invention is a self-supporting filamentary cathode for power tubes which has a large effective emitting surface, which may be made with relatively long spans of straight filaments that will not bow or buckle, or move off center, and which is rugged in construction and easy to manufacture.

The characteristic features of my invention are defined in the appended claims and one preferred embodiment thereof is described in the fol owing specification and shown in' the accompanying drawing in which Figure 1 is an elevational view, partly in section, of a power tube embodying my improved filamentary cathode, Figures 2 and 3 l are detailed views of my cathode taken along sectional lines 2-2 and 3-3, respectively, of Figure 1, Figure 4 is a detailed View of one of the filament conductors of my improved cathode, and Figure 5 is a detailed view taken along sectional line 55 of Figure 1.

For convenience of illustration my improved cathode is shown in a power tube with a conventional fluid cooled anode I enclosing and concentric with a grid electrode 2, and my improved cathode 3. The cathode comprises a plurality of parallel straight filaments 4, of high melting electron emissive metal such as tungsten or thoriated tungsten, arranged in a cylindrical surface. The support for the cathode comprises a series of metal blocks 5 arrangedin a circle at the lower end of the cathode, each block being carried on the end of a heavy support rod and current supply conductor 6, which in turn may be supported on conventional lead-in conductors, not shown.

The opposite end of the cathod stands free and, when constructed according to my invention, requires no brace or interelectrode spacer. The filaments are joined together at their upper ends to make a rigid unitary cage structure that will not move off center during operation, and yet the filaments are sufficiently free to expand Without" bowing or buckling. Each filament at its upper end is bent through an obtuse angle toward the center of the cathode, where all the filaments are bound together. Preferably, a short end section ib of each filament is straightened into a line parallel with the center line of the cathode, and against the side of a round metal plug 1. The diameter of the plug may be so chosen with respect to the wire diameter and number of wires that the end sections 41) li in side-by-side contact around the plug. A ring 8 may be pressed over the wires and plug to hold the end sections in place, and if desired, the plug, ring and filament and sections may be fused by arc welding.

To prevent relative movement between the filaments, as when the filaments may swing or rotate about their junction at the plug, a metal wedge or spacer is conveniently placed between each radial portion 4a of the filament and intermediate the operative cylindrical surface and center of the cathode. A cap 9, preferably comprising a disc or cone integral with the ring, may be provided with a downwardly extending flange IE, the flange being provided with a series of evenly spaced notches into which each filament is guided as the cap is fitted over the ends of the cathode. The cantilever anchorage of each filament effected by fixing the radial portion 4a of each filament wire at two spaced points, effectively prevents lateral movement of the upper ends of the filament wires.

The filament wires are preferably arranged in pairs so that filament heating current flows in one direction in two adjacent filaments or pair and in the opposite direction through the next pair. By operating two round wires in parallel, greater ffective emitting surface is provided than if the two wires were consolidated in a single round conductor. The larger number of fine filaments produces a more uniform electron emission from the cylindrical surface of the cathode and affords a higher tube perveance. The accurately bored holes are made in each metal block 5, the holes being just large enough to snugly receive the ends of the filament wires 4. A slight taper in the tungsten .040 inch in diameter.

current supply conductors 6 grouped and contwo holes may, if desired, be provided to tightly wedge the filaments in an upstanding position parallel to the center line of the tube. If desired the lower ends of the filaments may be passed a short distance through the blocks and. the ends riveted or welded.

Since current flows in the same direction through the two wires of any one pair, and in the opposite direction through the adjacent pair, there is a tendency, caused by electromagnetic fields produced by the filament heating current, for the wires of the pair to move together,

According to another characteristic feature of my invention, the wires are braced at one or more points to prevent bowing. The tie-wire ll passes between the two filament wires of one pair and is looped one or more times around each filament. An offset bend in the filament elfectively prevents the looped tie-wire from slipping lengthwise of the filament. The ends of the tie-wire may conveniently extend to another pair of filaments having the same voltage and polarity where the wire loops about this second pair and hence on to a third or fourth pair,,if desired. In the particular cathode illustrated, twenty-four filaments are shown, the

twelve pairs of filaments being tied together in groups of six so that four tie-wires engage all of the filaments.

. Good results have been obtained in making a cathode about 5.25 inches long, from the blocks to the plug 1, and 1.75 inches in diameter with twenty-four round filament wires of thoriated With alternate nected for single phase operation, the heating current for the filaments is about 420 amperes at 8.5 volts for heating the filaments to their normal operating temperature of about 2050 K.

In manufacture, the filaments are pre-formed with the bends at 40. and 4b and with the offset bends for the tie-wires. The tie-wires of tungsten with thier loops are likewise pre-formed and the filaments and tie-Wires threaded together, whereupon the lower ends of the filaments are inserted in the bored holes of the metal blocks 5 and welded in place. The metal plug 7 of tungsten is then held in place while the end sections 41) of the filaments are grouped around the plug and while the cap 9 of molybdenum and 1.1 inches in diameter is pressed over the assembly with the notches of the cap flange guided over the filaments. Finally the plug, ring and filament end section are fused together by an electric arc. The cathode assembly is flashed and carburized and is then mount ed with the grid and other electrodes of the tube. The finished tube is preferably operated with the filaments hanging, that is with the plug 1 below the conductors 6.

My improved cathode is self-supporting, has a large effective emitting surface, and comprises long spans of straight filaments that will not bow or buckle. My improved cathode, even without interelectrode spacers or a center support rod, is a rigid unitary structure that will not move off center during operation. My improved cathode is rugged in construction and easy to manufacture.

I claim:

1. A cathode comprising a plurality of parallel straight filaments arranged in a cylindrical surface, the end of each filament at one end of the cathode being bent radially inward and bound together in a single rigid mechanical junction on the axis of the cathode, metal spacers between adjacent wires intermediate said surface and said junction, the opposite ends of the filaments being connected to current supply conductors, and tie-wires intermediate the ends of the cathode between points on the filaments of like voltage and polarity.

2. A cathode comprising a plurality of parallel straight filaments in a cylindrical surface, the filaments being connected to and supported by current supply conductors at one end, and being bent radially inward at their other end and joined together at the axis of the cathode, and means to fixedly space the filaments at their said other ends comprising metal spacers between the filaments intermediate said surface and said axis.

3. A cathode comprising a pluralityof circularly arranged parallel straight filaments, bends in the ends of the filaments, with said ends joined together at the center of the filament circle, a metal cap with uniformly spaced notches in its rim, said cap being secured over the joined ends of the filaments with each filament passing through one of saidnotches.

4. A cathode comprising a plurality of cylindrically arranged uniformly spaced parallel filaments, the filaments being joined together at one end, the opposite ends of the filaments being connected to current supply conductors, two adjacent filaments being connected to one conductor and the next two filaments being connected to another conductor.

5. A cathode comprising a plurality of spaced parallel filaments arranged in a cylindrical surface, a plurality of current supply conductors at one end of the filaments, the filaments being arranged in side-by-side pairs, the two adjacent filaments of each pair being connected to one of said conductors, a short tie-wire to brace apart the filaments of each pair, each tie-wire being joined to each filament of its pair intermediate the ends of the filament.

ARTHUR K. WING, JR.

Images