US2296324A - Gaseous discharge device - Google Patents

Description

Sept. 22, 1942. w. E. BAHLS GASEOUS DISCHARGE DEVICE Filed July 29, 1941 AF MA F a 50 A TA I-IF 0B V 4 wwm W T ROC .9 mm 82F OPERATION. SERIES GRID RES/STANCE EOUALS l0 MEGOHMS MK wQS I wZRRvBW kw WERNER MQQET H0l/R6 INVENTOR 14 ENDRESBAHLS ATTOR Y Patented Sept. 22, 1942 GASEOUS DISCHARGE DEVICE Walter E. Bahls, Water-town, Mass, assignor 'to Radio Corporation of America, a corporation of Delaware Application July 29, 1941, Serial No. 404,573

Claims. (01. 250-275) My invention relates to gaseous discharge devices, particularly to devices of the type having a control element or grid for initiating or controlling discharges between the cathode and anode of the device.

Gas-filled electron discharge tubes having a socalled grid for controlling the initiation of a discharge between the cathode and anode, are non-conducting for all anode voltages below a certain critical anode value, for any given grid voltage. When the anode voltage rises above this critical voltage, an arc-like discharge suddenly commences and continues until the anode voltage is lowered below a value which will maintain the discharge. In commercial applications of tubes of this kind it has been found that over long periods of operation the starting anode voltage gradually increases so that finally the tube becomes unusable. Various grid constructions have been tried including complicated structures with shields for protecting the grid from heat radiating from the cathode. Unfortunately the anode voltage in these tubes continues to shift upwardly during life.

An object of my invention is an improved discharge tube of the gas-filled type in which a control element or grid initiates and controls discharges between the anode and the cathode and in which the anode starting voltage, for a given grid. voltage, remains substantially constant throughout the life of the tube. 1

The characteristic features of my invention are defined in the appended claims and one preferred embodiment thereof is described in the following.

specification and shown in the accompanying drawing in which:

Figure 1 is an elevational view, partly in section, of my improved grid-controlled, gas-filled tube;

Figure 2 is a cross-section on line 2-2 of Fig. 1,

Figure 3 is a detailed view of the control ele-' ment or grid of my improved tube; and

Figure 4 is a graph showing the starting anode voltage for zero grid voltage of my improved tube.

compared to other and more conventional tubes.

When the cathode electron emissivity is decreased, the starting anode voltage, for any given grid voltage, is increased. Also when the cathode electron emissivity is decreased, the tube voltage drop, or cathode-to-anode impedance, is increased. The operating data of grid-controlled gaseous discharge tubes of conventional con-- struction with oxide coated cathodes do not show appreciable change in anode-to-cathode voltage drop or discharge current during the life of the starting, the rate of loss of emission for any,

tubes, although the anode starting voltage may rise many fold. The internal construction of these conventional tubes is such that the emitting surface of the cathode is not uniformly spaced from the anode, the cathode either being disposed end-on or perpendicular to the plane of the anode so that one end is closer to the anode than the other, or the grid opening being so constricted that the anode can see only a portion of the cathode. Examination of these .tubes after several hours of use shows a spot of distinct discoloration on the surface of the oxide coating of the cathode in alignment with the grid aperture. It is my belief that this spot on the cathode is an area bombarded and rendered non-emissive by ionized gas molecules and that this area is localized because it is the area first reached by the positive field of the anode through the grid and from which electrons first emerge to ionize the gas. This area, therefore, is the first to be bombarded and damaged. Even though the overall emissivity of the cathode may not be materially changed, the grid voltage-anode voltage ratio, or control characteristic, at starting may materially change.

According to my invention the cathode is parallel to the plane of the anode and the grid opening is co-extensive with the cathode so that the field of the anode aifects the cathode equally throughout its length. By increasing the area of the cathode which may be ion bombarded during given unit area opposite the grid aperture is probably decreased in proportion to the amount of increased area. According to a further characteristic of my invention the control element is so shaped and disposed with respect. to the cathode and anode that although its opening is large enough to expose the entire cathode to the anode, it exercises full control of the discharge and provides a high degree of tube sensitivity. Further, the anode arc initiating voltage of my improved tube remains substantially constant, for a given grid voltage, throughout the life of the tube.

One commercial embodiment of my tube, shown in Figure 1, comprises a tubular indirectly heated oxide coated cathode I mounted parallel to the face of the anode 2, the anode and cathode being within a rectangular metal shell 3 closed at each end by mica spacers 4. Between the anode and cathode is mounted my novel control element or grid 5 comprising an oblong flattened ring mounted at its top and bottom ends in the spacers 4 and presenting an elongated opening to the cathode I. The grid opening is slightly wider than the cathode sleeve and is approximately equal in length to the coated area of the cathode. Little heat can be absorbed by the edges of the grid, from the cathode, and active barium metal, vaporized from the cathode, finds small condensing areas on the grid. Transverse metal partition 6 is disposed between the anode and grid and has an elongated slot or opening 1 slightly larger than and-in registry with the grid opening. The partition 6 is preferably joined directly to the side-walls of the metal shell 3 so that the cathode may be shielded from all anode field effects except those lines of force passing between the sides of the grid. In my tube the arc-like discharge is a single well defined flat flame through the narrow grid opening. The grid may conveniently comprise two metal straps, shown in greater detail in Figure 2, each with two off-set bends 9 unequally spaced from the ends of the strap. One strap is reversed with respect to the other so that one end of each strap extends beyond the end of the other. When secured together at their ends,'as by welding, one end of each strap bears against the inner face of the mica spacer, its opposite end extending through the other mica to rigidly support the grid structure in place and prevent endwise movement or turning. The electrodes of my novel tube may be mounted as a unitary assembly between the micas and attached to the lead-in conductors in the stem in a manner common in the manufacture of conventional radio tubes. Uniform predetermined spacing of each of the electrodes is insured by preformed holes in the micas, thus insuring in factory practice uniform electrical characteristics. A glass pant leg I I integral with the stem surrounds the anode lead and extends upwardly and through the bottom spacer to increase the leakage path between the anode and the grid and cathode leads and prevents discharge to the anode lead. Preferably two mica sheets are used at each end of the mount to minimize leakage between the electrodes caused by deposition of barium on the tube parts from the cathode.

By disposing the cathode, which is here shown as the tubular indirectly heated type, parallel to the anode and controlling the discharge between these two electrodes with my improved slat-like grid, ion bombardment of the cathode is uniform throughout its length and is of average low intensity. Tests show my tube maintains a substantially constant anode starting voltage throughout life. Curve C of Figure 3 indicates but a few volts rise in the anode starting voltage, for a zero grid bias, during 1800 hours of operation of the tube shown in Figure 1. This characteristic is compared to the characteristic, shown by curve B, of a tube similar in construction to the tube of Figure 1 but having a round grid and partition 6 opening smaller in diameter than about one quarter of the cathode length, the grid comprising a metal cylinder of strap metal of about the same size as the strap of the ring 5 of Figure 1. Curve A of Figure 3 shows the rapid rise in anode starting voltage where the grid comprises a metal sheet parallel to partition 6 with a small round opening of the same diameter as the opening in the grid of the tube corresponding to curve B. The series grid resistance in each case was 10 megohms.

Curve C was obtained from a tube now commercially known as the RCA 2051 having a conventional round indirectly heated cathode sleeve externally coated with barium strontium oxide and internally heated with a conventional 6.3 volt tungsten heater, the sleeve being .050 inch in diameter with a coated area about 19 mm. long, the cathode-to-grid spacing being about 4 mm. and the grid-to-anode spacing being about 5 mm., the grid opening being about 3.5 mm. wide and about 17 mm. long. The tube was filled with argon gas to a pressure of about 350 microns.

My improved gas-filled discharge .tube has a constant anode starting voltage, for a given grid voltage, and is easy and inexpensive to manufacture.

I claim:

1. A gaseous discharge device comprising a cathode with an elongated emitting surface, an anode spaced from and parallel to said surface, a flattened ring in a plane parallel to the anode and with its elongated opening in registry with said emitting surface, an electrostatic shield comprising a metal plate between said ring and one of the remaining electrodes, said partition having an elongated opening in registry with the elongated opening of said ring.

2. A gaseous discharge device comprising an elongated cathode, an anode spaced from and parallel to said cathode, a slat-like control element between said anode and cathode, said element comprising a metal ring oblong in crosssection with the opening through the ring in alignment with said cathode and the major axis of the ring being substantially co-extensive with the length of the cathode.

3. A gaseous discharge device comprising an anode, an elongated oxide coated cathode spaced from and parallel to said anode, a metal shield surrounding the anode and cathode and having a metal partition extending between said anode and cathode, the emitting surface of the cathode being exposed to the anode only through an elongated opening in said partition said opening being substantially co-extensive with said cathode, and a control element comprising a flattened ring spaced from and substantially co-extensive with the periphery of said opening.

4. A grid structure for a gaseous discharge device comprising a flattened ring, said ring being formed of two metal straps with oif-.-set bends near the ends of the straps, said ends being welded together to form a closed oblong loop with ears extending beyond the ends of the loop.

5. A control element for a gaseous discharge device comprising two metal straps, each strap having two off-set bends unequally spaced from the ends of the strap, one strap being reversed end-for-end with respect .to the other and the ends of the straps being secured together with the end of one strap extending beyond the corresponding end of the other.

WALTER E. BAHLS.

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