US2075876A - Cathode organization - Google Patents

Description

April 6, 1937. Q J VON wEDEL 2,075,876

CATHODE ORGAN I ZAT ION Original Filed Dec. 28, 1927 INVENTOR 634m J R hf m/v W505:

ATTORNEY Patented Apr. 6, 1937 UNITED STATES PATENT OFFICE CATHODE ORGANIZATION Application December 28, 1927, Serial No. 243,042 Renewed January 22, 1931 27 Claims.

The present invention relates generally to cathode organizations for discharge tubes, and more particularly to cathode organizations adapted to have the necessary heat for electron emission generated by alternating current. It contemplates in particular the so-called indirectly heated type cathode.

One object of the invention is to provide a cathode organization having special provision for rendering ineffective in large measure the alternating current field effects which tend to create hum producing disturbances in the operation of discharge tubes as radio and like detectors and amplifiers. Another object is the provision of special features permitting the use of currents at high potentials for cathode heating, as from the usual 110 volt house lighting system.

A further object is the inclusion of features permitting the use of current at the higher voltages in so-called gas filled tubes where heretofore this has not been possible on account of the close spacing required of the parts of the tube subjected to diiferences of potential sufficient to produce discharges across these short paths in the gas filling.

In the indirect method of heating a cathode for electron emission there is necessarily a drop of temperature between the active electron emitting surface and the heating element, with the result that to obtain the same temperature of the active emitting material as is obtained in the directly heated or filament type of cathode it is necessary to employ current of substantially greater intensity, other conditions being the same. This necessity for large current is further enhanced by the fact that much greater radiating surface and mass is had in the indirect heated method of construction, requiring much larger quantity of heat to maintain the active surface at the desired temperature. The larger current results in the production of more intensive electromagnetic and electrostatic fields within the interior regions of the tube where they are best located to have large effect on the operation of the tube. Since the alternating current of usual house supply systems is of such frequency as to be within the range of the useful frequencies of desired signal currents being handled by the tubes it is not feasible to eliminate the disturbances after they have been created, this because any attempt at such elimination results in eliminating some of the desired signal currents. It is therefore better to prevent the creation of the disturbances than to permit their creation with the hope of later elimination.

It is obviously most desirable to be able to heat the cathodes of tubes at potentials directly available from ordinary commercial current supply systems, as such supply systems are now more or less standardized in the matter of supply potentials, and such practice avoids the interposition of converting apparatus, such as transformers, rectifiers, filters and the like. There is also the advantage that the supply of current at high potentials involves the use of currents of small intensities, thus eliminating the necessity for maintaining supply wires short and of large cross section.

The diiilculty of using heating current at high potential is the necessity for crowding into the very small space usually available in a tube the necessary length of fine wire, and preventing in such crowding short circuits and other forms of current leakage. Also the increased electrostatic field intensities produced by the higher differences of potential are troublesome. This invention makes special provision for overcoming these difficulties.

High potentials in gas filled tubes oifer particularly difilcult situations at those parts of the leads carrying the high potential currents through gas filled portionsof the tube, because such gas facilitates the production of ionic discharges between high potential points, and such discharge is almost certain to result in immediate destruction of the more or less fragile lead-in or heating wires.

The invention is further explained in connection with the figures of the accompanying drawing, in which-like reference characters in the several figures represent like parts as far as possible.

Fig. 1 shows a cathode organization of the indirect heated type particularly suitable for heating current supplied at low potentials.

Fig. 2 shows a cathode organization within a suitable container or vessel in which special provision is made for obtaining indirect heating with current supplied at higher potentials.

Fig. 2a illustrates in detail several of the features of Fig. 2.

Fig. 3 shows a special arrangement of the cathode organization making provision for use in gas filled tubes.

Referring to Fig. 1 there is shown the usual glass stem S of a discharge tube, which acts as a carrier of the lead-in wires and a support for the multiple electrode organization. There is shown the usual cylindrical anode or plate P surrounding control electrode or grid G. The cathode structure is shown to include a thimble or cylindrical core 0 for carrying the emitting g i A coating. It is not necessary for the thimble to be cylindrical, as other cross sectional forms may have advantages under particular circumstances. The thimble is shown supported by a 5 suitable upright W with horizontal extension, this upright making conductive and supporting con-' tact with the metallic thimble at O. The upright W is conductive and connected to a lead-in wire through the stem S.

The thimble may be made of any one of the.

usual core materials for electron emitting oxides or compounds, such as platinum, molybdenum, nickel, cobaltum or alloys of them or with aluminium. Since no particular structural strength is required of the thimble, as is usually the case with directly heated cathodes, some of the softer metals, such as nickel and cobalt, may be employed with advantage, and are particularly suitable because they are not subject to corrosive disintegration to the same extent as some of the other metals. While the matter of disintegration of the metal of a comparatively large thimble is not so serious as the disintegration of the core of a filament in the directly heated type of cathode, on the other hand in many cases the corrosive compounds formed by the corrosive materials have the effect of lessening the electron emission of the cathode.

The heater system is shown to include a pair of wires H helically twisted or helically wound about each other, and conductively cross-connected at the top and connected to the suspension point 0, and therefore to supporting upright W, the two lower ends of this helically twisted arrangement being connected to lead-in wires of the stem S. This helically twisted arrangement requires that the two wires be effectively insulated one from the other to prevent undesirable leakage. Since these wires must be heated to 40 temperatures higher than the required temperature at the surface of the thimble, even to the extent of several hundred degrees, and therefore operate at rather high temperatures, for instance, in the neighborhood of 1000? C., the problem,

when an insulating coating is desired, of insulating one from the other in the presence of this high temperature becomes a special one. I have found that certain compounds such as beryllium oxide and aluminium which do not have high electron emission, when mixed with sintering materials, such as the fluorides, for example, strontium fluoride, form admirable insulating coatings for such twisted wires operating at the high temperatures required.

Previous attempts to accomplish similar results, using the silicates and like materials which are non-conducting at low temperatures, have resulted in practical failures at the high temperatures involved.

The heating wires may be nickel, tungsten or like materials having the desired resistance characteristics, and which will withstand the high temperatures involved, and more particularly those adapted to receive and hold the chemical compounds involved in the insulating coating. It is desirable that the core material not be one that forms a corrosive compound with the coating material. It is possible to avoid this situation by plating such a core with a metal not subject to such corrosive compound formation.

The coating compounds, or other compounds mentioned, may be formed on the heating wire as coatings by processes now well known for the formation of such coatings.

It is thus seen that if the two ex e nal l ads of the helically twisted heating wire H be connected to an alternating current transformer, whereby the heating system is supplied with alternating current, that the connection to the heating Wire of the upright W at point 0 provides a connection to the neutral potential point of the heating wire H, and furthermore provides a connection to the most neutral potential point in the cathode thimble, so that such arrangement makes an ideal connection between the grid circuit and the cathode system to avoid introducing varying potential effects on the grid by reason of such alternating current use. It will further be noted that the connection between the heater wire and the cathode thimble is very short, and therefore of negligible resistance, thus substantially eliminating a potential difference between these two elements. It is preferable to form the cathode thimble of one continuous cylindrical body rather than having an arrangement of spiral wires or ribbons, this because extremely low resistance is maintained between the neutral point of the thimble and distributed points in the thimble surface, to maintain low differences of potentials between such points.

It will also be noted that the metallic thimble completely surrounds the alternating current carrying heater wires, this including a complete closure of the top of the thimble, so that the electromagnetic and electrostatic fields are shielded from the regions outside of the thimble to a high degree. It is also to be noted that by reason of the twisted formation of the heated wires that the resultant fields are maintained extremely low, so that any fields straying from the interior region of the thimble are naturally weak, resulting as they do from a weak resultant field.

It is desirable to coat with electron emitting material only that portion of the thimble included within the extent of the grid, or even to have the grid structure extended slightly beyond the limits of the coating. Another feature worthy of attention is to extend the lower part of the thimble, which is open in the direction of the stem, somewhat below the lower extent of the grid structure, this in order to shield as far as possible the grid from the potentials on the lead-in wires of the heater system.

In Fig. 2 modifications are made which particularly adapt the heating arrangement and cathode system for energizing from a high voltage source, either direct current or alternat ing current, such as the ordinary volt light socket, and therefore without the use of the usual step-down transformer for alternating current operation. In this case the heating wire H comprises closely helically wound fine wire wound in bifilar form in two helically ascending parts in the grooves of a grooved sleeve B, shown in detail in Fig. 2a, each part having a connection to a leading in wire in the stem S, the details of the winding being better shown in Fig. 3. The two parts are electrically continuous one to the other, and may be conductively connected at the top mid point, and therefore the alternating current neutral point, to the thimble C and conductive support W which passes through the grooved sleeve to a leading in wire E (Fig. 2a) in the stem S. If the ultimate tube is intended for operation by filament heating directly from a commercial lighting and power system it is not advisable to connect the mid point of the heating wire to the wire W because such systems are usually grounded on one side, and any use of the tube in a system requiring grounding of the wire W would result in a serious short-circuit. The cathode or thimble C is however connected to support W at the point as shown.

The sleeve B must be an insulating material capable of remaining insulating at the very high temperatures mentioned in connection with insulating features in Fig. 1. I have found that such mixtures as beryllium oxide, thorium oxide, aluminium oxide, together with sintering matel0 rials, such as fluorides like barium fluoride and calcium fluoride, one such mixture bearing-the commercial name steatite, are particularly suited for forming such an insulating sleeve.

One suitable method of making such a sleeve is to place powders of these materials in a pressure mould having the desired groove formation,- the support W being passed through the center of the mould. After formation of the ingot by pressure it can then be heated to sufllcient temperature to sinter the materials into a hard mass. The sleeve may have an enlarged portion D of such size to fit snugly within the lower part of the thimble C, with holes K, K as shown through which the two ends of the heater wire may pass to the lead-in wires of stem S, shown in detail in Fig. 2a.

The support W serves as a connection to the neutral point of cathode C and, if desired, as a connection to the neutral point of the heating wire system. It also acts to support the heating wire, the heating wire insulator, and the cathode thimble, and at the same time spaces the thimble for proper relation to the enlarged portion D on sleeve B.

In view of the extremely restricted space available within the cathode thimble of an ordinary tube the length of wire required to satisfy the high potential contemplated for the heater and give the required heating surface is obtained by 40 winding helically wound flne wire in ascending helical form, and two such ascending helices are used, wound bifllar side-by-side in the same direction, in order to produce the field neutralizing effects described in connection with the helically 15 twisted form of heater of Fig. l and to bring the neutral point of the heater system adjacent the neutral point 0 of the thimble C.

It is particularly desirable to have the turns of the helices of the fine wire closely spaced in order to secure great length of wire in the small space available. Since winding a closely spaced helically wound wire on a sleeve would unquestionably result in there being many contacts between turns at the inner side of such winding, it is necessary to coat such a wire with an insulating coating as in the case and under the conditions of the heating wire of Fig. 1. The voltage between the adjacent turns of the spiral is low, so that a coating of this kind is not called upon to insulate against high potentials. The high potentials between the parts of. the heater system are taken care of by winding in the spaced grooves of the insulating sleeve.

It is apparent that the end of the cathode systern towards the stem is subject to greater heat loss by conduction than the removed end, so that unless special provision is made the cathode will not have uniform electron emission temperature throughout. If arrangement is made to heat the lower part of the system at a. higher rate this effeet can be overcome. One way of accomplishing the result is, in winding the helically wound wire, to stretch it in ascending so'that the turns are gradually more widely spaced; or the slope of the helices on the sleeve can be gradually increased in ascending. I have found another eifect which may also be employed usefully in this direction, which is using for the coating a compound that emits electrons, such as strontium nlckelate, thus producing electron currents between the two parts of the helical windings, which electron currents will be more intense at the stem end than the upper end because of a higher difference of potential at the stem end. These electron currents will increase the general heating current to make the total heating effect greater at the higher heat loss end.

It is thus seen that the arrangement of Fig. 2 provides for reducing the hum producing effects in like way to Fm. 1, except with the improvement that the connection to the neutral point is led directly through the middle of the heater winding instead of outside, as in Fig. 1, thus securing the best possible location for neutralizing the alternating current fields, and including those of the lead-in wire connections in the stem. This is particularly important in the case of the high heating potentials contemplated. Since the support wire W, passing through the middle, is conductively connected to the surrounding thimble C, both of which are of low resistance, an ideal shield organization is provided for, especially in the matter of reducing thimble eddy current voltages. v

Fig. 3 includes features particularly directed towards permitting high potential heating current in gas-filled tubes, it being the object to prevent ionic discharges between parts of the heater system as such discharges, if permitted,

will result in almost immediate destruction of the fine wires employed by reason of the impact energy of ions. The likelihood of such discharge is increased if electron emitting coatirm material is used in the insulating coating of the heater wire as such'discharges can then be produced at much lower differences of potential.

In tubes using quite low gas pressures the prevention of discharge can be had in whole or in part by spacing the difierence of potential parts suillciently short distances that they are about equal to the mean free wavelength path of the electrons for the particular gas and pressure of it used. Such spacing will usually not be effective in gas-filled tubes for the spiralled part of the heater because usually the distances become too small for practical construction, but may be of particular value in spacing the lead-in wires. For higher pressure gas-filled tubes, where physical spacing is not practical, it becomes necessary to resort to other expedients. I provide an arrangement for preventing ionic discharges by surrounding the diiference of potential parts with powders or materials eilfective to prevent ionic discharges taking place through them, such aspowders of the same materials previously mentinned for both the coatings of the heater wires and forming the insulating sleeve B. These powders T, Fig. 3, are so inserted inside the thimble C as to closely surround the parts to be protected, and when the tube is heated these powders form in whole or in part the compounds previously mentioned, and therefore give suificient non-com ductivity for electrical current to prevent short circuits or leaks of undesired degree between the difference of potential parts. On the other hand these compounds have a sufilciently high heat conductivity to permit of transferring the desired quantities of heat to the thimble C without requiring a too high temperature of the heater system. i

In order to extend this protection to the leadin wires there is provided on the stem 8 a mounting M through which the lead-in wires pass as shown. This mounting may be made of the 5 same material as the sleeve B, and should have means for making a flange or other suitable joint with the thimble C as shown, thus forming with V the thimble C a chamber completely filled with the desired material. This mounting at the same time serves to center the whole cathode structure. It is not necessary for the joint between the mounting M and thimble C to be a gas tight Joint, as the filling takes care of the presence of any gas. After the thimble C is in place the connec- 5 tion with the support wire W is made by fusing or other means, so that the support wire then serves to hold the arrangement as a compact unit. If tubes of large outputs are contemplated for use with alternating current as a source of cathode heating the troublesome field effects due to increased intensities of currents required may be further reduced by including in the tube, such as Fig. 2, two lead-in stems S at opposite ends, and

bringing the grid and plate leads through one stem and the heating current leads through the other stem. The grid and plate leads are, in this way, widely separated from the heater leads carrying the heavy heating alternating currents.

Having thus described my invention, what I claim is:

1. A cathode organization including a current carrying-heating element, a metallic support for an electron emitting coating closely surrounding said heating element, and a filler of effectively electrical non-conducting material at the operating high temperatures of said organization in the normally free spaces internally of said support, said filler containing an electron emissive compound in close proximity to points of maxi- 0 mum heat energy loss whereby the said metallic support is uniformly heated throughout its length.

2. A discharge tube having a cathode organization including a hollow metallic support for an external electron emitting coating, an electrode mounting stem in said tube, a conductive support erected in said mounting stem in conductive continuation'or alead-in wire, and passing centrally through said hollow coating support to a conductive and supporting connection therewith, O and a bifilar system of heating conductors in sulatedly wound about said conductive support.

3. In a discharge tube, the combination of a stem, lead-in wires through said stem, an electrode support carried by said stem and in conductive continuation of one of said lead-in wires, a cathode thimble carried by and surrounding said support, a cathode heater within said thimble, and means for spacing said heater element from said support and said thimble.

4. In a discharge tube, the combination of a stem, lead-in wires through said stem, a cathode support carried by said stem and in conductive continuation of one of said lead-in wires, an insulating element surrounding said support, a cathode heater element carried by said insulating cathode heater element consisting of a bifilar helical winding of wire carried by said insulating element and in conductive extension of a pair of said lead-in wires, a cathode thimble conductively mounted upon said support and surrounding said heater element, and means for spacing said heater element from said thimble.

6. In a discharge tube, the combination of a stem, lead-in wires through said stem, a cathode support carried by said stem and in conductive continuation of one of said lead-in wires, an insulating element surrounding said support, a cathode heater element consisting of a bifilar winding of helically wound wire carried by said insulating element and in conductive extension of a pair of said lead-in wires, a cathode thimble conductively mounted upon said support and surrounding said heater element, and means for spacing said heater element from said thimble.

7. A cathode organization including a current carrying heating element, a metallic support for an electron emitting coating closely surrounding said heating element, and a filler of beryllium oxide in the normally free spaces internally of said support. I

8. In a discharge tube having an indirectly heated cathode, a heater therefor having a noninductive bifilar helical winding of helically wound wire, said wire being coated with beryllium oxide.

9. In a discharge tube having an indirectly heated cathode, a heater therefor comprising an insulating support, and a pair of helical windings of helically wound wire wound about said insulating support and having the turns of said windings disposed in grooves in said support.

10. A cathode organization comprising a twisted pair of conductors having an insulated coating forming a heating element, the coated conductors being in physical contact and connected together at one end, and a support for an electron emissive coating surrounding said heating element.

11. In an electron-discharge device, a cathode comprising a helically wound type of heater filament supported along its length by a refractory supporting element of insulating material, said refractory element extending beyond the vicinity of said heater filament, electron-emissive means surrounding said heater filament, and means for supporting said refractory element at a point substantially removed from said filament, said electron-emissive means and said refractory supporting element being rigidly supported with respect to each other at one end while they are so supported at the other end with respect to each other as to permit relative movement caused by expansion.

12. In a discharge tube having an indirectly heated cathode, a heater therefor having a noninductive bifilar helical winding of helically wound wire, said wire being coated with a layer of strontium nickelate.

13. An electron tube of the auxiliary heater type having an electron emitter comprising a filament disposed within grid and plate electrodes consisting of a pair of spiralled sections disposed parallel one to another and having the magnetic fields thereof confined substantially within the central zone of said electron tube, means surrounding said filament includingelectron emitting surfaces specially related to said filament for substantially eleetromagnetically shielding said grid and plate electrodes from the field around said filament and supports adjacent each end of said means for fixing the spacial relation of said means with respect to said filament.

14. A cathode structure comprising a hollow elongated member having an external surface adapted to emit electrons upon heating, a heating filament within said member, said filament comprising two series-connected, helical portions, the terminals of said filament extending from one end of said hollow member and the respective 10 convolutions of the two helical portions lying in substantially the same circumferential plane in relative parallel spaced relationship, means closing one end of said hollow member, said means carrying a rod-shaped insulating member, at least one turn of each of said helical portions closely surrounding and engaging the surface of said rod-shaped member, whereby all the turns of each helical portion are held spaced from each other, an unobstructed annular space being pro- 20 vided within the inner walls of said hollow membeer and said filament, whereby said walls may be heated by direct radiation from said filament, said structure including means for closing the opposite end'of said hollow member.

15. A cathode structure comprising a hollow elongated metal member having an external surface adapted to emit electrons upon heating, an insulating plug closing one end of said hollow member, a heating filament within said member,

:10 said filament comprising two series-connected helical portions, the terminals of said filament extending from one end of said hollow member and the respective convolutions of the two helical portions lying in substantially the same cir- 35 cumferential plane-in relative parallel spaced relationship, said plug carrying a rod-shaped insulating member extending part way toward the other end of said hollow member, at least one turn of each of said helical portions closely sur- 40 rounding and engaging the surface of said rodshaped member, whereby all of the turns of each helical portion are held spaced from each other, an unobstructed annular space being provided between the inner walls of said hollow member 45 and said filament, whereby said walls may be heated by direct radiation from said filament.

16. A cathode structure comprising a hollow elongated member having an external surface adapted to emit electrons upon heating, a heat- .70 ing filament within said member, said filament comprising two series-connected helical portions, the. terminals of said filament extending from one end of said hollow member and the respective convolutions of the two helical portions lying in 5.3 substantially the same circumferential plane in relative parallel spaced relationship, means closing one end of said hollow member, said means carrying a rod-shaped insulating member, at least one turn of each of said helical portions closely to surrounding and engaging the surface of said rodshaped member, whereby all the turns of each helical portion are held spaced from each other, an unobstructed annular space being provided within the inner walls of said hollow member and said filament, whereby said walls may be heated by direct radiation from said filament, and means for closing the opposite end of said hollow member.

17. An electron emitting cathode of the indi- 70 rectly heated type comprising a tubular metal sleeve member exteriorly coated with thermionically active material and an interlorly positioned double helical heater element electrically insulated therefrom, the terminals of said coll ex- 75 tending from one end thereof and the respective convolutions of the two helices lying in substantially the same circumferential plane in relative parallel spaced relationship, a small rod solely of insulating material extending from said one end of said tubular metal sleeve partway toward the other end of said tubular metal sleeve, at least one turn of each of said helices closely surrounding and engaging the surface of said rod, whereby all of the turns of each helix are held spaced from each other, an unobstructed annular space being provided between the inner walls of said hollow member and said heater, whereby said walls may be heated by direct radiation from said heater.

. 18. A cathode organization comprising a heater element having parallel conductors electrically continuous at one end, an insulating coating thereon, the conductors being twisted about each other so closely that the over-all diameter of said heater element is only slightly larger than the sum of the coated conductor diameters, and a metallic cathode sleeve surrounding said heater and spaced therefrom, said sleeve having a thin wall relative to its diameter.

19. In a grid-controlled discharge tube, a cathode organization comprising a cathode sleeve having an outer emissive surface and a thin metal wall pervious to magnetic fields, a heater element disposed in central spaced relation within said sleeve and adapted to heat the sleeve by direct radiation and thereby heat the cathode surface to emissivity with low heater voltage compared to the usual supply voltages, said heater being formed of two parallel conductors closelv twisted about each other in insulated relation and electrically continuous at one end, and a control grid adapted to control the electron emission from said emissive surface.

20. A cathode heater comprising a helical winding and an insulating support therefor having the rigidity and refractoriness to support the turns of said winding in insulated spaced relation at a temperature of 1000 degrees centigrade, said support being composed substantially of a sintered mixture of metallic oxide selected from the oxides of beryllium, thorium and aluminium, and a sintering material.

21. In a thermionic tube of the heater type a tubular cathode, a heater filament within said cathode, means to support the two ends of said filament, and means integral with said filament to insulate said filament from itself and from said cathode, said means comprising a coating containing as an essential element crystalline aluminum oxide integrally formed on a refractory base heater.

22. In a thermionic tube of the heater type, a tubular cathode, a heater filament within said cathode, means to support the two ends of said filament and means integral with said filament to insulate said filament from itself and from said cathode, said means comprising a. coating containing as an essential element beryllium oxide integrally formed on a refractory base heater.

23. An electric discharge tube filament comprising a refractory metal base having an insulating coating thereon containing beryllium oxide as an essential element.

24. A heater cathode unit for electric discharge tubes comprising a refractory heater filament, a cathode positioned to be heated by said filament and an insulating body between said cathode and heater composed of beryllium oxide mixed with a sintering material.

25..A vacuum tube filament comprising a re- Iractory metal base having an insulating coating composed substantially oi beryllium oxide and a sintering material, said coating being integral with said base.

26. A filament for electric discharge tubes comprising a base of tungsten having an insulating coating containing beryllium oxide and a sintering material.

27, A refractory filamentary metal heating element for an electron tube having a coating comprising a mixture composed of an oxide of an element oi the class comprising aluminum and beryllium and a sintering material selected irom the group 01' compounds consisting of fluorides of barium, strontium and calcium, and steatite.

CARL J. R. H. you WEDEL.

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