US2089817A - Indirectly heated cathode - Google Patents

Description

Aug. 10, 1937. P, w STUTSMAN 2,089,817

INDIRECTLY HEATED GATHODE Filed Jan. 15, 1935 [NVE/VTOR PAUL 1M 5rursMA/v A TTORNE Y Patented Aug. 10, 1937 UNITED STATES PATENT OFFICE mnnmo'ru HEATED oa'rnonr.

Application January 15, 1935, Serial No. 1,931

; 8 Claims. (Cl. 250-475) This invention relates to indirectly-heated cathodes for electrical space discharge tubesfand more particularly to such cathodes which are heated by a heating filament.

In cathodes of the above type, it is often desirable to energize the heating filament by alternating current. Despite the fact that the electron-emitting member of the cathode is usually electrically separated from the heating filament,

it has been difficult to eliminate disturbing effects due to the alternating character of the current. These disturbing effects have been considered as being due to varying magnetic orelectrostatic fields or to variable leakage currents between the electron-emitting member of the cathode and the heating filament. These leakage currents may be due to electron emission between the electron-emitting member of the cathode and the heating filament, or to direct leakage paths between these two members. These various disturbances have manifested themselves as an audible hum in the output of systems in which such space discharge tubes were used.

An object of my invention is to produce a cathode of the type designated which is simple and inexpensive to manufacture, in which humproducing disturbing effects are substantially eliminated by the elimination of substantially all stray electrostatic and magnetic fields.

0 Another object of my invention is to decrease said hum-producing disturbances by substantially eliminating variable leakage currents between the electron-emitting member of the cathode and the heating filament.

The foregoing and other objects of my invention will .be best understood from the following description of exemplifications thereof, reference being had to the accompanying diagrammatic drawing, wherein:

Fig. 1 is a representation if apparatus for producing heating members for my novel cathode;

Fig. 2 is a perspective view of a section of my novel heating element showing the various elements thereof broken away;

Fig. 3 is a side view of one form of my novel cathode mounted on the stem of an electrical space discharge tube;

Fig. 4 is a cross-section taken along line 4-4 of Fig. 3; and

Figs. 5 and 6 are diagrammatic representations of two different methods of connecting the heating filament of my novel heater member.

My novel cathode utilizes as one of its novel elements a heating member made-from a "twin" 55 wire heater, such as illustrated in Fig. 2, which may be formed by a continuous manufacturing process, such as that illustrated in Fig. 1. The twin wire heater consists of two filaments I and 2 placed side by side and embedded in an insulating coating 3. The filaments I and 2 are first separately coated with individual insulating coatings 4 and 5 in any suitable manner, such as by the well-known dipping and baking process. The insulating coating which I prefer to use consists of aluminum oxide. I prefer to coat the filaments I and 2 with a thickness of coating 4 and 5 which is substantially equal to the diameter of the filament. For example, if a filament of 2.3 mils is used, the coated diameter thereof would be not less than 7 mils, and preferably slightly in excess thereof.

The filaments I and 2 carrying their respective coatings 4 and 5 may be mounted on spools 8 and I, such as that shown in Fig. 1, and these spools are then placed in a coating apparatus, such as that illustrated diagrammatically in Fig. 1. The coated filaments I and 2 are first passed over rollers 8 and 9 placed such as to bring the coated filaments I and 2 into contact with each other and maintain them in contact. The two separately coated filaments I and 2 are then passed through an insulating coating bath I0, in which insulation, preferably in paste or liquid form, is applied by means of a roller II. The two coated wires I and 2 with an additional outer coating of insulation are then passed through a furnace of some suitable kind, such as for example an electric oven I2. The usual coating apparatus of the type illustrated consists of a series of such coating baths I0 and ovens I2. After the coated pair of filaments I and 2 has been passed through the successive coating baths and ovens, the final insulating coating 3, as shown in Fig. 2, is formed thereon. As the finally coated wire comes from the apparatus, it passes over a guiding roll I3 and is wound up on a spool I4. If there are not sufiicient coating baths in a coating apparatus to give to the final heater element the requisite thickness of coating, it may be necessary to pass the heater element through the coating apparatus more than once. For example, in coating apparatus having three coating baths, I have found that it is desirable to pass the heater element through the apparatus three times.

The heater element, as it is finally assembled on the spool I4, consists of the two filaments I and 2 embedded in the insulating coating 3, as shown in Fig. 2". The outer surface of the heater element is elliptical in shape, as illustrated ing coating to any one of the heater wires is less than this amount, a decided increase in hum is produced.

In order to construct the cathode with the heater element as produced by the above process,

the coated heater element is cut off to the desired length. The insulation is cleaned oil the filaments at short distance at each end of the length cut off. The length is then folded over at its midpoint so as to form two halves I5 and I6 of the heater element which are parallel to each other, as shown in Fig. and 4. Since the bend shown at the top of Fig. 3 is rather sharp, the insulation has a tendency to break off from the filaments at this point. However, the insulation on the remaining portion of the heater element spaces the two filaments from each other and from the rest of the cathode structure. I prefer to bend the heater element so that the short axes of the cross-section of the halves I5 and I6 are in line with each other. The heater element bent in this manner is then slippedinside the cathode sleeve I I which may be provided on the exterior with some suitable electron-- emissive coating IS. The interior diameter of the sleeve I1 is made substantially the same as the total distance between the outer surfaces of the two halves I5 and I6, and preferably slightly larger than said distance to facilitate easy assembly. Due to the manner in which the heating element is bent, the sleeve I'I touches the heater element only at points I9 and 20 which lie at the outer extremity of the short axes of the cross-sections of the halves I5 and 5.. These points, as will be seen, are intermediate the filaments I and 2.

The cathode assembly in the manner described is preferably connected in the tube so that heating current flows through the filaments I and 2 in opposite directions to each other. For ex- 0 ample, one connection is illustrated in Figs. 3

and 5. In these figures, filaments I and 2 of the half I5 are welded to lead-in wires 2| and 22 sealed in stem 23 which may form part of an electrical space discharge tube. The two filaments I and 2 of the half I6 are welded together to a dummy wire 23 also sealed in the stem 23. The stem 23 is also provided with the usual additional supporting standards for supporting the other electrodes of the tube. The two lead-in wires 2i and 22 are connected externally to some suitable source of heating current, such as, for example, a heating transformer 25. The dummy wire 24 is not provided with any external connection. The heating transformer 25 mayhave its secondary 26 provided with a center tap 2'I which is connected to a lead-in wire 28 which in turn is connected to the cathode sleeve II by means of a conducting strip 29. The secondary 30 of the transformer 25 is adapted to be connected to a suitable source of alternating current. To the lead-in wire 28- are also made the other cathode connections of any circuit, of which the space discharge tube may be utilized. The resultant circuit for the heating filament,

illustrated in Fig. 3,'is shown in Fig. 5. 'The heating current fiows from one of the lead-in wires, for example 2I, through the filament I of both halves I5 and I6, to the .point where both the filaments I and 2 are connected to the dummy wire 24. Due to the fact that both of the filaments I and 2 are welded to this dummy wire, an electrical connection is made at this point so that the current then flows back through the filament 2 to the other lead-in wire 22. In this way the two filaments I and 2 are connected in series with each other. Instead of such a series connection, the two filaments I and 2 may be connected in parallel with each other, for example as shown in Fig. 6.

In each case I connect the filaments I and 2 so that heating current flows through the filaments l and 2 in opposite directions at the same time. Due to the fact that the filaments I and 2 are spaced close to each other, there is very little resultant magnetic field which extends out beyond the immediate vicinity of the two filaments. Also any potential which exists at any point on filaments I and 2 has an equal and opposite potential at an adjacent point on the opposite filament, and thus, due to the fact that these filaments are placed very close together, there is very little resultant electrostatic field which reaches out beyond the immediate vicinity of these two filaments.

The structure which I have described above also eliminates variations in any leakage current which may occur between the sleeve II and the heater wires I and 2. If any leakage current tends to occur from the sleeve II to the wire I through the point of contact 20, a similar leakage current will occur between the wire 2 and the sleeve I I through the same point of contact 20. Since with respect to any such point 20, the wires I and 2 are substantially the same distance therefrom and are at equal and opposite potentials, the leakage current which occurred at each instant will be substantially equal and opposite to each other. If there is any resultant leakage current between the sleeve 20 and the wire pair I and 2, this resultant current will be constant in value and will not follow the variations in voltage applied to the cathode heater. Of course it is to be understood that the point 20 may not always be exactly on the center line'between the wires I and 2. However, due to the fact that the distance from the wires I and 2 to the outside of the insulating coating 3 is preferably greater than the distance between these two wires, the point of contact between the insulating coating 3 and the interior of the sleeve I! can vary considerably on either side of the center point, as indicated at 20, and still have the wires I and 2 substantially equidistant from the point of contact. Slight diiierences in distance between the point of contact and the wires I and 2 will not introduce any serious resultant variation in the leakagecurrents, and therefore any structure in which the contact between the insulating coating and the cathode sleeve occurs at a limited area, which is substantially equidistant from two heating wires at substantially equal and opposite potentials, comes within the purview of my invention. Due to the symmetrical arrangement of the heater wires I and 2 with respect to the interior of the cathode sleeve II in which the potentials on the various wires are substantially equal and opposite to each other, the resultant emission current, if any, between the interior of the cath- .5 netic and electrostatic fields around my novel heater, and have likewise substantially eliminated variations in leakage currents between the heater and the cathode sleeve. Such a cathode .when utilized in an electrical space discharge device therefore introduces a minimum of hum into the operation thereof.

Of course it is to be understood that this invention-is not limited to the particular details of the arrangement as described above as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. A cathode for an electrical space discharge device comprising a cathode sleeve having an external electron emissive surface, a heater interiorly positioned within said sleeve, said heater comprising a pair of heater wires embedded side I by side in a unitary insulating covering and connected together at one end thereof, said pair of wires-with its insulating covering being bent back'on itself to form two parallel closely-adjacent heater sections, said pair of heater wires being connected to carry substantially equal heating currents therethrough in opposite directions.

2. A cathode for an electrical space discharge device comprising a cathode sleeve having an external electron emissive surface, a heater interiorly positioned within said sleeve, said heater comprising a pair of heater wires embedded side by side in a unitary insulating covering and connected together at one end thereof, said pair of wires with its insulating covering being bent back on itself to form a plurality of parallel closely-adjacent heater sections, said pair of heater wires being connected to carry substantially equal heating currents therethrough in opposite directions.

3. A cathode for an electrical space discharge device comprising a cathode sleeve having an external electron-emissive surface, a heater interiorly positioned within said sleeve, said heater comprising a pair of heater wires embedded side by side in a unitary insulating covering and connected together at one end thereof, said pair of wires with its insulating covering being bent back on itself to form two parallel closely-adiacent heater sections, the insulating covering of each section being adapted to contact with the interior of said sleeve along a limited part of its cross-sectional perimeter at one side thereof, the region of contact being substantially equidistant through said insulating covering to each of said heater wires, said pair of heater wires being connected to carry substantially equal heating currents therethrough in opposite directions.

4. The method of producing cathodes for electrical space discharge devices which comprises coating individually with an insulating material two wires, each of a length many times the length of wire to be incorporated into any one of said cathodes, placing said coated wires side by side and coating them with an additional insulating coating, subdividing said coated pair of wires into a plurality of shorter element pairs, each of which is of the length to beincorporated into one of said cathodes, bending each of said element pairs into a plurality'of parallel portions, and inserting it into a cathode sleeve.

5. The method of producing cathodes for electrical space discharge devices which comprises coating individually with an insulating material two wires, each of a lengthmany times the length of wire to be incorporated into any one of said cathodes, placing said coated wires side by side and coating them with an additional insulating coating, subdividing said coated pair of wires into a plurality of shorter element pairs, each of which is of the length to be incorporated into one of said cathodes, connecting the two wires of each of said element pairs together at one end thereof, and inserting said element pair into a cathode sleeve.

6. The method of producing cathodes for electrical space discharge devices which comprises coating individually with an insulating material two wires, each of a length many times the length of wire to be incorporated into any one of said cathodes, placing said coated wires side by side and coating them with an additional insulating coating, subdividing said coated'pair of wires into a plurality of shorter element pairs,

each of which is of the length to be incorporated I into one of said cathodes, connecting the two wires of each of said element pairs together at one end thereof, bending each of said element pairs into a plurality of parallel portions, and inserting it into a cathode sleeve.

PAUL W. STUTSMAN.

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