US2173498A - Hot cathode arrangement for a cathode ray tube - Google Patents

Description

Sept. 19, 1939.,

K. SCHLESINGER 2,173,498 HOT GATHODE ARRANGEMENT FOR A CATHODE RAY TUBE Filed Nov. 10, 1936 2 Sheets-Sheet I 4 5 2 r I f ,8 8

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HOT CATHODE ARRANGEMENT FOR A CATHODE RAY TUBE Filed Nov. 10, 1936 MQM 2 Sheets-Sheet 2 Patented Sept. 19, 1939 UNlTED STATES HOT CATHODE ARRANGEMENT FOR A OATHODE. RAY TUBE Kurt Schlesinger, Berlin, Germany, asSignOr to Radioaktiengesellschaft D. S. Loewe, Berlin- Steglitz, Germany Application November 10, 1936, Serial No. 110,043 In Germany November 16, 1935 10 Claims.

In electron-optical high-vacuum tubes, in which an electron-optical reproduction of the surface of the cathode is effected, there exists the requirement for an emissive surface of a defined form and size, which will provide an even density of the emission current from all points of its emissive area and does not change in the course of time. Moreover in the case of television tubes this density of the emission current must be susceptible of an intensity control without any appreciable effect on the form and size. The tube constructions known at the present time operate, as a rule, with an oxide surface, which is sunk into a metallic surface and is reproduced on the luminous screen by the electron-optical system mounted between said surface and said screen. Oxide spots of this nature do not always remain homogeneous as regards emission nor constant as regards form and the strength of emission. Further, the current density in the case of very bright television cathode ray tubes is much greater than usual in amplifying tube constructions. The sizes of the, emissive surfaces of these cathodes cannot be increased beyond a certain dimension which is prescribed by the optical system, and currents of approximately 1 mA. are frequently taken from a cathode of .5 mm. in diameter, i. e., a current density of 4 mA.: mm. and more.

The object of the invention is to provide a hot cathode arrangement, which is particularly suitable for electron-optical reproduction.

The invention will be described with reference to the accompanying drawings which are exemplary diagrams of arrangements according to the invention, and of which Fig. 1 shows a simple electrode system of a cathode ray tube together with some external sources connected thereto, embodying the invention;

Figs. 2 and 3 show modifications of the voltage supplying circuits for the cathode heating filament and for an auxiliary electrode,

Fig. 4 shows a particular structure of the cathode arrangement with a special discharge path incorporated therein for obtaining a rectified voltage for the mentioned auxiliary electrode,

Fig. 5 illustrates the light intensity control in a television tube designed according to the invention. 1

In the arrangement according to the invention, the optical object, i. e., the object of the optical reproduction, is represented by the aperture of a metallic screen. I in Fig. '1, they size of which corresponds to the requirements of the optical system, i. e., amounts forexample to 1/2 mm. Behind this surface I an emissive layer 2 is provided, the surface of which may be much larger than the object point I and which is situated at a small distance from I. A metallic body,

for example a nickel body 2, is heated to the temperature of emission, for example by a built-in filament 3, and has a boring 4 in its front surface, which boring contains the oxide. The spacing between i and the edge of 4 is of the order of 1 mm.

If a perforated cathode of this nature is set into operation, the emission which passes to the outside is, as a rule, much too weak for practical purposes. A suction anode 5 provided in front of the perforated diaphragm I is unable to extract sufiicient emission from the cathode 2,2, even if supplied by the source 6 with a positive potential relatively to the cathode as high as approximately 200500 volts. These conditions,

however, change upon application of an auxiliary battery 7, the potential of which requires to amount of only to a few volts positive, dependent on the spacing between I and 2. With a spacing of approximately 1 mm. between I and 2 a bias of +4 volts is sufiicient, whilst an additional Certainly this does not mean to say that the form and size of the image point, which, in an electron ray tube according to Fig. 1, is projected on the luminous screen 12 with the assistance of an electric lens 8/9, are likewise constant. While the size of the object point is certainly not altered by the variations in the space charge battery l, the angle of divergence of the rays leaving this point is nevertheless altered and in the case of excessively diverging bundles of rays the electron-optical lenses fail, as being unable to manage with an excessively large angle of aperture. According to a preferred feature of the invention, the surface of the perforated sheet I is so shaped that the combination of this lid with an anode 5 opposed thereto supplies a concentration field, which acts in the manner of a lens that is situated very close to the object and tends to concentrate the rays onto the point of the main lens, i. e., into the aperture of the main anode. A conical or spherical grind l3 solves this problem, affording a preliminary concentration which may be separately varied by varying the bias of 5 at the wiper tapping 24 without simultaneous variation in the sharpness of reproduction.

The radius of a spherical grind it is derived by rough calculation after entering the spacing 1' between cathode and anode and on the assumption that the -volt electrons arrive in the aperture i without any appreciable speed of their be uselessly dissipated in the resistances.

own, i. e., so that they exactly follow the concentration field of the anode 5. If 0 is the diameter of the ground surface I3, the height d of the edge thereof is found to be represented by the figure:

For a tube in which 0 is equal to 10 and 1' equal to 50 mm., the required value of d is found to be 1/2 mm., which means a comparatively fiat grind. The spacing between the accelerating electrode 5 and the aperture I is selected to be large in comparison with d. The aperture in the suction anode 5 is also selected to be large in relation to the cross section of the bundle of rays at this point-in the stated example 5 mm. Centering of the tube system is facilitated by a diaphragm of this nature which is situated remote from the cathode and has a large aperture, whilst the directional fields inclined in relation to the 'axis and resulting in the opposite case from defective centering'are stronger than the lens fields which are to be expected in anexactly centered optical system, so that astigmatism and coma are caused.

It is essential that the location of the first refractive power, viz., the condenser lens 5 /5, coincides with the location of the object i to be re produced. It is only under this condition that the adjustment of the width of the bundle of rays by the condenser does not vary the size of the image I2 projected by the main lens 8/8 of I. In the previous arrangements having a cathode surface which was arranged'at a certain distance behind the condensing lens, i. e., with direct reproduction of the surface 2, the adjustments of the condenser lens and the main lens were 'coupled with each other to a very considerable extent.

Tubeshaving a cathode free of space charges, as described, have the disadvantage that the currents traversing the circuit of the space charge battery "I are 'of a strength which is a multiple of the strength of the ray currents leaving the aperture 1. Upon operating the cathode ray tube with high anode potentials and upon the use .of potentiometers for obtaining the bias of 1. a considerable electric power would require to The urgent necessity accordingly arises to be able to take the powerful space charge current of the circuit 1 from a special potential source, so that the high-tension battery "fi/ID is very greatly relieved. According to a further feature of the invention, the space charge potential 7 is taken from the heating battery M, and if this is not a direct potential source but an alternating current generator the bias, in accordance with still a further feature of the invention, is derived from this generator by rectification, the rectification being performed .either by means of a separate external rectifier or preferably by means of a special hot-cathode discharge path within the tube itself and with its own built-in heating device.

In Fig. 2 there is shown, to begin with, the most simple case of a perforated cathode heated by direct current. The incandescent member heated by'a spiral filament 3, the filament 3 being so dimensioned that the heating voltage to be taken from the source M is of the amount neces sary for space charge elimination, for example 6 volts. The perforated plate I is simply connected in this case with the positive pole of the heating battery I 4, whilst the negative pole is filament 3 of the cathode ray tube.

linked up with the cathode surface 2 and serves as the zero point which all further potentials of the tube are referred to. The anode battery 5/! of the cathode ray tube is merely loaded by the weak ray current and accordingly operates with comparatively little power consumption.

Fig. 3 shows a perforated cathode with alternating current operation and external rectifier. A transformer winding I5 provides for the alternating current heating of the filament 3 with approximately 6 volts peak potential. By means of a detector I6 allowing the passage of current in the direction indicated a condenser I? of about 1 mf.-in the case of more than 500 mA emission the condenser I'I requires to have a higher capacityis charged positively to peak potential and its positive charge is communicated to the perforated diaphragm I. The crystal detectors employed in the radio art, for example the cuprous oxide detector Sirutor (passage of current up to .5 mA, permissible voltage up to 20 volts), are excellently suited to serve as the detector I6, but any other rectifier may of course be employed in place of a crystal detector.

Particularly convenient is the form of embodiment shown in Fig. 4, in which a small hot cathode discharge path is employed for rectification of the filament voltage, the hot cathode of which discharge path receives its temperature from the An insulating tube I9, which contains a spiral filament 3, is fitted on a cylindrical copper member I8. This cylindrical member heats simultaneously the emissive cathode 2 and an auxiliary cathode 20, which two cathodes are insulated against each other and have their leads passed out separately. Opposite to the auxiliary cathode 20 there is situated an auxiliary anode 2|, which is connected directly with the base I8. A cap I is fitted over the member I8 serving as a guidiing member, the plate terminating which cap acts as the perforated diaphragm and the space charge grid and may also be furnished with the aforesaid electron-optically effective grind I3, A cathode of this nature accordingly has in itself 5 poles, further the indicated connections between the auxiliary cathode 20 and the outer 'member I, and between the auxiliary anode 2I or the base member I8 and the filament 3. There results at the '1 mf-condenser IT a voltage by which the outer cylinder I is made positive in relationto the cathode, and which is of the amount of the filament peak voltage, ensuring the desired elimination of the space charge.

It is not absolutely necessary to employ the filament voltage in equal amount for the elimination of the space charge. Higher space charge potentials may for instance be obtained by employing additional windings at the heating transformer I5, or by theme of additional batteries or the like.

It is also possible to perform the light intensity control in the case of television tubes at the space charge electrode I. On account of the close proximity of this diaphragm to the hot cathode a light intensity control of this nature is extremely sensitive and is already successful with a control voltage the total amount of which is approximately 6 volts.

The light intensity control of a perforated cathode may also take place by means of a special control electrode according to Fig. 5. In this case the hot cathode 2 has a perforated diaphragm I opposed thereto, which is maintained at a constant potential relatively to the oathode, so that the aperture of l becomes the outlet point for generous emissions of low potential. A special control electrode 22 is situated at a distance of about 1 mm. in front of the space charge aperture I and possesses itself an aperture of approximately the same or somewhat larger diameter, so that it remains very nearly currentless even if its potential is sl ghtly postiive in relation to the cathode. The control diaphragm is coupled with the receiver 23 and effects the intensity control of space charge cathodes of this kind. A reproducing electronoptical system of the kind known per se, follows, in the direction away from the cathode, on an element of this nature. The first plate of this electron-optical system is provided with a higher potential than the two diaphragms l and 22, so that the emerging current is conducted immediately into the electron-optical system.

I claim:

1. In electric apparatus comprising a cathode ray tube including a picture receiving surface and an electron-optical system adapted to produce an electron image on said picture receiving surface: the combination comprising a hot cathode mounted in said tube more remote from said picture receiving surface than said electronoptical system, said cathode having an extensive emissive surface facing said electron-optical sysan apertured shield mounted in front of said emissive surface at a small distance therefrom, the aperture in said shield being centrally aligned with said emissive surface and substantially smaller than said emissive surface, a suction anode mounted in axial alignment with said shield at the side thereof remote from the oathode for causing said emissive surface to emit electrons through said aperture to adapt said aperture to form the object of the electronoptical reproduction to be efiected by said electron-optical system, the surface of said shield facing said suction anode being shaped to provide in combination with said suction anode a preliminary concentrating field for concentrating the electrons leaving said aperture into said electron-optical system, and means for maintaining said shield a few volts positive in relation to said cathode for eliminating space charges from the surroundings of said cathode.

2. In electric apparatus comprising a cathode ray tube including a picture receiving surface and an electron-optical system adapted to produce an electron image on said picture receiving surface: the combination comprising a hot cathode mounted in said tube more remote from said picture receiving surface than said electronoptical system, said cathode having an extensive emissive surface facing said electron-optical systern. an apertured shield mounted in front of said emissive surface at a small distance therefrom, the aperture in said shield being centrally aligned with said emissive surface and substantially smaller than said emissive surface, a suction anode mounted in axial alignment with said shield at the side thereof remote from the cathode for causing said emissive surface to emit electrons through said aperture to adapt said aperture to form the object of the electron-optical reproduction to be effected by said electronoptical system, the surface of said shield facin said suction anode forming part of a hollow cone to provide in combination with said suction anode a preliminary concentrating field for concentrating the electrons leaving said aperture into said electron-optical system, and means for maintaining said shield a few volts positive in relation to said cathode for eliminating space charges from the surroundings of said cathode.

3. In electric apparatus comprising a cathode ray tube including a picture receiving surface and an electron-optical system adapted to produce an electron image on said picture receiving' surface: the combination comprising a hot cathode mounted in said tube more remote from said picture receiving surface than said electronoptical system, said cathode having an extensive emissive surface facing said electron-optical system, an apertured shield mounted in front of said emissive surface at a small distance therefrom, the aperture in said shield being centrally aligned with said emissive surface and substantially smaller than said emissive surface, a suction anode mounted in axial alignment with said shield at the side thereof remote from the cathode for causing said emissive surface to emit electrons through said aperture to adapt said aperture to form the object of the electron-optical reproduction to be effected by said electron optical system, the surface of said shield facing said suction anode forming part of a hollow sphere to provide in combination with said suction anode a preliminary concentrating field for concentrating the electrons leaving said aperture into said electron-optical system, and means for maintaining said shield a few volts positive in relation to said cathode for eliminating space charges from the surroundings of said cathode.

4. In electric apparatus comprising a cathode ray tube including a picture receiving surface and an electron-optical system adapted to p-roduce an electron image on said picture receiving surface, said electron-optical system including an anode member: the combination comprising a hot cathode mounted in said tube more remote from said picture receiving surface than said electron-optical system, said cathode having an extensive emissive surface facing said electronoptical system, an apertured shield mounted in front of said emissive surface at a small distance therefrom, the aperture in said shield being centrally aligned with said emissive surface and substantially smaller than said emissive surface, a suction anode mounted in axial alignment with said shield at the side thereof remote from the cathode for causing said emissive surface to emit electrons through said aperture to adapt said aperture to form the object of the electron-optical reproduction to be effected by said electronoptical system, the surface of said shield facing said suction anode forming part of a hollow 'sphere to provide in combination with said suc tion anode a preliminary concentrating field for concentrating the electrons leaving said aperture into said electron-optical system, the thicknessof said shield at its outermost edge having a value d which is at least approximately determined by the formula O =4dr wherein O is the radius of said shield and 1' the distance between said shield and said anode member, and means for maintaining said shield a few volts positive in relation to said cathode for eliminating space charges from the surroundings of said cathode.

5. In electric apparatus comprising a cathode ray tube including a picture receiving surface and an electron-optical system adapted to produce an electron image on said picture receiving surface: the combination comprising a hot cathode mounted in said tube more remote from said picture receiving surface than said electron-optical system, said cathode having an extensive emissive surface facing said electron-optical system, an apertured shield mounted in front of said emissive surface at a small distance therefrom, the aperture in said shield being centrally aligned with said emissive surface and substantially smallerthan said emissive surface, a suction anode mounted in axial alignment with said shield at the side thereof remote from the oathode for causing said emissive surface to emit electrons through said aperture to adapt said aperture to form the object of the electron-optical reproduction to be effected by said electron-optical system, the surface of said shield facing said suction anode being shaped to provide in combination with said suction anode a preliminary concentrating field for concentrating the electrons leaving said aperture into said electronoptical system, means for maintaining said shield a few volts positive in relation to said cathode for eliminating space charges from the surroundings of said cathode, means for applying to said suction anode an electron accelerating potential, and means for varying said accelerating potential to adjust the strength of said preliminary concentrating field.

6. In electric apparatus comprising a cathode ray tube including a picture receiving surface and an electron-optical system adapted to produce an electron image on said picture receiving surface: the combination comprising a hot cathode mounted in said tube more remote from said picture receiving surface than said electronoptical system, said cathode having an extensive emissive surface facing said electron-optical system, an apertured shield mounted in front of said emissive surface ata small distance therefrom, the aperture in said shield being centrally aligned with said emissive surface and substantially smaller than said emissive surface, a suction anode mounted in axial alignment with said shield at the side thereof remote from the cathode for causing said emissive surface to emit electrons through said aperture to adapt said aperture to form the object of the electron-optical reproduction to be effected by said electron-optical system, the surface of said shield facing away from said cathode being shaped to provide a preliminary concentrating field which in itself would concentrate the electrons leaving said aperture into a point disposed between said shield and said electron-optical system, means for maintaining said shield a few volts positive in relation to said cathode for eliminating space charges from the surroundings of said cathode, means for applying to said suction anode an electron-accelerating potential, and means for varying said accelerating potential to weaken said preliminary concentrating field so that said field concentrates the electrons leaving said aperture into said electronoptical system.

'7. In electric apparatus comprising a cathode ray tube including a picture receiving surface and an electron-optical system adapted to produce an electron image on said picture receiving surface: the combination comprising a hot cathode mounted in said tube more remote from said picture receiving surface than said electron-optical system, said cathode having an extensive emissive surface facing said electron-optical system, an apertured shield mounted in front of said emissive surface at a small distance therefrom, the aperture in said shield being centrally aligned with said emissive surface and substantially smaller than said emissive surface, means for causing said emissive surface to emit electrons through said aperture to adapt said aperture to form the object of the electron-optical reproduction to be effected by said electron-optical system, an alternating current source for supplying heating current to said cathode, auxiliary electrodes mounted inside said cathode ray tube, said auxiliary electrodes comprising a further hot cathode to form an auxiliary dicharge path energized by said alternating current source to produce a direct current voltage of a few volts and means for connecting the negative pole of said direct current voltage to said cathode and the positive pole of said direct current voltage to said shield for eliminating space charges from the surroundings of the first said cathode.

8'. The invention set forth in claim 7, wherein one single heating member is provided for heating both said hot cathodes.

9. In electric apparatus comprising a cathode ray tube including a picture receiving surface and an electron-optical system adapted to produce an electron image on said picture receiving surface: the combination comprising a hot cathode mounted in said tube more remote from said picture receiving surface than said electron-optical system, said cathode having an extensive emissive surface facing said electron-optical system, an apertured shield mounted in front of said emissive surface at a small distance therefrom, the aperture in said shieldbeing centrally aligned with said emissive surface and substantially smaller than said emissive surface, means for causing said emissive surface to emit electrons through said aperture to adapt said aperture to form the object of the electron-optical reproduction to be effected by said electron-optical system, means for maintaining said shield a few volts positive in relation to said cathode for eliminating space charges from the surroundings of said cathode and means for applying shading control potentials to said shield.

10. In electron apparatus comprising a cathode ray tube including a picture receiving surface and an electron-optical system adapted to produce an electron image on said picture receiving surface: the combination comprising a hot cathode mounted in said tube more remote from said picture receiving surface than said electronoptical system, said cathode having an extensive emissive surface facing said electron-optical system, an apertured shield mounted in front of said emissive surface at a small distance therefrom, the aperture in said shield being centrally aligned with said emissive surface and substantially smaller than said emissive surface, means for causing said emissive surface to emit electrons through said aperture to adapt said aperture to form the object of the electron-optical reproduction to be effected by said electron-optical system, a shading control electrode mounted in axial alignment with said shield at the side thereof remote from said cathode, means for maintaining said shield a few volts positive in relation to said cathode for eliminating space charges from the surroundings of said cathode, and means for applying shading control potentials to said shading control electrode.

KURT SCI-ILESINGER.

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