TELEFUNKEN CATHODE ARRANGEMENT AND CATHODE-RAY TUBE HAVING SUCH A CATHODE ARRANGEMENT:In a cathode-ray tube cathode arrangement having a cup-shaped control electrode provided with an aperture in its end wall, a cup-shaped cathode sleeve having an end face provided with emission material and arranged in the control electrode with its end face adjacent the aperture, and a disc of insulating material arranged within the control electrode to support the cathode sleeve therein, with the end face of the cathode sleeve spaced from the apertured wall of the control electrode by a suitable spacing means, the ceramic disc is provided with a plurality of wedge surfaces and associated recesses to permit the disc to be inserted into the control electrode past a plurality of projections formed on the inner wall of the control electrode and to then be turned so as to become removably wedged into place between the spacing means and the projections.
In cathode-ray tubes, particularly oscillograph tubes and television picture tubes, the cathode consists of a cup-shaped cathode sleeve of metal, in the interior of which there is provided an electrical heating element and on the outer end of which there is provided an emission material. The insulated mounting of such a cathode sleeve is generally effected by means of a ceramic disc which is secured in a cup-shaped control electrode (Wehnelt Cylinder). In the center of the end face, the control electrode has an aperture for the passage of electrons which is opposite the emission surface and separated therefrom by a short distance. The short distance between the emission surface and the control electrode aperture is usually maintained by a spacer having a shoulder and inserted in the control electrode, or by shoulders, for example in the form of an annular constriction provided in the control electrode, against which the insulating disc is pressed with the cathode sleeve secured in its central aperture. The pressing of the insulating disc against the shoulder determining the spacing is effected, as is known, by pressing in beads, bending tongues at an angle, twisting tongues or inserting resilient clips.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide a novel cathode arrangement of the aforesaid kind which renders simple and reliable holding of the cathode sleeve possible inside the control electrode and which permits replacement of the cathode sleeve inside the control electrode in a simple manner.
According to the invention, it is proposed that projections should be provided on the inner wall of the control electrode and that the insulating disc should have wedge surfaces which gradually vary the thickness thereof in the circumferential direction and which, on rotation of the insulating disc, cause a wedgelike locking thereof between the shoulder and the projections.
The mode of securing the insulating disc inside the control electrode according to the invention has the advantage that the cathode sleeve together with the insulating disc can be removed from the control electrode again at any time and replaced by another one. The replacement of the cathode is particularly important when a completely or partially manufactured electron tube proves unservicable and has to be reopened in order to remove the damage causing the disturbance. This damage may have occurred to the cathode itself or elsewhere, for example to the luminescent screen of a television picture tube. Since contact with air generally damages the formed cathode, the whole electrode system would have to be thrown away if the cathode could not be replaced. The possibility of replacing the cathode is therefore particularly desirable if the electrode system is extensive and expensive. A preferred use for the cathode arrangement according to the invention is therefore in television picture tubes and more particularly in color television picture tubes. The electrode systems of color television picture tubes consist of a plurality of, preferably three, electron-beam generating systems, and are relatively expensive, so that replacement of the three cathodes in the event of repair to the tube saves considerable expense because the remaining cathode system can be used again after insertion of new cathodes. Additional electrodes for producing convergence of the three electron beams (convergence electrode) are generally secured to the electrode systems for color television picture tubes, and these consist partially of expensive materials and would have to be thrown away with the electrode system if replacement of the three cathodes were impossible. The invention therefore leads to particular advantages in conjunction with color television picture tubes.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 to 5 show an example of a preferred embodiment of the invention in various views, or different parts thereof.
FIG. 1 shows a view from below of the cathode arrangement according to the invention;
FIG. 2 shows a cross section therethrough;
FIG. 3 shows a side view thereof;
FIG. 4 shows a side view partially in section and
FIG. 5 shows a portion of a cross section of the cathode arrangement according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The circular cylindrical control electrode 1, also called a Wehnelt Cylinder, consists in known manner of a cup-shaped structure, the end face 5 of which has a central aperture 12 for the passage of the beam. Opposite the aperture 12 for the passage of electrons, with short spacing, is the emission layer 11 of the cup-shaped cathode sleeve 2 in the interior of which, an electrical heating element is mounted with insulation in the usual manner. The mounting of the cathode sleeve 2 inside the cavity of the control electrode 1 is effected with the aid of an insulating disc 6 consisting preferably of ceramic. The securing of the cathode sleeve 2 to the insulating disc 6 is effected in any known manner, with precision, so that the emission surface 11 of the cathode sleeve 2 extends to a specific extent over the area 10 of the insulating disc. In order to set a defined spacing between the end face of the cathode sleeve coated with emission material 11 and the inner face of the end wall 5 of the control electrode 1, a spacing ring 4 is provided which is inserted in the control electrode 1 and against which there is pressed the insulating disc 6. Projections or shoulders, particularly an annular shoulder, could naturally be provided on the cylindrical inner wall of the control electrode 1 instead of the spacing ring 4, and the insulating disc 6 be pressed against them.
According to the invention, projections 3 which extend inwards and which consist of mechanically produced impressions in the material, are provided at the inner wall of the control electrode 1. These projections are preferably impressed by means of a semicircular punch so that the material is partially torn as illustrated in FIG. 5.
Furthermore, the insulating disc according to the invention has inclined wedge surfaces 8 at its circumference. In particular, two or three of these wedge surfaces are provided which extend over a portion of the circumference. The wedge surfaces are provided by the fact that the thickness of the insulating disc varies gradually in the region of the wedge surfaces. The individual portions of wedge surface at the circumference of the insulating disc, together with the opposite plane face of the insulating disc, does form wedges to a certain extent. The wedge surfaces are provided at the face 9 in the insulating disc 6 remote from the emission surface 11. The face 10 of the insulating disc 6 adjacent to the emission surface 11 is plane, at least in the regions where the insulating disc rests on the shoulders or the distance ring 4, and is perpendicular to the longitudinal axis of the cathode sleeve 2.
The wedge surfaces according to the invention on the insulating disc may also be regarded as approximately sections of the surface of a screw thread. Associated with each wedge surface is a recess 7 in the insulating disc 6 starting at the circumference. The purpose of these recesses 7 is to enable the insulating disc to be inserted in the control electrode 1 which is effected in such a manner that the projections 3 on the control electrode slide through the recesses 7 during insertion. If, after the insertion of the insulating disc in the control electrode, this rests on a shoulder (not shown) or on the spacing ring 4, the ceramic disc is turned whereupon the wedge surfaces 8 engage below the projections 3 and as the turning continues, the insulating disc is firmly wedged between the spacing ring 4 and the projections 3. Each wedge surface in the insulating disc is associated with a projection on the control electrode. The arrangement of the wedge surfaces and of the projections is preferably effected so that they are distributed symmetrically round the circumference.
If the projections 3 extending inwards in the wall of the control electrode of the invention are formed in such a manner that the material tears partially as illustrated in FIG. 5, sharp edges are obtained on these projections 3 and, as a result of increased friction against the insulating disc, prevent this from being accidentally loosened. As a result of deliberate turning of the insulating disc 6 in the opposite direction, the insulating disc together with the cathode sleeve 2 can be loosened again and removed from the control electrode 1.