1. Field of the Invention
The invention concerns an electron gun for cathode ray tubes of the type with focusing of the beam by magnetic lenses or electrostatic lenses having high resolution and high luminance.
2. Description of the Prior Art
A cathode ray tube (CRT) is the unit formed by a chamber under vacuum within which is placed an electron gun producing an electron beam that bombards a luminescent screen.
The electron gun has a set of electrodes and, if necessary, deflection plates. The source of electrons is conventionally formed by a cathode with alkali/earth oxides heated by a filament. The thermionic emission of electrons is stimulated by an electrical field created between the cathode and the acceleration electrode. A control or Wehnelt electrode, formed by a metallic cup-like piece surrounding the cathode and having a small-diameter hole in the axis of the tube, enables the modification of the electrical field and enables checking the intensity of the electrons emitted and, consequently, checking the beam current and, finally, checking the luminance of the screen trace.
In order to limit the divergence of the beam, conventionally an electron gun is used, such as the one shown in FIG. 1. This gun has an electron source electrode or emitting cathode 1, a Wehnelt electrode or control electrode 2, a first acceleration electrode or anode 3 and then an electrode 4 which, as the case may be, may be a final acceleration electrode or a focusing electrode.
Each electrode is subjected, in operation, to a bias voltage, respectively V1, V2, V3, V4 and, possibly, V5 if there is both a final acceleration electrode and a second focusing electrode available.
A pre-focusing of the electron beam is performed to obtain a beam with a small diameter throughout its length as well as a low inclination of the rays with respect to the axis of the beam. This pre-focusing is got by the effect of the pre-focusing lens formed by the electrodes 3 and 4. A main focusing lens is formed, as the case may be, by a magnetic lens placed outside the neck of the tube or by an electrostatic lens formed, for example, by the electrodes 4 and 5.
Bias voltages V3 and V4 are applied, in operation, in a voltage ratio of V4/V3 which, depending on the type of gun, may be high (about 15) or low (about 5 to 8). Bias voltages V5 and V4 are applied during operation, in a voltage ratio V5/V4 which is low (about 4 to 6).
The anode electrode 3 has a plane side or, preferably, a plane input disk 13, drilled at its center with a circular hole 17. The first focusing electrode 4 has a plane side or, preferably, a plane input disk 14, also drilled with a circular hole 18 at its center. The input disks 13 and 14 of the electrodes 3 and 4 are parallel to each other and are orthogonal to the axis of the electron beam 21 going through their center. The first focusing electrode 4 has a plane side or, preferably, a plane output disk 15, and the second focusing electrode 5 has a plane side or, preferably, a plane input disk 16. The two disks 15 and 16, each drilled with a circular hole, 20 and 19 respectively, at their center, are parallel to each other and orthogonal to the axis of the electron beam 21 going through their center.
The pre-focusing lens is therefore formed by the parallel disks 13 and 14 of the electrodes 3 and 4 provided with their holes. The second or main lens is formed by the parallel disks 15 and 16 of the electrodes 4 and 5 provided with their holes.
The voltage ratio V5/V4 may be low in comparision with the voltage ratio V4/V3. However, owing to its geometrical constitution, the power of the said second lens is sufficient to provide the main focusing. The input hole 18 of the electrode 4 is formed by a circular hole of small . diameter. This hole causes a curvature in the field lines and a narrowing of the beam at its level. The second lens or main lens forms the image of an object on the screen. The position and diameter of this image vary little with the throughput of the cathode, thus providing little variation in the dimension of the spot on he screen. The deflection of the electron beam is got by means of any standard deflector (not shown in FIG. 1). A control voltage V2 is applied to the Wehnelt electrode. For a certain value of V2, it is possible to cancel the cathode current. This value of V2 is called the cut-off voltage.
The cut-off voltage of the electron beam depends on the dimensions of the elements forming the gun, especially the distances between the Wehnelt-cathode electrodes and the Wehnelt-anode electrodes and the diameters of the holes of the input side of each of these electrodes.
However, in practice, this cut-off voltage never takes exactly the same value for each tube manufactured, owing to manufacturing tolerances. The variation in this cut-off voltage, between different tubes of one and the same model, makes it necessary to do a setting. This setting may be done on the voltage V2 of the control electrode. However, it is generally preferred to do this setting on the bias voltage V3 of the anode, for it is easier to obtain an adjustable supply for the bias voltage of the anode than for the voltage of the control electrode..
It so happens that for electron guns for high-performance CRTs, the adjusting of the cut-off voltage, through a setting of the bias voltage of the anode causes deterioration, either in resolution or in brilliance depending on whether the gun has a higher or lower cut-off voltage in the setting range. The variation in electro-optical characteristics from one tube to another, in the same manufactured batch, is thus quite large and may raise problems for high quality equipment, where the performance characteristics have to be very stable and uniform.