US2248557A - Cathode-ray tube system - Google Patents

Description

y K. SCHLESINGER 2,

CATHODE-RAY TUBE SYSTEM Filed June 21, 1933 mvm Patented July 8, 1941 UNITED STATES PAT EN T O FF ICE 1 I e v n 7 2,248,557

CATHODE-RAY TUBE SYSTEM Kurt Schlesinger, Berlin, Germany, assignor, by *mesne assignments, to Loewe Radio, Inc., a corporation of New York Application June 21, 1938, Serial No. 214,914

' In Ge'rmany July 24, 1937 -2 Claims.

pears desirable not torapply this high potential to the cathode ray system direct, but to connect the same only in the vicinity of the luminous screen. If the reverse method were adopted, the deflection of the electronic ray would be rendered much more diflicult, as the electrons already possess a very considerable stiffness when leaving the final anode of the system. Moreover the problem of insulation causes difiieulties in the design of the system. i I V The use of after-accelerationbetween anode and screen is very old. It was believed heretofore that it was necsesary to perform this afteracceleration in a comparativelyjlarge number of stages, i. e. to allow the sameto increas'e progressively. There have accordingly been set forth by other sources (Erich SchwartzzFFernstehen und Tonfilm, Television and Sound Films, 1935,'pages 37 to 40 and 47 to 49) after-acceleration systems, which were formed by resistance layers, and in which it was the object to attain a gradual increase in the speed of the electrons towards the luminousscreen. According to Knoll the same effect was tobe accomplished by a plurality of superimposed rings, between. which there were applied external resistances. The disadvantage of all of these arrangements is the strong detrimental generation of heat, 1. e. the great power dissipation of the resistance accelerators. Experiments conducted by the applicant have shown that agradual step-bystep acceleration of this kind is unnecessary. It is suflicient to apply the initial potential and the final potential each to an annular electrode, i. e. to design the system merely with two poles. Owing to the lack of any built-in resistances a tube of this kind remains perfectlycold in operation. The only difficulty occurring in these systems is the detrimental lens-effect of the bipolar system. This lens effect actsin distorting fashion on the form of the picture, iasthe electronic ray is'already deflected behind the system anode, the same leading to a barrel-shaped distortion of the form, :as' it increases towards the The applicant has found that it is possible with prescribed form of picedge of the picture,

ture to eliminate this lens efiect entirely if the amount of the part-potential of the input-system, i. e. the fraction of the total potential that ap l e t th a are ec r de-onheinput.

side, is selected to be sufficiently high. This intermediate value is critical and, asthe applicant hasfound, may not in the case of the dimensions set forth below be less than one-third of the screen potential.

As further difiiculty associated with the after- .acceleration it was found that if a dividing of the potential was performed between the anode potential of the system and the potential of the luminous screen, the internal resistance of the potentiometer caused considerable variations in the anode potential dependent on the intensity of the ray current. This will be dealt with at [a later point.

.The invention is illustrated by the appending drawingof which Fig. 1 shows the diagram of a tube according to the invention and circuits connected therewith, whilst Fig. 2 shows means, for supplying electrodes: of said tube with adapted voltages.

' In Fig. 1 there is shown by way of explana- .tion the assembly of a projection tube with division of the potential. In the most general, instancethe tube will possess in the vicinity of the luminous screen I an annular electrode 2,

which is connected with the point of maximum potential of the anode potential source.,3. The tube system is represented diagrammatically by the tube anode 4, the tubular member 5 which forms together with 4 a lens, the control electrode 6 and the cathode l. The cathode 115 connected with thenegative pole of the potential source 3, and the control electrode 6' with the receiving apparatus 8'. The deflecting elements,

' represented by the deflecting plates 9 and lflgare situated behind the anode l.

istics vary upon the deflection of the ray dely by the centre of the plate. If this lens field is sufficiently strong and the ray is sufliciently soft after leaving the'plates, considerable deformations of the rectangular image accordingly result on the screen.

As first remedy the applicant experimented with the provision of a rotational-symmetric annular electrode. The annular electrode is designated 12 in Fig. 1 and is raised normally to the potential of the anode 4. It is assembled coaxially to the tube and the screen electrode and comprises a plate 12a with central opening and an annular fitting 21). The openings in the two are in the ratio 1:2, and the height of the cylindrical fitting is such that the field, regarded from the screen, is cylindrical-symmetrical, so that the geometric form of the deflecting ele- 'ments no longer affects the disposal of the field after leaving the annular element. As shown in practice, a height of the cylinder amounting to one-quarter of the diameter of the cylinder is sufficient for this purpose, so that the height of the cylinder, therefore, is approximately equal to the radius of the aperture in the cover l2a.

If there is employed a system terminating means [2 of this kind in conjunction with a cylindrical wall coating 2, there is rendered possible in the first place the use of deflecting elements of desired form, for example plates or iron plate boxes, for mixed electrical and magnetic deflection, without interference with the circular form of the image point by the after-acceleration.

There now occurs, however, the following problem: If the component potential ea is selected to be very low, there is certainly obtained a very great sensitivity of deflection, but a considerable distortion of the picture form. This usually makes itself noticeable as a barrel-shaped distortion, the four corners of the previously rectangular picture being forced towards the centre. The reason for this barrel-like distortion in the case of an anode part-potential which is too small is the increasing concentrating effect of the after-acceleration lens system comprising the electrodes l2 and 2 at the edge of the crosssection of the pyramid of deflection. If vice versa the anode potentiometer is displaced in the sense of increasing system anode potential, sensitivity of deflection is lost, the requirement for deflection power is very considerably increased, and the tube system is overloaded by excessive suctional or anode potentials and correspondingly increased biasses and also by insulation stress. The applicant has found that there is a most favourable value for the component potential of the anode 4 at which, with prescribed size of the picture, the error in the shape, i. e. the barrellike distortion, just disappears. Excess beyond this value upwards is detrimental from the point of view of power for the reasons stated, whilst excess beyond this value downwards is prohibited by the distortion of the picture then occurring. A condition relative to the fact that an optimum adjustment of this nature exists at all is that the field behind the deflection is rotational-symmetric. The potentiometer value found is located almost exactly at one-third of the total potential 3, a

deviation downwards, for the reasons stated, be

ing more critical than a deviation upwards. According to the invention, the bias value of the control electrode 6, upon these adjustments, is

always readjusted to constant current, whereby tensity the ray current increases the existing potentials in respect of the no-load conditions i=0 (black in the image) are not maintained, the potential values being shifted to a greater or smaller extent. It is clear that the absolute value of the resistance I4 is limited in the downward direction by the efficiency of the rectifier of the power supply apparatus supplying the anode potential 3. For example, with 20,000 volts it is hardly possible to pass a current of more than a few milliamperes through the potentiometer, as this current in any case is uselessly consumed and potentiometers for these high potentials become more and more ungainly and difficult to build with increasing output. A bridging of the potentiometer with condensers is also useless, as in television operation an enduring stationary light intensity of the image must always be reckoned with, in which connection a signal duration can always occur, which is greater than the time constant capable of being attained by the condensers.

The applicant, therefore, in the connection system shown in Fig. 2, has tried a circuit, which produces with a minimum of internal resistance the described critical ratio between main potential and component potential and at the same time dispenses entirely with a potentiometer with its trouble-some shunt current requirement. The special connection system in Fig. 2 is similar in its design to the known potential-doubling system consisting of a transformer 15 and two rectifying tubes 16 and I! connected to part-windings of the secondary, and differs from the same only insofar as the first tube utilises less than a half of the complete number of turns of the secondary. The condenser [3a is charged, for example, to 5,000 volts, and the condenser Ma. to 10,000 volts, so that the ratio of 1:3 in respect of the anode part-potential is supplied without variation. A special potentiometer is accordingly unnecessary, and the screen electrode 2 can be connected with the tube I'l directed by the anode 4 of the system being connected with the lower-voltage tube It. The size of the condensers Ba and Ma. depends on the degree of steadying and the energy requirement of the connected tube circuits. In the case of a good-projection tube, in the ideal instance, the current to all intermediate electrodes should disappear, and the total current entering the cathode I should reappear at the screen or the screen ring 2. This ideal requirement is practically never capable of being attained with tubes having a high ray power of this nature. It can be accomplished, however, that the maximum current loss amounting to approximately 50% leaks 01f practically entirely to the intermediate anode 4, and not previously to the elements: control electrode, tubular member, etc. It is accordingly to be expected that the ratio of the outputs of screen circuit and anode circuit amounts to approximately 2:1, and the condenser I30. will accordingly be made at least twice as large as the condenser [4a. In practice values of .1-.2 mf. for the condenser I31: and .05 mi. for the condenser Ma have been found to be fully adequate upon connection of a projection tube with a ray power of 30-40 w. The tube concerned had a cathode current of 2 milliamperes, of which approximately one-half reached the screen, and operated at 20 kilovolts screen potential with a potential distribution in the stated ratio of 1:3, i. e. with an anode part-potential of 6,500 volts at the system anode 4.

It is clear that in the case of differently dimensioned system assemblies, i. e. in the case of apparatus.

other reciprocal conditions in the after-acceleration lens l2 and 2, other values may be necessary with respect to the component potential, which can be fulfilled simply by varying the tapping m at the transformer of the power supply The important advantage of this divided potential-doubling connection, viz. the constancy of the potential amounts owing to the small internal resistance of the circuits and the possibility of managing with merely one transformer for the tube, is maintained also in the cas of these variations.

It is to be observed that the edges of both the system cover I22) and the wall coating 2 must be completely rounded off by bending over or by the inclusion of circular wires in order to avoid the occurrence of corona discharges under the action of the considerable difierence in potential between 2 and I2.

I claim:

1. In cathode ray apparatus, a cathode ray tube having included therein an electron emitting cathode, a first accelerating electrode, a target element, a second accelerating electrode closely adjacent the target, and electron beam deflecting means intermediate the accelerating electrodes, and auxiliary electrode means electrically connected with the first accelerating electrode and positioned intermediate the deflecting means and the second accelerating electrode, a

source of energizing voltage for energizing said electrodes comprising a source of alternating current, a transformer having primary and secondary windings, said source of alternating current being connected across the primary winding, ,1.

denser connected between the first and second accelerating electrodes and a second condenser connected between the first accelerating electrode and the cathode, said second condenser being of the order of at least twice the capacity of the first condenser, whereby a ratio of voltage b tween the auxiliary electrode and the second accelerating electrode of the order of two times that between the cathode and the first accelerating electrode is obtained.

2. In cathode ray apparatus, a cathode ray tube having included therein an electron emitting cathode, a first accelerating electrode, a target element, a second accelerating electrode closely adjacent the target, and electron beam deflecting means intermediate the accelerating electrodes, and auxiliary electrode means electrically connected with the first accelerating electrode and positioned intermediate the deflecting means and the second accelerating electrode, a source of energizing voltage for energizing said electrodes comprising a source of alternating current, a transformer having primary and secondary windings, said source of alternating current being connected across the primary winding, a connection from one terminal of the secondary winding to the first accelerating electrode, a first diode rectifier having its anode connected to the other terminal of the secondary winding and its cathode connected to the second accelerating electrode, a second diode rectifier having its cathode connected to a point on the transformer secondary winding intermediate the terminals thereof and its anode connected to the cathode of the cathode ray tube, a connection from said cathode to a point of fixed potential, a first condenser connected between the first and second accelerating electrodes and a second condenser connected between the first accelerating electrode and the cathode, said second condenser being of the order of at least twice the capacity of the first condenser, whereby a ratio of voltage between the auxiliary electrode and the second accelerating electrode of the order of two times that between the cathode and the first accelerating electrode is obtained.

KURT SCHLESINGER.

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