US2297429A - Cathode ray tube - Google Patents

Description

H. W. PAEHR CTHODE RAY TUBE Sept. 29, 1942.

lf'iled Jan. 26. 1940 H II Patented Sept. 29, 1942 CATHODE RAY TUBE Hans Werner Paehr, Berlin-Charlottenburg, Germany; vested in the Alien Property Custodian Application J anuary 26, 1940, Serial N0. 315.635 In Germany February 9, 1939 Claims.

The invention relates to cathode ray tubes and in particular to the electron gun arranged Whin a cathode ray taube.

It is an object of the invention to provide a cathoole ray tube having a cathode ray of small divergence and small cross section in the plane of the screen. It is a further object to utilize an electron gun produclng such a ray in a taube having a comparatively large screen and a short distance between the cathode and the screen so that a wide angle of cleflection must be used to scan the entire screen surface.

It is a further object to improve the electron gun systems hitherto in use with respect 130 control sensitivity and sharpness of the Spot. Other objects of the invention will be apparent from the following description in connection With the drawing representing in Fig. 1 a longitudinal section through the neck portlon 0f the cathode ray tube and in Fig. 2 a side view 01 a complete taube.

The most simple and in first approximation also true explanation of the electron optical arrangement 0f a cathode ray tu'be is that the emitting surface of the cathode is reproduced upon the screen of the tube by means of an electron lens. It has however been ascertained that the sharpest spot is obtained no1: as an image of the cathode surface laut as an image of a cross section of the cathocl ray immediately in front of the cathode ab a point where the cathode ray has its smallest diameter. 'Ihis cross section is usually called the cross-over point.

Up to the present filme the development 0f electron gun systems has been in the directiou to increase as far as possible the ratio between the emitting area of the cathocle and the area 0f the cross section of the cathocle ray in the cross-over point in order to reduce the specific emission load of the cathode surface and to obtain in spite thereof a high int-ensity in the cathocle ray. This development was in the direction to large angles of divergency.

According to the invention it has been founcl thao it is not preferable to use a ray With a large angle cf divergency but that it is better to use a cathode ray With a divergency as sma1l as possible.

The invention is of particular importance for television receiving tubes with intensity control by means of a variation of the potential of a Wehnelt cylinder and furthermore in connection with tubes having a large angle of defiection of 45 and more towards each side from the aXis of the tube. Such tubes have been suggested in order t0 obtain a short total length of the tube. Tubes of this type make use of inhomogeneous deflecing fields in order to obtain the necessary deflection upon a screen With a large radius of curvature. The consequence is that the luminescent spot is not sharply focused near the edges of the screen. It has however been found that this drawback can be overcome if a very narrow athode ray is used having a divergency which is so small that the inhomogenity -0f the deflecting field can be neglected within the space between the boundaries of the cathode ray.

Experiments about the requirements for the construction of a suitable electron gun have shown that an improvement of the directional efiect can be obtained -by increasing the ratio between the cut 01T voltage at the Wehnelt cylinder and the anode volta-ge. An increase of this ratio results however in a loss of control sensitivity. The invention points a way to improve the directional efiect and 170 obtain a cathode ray bundle 0f small divergency without a simultaneous loss of control sensitivity.

Aecording to the invention the Wehnelt cylinder includes a front portion which has at least partly the form of a cone o1 funnel opening towards the anode of the taube. It has been found that the inclination of the sides of the cone is very essential for obtaining the desired eflect. According to the invention the opening of the cone has an angle of approximately In Figs. 1 and 2 the cathode ray taube 6 has a neck portion l of cylindrical cross section. The neck portion has a foot l supporting a cathode 2 and a Wehnelt cylinder 3. The cathode is of the hairpin type carrying a small ba1l at the point of the bend. The Wehnelt cylinder consists of a cylindrical portion surrounding the cathode and a conical portion immediately in front of the cathode. The conical portion has a central aperture and opens towards the screen of the tube. The anode 4 has the form of a Wall coating ending shortly before the Wehnelt; cylinder. The concentrating fields is produced by a coi1 5 on the outsicle of the neck of the taube. systems are not represented in the drawing.

A system as described in connection with Fig. 1 produces a cathode ray of unexpected sma1l divergency and has the advantage that the divergency remains practically constant when the current is increased while hitherto an increase of the current has always resulted in an increase of the angle of divergency. The gun system of the invention produces therefore a spot of more uniform diameter in intensity controlled tubes.

The deflecting The experiments have shown that an angle of the cone of 90 and less, or of 160 and more has given much lass favourable results.

The opening Within the Wehnelt cylinder shall be as small as possible; a diameter of 1 mm. Will however be the lower limit for practical reasons. The height of the cone in the direction of ehe tube axis shall be at least three or better four times as lal'ge as the diameter of the sma1l aperture f the Wehnelt cylinder. Instead of a conical Wall portion it is possible to use a heavy plate having a conical depression on the side towards the anode. The Wehnelt cylinder shown in the drawing is produced, for example, by pressing or stamping.

It is particularly preferable 130 use this form of Wehnelt cylinder in combination with a cylindrical anode of large opening. This is contrary to usual systems in which the anode has the form of a flat disc with a small aperture. The inner diameter of the anode cylinder shall be at least larger than the outer diameter of the Wehnelt orcontrol electrode.

What I claim is:

1. A cathode-ray device adapted to provide an angle of ray deflection of the oraler of 45 or higher comprising a cathocle, a conical control electrode apertured at its apex and positioned with said apex acljacent said cathoole, and an anode comprising a hollow cylinder adjacent said conical electrode and unobstructed therefrom, the diameter of said cylindrical anode being greater than the outer diameter of said conical electrode whereby to provlde a cathode ray bundle of small divergency without simultaneous loss of control sensitivity.

2. A cathode-ray device adapted to provide an angle of ray deflection of the order of 45 or higher comprising a cathode, a control electrode having a cylindrical portion surrounding said cathode and a conical portion apertured at its apex and positioned. With said apex adjacent said cathode, and an anode comprising a hollow cylinder adjacent said conical electrode and unobstructed therefrom, the diameter of said cy1in drical anode being greater than the outer diameter of said conical electrode whereby to provide a cathode ray bundle 0f small divergency Without simultaneous loss of control sensitivity.

3. A cathode-ray device adapted to provide an angle of ray deflection of the order of 45 or higher comprising a cathode, a conical control electrode having an apex angle between and 160 and being apertured at its apex and positioned with said apex adjacent said cathode,

and an anode comprising an unobstrucced hollow cylinder, the diameter of said cylindrical anode being greater than the outer diameter of said conical electrode whereby to provide a oathode ray bundle of small divergency without simultaneous loss of control sensitivity.

4. A cathode-ray device adapted to provide an angle of ray deflection of the Order of 45 or higher comprising a cathode, a conical control electrode having an apex angle of approximately and being apertured at its apex and positioned With said apex adjacent said cathode, and an anode comprising an unobstructed hollow cylinder, the diameter of said cylindrical anode being greater than the unter diameter of said conical electrode whereby to provide a cathode ray bundle of small divergency without simultaneous loss of control sensitivity.

5. A cathode-ray device adaptecl to provide an angle of ray deflection of the order of 45 o1 higher comprising a cathode, a conical control electrode having an apex angle cf approximately 120 and being apertured at its apex and positioned with said apex adjacent said cathode, the depth of said conical electrode being ab least th.ree times the diameter of the aperture of said conical electrode, and. an anode comprising an unobstructed hollow cylinder, the diameter of said cylindrical anode being greater than the outer diameter 01 said conical electrode Whereby to provide a cathode ray bundle of small divergency without simultaneous loss of contro1 sensitivity.

HANS WERNER PAEHR.

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