US2476961A - Electric discharge tube - Google Patents

Description

July 26, 1949. G. DIEMER ELECTRIC DISCHARGE TUBE Filed March 6, 1947 Patented July 26, 1949 Gesinus Diemer, Eindhoven, Netherlands, as-

signor to N. V. Philips, Gloeilampenfabrieken,

Eindhoven, Netherlands Application March 6, 1947, Serial No. 732,724 In the Netherlands April 8, 1946 1 Claim.

The invention relates to an electric discharge tube the electrode system of which comprises at least one cathode, one anode and two intermediate grids, the active sections of one grid being arranged behind those of the other grid, as viewed in a direction from cathode to anode.

One of the parameters of such discharge tubes is the anode-current-control-grid voltage characteristic curve (Ia-Vg-characteristic curve). In the case of a triode this varies, as is well-known, according to a 3/2 power curve, whereas with other tubes it differs more or less from the 3/2 power curve. However, the Ia-Vg-characteristic curve is to be looked upon for a comparatively small part, as substantially straight. Due to this, only a small portion of the negative grid voltage region can be used, if a given permissible distortion percentage may not be exceeded.

It has already been endeavoured to construct amplifying tubes so that the said distortion occurs to a smaller extent, which becomes manifest in the Ia-Vg-characteristic curve in such manner that this characteristic curve is substantially straight over a larger portion. It is known to use tubes for this purpose in which the control of the anode current is effected by deflection of an electron beam. Tubes of this kind are, however, comparatively complicated and generally require higher voltages for the optimum effect.

The said disadvantages now may be obviated by using a tube according to the present invention. As a matter of fact, many researches have revealed that it is possible to give a definite section of the Ia-Vg-characteristic curve a straighter form than the 3/2 power curve, and more particularly the section lying in the working range, in an electric discharge tube in which the cathode and the anode have in between them at least two grids, the active sections of one grid being located behind those of the other grid, as viewed in a direction normal from cathode towards the anode, if, according to the invention, the ratio between the spacing and the pitch of the two grids is at least 2, whilst the wire thickness of that of the two grids which is remotest from the cathode exceeds of the pitch of the said grids. The larger the pitch and the spacing of the grids, the thicker must be chosen the wires of the said grid.

In order that the invention may be more 2 clearly understood and readily carried into effect, it will now be explained more fully with reference to the accompanying drawing, in which Figs. 1; 2 show characteristic curves and Figs. 3 and 4 show grid arrangements of a tube according to the invention; and

Fig. 5 is a side elevational view of an electric discharge tube with a portion of the shell broken away to show the arrangement of the electrodes within the shell.

Referring to Fig. 1, l designates a static Ia- Vg-characteristic curve of a pentode varying approximately according to the 3 /2 power, the grids being arranged one behind another, whereas 2 designates a characteristic curve improved in accordance with the invention. The section ab of line 2 which corresponds with the working region of the tube is straighter than the corresponding section of line I. This is realized in that, as may be seen from Fig. 2, the screen-grid tends to absorb more current in this region (line 4), since the voltage variation of the first grid results in a displacement of the foci of the electron beams and a greater divergence of these beams, so that comparatively more electrons find their way to the screen-grid. In this rang the screen-grid current, consequently, increases to a greater degree than line 3 and the anode current to a smaller degree than according to the curve I of Fig. 1, which enables the section (1-1) to become substantially straight.

This effect is obtained if, as is shown in Fig. 3, the distance of grid 6 from grid I is at least twice the pitch .9 of the grids, whilst the wire thickness of the screen-grid 1 is at least /6 of the pitch. As is common practice in pentodes, the distance of the cathode 5 from the first grid 6 is preferably equalized with the pitch of this grid. The wire thickness of the first grid may have a customary value, for example, of about 60 microns.

It is known per se to arrange the grid wires of tetrodes or pentodes one behind the other. This was, however, always done to minimize the screen-grid current. However, in a tube according to the invention the arrangement is chosen to be such that the screen-grid absorbs more current at the negative control grid voltages in the working rang of the tube than with the usual arrangement, in which the grids are positioned one behind the other. The form of the electron paths is shown for different negative control grid voltages in Figs. 4A, B and C, of

which Fig. 4A shows the electron path to the left of point a in Fig. 1. In this case the flow of the electrons is concentrated to 'form very narrow beams, so that the screen-grid becomes substantially currentless and hence the lower part of line 2 in Fig. 1 will be substantially coincident with line I. Fig. 4B shows the electron path in' a the working range ab, comparatively many electrons finding theirway to the screen-grid,

with theiresulttthatethe.anodeccurrent. increases to a smaller. degree thanmormally; l I

Fig. 40 illustrates the electron path in the rangerto the right of point b of Fig. 1; in this.

case the screen-grid again absorbs comparatively little current, so that the static Ia-Vg charactenistic curve again tends to exhibit a.ste,eper course.

This need not afiect, however, the dynamiclae Vg-characteristic curve. In a given embodiment of an outputpentode according to the invention he pitch is 0.44 mm. as is also thedistanceofthe cathode from the first grid and the distance of the first grid from the second grid is equalto g twice the pitch s=0.88 mm. whilst the wire thick I to anode, being located behind' those of the first. grid, wherein the ratio between the spacing and} the pitch of the two grids is not less than 2, and

'thewire thickness. of that. of...the. s.econd is of the orderof%ifoffthe='pitch;f

' GESINUS DIEMER.

7 REFERENCES CITED 7 The following referenlces are of record in the file of this patent? 1 so. UNITED STATES PATENTS Number: V 7 Name Y V Date 7 2,214,808 Burnside Sept. 17, 1940 Zinke Oct. 8, 1940

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