US2206491A - Thermionic valve - Google Patents

Description

July 2, 1940. 6, w. WARREN 2,206,491

THERMIONIC VALVE Filed Feb. 15. 1939 Inventor GEOFFREY W- WARREN Patented July 2, 1940 UNITED J srrA-rss THERE/[IONIC VALVE Geoffrey William Warren, Eastcote, Pinner,

England, assignor to M-O Valve Company Limited, London, England Application February 15, 1939, Serial No. 256,482

' In Great Britain February 16, 1938 1 2 7 Claims.

This invention relates to thermionic values of the type comprising within an evacuated envelope and along the same discharge path in the order stated a cathode, a control grid, an auxiliary electrode, an accelerating ezectrode and an anode, the gaps in the said auxiliary and accelerating electrodes being similar and alined one with the other so that'when the said auxiliary electrode is maintained at a potential negative (or possibly slightly positive) to the cathode, and the accelerating electrode is positive to the oathode, the auxiliary electrode drives the electrons through the gaps in the accelerating electrode and prevents them from striking the accelerating electrode. The auxiliary and accelerating electrodes together form a screen between the control grid and the anode.

The object of my invention is to produce valves of this type having improved operating characteristics and in particular to prevent secondary electrons from the anode returning to the accelerating electrode when the anode is negative to the accelerating electrode.

In some known valves secondary emission from the anode has been suppressed by means of an additional electrode or electrodes maintained at negative or zero potentials with respect to the cathode and situated between the accelerating electrode and the anode either within or adjacent to the discharge path. The eflc'ect of such additional electrodes is to produce a potential minimum in the discharge path between the accelerating electrode and the anode and my invention consists broadly in so designing the auxiliary electrode and arranging the accelerating electrode and anode relative to. it that a potential minimum is obtained in the discharge path between the accelerating electrode and anode without the use of any such additional electrodes.

In order that this result should be obtained it is necesary that the potential of the auxiliary electrode should influence substantially the field in the space between the accelerating electrode and the anode; that is to say, the accelerating electrode must not shield the said space effectively from the auxiliary electrode. This condition may be fulfilled by making the distance between the auxiliary and accelerating electrodes sufiiciently small compared with the distance be tween the accelerating electrode and anode; or by providing the auxiliary electrode with a part extending beyond the accelerating electrode, either in the direction of the anode or in a direction perpendicular to that in which the elecder 20 mm. in diameter.

trons mainly travel; or by combining these two features.

Valves made accordin'gto my'invention possess several advantages over the usual forms of tetrode and pentode valve. valves of anode impedance greater than one megohm may readily be obtained, while at the same time the current to the accelerating electrode may be less than three per cent and in some cases less than one per cent of the anode current.- It has; beenfound. that this latter feature results in a considerable reduction in the noise level compared with valves of the more usual types.

In order that my invention may be fully understood, I will now describe four embodiments of it by way of example making reference to the figures of the accompanyingdrawing in which Figures 1 and 2 are respectively a. perspective view and a cross section of the electrodes of one embodiment,

Figure 3 and Figure 4 are cross-sections of two other embodiments, and

Figure 5 is a longitudinal section of the fourth embodiment.

In all the figures, l is a thermionic cathode, 2 is a control electrode, 3 is the auxiliary electrode and is connected to the cathode l, 4 is the accelerating electrode and 5 is the anode.

' In Figure 1, the cathode .l is indirectly heated and has the form of a nickel tube 26 mm. long and 1.2 mm. diameter coated with a thin layer of alkaline earth oxides. The control electrode 2 is a helical grid of oval cross section having an internal minor axis of 2 mm. and wound on support wires 1; the anode 5 is a hollow nickel cylin- The auxiliary electrode 3 is a hollow hexagonal prism made from nickel sheet 0.1 mm. thick and is pierced by apertures 8, 2 mm. in width, in the opposite faces of the prism, which are parallel to the plane containing the grid support wires, the distance between these opposite faces being 3 mm. Additional screens, not shown, are placed at each end of the electrode system and connected electrically to the electrode 3, to reduce the electrostatic capacity between the anode 5 and control grid 2. The accelerating electrode 4 consists of four straight wires, 0.3 mm. in diameter, the axes of which lie at the intersections of two parallel planes, 2 mm. apart and perpendicular to the plane containing the grid support wires, with two parallel planes, 5 mm. apart and parallel to the plane containing the grid support wires; the

Thus, for example,

axes are symmetrically arranged with respect to the axis of the cathode.

In Figure 3 the auxiliary electrode is provided with fins 9 extending towards the anode. Otherwise the arrangement is the same as in Figures ciency is thereby obtained, particularly at very high frequencies, since the paths of the electrons through the control grid are straighter and the transit time is reduced.

In the foregoing ernbodiments,'-"the influence of the potential of the auxiliary electrode on the field in the space between the, accelerating electrode and the anode is due largely to the extension of the auxiliary electrode beyond the accelcrating electrode. In Figure 5 the auxiliary electrode 3, and accelerating electrode 4, are in the form of two helical grids the turns of which are alined; the distance between these electrodes is made so small that the aforesaid potential minimum exists'between the accelerating electrode and anode. For example the grids 3 and 4 may be wound with wire 0.1 mm. diameter to a pitch of 1.5 mm. and may be separated by a distance of less than 0.7 mm.

I claim:

1. A thermionic valve comprising within an evacuated envelope and along the same discharge path a cathode, a control grid, an auxiliary electrode, an accelerating electrode and an anode,

the gaps in the said auxiliary and accelerating ating electrode and anode.

2. A therminoic valve comprising within an evacuated envelope-a cathode in the form of a metal tube coated with electron emitting material, a helical control grid surrounding the cath- "ode, an auxiliary electrode in the form of a hol-

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