US2113801A - Electron discharge device and circuits therefor - Google Patents

Description

April 12, 1938. I. SHOENBERG ET AL ELECTRON DISCHARGE DEVICE AND CIRCUITS THEREFOR Filed July 27, 1954 4 Sheets-Sheet 1 I57: 3 12". magi? 3 RODDQ April 12, 1938. SHOENBERG ET AL ELECTRON DISCHARGE DEVICE AND CIRCUITS THEREFOR 4 Sheets-Sheet 2 Filed July 27, 1934 s. 2912 .ve gi April 12, 1938. I. SHOENBERG ET AL ELECTRON DISCHARGE DEVICE AND CIRCUITS THEREFOR Filed July 27, 1954' 4 Sheets-Sheet 5 'I. 671 oenbe Pg 6'. 8.3044 8 .ROMH

April 12', 1938. l. SHOENBERG ET AL 2,113,801

7 ELECTRON DISCHARGE DEVICE AND CIRCUITS THEREFOR Filed July 27, 1934 4 Sheets-Sheet 4 Jam: 5730806813, 619502 Jana 3012', ofikiney Wadda Patented A r. 12, 1938 I UNITED STATES PATENT OFFICE ELECTRON DISCHARGE DEVICE AND CIR- CUITS THEREFOR Application July 27, 1934, Serial No. 737,192 In Great Britain August 2, 1933 6 Claims.

The present invention relates to electron discharge devices of the kind comprising a cathode, a control grid and an anode and, between the control grid and the anode, a screening electrode; and to circuits utilizing such devices.

It is known that when valves of this kind are operated with large anode voltage swings and with a, fixed positive voltage on the screening electrode, secondary emission tends to occur from the anode to the screening grid when, in the course of its oscillation, the anodevoltage falls more than a certain small amount below the fixed voltage of the screening electrode. For many purposes the flow of such secondary emis sion current from the anode to the screen is undesirable and proposals have been made to prevent it.

In one known arrangement a third grid, usually connected to the cathode, is arranged between the screening electrode and the anode.

According to the present invention, a discharge device of the kind specified comprises an auxiliary electrode located at least mainly clear of the path of the electron stream and adapted when maintained at a low potential, for example at cathode potential, to modify the electrostatic field in the space between the anode and the screening electrode in such a manner that, even in the absence of electron flow between the cathode and the anode, the rate of change of potential gradient from the anode to the screening electrode is made positive or has its otherwise positive value increased over substantially the Whole anode surface when the anode potential is lower than that of the screening electrode, and in such a manner that under operating conditions with the control grid at cathode potential and the screening electrode at a normal operating potential the anode voltageanode current characteristic exhibits no inflexion. The space charge itself also modifies the electrostatic field in the space between the anode and the screening electrode and this mod-' ification together with that produced by the auxiliary electrode has been found sufiicient, under operative conditions, to prevent the flow of secondary electrons.from the anode to the screening electrode, even though that due to space charge alone may be insuflicient.

Although in some forms of discharge devices according to the present invention the distortion of the electrostatic field produced by the auxiliary electrode may not be such as to produce an actual potential minimum in the space between the anode and the screen, in the absence take place.

(Cl. 2502'l) of electron flow, over the whole range of working voltages and corresponding working anode currents, nevertheless the combined effect of the means for modifying the electrostatic field and the space charge is always to produce a combined potential minimum throughout the whole range of working anode voltages and corresponding working anode currents over which secondary emission to the screening electrode might This is usually regarded as the range over which the anode voltage is less than the screen voltage.

One form of electron discharge device according to the present invention comprises a cathode surrounded by a control grid and a screening grid, the screening grid being surrounded in part by an anode and in part by an auxiliary electrode insulated from said anode, wherein said anode and said auxiliary. electrode are each in a plurality of sections, the sections of the anode being interspersed with the sections 20 I of the auxiliary electrode.

Various forms of discharge devices which can be made to exhibit the feature of the present invention will be described hereinafter and the theory which is at present considered to explain the observed phenomena will be set forth. It is to be understood, however, that this theory may require modification in the light of further knowledge.

The invention will be described with reference 30 to the accompanying drawings, in which- Figures 1, 6 and 7 are diagrammatic views in plan of certain forms of discharge devices according to the present invention,

Figures 2 to 5 are curve diagrams serving to explain certain characteristics of discharge devices according to the invention,

Figures 8, 9 and 12 are diagrammatic views in plan of further forms of devices according to the invention,

Figures 10 and 11 are diagrammatic views in elevation and plan, respectively, of a further form of the invention,

Figures 13, 14, 15 and 16 are views in side elevation, front elevation, sectional elevation and plan respectively of a preferred constructional embodiment of the invention,

Figure 17 is a perspective view of an anode of the device of Figures 13 to 16, Figures 18 and 19 are perspective views show- 0 ing methods of supporting anodes in arrangements according to the present invention,

Figures 20, 21, 22 and 23 are views in elevation, sectional plan, sectional elevation and underside plan respectively of a second preferred constructional embodimentof the invention, and

Figures 24 and 25 show circuit arrangements embodying discharge devices according to this invention.

Like parts are indicated in the various figures by the same references.

Referring to Figure ,1, there is shown diagrammatically the electrode arrangement of one form of device according to the present invention. A cathode a is of rectangular cross-section and is elongated (in a direction perpendice ular to the paper). Two control grids b1 and b: are arranged one on each side of the cathode a and are spaced therefrom by means of insulating strips it, one strip being arranged near one end of the cathode and another near the opposite end. Thus the strips k are located substantially outside the electron stream. Two screening grids c1 and 02 are arranged outside the control grid and are spaced therefrom by means of further insulators k. This assembly may be clamped together in any convenient way and mounted within a casing e which constitutes an auxiliary electrode. This auxiliary electrode has portions 1 extending towards but not to any great extent into the discharge space. Anodes di and dz are provided as shown. It will be assumed for the present that the anodes (Z1 and 112 are electrically connected together and that the two grids b1 and b2 on the one hand and c1 and c: on the other'hand are also connected together.

If the anodes d1, (is be maintained at a positive potential relatively to the cathode a. and if the screens 01 and 02 be maintained at a potential which is higher than the anode, the potential distribution in the space between each anode section di and d2 and the corresponding screen section 81 or 02 along lines normal to the screen and anode will be somewhat as representedin Figure 2, where voltages are plotted as ordinates against distances as abscissae. The position of the anode is represented by the line D and the position of the screen by the line C, the heights of these lines representing the potentials at the anode and screen respectively. The dotted curve shows the distribution of potential along lines between the anode and screen approximately normal to these electrodes in the absence of electron flow and without the auxiliary electrodes. In the case of infinite plane parallel anode and screen this curve go is a straight line. In other cases it is curved somewhat as shown. Owing to the action of the auxiliary electrode including the portions 1 thereof, it will be seen that the distribution of potential between the anode and screen represented by the curve g is such that is made more positive proceeding from the anode to the screen, where V is the potential at any point and x is the distance of this point from the anode. That is to say the rate of change of the potential gradient is more positive, the potential gradient being of course It will be seen also that the potential gradient in the neighbourhood ofthe anode is very small as shown in Figure 3 where the curve a of Figure 2 is shown dotted and the curve? shows the effect of a further drop in potential in the space. Such a further drop can of course be produced by space charge due to the primary electrons from the cathode and the secondary electrons from the anode. It has been found however that unless, in the absence of electron emission, the value of 1 is positive to a substantial degree, the space charge is incapable of forming the potential minimum at least in those cases which have so far been investigated, with normal working currents.

In Figure 2 there is shown in dotted lines the distribution of potential 91 when the screen indicated by C1 is closer to the anode. It will be seen that the potential gradient close to the anode is much higher than in the case above considered and consequently a greater additional drop in potential will be required to produce the desired potential minimum. This curve indicates the importance of arranging the anode at a sufficient distance from the screen taking into account the voltages and currents at which the electrodes are to work.

Figure 4 shows the distribution of potential when the anode and screen are at the same potential. Here also is positive and therefore a potential minimum is produced even in the absence of electron flow.

This' minimum, produced electro-statically when the anode voltage is equal to that of the screen, if it be of sufficient depth, prevents the flow of secondary electrons between the anode and screen in both directions under conditions in which the depression due to space charge alone, owing to high electron velocities, is insuflicient. When the anode voltage is much lower than that of the screen, the increase in space charge eflects due to reduction in electron velocities causes such an increase in the depression in potential that although there is now no electrostatic potential minimum, there is sufllcient total depression of the potential below that of the anode to prevent the flow of secondary electrons from the anode to the screen.

Along a line in a direction at right angles to the direction of the electron stream (that is parallel to the anode surfaces) the distribution of potential in the space between the anode and the sc'reen will of course be such that there is a maximum about midway along the line and low values at the two ends. The potential distribution in the space is thus in the form of a saddle.

The modification of the potential distribution in the anode-screen space produced by the auxiliary electrode has a focusing or concentrating eifect upon the electron stream in the screen to anode space. The result of this concentration of the electrons is that the space charge developed is considerably increased and the formation of a potential minimum by space charge is assisted.

- I Thus if secondary electrons are emitted bythe anode at a velocity of about 20 volts, for example, they may travel some distance towards the screening grid until they reach a position in which, if the space charge is sufliciently dense, they are brought to rest by the potentials arising from the space charge andthe electrostatic field. They then return to. the anode and give rise to more secondaries. The process may be repeated so that it may be expected that in the neighbourhood of the anode surface there willbe a very dense space charge of comparatively slowly moving electrons. Consequently any algebraic decrease of in the neighbourhood of the anode produced by modification of the electrostatic field will be accentuated by the space charge due to the primary electrons and also by the space charge due to the secondary electrons, and the electrostatic and space charge modifications are arranged together to give rise to a combined potential minimum. It is this combined potential minimum which is capable of preventing the flow of secondary electron current from the anode to the screening grid.

For most purposes a tetrode is required to have an anode voltage-anode current characteristic of the kind shown in Figure 5 wherein the anode voltage EA is shown plotted against anode currentIx for various values of the control grid voltage Eg. It is usually desirable that the knee m should be as sharp as possible and should occur at as low a voltage as possible, and that there should be no infiexion or kink, or at least no marked inflexion, in the characteristic. A kink such as appears at n in the curve Fig: 4 and to a less extent in the curve E9 -3 is not material because it is at a point away from any practical working point. However, it has been found possible to produce devices according to this invention in which no noticeable kink is present in any characteristic. A practical load line for a resistive load is shown at o and in the case of an inductive load the load line may have the form shown dotted at 01. In neither case do appreciably inflected portions fall within the working range of anode voltages and currents. It will be assumed that the type of characteristic shown in Figure 5 is desired (although it will be understood that for certain purposes other forms of characteristic may be preferred) and certain ways in which departures from the desired characteristic are obtained will be explained in order that the effects of the several variables may be appreciated.

If either the anode d1, (Z2 is too small in area, the auxiliary electrodes e are too close to the electron stream or the projections or slats 1 extend too far into the electron stream, the characteristic has a knee m of large radius of curvature and the value of anode voltage above which the current is nearly constant is not well defined and rather high. If the area of the anode is increased or if the auxiliary electrodes are arranged further away from the electron stream, the desired form of characteristic can be obtained. Further increase in area of the anode or in the distance of the auxiliary electrodes from the electron stream gives rise to a characteristic in which the knee m is very sharp and the anode current falls beyond'the knee and then rises again so that the curve exhibits an inflexion. This form of characteristic indicates that secondary emission is reaching the screening grid.

The way in which the devicewith what has been referred to as the desired characteristic is believed to operate will now be described.

The screening grid will be assumed to be held at some suitable positive potential relative to the a potential.

' whole area of the virtual cathode.

cathode, the auxiliary electrode will be assumed to be at cathode potential and the anode voltage will be regarded in the first place as at cathode Under these conditions the electrons projected towards the anode will come to rest before reaching the anode on account of the electrostatic field, the low anode potential and the space charge. If the anode voltage be now increased to a value equal to say one half the knee voltage, the space charge will still be suilicient to produce zero potential in the space between the anode and the screening grid. In this region of zero potential all electrons are brought to rest and the region may be regarded as a virtual cathode. The number of electrons flowing from the virtual cathode to the anode or to the screen can only travel forward and eventually strike the anode even though they are retarded by the potential minimum. The knee is sharp because, as theanode voltage is increased, the virtual cathode becomes a region of minimum, but not zero potential, substantially simultaneously over the If the size of the anode is decreased, or if the auxiliary electrodes are placed closer to the stream, the virtual cathode will persist at agreater anode voltage around the edges of the beam than in the centre, thus giving rise to a knee of larger radius of curvature. I

If the anode be unduly large, the secondary space charge consisting of electrons moving slowly in random directions can diffuse to the portions of the anode not struck by primary electrons or at least not struck by any considerable number of primary electrons. The space charge is in this case reduced in intensity and may be insuflicient to prevent the flow of secondary electrons to the screening grid. Even if the anode is not unduly large it is usually necessary to take steps to prevent the secondary electrons from difiusing around to the back of the anode with similar results to those produced by an unduly large anode. For this purpose the auxiliary electrodes already described are arranged to extend to points close to the edges of the anode. With anodes of small size it may be found unnecessary to extend the auxiliary electrodes so close to the anode.

Unless the anode is of considerably greater length than breadth it is usually desirable to provide the auxiliary electrodes, as already mentioned, close to both pairs of opposite edges. With long and narrow anodes, or Where less perfect screening from secondary emission is permissible, it may be suflicient to provide the auxiliary electrodes close to the longer edges of the anode only.

If it is required that the knee voltage should be reduced, it is necessary to reduce the primary space charge so that the virtual cathode formed thereby may collapse at a lower anode voltage. This can be done by increasing the screening grid voltage or by reducing the distance between the anode and the screening grid. The limit to which these two changes can be taken depends, in the case of the former, upon the ability of the primary space charge, with the assistance of the secondary space charge and the electrostatic comparison with the pitch of the screening grid. for example about four times or less, the electron field,- to maintain a sufilciently pronounced potential minimum to prevent the secondary electronsirom travelling to the'screening grid and, g

in the latter case, upon the fineness of structure or the screening grid. 7

It has been found that ii. the distance between the anode and the screening grid be too small in distribution over the surface of the anode is markedly non-uniform. The-eifect of this is that secondary electrons can return to the screeninggrid through those parts of the space which lie in i'rontof the screening grid wires because in these parts there is very little primary space charge and consequently only a small screening effect due to space charge.

' This eflect of the non-uniformity of the electron stream can be reduced or removed by dlvid-" ing the anode into a plurality of parts and prefer-' ably by inserting auxiliary electrode plates, which may be connected to the auxiliary electrodes already described between the two or more parts -of theanode.

So far it has been assumed that the auxiliary.

- ing from the cathode potential. An auxiliary electrode at cathode potential in a given position can often be replaced, with equivalent results, by an auxiliary electrode located closer to the electron stream and having a potential above that of the cathode why an auxiliary electrode located further from the electron stream and having a potential lower than that of the cathode.

Further it is not necessary that the auxiliary electrode potential should be maintained constant. In some cases the auxiliary electrode potential may be allowed to vary with the potential of the anode or control grid. v Thus .oneach of Figs. 24 and 25 there is shown diagram atically a valve having an electrode arrangement of the, kind shown in Fig. 1, the electrodes being disposed within an envelope 1'. In Fig. 24 it is arranged'that the auxiliary 'electrode potential varies with the control grid po-' tential, and in Fig. 25 the auxiliary electrodepotentlal is arrangedto vary with the anode potential. In'both these figures the control grids b1, b2 are maintained 'at a negative potential rela-' tive'ly to the cathode a by a source 30. Input electrical variations are applied to the control grids bl, be through transformer 31. The anodes (11, (in and screen grids c1, 02 are maintained at suitable positive potentials relatively to thecathode a by a, source 32. The screening grids are connected to the positive terminal of the source through a resistance 33 and to the negative terminal of the source through a condenser 34 in the usual way. The anodes d1, (12 are connected to the positive terminal of the source through a resistance 35. The auxiliary electrode e is maintained at cathode potential by a leak resistance 36. The output is taken at terminals 31.

In'Fig. 24 the auxiliary electrode e is coupled to the control grids b1, in through a condenser 38 so that the auxiliary electrode potential varies with the control grid potential. In Fig. 25 the auxiliary electrode e is coupled through a condenser as to the anode d1, at so that the auxiliary electrode potential varies with the anode potential.

stantially simultaneously over the greater part of the region as the anode potential is increased, 15

and i (2) The distance between the anode and the screening grid 'should not be less than about twice the pitch of the screening grid, that is to say the mesh of this grid or the equivalent structure 20 size.

Condition (1) is not essential since if it is not fulfilled the principal effect is to influence the shape of the knee, of the characteristic. The. eil'ect of this 'may only be that the amplificationf25 obtainable is not so free from distortion as with a sharper knee. A knee'which is slightly more rounded than it need be may be advantageous in some cases as it has been found possible to obtaina higher impedance above the knee voltage under these conditions.

Instead of allowing the auxiliary electrodes e to extend alongside of the grids and cathode as shown in Figure l, the auxiliary electrodes may be terminated just short of the screening grid, for example at the inwardly projecting slats l as shown in Figure 6., The electrostatic field in the space between the anode and screening grid is not aiIected by this'shortening of the auxiliary electrodes. Auxiliary electrodes 11'. preferably 4 electrically connected to the screening grids c1 and C2, maythen be provided around the grids and cathode. In the construction shown in Figure 6, the auxiliary electrode is extended around the back of the anode as shown at q.

'A modification of the arrangement of Figure l is shown in Figure 'l where the projections l are omittedand the auxiliary electrode e is shaped so asito, influence the electrostatic field in the anode-screen space inthe desired manner. As in 5Q Figure16, the auxiliary electrode is continuedbehind the anode at q and it also acts as a clamp for, thegrid and'cathode assembly. For this purpose theelectrode e is formed in two halves with flanges T which can be bolted or otherwise fixed 55 together, suitable insulating means k1 being provided between the electrode e and the grids c1 and C2.

Another construction according to the invention is showndiagrammatically in Figure 8. In 00 Figure 8, the elongated cathode a is surrounded by a control grid b which is in turn surrounded by a screening grid 0. Around the screening grid 0 is arranged an anode in two sections di and dz each of part cylindrical shape and between the 5 anode sections 11 and d: are arranged further part cylindrical electrodes'er and 62 constituting sections of the auxiliary electrode. The two grids b and c are of cylindrical shape and these two electrodes together with the anode and auxiliary 7 electrode preferably extend over substantially the whole length of the cathode a. In this case it will be seenthat the sections :2; and ez of the auxiliary electrode constitute continuations of the anode sections d1 and'dz and lie in the same cylindrical 7 surface as the anode sections, this surface being co-axial with the cathode a and the two grids b and c. The sections of the auxiliary electrode are connected electrically together and are either connected to the cathode, for example within the envelope 1 of the device, or else they are brought out to a suitable external terminal by which they can be maintained at a low potential usually not differing greatly from cathode potential.

The auxilary electrodes e1 and en serve to modify the electrostatic field in the anode-screen space in such a way that dx is sufficiently positive proceeding from the anode to the screen and when electron current is flowing between the cathode and the anode a space charge is set up in the space as already described. In the regions of the space opposite the rods 3 which support the grids, however, an electron shadow is cast and in these regions what may be termed the electron screen which serves to stop secondary electrons is absent or insufliciently dense. To overcome this difliculty there are provided shield members it which are so placed that they stop secondary electrons which might otherwise reach the screening grid 0.

The shields t may be of metal or of insulating material. In the former case they may be of solid metal or in the form of a metal coating on an insulating backing. The metal surface may be connected to the auxiliary electrodes c1 and ea or to some other point of relatively low potential or they may be left unconnected. In the case where insulating shields are used they may be of glass or mica for example and in operation each shield acquires a charge upon its surface suflicient to make its potential approximately equal to that of the cathode.

In a modification of the arrangement of Figure 8 shown in Figure 9, the anode and auxiliary electrode are still further subdivided into parts d1, d2, (13 etc. and e1, ez, ea respectively the former parts being interspersed with the latter.

In Figures 10 and 11 there is shown a modification of the arrangement of Figures 8 and 9, the subdivision being along the length of the cathode. Thus each anode section a1, a2 and each section of the auxiliary electrode e1, ez, c; is in the form of a short cylinder.

Figure 12 illustrates a modification of the arrangement of Figure 8 in which the shields t and the auxiliary electrodes e1, e: are combined and the whole electrode e1, e2, t may be regarded as the auxiliary electrode.

In the arrangements of Figures 1, 6, 8, 9 and 10, the auxiliary electrodes are themselves or have parts it (Figures 1 and 6) which form continuations of the anode.

In Figure 12 the auxiliary electrode although not constituting a continuation of the anode surface nevertheless has edges located close to edges of the anode so that the anode sections 111 and d2 together with the auxiliary electrodes form substantially a box-like structure extending around the grids and the cathode. It will be understood that the auxiliary electrode e of Figures 1, 6 and 7 may extend completely around the anode and grids, that is around the edges thereof parallel to the plane of the paper and this is usually desirable when the anode and grids are,of substantially square or circular shape. "When these electrodes have a much greater length (in the direction of the length of the cathode) than breadth, however, it is often unnecessary to arrange that the auxiliary electrode completely surrounds the grids and anode.

In discharge devices of the kind described in which the auxiliary electrodes extend close to the edges of the anode, it is often convenient to support the anode from the auxiliary electrodes with the aid of suitable insulators which may be of mica or ceramic material for example. Two ways in which this may be done are illustrated in Figures 18 and 19. Referring to Figure 18, the inner edges of the auxilary electrodes c1 and er are'slit in such a way that two tongues vi and 122 can be bent outwardly into a horizontal plane near the top of the two auxiliary electrodes. Two similar tongues are also bent out into a horizontal plane near to the bottom of the two auxiliary electrodes but this is not shown in the drawing. The anode d, in the form of a flat rectangular plate, has a rod 10 preferably of square section fixed along thecentre line. Two insulators in the form of rectangular bars of which the upper is shown at :c are each provided at their centres with a square hole and have notches cut in their ends. The rod 10 which projects from the upper and lower ends of the anode is inserted in the holes in the insulators a: and

.the insulators are slipped into position along the auxiliary electrodes, the inner edges of the auxiliary electrodes engaging in the notches in the ends of the insulators. The tongues v1 and 112 are then bent outwardly and around the insulators as shown at D2 to hold them in position.

In a modified construction shown in Figure 19 thin strips of insulating material Z1 and Z: are fixed to the anode d, for example with the aid of rivets, near to the top and bottom thereof. These strips are arranged to project outwardly beyond the vertical edges of the anode. The anode is placed in position upon tongues 01,112, m and v4 bent outward from the plane of the auxiliary electrodes as above described and these tongues are bent over the projecting ends of the strips as shown at m and v: to hold them in position.

Instead of, bending tongues outfrom the body of the shields, suitable tongues may of course be fixed to the shields, for example by welding.

The method of assembly above described can also be applied to the securing of a plurality of anode elements to one another in cases, such as have been described above, in which the anode is divided into a plurality of parts. When the anode elements are separated by an auxiliary electrode the adjacent sections of the auxiliary electrode can also be supported from one another by the use of insulators as above described.

' In a similar way any one electrode can be supported from any other electrode so long as'these electrodes have parts extending sufiiciently near together.

Referring now to the constructional embodiment of the invention shown in Figures 13 to 17, two control grids b1 and b2 and two screening grids c1 and 02 are mounted in suitably spaced relation from one another and from the cathode a by means of suitable insulators k. The two control grids are connected together and to a lead I passing through the top of the envelope 1. The grids are composed of thin wires extending between supporting rods and between the two upper and between the two lower wires of each grid are passed insulating strips 2 for example of mica. Two anodes :11 and d2 are fixed to the mica strips 2 by means of tongues 3 which project through so that together with the screening grid 0 they the mica and are bent over. Associated with each anode is an auxiliary electrode composed of a fiat portion 4 having a large rectangular aperture,

iliary electrodes are caused to bear through insulating strips it upon the grid and cathode assembly and the two auxiliary electrodes are then' united by metal cross straps l, which may be attached by spot welding, and the cathode and grid assembly is thus held firmly in position. Four metal members 8 are then spot welded to the auxiliary electrode and serve to prevent movement of the mica strips 2.

The shields 25 and 26 may if desired be extended completely envelop the control grid b excepting for apertures for the passage of the cathode and control grid leads.

The two anodes d1 and dz and the two control grids b1 and bz may be connected together or if desired they may be brought out to separate terminals so that the device can be used as a pushpull amplifier. The auxiliary electrode structure may be connected to the cathode within the envelope or if desired may be brought out to a separate terminal so that it can be maintained at any desired low potential. The potentialof the auxiliary electrode is usually fixed but as already stated need not necessarily be so as it may be caused to vary with the anode potential for example.

Referring to Figures 20-23, there is shown a constructional embodiment of the type of device shown in Figure 8. The electrodes are mounted between two discs of insulating material 9 and I0 for example of mica which have serrated edges adapted to bear against the inner walls of the envelope f. The anodes d1 and dz and auxiliary electrodes e1 and ez are mounted upon the mica discs 9 and II! with the aid of lugs l I which pass through holes in the discs and are bent over. Thus the electrodes d1, dz, e1, 62 serve to form a rigid structure with the discs 9 and Ill. The cathode a is located in apertures in the centres of the discs 9 and I0 and the grid supporting rods s also pass through holes in the discs 9 and Ill. The cathode is or? the indirectly heated type and the heater is brought out through leads l2 and I3, the cathode itself being connected to lead I 4. The anode parts di and dz are connected together by a wire l5 and to a lead IS. The auxiliary electrode parts e1 and ez are connected together and to shields t by a wire I! and to a lead l8 and also to the cathode a. The grids b and c are connected respectively to leads I9 and 20. The shields t are provided in the shadow cast by the rods s for the purpose described in connection with Figure 8. These shields are fixed to the mica discs 9 and I 0 with the aid of lugs 2|.

The anode sections d1 and dz may be provided as shown with slats or projections 22 in the form of small part annular members of sheet material. These slats are preferably arranged fairly near to the upper and lower end of each section di and dz. In one example they are distant from the ends of the sections d1 and dz by about one fifth the whole length of the sections. The purpose of these slats is to intercept electrons travelling oblquely in the anode-screen space which might in the absence of the slats reach the screen around the outside of the space charge area.

It has been found that in addition to the various factors influencing the behaviour of discharge devices according to this invention already mentioned, there are certain other points which should be taken into consideration. If the screening grid be arranged too close to the control grid or be too open, the control grid may produce a reduction in potential in the anode-screen space.

This reduction in potential enhances the eflect of the auxiliary electrode and must be taken into account when designing the device or else flrstly the knee of the characteristic may be of unduly large radius of curvature and secondly the impedance of the device above the knee voltage may be lowered. The former effect is believed to arise because the virtual cathode disappears at diiferent voltages at difierent points in the anodescreen space. The latter effect is believed to arise from the influence of the control grid in preoperate unsatisfactorily ii the grids are not aligned. The efiect produced is equivalent to that produced by the use of a screen which is too open.

We claim: 1. A circuit embodying an electron discharge I device having, within an evacuated envelope arranged in the order named, a cathode, a control grid, a screening grid and an anode, said anode extending over a part of the space around said cathode and having'also within said envelope an auxiliary electrode extending over a further part of said space, said circuit comprising means for maintaining said control grid at a negative potential relative to said cathode, means for applying electrical potential variations to said con trol grid, means for maintaining said anode and screening grid at positive potentials relative to said cathode, means for maintaining said auxiliary electrode at a potential in the neighbourhood of cathode potential, means for deriving output electrical variations from a circuit between said anode and said cathode and means for applying a varying electrical potential to said auxiliary electrode.

2. A circuit embodying an electron discharge device having, within an evacuated envelope arranged in the order named, a cathode, a control grid, a screening grid and an anode, said anode extending over a part of the space around said cathode and having also within said envelope an auxiliary electrode extending over a further part of said space, said circuit comprising means for maintaining said control grid at a negative potential relative to said cathode, means for applying electrical potential variations to said control grid, means for maintaining said anode and screening grid at positive potentials relative to said cathode, means for maintaining said auxiliary electrode at a potential in the neighbourhood of cathode potential, means for deriving output electrical variations from a circuit between said anode and said cathode and means for coupling said auxiliary electrode to said control maintaining said control grid at a negative po-' tential relative to said cathode, means for applying electrical potential variations to said control grid, means for maintaining said anode and screening grid at positive potentials relative to said cathode, means for maintaining said auxiliary electrode at a potential in the neighbourhood of cathode potential, means for deriving output electrical variations from a circuit between said anode and said cathode and means for coupling said auxiliary electrode to said anode whereby the potential of said auxiliary electrode is varied in dependence upon variations of the potential of said anode.

4. A circuit embodying an electron discharge device having, within an evacuated envelope arranged in the order named, a cathode, a control grid, a screening grid and an anode, an auxiliary electrode also arranged within said envelope but displaced from the direct electron path between cathode and anode, said circuit comprising means for maintaining said control grid at a negative potential relative to said cathode, means for applying electrical potential variations to said control grid, means for maintaining said anode and screening grid at positive potentials relative to said cathode, means for maintaining said auxiliary electrode at a potential in the neighbourhood of cathode potential, means for deriving output electrical variations from a circuit between said anode and said cathode and means for applying a varying electrical potential to said auxiliary electrode.

5. A circuit embodying an electron discharge device having, within an evacuated envelope arranged in the order named, a cathode, a control grid,-a screening grid and an anode, an auxiliary electrode also arranged within said envelope but displaced from the direct electron path between cathode and anode, said circuit comprising means for maintaining said control grid at a negative potential relative to said cathode, means for applying electrical potential variations to said control grid, means for maintaining said anode and screening grid at positive potentials relative to said cathode, means for maintaining said auxiliary electrode at a potential in the neighbourhood of cathode potential, means for deriving output electrical variations from a circuit between said anode and said cathode and means for coupling said auxiliary electrode to said control grid whereby the potential of said auxiliary electrode is varied in dependence upon variations of the potential of said control grid,

6. A circuit embodying an electron discharge device having, within an evacuated envelope arranged in the order named, a cathode, a control grid, a screening grid and an anode, an auxiliary electrode also arranged within said envelope but displaced from the direct electron path between cathode and anode, said circuit comprising means for maintaining said control grid at a negative potential relative to said cathode, means for applying electrical potential variations to said control grid, means for maintaining said anode and screening grid at positive potentials relative to said cathode, means for maintaining said auxiliary electrode at a potential in the neighbourhood of cathode potential, means for deriving output electrical variations from a circuit between .said anode and said cathode and means for coupling said auxiliary electrode to said anode whereby the potential of said auxiliary electrode is varied in dependence upon variations of the potential of said anode.

ISAAC SHOENBERG. CABOT SEA'I'ON BULL. SIDNEY RODDA.

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