US2228980A

US2228980A - Electron discharge device

Info

Publication number: US2228980A
Application number: US300366A
Authority: US
Inventors: Steimel Karl; Scheel Joachim
Original assignee: Telefunken AG
Current assignee: Telefunken AG
Priority date: 1938-09-27
Filing date: 1939-10-20
Publication date: 1941-01-14
Anticipated expiration: 1958-01-14

Description

Jan. M N41; K. STEIMEL ETAL ELECTRON DISCHARGE DEVICE Filed Oct. 20, 1939 [I III III IN VEN TORS LR K E% M MC R m m SM T L fl C AA KO J Y B Patented Jan. 14, 1941 UNITED STATES PATENT OFFICE Berlin, Germany,

assignors to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H, Berlin, Germany, a corporation of Germany Application October 20, 1939, Serial No. 300,366 In Germany September 27, 1938 3 Claims.

The invention relates to an electric discharge tube of the pentode type having at least a thermionic cathode, three grid electrodes surrounding the cathode, and an anode. Tubes of this type 5 are so esigned that the innermost grid neXt to the cathode serves as control electrode, the fol lowing grid is the screen grid, and the third grid acts as a suppressor or retarding grid for suppressing the exchange of secondary electrons be- IO tween the anode and the screen grid. The screen grid provides a decoupling or screening between the anode and the control grid and assures a decrease of the capacity between the said two electrodes. Usually the grids are wire helices car- 15 ried on one or more support or grid side rods, and

the side rods are in alignment.

If such tubes are employed in circuits with variable amplification,thefield of the anode and screen grid through the control grid is often made nong9 uniform for the purpose of obtaining a flat curve of the plate current characteristic in reference to the control grid potential. The non-uniform field is brought about in general by varying the pitch of the grid helix, and it is customary to have the 25 largest pitch of the grid at the middle of the cathode and gradually to diminish the pitch towards the ends of the cathode. If the pitch of the control grid is non-uniform, often the screen grid is also non-uniform, and in certain cases the 30 retarding grid has a non-uniform pitch. If more than one grid is of non-uniform pitch, the arrangement is generally such that both the screen grid and the retarding grid have, like the control grid, the largest pitch at the middle of the 35 cathode. In many cases the screen grid cannot be as closely wound with uniform pitch as could be desirable for the shielding or capacitive decoupling between the control grid and the cathode. If the screen grid has a fine mesh, it absorbs much current, which causes a strong rustling noise in the tube. In order to reduce this noise as much as possible, the current distribution between the screen grid and the anode should be such that little current goes to the screen grid. This current dis- 4 tribution may be obtained by winding the screen grid with wide spacing; for instance, by choosing the pitch of the screen grid winding twice as large as that of the control grid, so that the thickness of the screen grid wire is approximately only one- 50 tenth of the pitch. In one example of construction the pitch of the control grid was 0.27 mm., the pitch of the screen grid 0.6 mm. and the thickness of the screen grid wire was 0.05 mm.

If the pitch of the control grid winding at the m middle of the cathode is wider than at the ends,

the pitch of the screen grid should be in the same proportion, otherwise, the middle of the screen grid would absorb an inadmissibly large amount of current. In one example of construction the pitch of the control grid which was 0.27 mm. 5 at the ends of the grid was increased in the center thereof to 6.5 mm; the screen grid was wound at the ends with a pitch of 0.6 mm. and in the center with a pitch of 1.0 mm. The change of the pitch was in approximately the same propor- 10 tion in both grids.

The retarding or suppressor grid cannot be expected to compensate for the poor shielding action or the large pitch part of the screen grid. The retarding grid should be constructed in the same manner at the place where the screen grid has the large pitch, since otherwise an excessively dense space charge would collect in front of the retarding grid; therefore if the retarding grid is of nonuniform pitch, the center must be of such a large pitch that the suppressor grid contributes little to the shielding, or ii of uniform pitch, it must be of this large pitch throughout its length.

The principal object of the invention is to provide a tube particularly of the suppressor grid type, which has desirable characteristics due to nonuniform pitch and also good shielding between the anode and control grid circuits.

In accordance with the invention the increase incapacity between the control grid and the anode 3 resulting from the use of wound grids of nonuniform pitch or of uniform but large pitch is compensated for or counteracted by interposing between the retarding grid and the anode at least one shielding electrode which extends across the grid rod sector of the electrode interspace in the electron shadow of the grid rods, where the inter electrode space is not filled with the discharge current.

The invention will best be understood by referonce to the following description taken in connection with the accompanying drawing which shows schematically examples of construction in accordance with the present invention, ancl'in which Figure 1 is a cross-section through one form of pentode with shields separate from the suppressor grid; Figure 2 is a cross-section of a tube with the shields attached to the suppressor grid and in effect constituting grid rods; and Figure 3 a longitudinal section of the tube shown in Figure 1.

Referring to the drawing, Figure 1 represents schematically a cross-section and Figure 3 a longitudinal section through an electrode system having a cathode l, a control grid 2, a screen grid 3, and

a suppressor or retarding grid 4. These grids are preferably fine wire helices, oval in cross-section, and each helix is fixed to two supports or side rods at the ends of the largest diameter of the helix. The grids are coaxial with and surround the cathode and are surrounded by an anode 5, which may be oval or circular in cross-section. The screen grid 3 is wound at least in part with a comparatively large pitch; The discharge current will be distributed in a non-uniform fashion over the circumference of the cathode and is concentrated to a certain extent by the oval shape of the grids and by the beam forming action of the grid side rods so that two electron beams are produced which extend in opposite directions from the cathode and perpendicular to the plane of the side rods of the grids.

In accordance with the invention two shield electrodes 6 are interposed between the anode 5 and the retarding grid 4. These shields, which are curved to match the curvature of the electrode system, are opposite the gaps or more open portions of the grids and substantially cover the gaps, as shown in Fig. 3, and are of such width that the shields extend across only the spacial sector which is defined by the electron shadows of the grid rods and which are between the sec tors occupied by the: two electron beams. The shields are given a constant potential, and may for instance, be connected to the cathode or to the retarding grid inside the tube. Since the shield electrodes are situated outside the discharge path, they have practically no influence upon the discharge. On the other hand, they cut off entirely the lines of force of the electrostatic field between the anode 5 and the control grid 2 in the special sectors covered by said shields. As a result, the capacity between the anode and the control grid will be reduced, and the detrimental increase in capacity coupling of the anode and control grid circuits due to the non-uniform winding of the screen grid is compensated for and practically eliminated.

Figure 2 shows a cross-section through an electrode system containing the same parts as the system shown in Figure 1. The shielding sheet metal electrodes 6 are connected in this case directly to the support rods of the retarding grid, and therefore, have the potential of the retarding grid. It is obvious that the support rods may be omitted, and the shielding electrodes alone may act as supports for the winding of the retarding grid.

A shield electrode in the electron-free space between the retarding grid and the anode can obviously be provided even where the electrode forms differ from those shown as examples of construction. If, for instance, the grids are each supported at one side only by a supporting rod, it is only at this side that the shielding electrode is needed. On the other hand, three or four electrodes may be distributed over the circumference where through an identical number of grid rods at each grid an equal number of discharge-free spaces are provided.

The arrangement of the shielding wall between the retarding grid and the anode is to be preferred over the arrangement in which the shields are arranged at the cathode side of the retarding grid, since at the aforesaid place the discharge will practically not be influenced at all while the shield inside the retarding grid could produce electronic space charges in certain cases.

The capacity between the anode and the control grid may be reduced if the anode is made of sheet metal segments which extend only across the sector in which the discharge flows, but this arrangement has some disadvantages, since on the one hand the shielding of the discharge space towards the outside is no longer as favorable and disturbances may readily occur due to exterior fields, and, on the other hand, the mechanical stability of such an anode composed of segmentlike metal sheets will be lower. It can also be said that in this case the structure of the system will be more complicated, since the type of anode which acts as a support for the entire system is not present.

We claim:

1. An electron discharge tube comprising a thermionic cathode, an anode, three successive grids between said cathode and anode, each of said grids having a side rod and said grids having their side rods in alignment between said cathode and said anode, the first grid next the cathode having a portion more open than the remainder of the grid, the second grid from the cathode having a portion more open than the remainder of said grid, said more open portion of said sec- 0nd grid being opposite the more open portion of said first grid, and a sheet metal shield electrode in the interspace between said second grid and said anode opposite the more open portion of said second grid and extending transversely of the plane of said grid rods and across only the spacial sector defined by the electron shadow of the grid rods.

2. An electron discharge tube comprising a thermionic cathode, an anode, three helical grids of different diameters and oval cross-section, each having two side rods at opposite ends of the major axis of said grid, said grids being mounted between said cathode and anode with their side rods in a common plane, the first and second grids from said cathode having a variable pitch in the same proportion and with the most open portions of said grids-opposite each other, and a sheet metal shield electrode between said second grid and said anode in the electron shadow of the side rods of said grids and extending transversely of the plane of said grid rods opposite the more open portion of said second grid and across only the spacial sectors defined by the electron shadows of said grid rods.

3. An electron discharge device comprising a thermionic cathode, an anode, a helically wound control grid next to the cathode having a portion of large pitch, a screen grid next said control grid and having opposite the large pitch portion of said control grid a portion of equally large pitch, a suppressor grid next to the anode and having opposite the large pitch portion of said screen grid a portion of equally large pitch, each of said grids having a side rod and said grids being mounted with their side rods in alignment, and a sheet metal shield electrode between said anode and said screen grid opposite the large pitch portion of said screen grid and outside the grid rods of the suppressor grid, said shield electrode being in the electron shadow of said grid rods and extending transversely of the plane of said rods across only the spacial sector defined by the electron shadows of said grid rods.

KARL STEIMEL. J OACHIM SCHEEL.