US1921061A - Vacuum tube - Google Patents

Description

R. M. WISE Aug 8, 1933.

VACUUM TUBE Filed Aug. 18, 1932 Patented Aug. 8, 1933' VACUUM TUBE Roger M. Wise, Emporium, Pa. Application August 18, 1932. Serial No. 629,366

1 Claim.

This invention relates to thermionic devices and more particularly to vacuum tubes, such as are commonly used in radio receivers and like apparatus.

An object of the invention is to provide a novelly constructed device of this character which is highly eihcient in operation and which readily lends itself to economic commercial manufacture.

A further and more specific object of the invention is to provide in a vacuum tube a shielding electrode which is arranged to coact with an accelerating or screen grid to shield the plate and prevent electrostatic interaction thereof with the control grid.

A still further object of the invention is to provide a shielding grid'which in connected to the filament of the tube, and therefore, has the same potential as the filament, whereby flow of space current between the filament and shielding grid is prevented.

Another object of the invention is to provide in a vacuum tube an accelerating or screen grid and a shielding grid separated therefrom, the latter grid being connected to the filament of the tube and remaining at filament potential regardless of change in potential of the screen grid due, for instance, to radio frequency currents flowing in the screen grid circuit and which would otherwise operate to reduce the shielding affect of the shielding grid.

Other objects will appear hereinafter. The invention is illustrated in a preferred form on the accompanying drawing in which the single figure is an elevational view of the device partly in section for clarity of illustration.

Referring to the drawing, the device comprises the usual cup-like base 1 of insulating material having contact prongs 2 attached to the lower surface thereof, and the usual glass envelope 3 mounted upon the base and sealed thereto. The device also comprises the usual glass support 4 extending into envelope 3 from the lower end thereof and having the glass press portion 5 into which the conductors and supporting elements for the tube electrodes are inserted for supporting the same, the various lead-in conductors passing downward through the base and being connected to prongs 2 in the usual manner.

In the present device, the cathode 6 takes the form of a centrally located element. The control grid 7 is cylindrical in shape and is connected to the usual metallic cap 8 disposed on top of the glass envelope. The accelerating or screen grid 9 is also cylindrical in shape and surrounds the control grid. The accelerating grid is supported by and electrically connected to a metallic cup 10 in which is disposed an insulating disk 11. This disk serves to support the control grid 7 and, at the same time, insulates that grid from the accelerating grid. The plate 12 is also cylindrical in shape and surrounds theaccelerating grid. The construction of the tube thus far described is generally old.

In accordance with the present invention, 6 there is provided a cylindrical shielding grid 13 which surrounds the anode and has its ends extended beyond those of the anode. As disclosed herein, this element takes the form of a cylindrical mesh or screen body 14 having a metallic ring 15 inserted in one end thereof and attached thereto at the peripheral edge of the screen body. To this end, the peripheral edge of ring 15 is flanged, as at 16, and connected to the upper peripheral edge of screen 14. The centrally disposed aperture 17 in ring 15, has its edges substantially coincident with the upper peripheral edge of the cylindrical accelerating grid 9, but spaced therefrom for the purpose of insulating the elements from' each other. It will 0 be observed that the elements forming the shielding grid 13, together with the accelerating grid 9, substantially enclose the anode, thereby serving to substantially prevent electrostatic interaction between the control grid and 5 the anode.

The cathode support and lead wire is shown at 18. The screen or shielding grid is supported by the support wire 19. Support wire 19 is connected by wire 20 to wire 21 which is one of the lead wires to filament 22. Thus, the shielding grid is connected directly to the filament and has the same potential as the filament, independently of the cathode potential. Obviously with these two electrodes at the same potential, there is no flow of electrons between the filament and the shielding grid. As compared with the construction wherein the shielding grid is connected to the cathode, this arrangement presents the advantage that in most amplifying circuits there will be no flow of electrons between the filament and the cathode on one hand and the shielding grid on the other hand, as it is customary to supply a positive potential to the cathode with respect to the filament for the purpose of maintaining the grid more negative than the cathode.

This invention is not limited to the construction as shown in the drawing but may assume numerous other forms. In the device known as 1-10 the screen grid pentode, for instance, a spiral grid similar to the accelerating grid 9 may be inserted between grid 9 and plate 12. This grid is known as a suppressor grid and its function is well known in the art. It is most conveniently connected both electrically and mechanically to the metallic ring 15 and is usually extended so as to form, with ring 15 and screen 13, a substantially complete cage about plate 12. In this construction also, the above mentioned advantage is obtained, that no flow of electrons occurs between the filament or cathode and the shielding grid.

When the filament is energized by direct current, the connections from the energizing source should be such that the shielding grid is connected to the most negative part of the filament.

Substantially the full benefit of my invention is obtained when the shielding grid is connected to any point on the filament, and when an alternating current energizing source is used, I have found that it is sometimes desirable to connect the shielding grid to the midpoint of the filament to reduce hum.

It has heretofore been proposed to provide a shielding grid somewhat similar to that of the present device, which shielding grid has, however, been connected to the accelerating or screen grid and has the same potential as that grid. When such a device is operated with an accelcrating grid potential almost as great as the anode potential, its operation is sometimes det rimentally afiected by the attraction of electrons to the shielding grid from the anode, thereby reducing the anode or plate current. This action is largely due to the fact that the electrons have such a high velocity when they reach the plate that they are prone to pass the plate and impinge upon the shielding grid. The present device eliminates most of this deleterious action. since the shielding grid is at a widely different potential from that of the anode and is, in fact, at a substantially more negative potential than the anode. Such action as that above described cannot, therefore, take place in this device, to any substantial extent.

A vacuum tube constructed in accordance with the teaching of the present invention has an inter-electrode capacity between the control grid and the anode of approximately .025 micromicrofarads which is quite as low as that of prior art devices, where the shielding grid is connected to the accelerating grid. It will be seen then that the present device is a distinct improvement in the art and that it functions efficiently to attain the desired end. Since it involves little variation in mechanical construction from prior devices, it may be manufactured as economically as such devices.

While there is disclosed herein a single preferred embodiment of the invention, for the purpose of illustration, various changes and modifications within the spirit and scope of the invention may be made and are contemplated.

I claim:

In a thermionic device a cathode, a filament independent of said cathode and adapted to heat the cathode, an anode, a control grid, and an accelerating grid, said grids being interposed between said cathode and said anode with said accelerating grid adjacent said anode, and shielding means at least partially surrounding said anode and connected to said filament whereby flow of space current between said filament and said shielding means is prevented.

ROGER M. WISE.

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