Edward W. Herold's first patent, the type 48 power tetrode

RCA tube designer Edward W. Herold's first patent,

the type 48 power output tetrode :

 

 

Patented Oct. 22, 1935

UNITED STATES PATENT OFFICE

 

2,018,362

ELECTRON DISCHARGE TUBE

 

Edward W. Herold. Bloomfield. N. J. assignor to

Radio Corporation of America, a corporation of Delaware

Application September 9. 1932, Serial No. 632,285

3 Claims. (CI. 250-27.5)

This invention relates to a four electrode discharge tube of the screen grid type constructed to produce the results obtained with a pentode type of tube, and more particularly to a novel type or plate or anode structure which is substantially free from secondary electron emission so that the usual auxiliary means for suppressing this emission becomes unnecessary.

In the pentode type of tube there are three grids, the first or inner grid to which the signal is applied, a second or accelerator grid and a third or suppressor grid. In such a tube the electron flow for a given control grid voltage is largely governed by the accelerating voltage applied to the second grid. Not all or the electrons accelerated by the second grid flow to it, in fact, most of them pass thru the second grid and eventually are received by the anode. These electrons under the influence of the anode voltage produce an emission of secondary electrons. The suppressor grid lying between the second grid and the anode and in the ordinary output pentode, connected internally to the cathode prevents for the most part the flow of these secondary electrons to the accelerator or screen grid. The presence of this suppressor grid, however, impedes the flow out the primary electron current as well as the flow of the secondary electrons in the reverse direction. This effect, while present at all anode voltages, is very striking at anode voltages of the order of 100 volts or less. For this reason the ordinary pentode is an inefficient output tube for use at these lower voltages.

The present invention provides a structure which while suppressing the flow of secondary electrons does not appreciably impede the flow of the primary current. This results in a tube which is much more efficient than a pentode at low anode voltages.

An object of the present invention is to provide an anode such that the electrostatic field surrounding those portions of the anode on which the primary electrons impinge will be sufficiently low that secondary electrons very largely return to the anode. Another object of the invention is to provide an anode structure which in itself combines the functions of the anode and of the suppressor grid of a pentode.

Another object is to provide an anode structure which will inherently eliminate secondary emission therefrom, making unnecessary the use of a suppressor grid.

Another object is the provision or a structure offering little interference to a beam of electrons approaching it but strongly resisting a flow of secondary electrons emitted from it under the influence or the first beam.  

 

The invention will be better understood by having reference to the following specification and the accompanying drawing wherein :

Fig. 1 is a plan view, partly in section, of an anode structure according to the invention;

Fig. 2 is a cross sectional view taken on line II — II of Fig. 1;

Fig. 3 is a perspective view of the grille member detached from the main anode plate;

Fig. 4 shows a tube assembly using the anode of Figs. 1 and 2, and

Fig. 5 is a plan view of the tube illustrated in Figs. 4.

Referring now particularly to Figs. 1 and 2 of the drawing, the anode consists of a pair of similar trough-shaped plates 1 which at their ends are crimped to and supported by a pair of support rods 2. On each of the inner opposing faces of the plate members 1 is positioned a series of thin metallic slats 3 which may either form an integral part of the plate member or else may be placed closely adjacent and electrically connected thereto. These slats extend between the upper and lower edges of the plate members and are arranged in parallel relation and also with their planes parallel to the lines of electron flow from cathode to anode. As shown in Figs. 2 and 3 the slats or grille member may be fashioned from a continuous metal strip in the following manner: the terminal 4 of the strip, of a length equal to the width between slat sections, is shaped at right angles with respect to its adjacent slat section. The strip is then run back and forth in parallel relation to the extent of the length of the anode plate member with the desired spacing between slat sections.

When the proper number of slat sections have thus been formed the strip is carried over the top of the formed slat sections and welded or otherwise fastened to the top connecting pieces 5 between sections, then around to the bottom of the slat sections and welded or otherwise fastened to the bottom connecting pieces 6, with the strip terminating at 7.

The construction of the anode above described is such that the primary beam of electrons for the most part flows between the metallic slats and to the main body of the anode. That is, the electrons received by the anode are received into a series of boxes, the fronts of which are open. Since the back and the sides of the boxes are electrically of one piece of metal, the electrostatic field at the back of the box, that is at the anode surface proper, is very small and secondary electrons released from this surface are for the most part retained within the box and drawn back to the anode instead of being drawn to the other elements of the tube. At the same time this structure offers little increased resistance to the flow of the primary beam since the accelerating field is that due to the slats.

A further reduction in secondary emission is obtained by a blackening of the above anode structure by means of carbonizing or in any equivalent manner. There is thus provided a simple efficient structure which enables the tube to function as a pentode altho it has only two grids, and enables a tube to be constructed which for a low anode voltage, is more efficient than a pentode.

In some cases it may be desirable instead of slats to use wires projecting towards the cathode or to use any other equivalent means producing the same or similar reduction in field strength at the anode surface.

It is also possible, in certain cases, to place the slats parallel to the grid wires and in the “electron shadows” cast by the grid wires, so that all of the primary electrons from the cathode stream would be received into the boxes of the plate and none intercepted by the slats.

In the case of the tubes to be used at comparatively low frequencies and in particular for audio frequencies it may be found desirable to use the slats alone as anodes, in other words, to eliminate the solid part of the anode. This results in a structure the projection of which at the cathode, is very small and which permits excellent thermal radiation from the cathode. It may be found necessary to adjust the widths of the slats in accordance with the voltage applied to them in order that a sufficiently low percentage of the electrons will pass thru the slats without being drawn to them.

In designing an anode of this type it is desirable to have the slats sufficiently close to produce the desired uniformity of a field and yet not so close that the edges of the slats themselves emit appreciable secondary emission. It has been found that slats approximately 120 mils wide, spaced 90 mils apart and made of 3 mil carbonized nickel give very satisfactory results. These slats are placed edgewise against the plate and have their length essentially at right-angles to the individual grid wires; this is to allow the electrons which have been deviated by the grid wires to make as unrestricted an entry as possible into the boxes or cavities formed by the slats.

In Fig. 4 I have shown a tube construction which employs the novel anode structure and comprises highly evacuated vessel, preferably a glass bulb 8 having at the top a cylindrical dome 9 and the usual base (not shown) at the bottom. The bulb has a re-entrant stem 10 with a flat press 11 on which the electrodes are mounted. The anode 1 is carried on the stem by two support rods 2 which extend from the press and engage the anode wings 12. Within the anode and coaxially therewith is positioned the uni-potential cathode 13, containing the heater element 14. Surrounding the cathode and coaxial therewith is the control grid 15 carried by a pair of support rods 16 projecting from the press, at the other end circuit the grid a circular sleeve member 17 is carried by side rods 18 of heavy copper joined to the support rods 16 and about which the grid wire is wound. The sleeve member extends into the dome portion of the envelope above the electrode structure and it functions as a heat radiator whereby the grid wire may be kept sufficiently cool to prevent harmful primary emission. The grid construction just described is the invention of D. Y. Smith, and forms the subject matter of a co-pending application, Ser. No. 632,297, filed Sept. 9,1932, and assigned to the same assignee as the present application. Surrounding the control grid and coaxial therewith is the second or screen grid 19 which may be of usual construction. Two insulating bridges or spacers 20 and 21 extend between the rods 2 near the upper and lower ends of the plate electrode 1. The top spacer 20, preferably of sheet mica, fits into the dome 9 well enough to prevent lateral movement of the electrode mount. The lower spacer 21 is preferably an oblong sheet of mica. The two spacers are held in place at the ends of the plate by metal bands 22 which clasp the spacers and are secured to the wings of the plate. The cathode sleeve and the side rods of the two grids fit into holes in the mica spacers and are held in fixed spaced relation by the spacers.

The leading-in conductors 23, 24, 25, 26 and 27 for the anode, screen grid, heater, cathode and control grid respectively, are connected to the base pins or terminals (not shown) in the usual manner.

While I have shown and described the preferred embodiment of my invention, it is, of so course, understood that other structural arrangements may be used without departing from the scope of the invention as covered in the appended claims.

What is claimed is:

1. An anode structure for an electron discharge tube comprising a flat plate member, a pair of vertical support rods secured to the sides thereof, and a thin continuous metal strip, which commences at one end of the plate with its edge fastened to the plate surface, wound zig—zag fashion between opposite ends of the plate in parallel relation to said support rods, and forming a plurality of open-ended rectangular troughs, adjacent ones of said troughs being open at opposite ends, the remainder of the strip being carried along the opposite ends of the plate to close the open-ended troughs.

2. An electron discharge tube comprising a cathode, a helical control grid surrounding the cathode, a helical screen grid surrounding the control grid, an anode electrode in the form of a plate surrounding the screen grid, and a plurality of parallelly arranged metal strips mounted on the inner surface of and at substantially right angles to said anode electrode.

3. An electron discharge tube comprising a flattened indirectly heated cathode, a flattened helical control grid surrounding the cathode, a flattened helical screen grid surrounding the control grid, a flattened anode electrode in the form of a plate surrounding the screen grid, and a plurality of parallelly arranged metal strips mounted on the inner surface of and at substantially right angles to each of the opposing faces to of said anode electrode.

Edward W. Herold

 

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