US2073946A - Electron discharge device - Google Patents

Description

March 16, 1937.

B. SA-LZBERG ELECTRON DISCHARGE DEVICE Filed Sept. 22, 1933' INVENTOR BERNARD SALZBERG V ATToRN E Y and press It and the usual base ll.

Patented Mar. 16, 1937 ELECTRON DISCHARGE DEVICE Bernard Salzberg, New York, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application September 22, 1933, Serial No. 690,504

4 Claims.

My invention relates to improvements in electron discharge devices, and its principalobject is to provide an improved electron discharge device, particularly of the type having three or more electrodes and useful as an amplifier, which has more desirable characteristics and particularly a more nearly constant amplification factor than such tubes constructed in the usual way.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing in which:

' Figure l is a longitudinal section of an electron discharge device embodying my invention;

Figure 2 is a horizontal cross section of the electron discharge device shown in Figure 1 taken along the line 2-2 of Figure 1;

Figure 3 is a horizontal cross section of a modifled form of the electron discharge device shown in Figure 1; 3

Figure 4 is a horizontal cross section of another modified form of the electron discharge device of Figure 1;

Figure 5 is a. diagram for-graphically comparing certain characteristics of an electron discharge tron discharge device of the conventional type.

Referring to the drawing, the electron discharge device shown in Figure 1 has a dome shaped evacuated envelope 9 with the usual reentrant stem I The envelope or bulb 9 encloses a unipotential cylindrical thermionic cathode [2 having a heater l3, a tubular helical cold grid electrode supported from the press It] on grid side rods l5 and I6; and a tubular ld anode or plate electrode 11, with side flanges Hi to which the plate support rods l9 are attached, the grid and anode being coaxial with and surrounding the cathode. The anode carries from the flanges It the upper mica spacer 20 by the metal straps 2| and the lower mica spacer 22 by similar metal straps 23.

In the tube shown in Figure 1 the helical grid 14 is, as best shown in Figure 2, elliptical in cross section, the anode I! being of the conventional type and circular in cross section; in the modification shown in Figure 3 the grid 24 is made like the helical grid H, but is circular in cross section, the plate 25 being of elliptical cross section; and in the modification shown in Figure 4 the grid 26 and the anode 21 are both elliptical in cross section, and are set with their major axes perpen dicular to each other. In all these embodiments of my invention the spacing between the grid and anode is at a minimum at the side rods, and each construction illustrated produces a tube having improved characteristics, particularly an ampli- (Cl. 250-275) I fication factor which is substantially constant over a wide range of grid bias voltages.

The preferred explanation of the improved results observed in tubes embodying my invention can best be followed in connection with Figure 5, which is a graphical comparison of an electron discharge device madein accordance with my invention with an elliptical grid and a circular plate, and indicated by diagram A, with the usual electron discharge device with a circular grid and a circular plate and indicated by diagram B. Both diagrams in .Figure 5 represent a cross section of a tube having a straight cylindrical cathode I2, a tubular plate l1 circular in cross section and coaxial with the cathode, and helical grids of wire wound on two grid side rods I5 and' IS. The grid l4 in diagram'A is elliptical in cross section in accordance with my invention, and the grid 24 in diagram B is circular in crosssection, as in the usual three electrode amplifier.

The flow of the electron stream from the oathode to the plate is hindered to some extent by the grid side rod and the electrostatic field around it, hence that part of the plate behind or in the electron shadow of the grid side rod receives less current than the other parts. In Figure 5 the dotted line circles concentric with the side rods l5 and marked low voltage represent the effective fields around the side rods when a low negative voltage is impressed on the grid. That part of the plate behind the grid side rod is in the electron shadow of this field, which hinders the flow of electrons from the cathode, and in efiect reduces the efiective emitting surface of the oathode. The extent of this shadow is represented in the diagrams by the portion of the anode II which lies between the dotted lines extending from the cathode to the plate and tangent to the circles representing the efiective fields around the grid side rods. If the grid bias is increased so that its effective field is increased, as represented by the dotted line circles concentric with the side rods IS, the shadow" on the plate is materially increased, as graphically shown by the dotted lines tangent to these dotted line circles. As the grid bias or impressed negative voltage is increased the shadow eifect is materially increased and the effective emitting surface of the cathode reduced.

The shadow effect causes the amplification factor or mu of the tube to be diiferent for different impressed grid voltages. In the usual tube having a circular grid and plate, the mu for the various sectors of the grid is different, being greatest at the side rods, as the large side rods have a greater controlling effect on the electron stream than the small grid wire. The eifecthe cathode reduced until at some negative bias the emission from those parts of the cathode adjacent the grid rods cannot reach the plate and is practically cut ofl. Whereupon those portions of the grid causing this cut-oi! 8183110 longer effective in controlling variations in the electron flow to the plate. The portions of the grid which first reach the cut-off stage are the side rods and the parts nearest the side rods, and'since the mu is the greatest at these portions of the grid, the effective amplification factor of the grid is made to depend on the lower mu sections and the effective amplification of the tube is therefore decreased. The higher the impressed negative voltage, the greater the shadow effect and the lower the effective amplification factor of the tube. This characteristic is particularly objectionable in an amplifier, such as an audio amplifier designed for constant mu tubes, since the amplifier is depended upon to reproduce faithfully in its output circuit the variations in voltage applied to the grid of the tube in the input circuit. If the morenegative impressed grid voltages are amplified to a lesser extent than the less negative impressed grid voltages, as is done in the conventional tube, distortion in the output circuit of the tube will result, hence the grid swings in the usual tube having a circular anode and grid, must be limited to a narrow range to avoid this distortion.

My invention reduces this undesirable shadow effect to a minimum and makes the amplification factor more nearly constant, as appears from the upper diagram A of Figure 5 which shows that with the elliptical grid and circular plate the shadow effect caused by the effective field around the grid side rods is not only materially less for both low and high bias than with a circular grid and circular plate, but also that the shadow effect does not change as much with changes in the bias of the grid. In a tube embodying my invention the side rods I5 and iii are spaced at the minimum distance from the plate l1, and do not exert as much retarding effect on the electrons leaving the cathode as the side rods of the usual circular grid. As those portions of my grid which correspond to the higher mu sectors of the conventional circular grid are closer to the plate than the sectors corresponding to the lower mu sectors, the various sectors have substantially the same mu, and also due to the resulting larger effective emitting area of the cathode, the mutual conductance of the tube is increased.

In accordance with my invention the plate may be circular in cross section and the grid elliptical, or the plate elliptical and the grid circular, or both elliptical with their major axis perpendicular, but in all cases the spacing between the plate and the grid electrode is a minimum at the grid side rods.

An elliptical type grid permits the greatest separation between the cathode and side rods for a given transconductance. Placing the grid side rods further away from the cathode decreases the chance of short circuits between the grid and the cathode, decreases grid emission, as the grid heating is less, and permits the distance between the grid rod holes in the mica spacers to be increased, whereby the spacers are stronger than in the usual type of construction.

My invention is of considerable utility in multi-grid tubes, such as space-charge grid tubes and similar tubes in which the grid nearest the cathode is at a positive potential with respect to the cathode. The side rods of such positive grids of ,the round type draw a high current which is out of proportion to the value as a control section of the grid. The side rods of an elliptical space-charge grid are much further away from the cathode, have less effect, and draw appreciably less current with the result that the amplification factor is more nearly constant.

In addition to the advantages of the elliptical grid structure for the grid nearest the cathode, it is found that this structure is'also of advantage for other grids in the tube. It permits the side rods of the various grids to be separated suiliclently to meet the requirements of good mechanical design, and at the same time permits the wires of the different grids to be as close together as is desirable for electrical reasons for the best operation of the various grids.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is:

1. An electron discharge device having a straight cylindrical cathode, a tubular grid electrode surrounding and coaxial with said cathode and having a side rod parallel to said cathode, a tubular plate electrode surrounding said grid electrode and coaxial with said cathode, said grid electrode being elliptical in cross section, and the minimum spacing between said electrodes being at said side rod.

2. An electron discharge device having a straight cylindrical cathode, a tubular grid electrode surrounding and coaxial with said cathode and having side rods parallel to said cathode, a tubular plate electrode surrounding said grid electrode and coaxial with said cathode, said grid electrode being elliptical in cross section, and said plate electrode being circular in cross section, the spacing between said electrodes being at a minimum at said side rods.

3. An electron discharge device having a straight cylindrical cathode, a helical grid electrode of elliptical cross-section surrounding and coaxial with said cathode and having side rods parallel to said cathode for supporting said helical grid, and a tubular plate electrode surrounding said grid electrode and coaxial with said cathode with the spacing between said electrodes a minimum at said side rods.

4. An electron discharge device having a straight thermionic cylindrical cathode, a cold grid electrode surrounding and coaxial with said cathode, side rods on said cold grid electrode parallel to said cathode, and a cold tubular electrode surrounding said cold grid electrode, said cold grid electrode being elliptical in cross section, and said cold electrodes being positioned with the minimum spacing between said electrodes at the grid electrode side rods.

BERNARD SALZBERG.

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