US2154141A - Vacuum tube - Google Patents

Description

April 11, 1939. 'H A, WHEELER VACUUM TUBE Filed Feb. 2. 1957 9 INVENTOR.

HAROLD A. WHEELER ATTORNEY.

Patented Apr. 11, 1939 UNITED STATES VACUUM TUBE Harold A. Wheeler, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application February 2, 1937, Serial No. 123,557

8 Claim.

This invention relates to electron discharge devices, and more particularly to thermionic vacuum tubes adapted to be used in high-frequency circuits.

Recent developments in high-frequency signaling systems, as, for example, television signalmodulated carrier receivers, have given rise to the problem of developing vacuum tubes which operate satisfactory in ultra-high-frequency circuits generally, and, more particularly, in circuits for relaying or amplifying substantially without distortion a band of frequencies extending over an exceedingly wide range. --Thus, in a broad frequency band amplifier suitable for use in the signal-amplifying channel of a. television receiver, it is usually necessary to transmit and amplify, by means including one or more vacuum tubes, a band of frequencies extending from a low frequency of the order of 60 cycles or less to a high frequency of the order of 2 megacycles or more.

None of the various types of vacuum tubes at present commercially available is entirely satisfactory for use in such circuits. The principal limitation of the present known forms of tubes is that the amplification which may be obtained therein with tolerable distortion at all frequencies to be amplified is limited. This limitation is due primarily to high interelectrode capacitances of suchtubes, accompanied by only nominal values of mutual conductance. A further limitation pertaining primarily to the use of vacuum tubes in ultra-high-frequency circuits resides in the fact that most tubes of conventional design are characterized by a substantial amount of stray inductance in the electrode structure thereof, and particularly in the cathode lead-in connection, which inductance increases the apparent capacitance across the input or output electrodes of the tube.

It is an object of the present invention, therefore, to provide an electron discharge device so constructed and arranged that the magnitudes of the interelectrode capacitances and the inductances of the various electrodes and their lead-in connections are exceedingly small.

It is another object of the invention to provide an electron discharge device of the above character in which no suppressor grid is provided, but which is capable of being operated in a manner such that the eflect of a suppressor grid is procured.

It is a further object of the invention to provide an electron discharge device having the characteristics set forth in one or both of the preceding objects, which is so designed that the anode and screen electrode dissipation is not excessive under normal operation conditions andwhich has suillcient mutual conductance to ensure a substantial amount of amplification 5 therein. 7

It is an additional object of the invention to provide an electron discharge device having the above characteristics which is of simple and economical construction and lends itself readily to manufacture in commercial quantities.

Briefly, the above objects are attained in accordance with the present invention by providing an electron discharge device which comprises an elongated evacuated envelope of glass or like insulating material in which are supported the anode and an electrode structure comprising, in the order named, a cathode, a control electrode, and a screen grid. The anode is constructed in the form of a low-resistance narrow band of conducting material extending around the inner surface of the envelope and surrounding the electrode structure. The width of this band is made only a. minor fraction of the axial length of the cathode in order to reduce the anode-cathode or output electrode capacitance of the tube. .There is also provided a layer of high-resistance conductive material extending around the inner surface of the envelope and adjacent and at either side of the anode. The resistance of this layer is extremely high; in the ideal case it should have a conductivity just sufficient to prevent the accumulation of a negative surface charge on the inner surface of the envelope, although as a practical compromise it may be designed with such a resistance that the outer edges of the layer are'maintained at substantially zero potential The potential gradient along the layer drops sharply in the vicinity of the anode and more gradually towards the outer edges. The average potential of the layer is slightly positive, but negative with respect to the anode so that it produces a suppressor action and, in some cases, the conventional suppressor grid may be omitted, thus further reducing the interelectrode capacitances of the device. The resistance of the coating is substantially greater than its capacitive reactance with respect to the cathode, so that the coating introduces between the anode and cathode an admittance which is negligible compared with that of the anode and its unavoidable capacia tance. In the preferred construction, the screen and control electrodes provide an amplification factor between these two electrodes which ensures a reasonable value of mutual conductance therebetween without excessive screen electrode dissipation when operating with the maximum permissible space current.

The mechanical construction of the device is such that the length of all lead-in connections is reduced to an absolute minimum and the spacing therebetween is a maximum. The anode is terminated at a small cap on the side of the tube envelope displaced as far as possible from the control electrode terminal, which latter comprises a cap supported on the end of the tube envelope. At the same time, the configuration of the tube envelope and the electrode structure is such that the tube can be readily mounted in the standard type tube sockets.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, will best 'be understood by reference to the following specification taken in connection with the accompanying drawing, in which Fig. 1 illustrates an amplifier circuit included to aid in the understanding of the nature of the problem herein involved; and Fig. 2 illustrates an electron discharge device constructed in accordance with the present invention.

Referring now more particularly to Fig. 1 of the drawing, there is shown an amplifier more clearly illustrating the problem which the electron discharge device of the present invention is devised to solve. The amplifier comprises a pentode tube l0 having its input electrodes, comprising a cathode H and a control grid l2, coupled to a source of alternating voltage l3, and its output electrodes, comprising an anode l4 and the cathode ll, coupled by means including a resistor l5 to the input electrodes of a second pentode tube IS. The output capacitance of the tube Hi, the input capacitance of the tube l6, and the stray capacitances of the coupling network, all considered as an equivalent lumped shunt capacitance across the output electrodes of the tube Ill, are indicated by the dotted-line condenser H. The effect of this equivalent capacitance on the gain of the tube In may be partially or totally compensated in a conventional manner by including an inductance element l8 of appropriate size in series with the load resistor l5. The usual coupling condenser and leak resistor, indicated at l9 and 20, respectively, complete the coupling network. Potentials of the correct values are applied to the screen and anode electrodes of the tubes In and I8 through the terminals indicated at +Sc and +3, respectively.

It is well known that, with the above conventional circuit arrangement, the gain or voltage amplification obtained between the source l3 and the input electrodes of the tube I6 is determined by the ratio of the mutual conductance of the tube ill to the total output admittance, the admittance including the vectorial sum of the anode-cathode conductance, the load admittance, and the susceptance of the total effective shunt capacitance. As pointed out above, this shunt capacitance includes, in parallel, the stray capacitance of the coupling network, the output electrode capacitance of the tube In, and the input electrode capacitance of the tube l6. Obviously this capacitive component of the admittance increases as a function of frequency to increase the total admittance and, since this factor is in the denominator of the ratio, the gain across the v capacitances be small for another reason which is also based onthe ratio discussed above. With the amplifier shown, the capacitances of the coupling network and tube determine, in each amplifier stage, the phase shift at any given frequency. The magnitude of the phase shift, of course, increases with frequency since the capacitive susoeptance increases with frequency. It

will be apparent, therefore, that, if the capacitance in a given circuit is reduced while the load resistance is maintained constant, the phase shift will be decreased, thereby to reduce phase distortion.

Referring now more particularly to Fig. 2 of the drawing, there is illustrated an electron discharge device or thermionic vacuum tube constructed and arranged in accordance with the present invention to procure the desired characteristics of substantial mutual conductance and low interelectrode capacitance, as set forth above. This tube may be connected in the circuit of Fig. 1 in an obvious manner. Briefly described, the tube comprises an elongated cylindrical envelope 2| supported on a base 22 with the bottom of the re-entrant press 23 thereof closely adjacent the upper ends of terminal prongs 24 for the various electrodes. The prongs extend through and are secured to the base 22 and are arranged in a circle in accordance with conventional practice to be Extending from the press 23 are wires 25 and 26,

which serve to support the electrode structure within the envelope. This structure comprises, in the order named, a cathode 21, a control grid 28, a screen grid 29, and a suppressor grid 30. The electrode structure is of conventional design, in that the girds 21, 28, and 29 are in the form of helices and are supported between washers 3| and 32, of insulating material, by groups of vertical supporting wires 33, 34, and 35, respectively. Also supported on the washer 32 is a metallic shield 36 extending outwardly almost to the inner surface of the envelope and carrying at its upper end a mica washer 31 for centering the electrode structure within the tube. This shield is connected to the supporting wires 25 and 26 which terminate at one or more of the terminal prongs 24 to provide a ready means whereby the shield may be grounded. Similarly, the cathode, the cathode heater, the screen grid, and the suppressor grid are individually connected by lead-in wires to appropriate ones of the terminal prongs 24. It is pointed out that the whole assembly is designed to render the lead-in wires as short as possible in order to minimize the inductances thereof and the capacitances therebetween. The control grid is connected by a short lead-in wire 38 to a connector cap 39 on the end of the tube envelope for the same reason, that is, materially width of the band a minor fraction of the axial length of the cathode, preferably not more than to reduce the capacitance of this electrode to the other electrodes of the tube structure. v The desirability of reducing the anode-cathod capacitance of the tube is pointed out above and this is accomplished in accordance with the present invention by providing an anode in the form of a low-resistance narrow bandv of conducting material extending around the inner sur'-' face of the envelope 2| and surrounding the electrode structure. Preferably, this band comprises free metallic silver or graphite intermixed with a suitable binder and coated or painted on the inner surface of the envelope 2|. Since the anode-cathode or output electrode capacitance of the tube is determined by the extent of the anode surface, it is apparent that this surface should be as small as electrical requirements will permit. This is accomplished by making the one-third. The anode is terminated at a cap 4| supported on the side of the envelope 2| by means comprising a short connector wire 42 and,

in order to secure the maximum separation between this cap and the cap 39, thereby to re-' 'duce the capacitance therebetween without unduly lengthening the lead-in wire 42, it is positioned adjacent the lower end of theelectrode emitted electron stream is not sufficiently .attracted by the positive potential of the anode 40 to cause the same to collect on the anode and, I

in the absence of any further means in the tube, tends to collect on the inner surface of the envelope adjacent the anode to build up a negative potential which tends to repel the electron stream back to the screen and the cathode. The efiect is that of an excessive suppressor grid action,-but

one which is extremely variable with operating conditions and surface contaminations which may vary widely with different tubes. This phe nomenon causes a reduction and nonuniformity in themutual conductance of the tube'with the final result that the over-all amplification is materially reduced.

either side thereof means comprising a thin discharging layer 43 extending around the inner surface ofthe envelope 2|, which, with the anode, completely surrounds the electrode structure. This latter layer is of an extremely high-resistance material, preferably comprising comminuted graphite intermixed with a suitable binder and applied as a thin coating on the surface of the envelope; A colloidal suspension of graphite commercially available as Aquadag" is one ma-, terial suitable for this purpose. In any case, the

electrical conductivity of this layer should bevery' much less than the electrical conductivity ofthe' material comprising the anode 40., Due to hijghf resistance of the layer, an exceedingly high mt r tial gradient exists along the layer near theedges the band so that the major portion of the laye'r' i at a low positivepotential, whichniayal ilroacn zero potential, during'the operation of the tube. As a result, no negative surface charge is built up on .the envelope adjacent the anode 40, and thejhigh positive potential of the anode is more effective to draw electrons to the anode.

As mentioned briefly above, the principal adlayer 43 introduces no appreciable increase in the anode-cathode admittance of the tube.

If desired, the tube illustrated in Fig. 2 and described above may be modified further to reduce the interelectrode capacitance of the tube by omitting suppressor grid 30 from the structure.

' The omission-of this grid, however, is most practical when the tube is operated in the particular manner described below. As iswell known, the suppressor gridfunctions to prevent secondary electrons emitted from the anode from reaching the screen grid, by creating a repelling field for these electrons in the region between the suppressor grid and the anode. The same repelling field may be produced by maintaining, during operation of the tube, an electron density between the screen grid and the anode which is above a predetermined and substantial value. With the structure shown, the wide spacing of the screen grid and anode and the concentration of the electron stream by the narrow anode band materially assists in obtaining such electron density as to create the required repelling field.

The discharging layer 43 also plays an important part in the suppressor action. As mentioned above this layer assumes a potential negative with respect to the anode and thus reduces the potential of the space between the anode and the screen, retarding the electrons in this space.

The discharging layer 43 may be regarded simply as a suppressor electrode which is in effect .very similar to'that of a conventional suppressor grid, but having a different configuration.

The maximum mutual conductance of the tube is, of course, limited largely by the energy dissipation from the screen grid and the anode, and

fthe amount of dissipation from each of these In order to obviate the difliculty just described there is provided" adjacent the anode 4|] and at electrodes is equal to the product of the space current and the voltage on the respective electrodesgv With atube ,of the structure shown, the

trolled by the control grid between the cathode and thescreen grid. Thus, if this grid has too closemesh, an excessively high screen-grid potential' is? required to maintain the maximum permissible space current, with the result that r of the anod -band in either direction awayarrqing 1 the screen-grid dissipation becomes excessive.

ILIpn'the-otherhand, this grid has too wide mesh, the pitch of the control-grid helix necessarily approaches the spacing of this grid to the cathode, and thereby provides a gradual cutofl characteristic which always corresponds to a reduction of the transconductance that may be obtained with a given value of space current. In order to compromise the two conflicting requirements as set forth above, the pitch of the control grid 28 is proportioned to provide an amplification factor of optimum value between this electrode and the screen grid 29. It has been found that satisfactory operation in certain structures is obtained if the grid 28 is constructed to provide an amplification factor between these electrodes of the order of 10.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modiflcations may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An electron discharge device comprising an evacuated envelope of insulating material, an electrode structure supported within said envelope and comprising, in the order named, a cathode, a control electrode, and a screen grid, an anode comprising a narrow band of conductive material extending around the inner surface of said envelope and surrounding said electrode structure, and a layer of high-resistance material extending around the inner surface of said envelope adjacent said anode.

2. An electron discharge device comprising an evacuated envelope of insulating material, an electrode structure supported within said envelope and comprising, in the order named, a cathode, a control. electrode, and a screen grid, an anode comprising a narrow band of conductive material extending around the inner surface of said envelope and surrounding said electrode structure, and a layer of high-resistance mate rial extending around the inner surface of said envelope on either side of said anode, said anode and said layer surrounding said electrode structurethroughout its axial length.

3. An electron discharge device comprising an evacuated envelope of insulating material having a section of circular cross section, an electrode structure supported within said envelope and comprising, in the order named, a cylindrical cathode, a control electrode, and a screen grid, an anode comprising a band of conductive material extending around the inner surface of said envelope substantially concentric with said cathode, the width of said anode being a minor fraction of the axial length of said cathode, and a layer of high-resistance material extending around the inner surface of said envelope section on either side thereof, said layer surrounding said electrode structure throughout its axial length.

4. An electron discharge device comprising an evacuated envelope of insulating material, an electrode structure supported within'said envelope and comprising, in the order named, a cathode, a control electrode, and a screen grid, an anode comprising a narrow band of conductive material extending around the inner surface of said envelope and surrounding said electrode structure, and a layer of conductive material extending around the inner surface of said envelope adjacent said anode, said layer of conductive material having substantially lower electrical conductivity than the material comprising said anode and being electrically connected to said anode.

5. An electron discharge device comprising an evacuated envelope of insulating material, an electrode structure supported within said envelope and comprising, in the order named, a cathode, a control electrode, and a screen grid, an anode comprising a narrow band of conductive material extending around the inner surface of said envelope and surrounding said electrode structure, and a layer of conductive material extending around the inner surface of said envelope on either side of said anode, said layer of conductive material having substantially lower electrical conductivity than the material comprising said anode and being electrically connected to said anode.

6. An electron discharge device comprising an evacuated envelope of insulating material, an electrode structure supported within said envelope and comprising, in the order named, a cathode, a control electrode, and a screen grid, an anode comprising a narrow band of conductive material extending around the inner surface of said envelope and surrounding said electrode structure, and means for preventing electrons emitted from said cathode from collecting on the inner surface of said envelope adjacent said anode.

7. An electron discharge device comprising an evacuated envelope of insulating material, an electrode structure supported within said envelope and comprising, in the order named, a cathode, a control electrode, and a screen grid, an anode comprising a narrow band of conductive material extending around the inner surface of said envelope and surrounding said electrode structure, and means comprising a coating of high-resistance conductive material extending around the inner surface of said envelope and connected to said anode for preventing electrons emitted from said cathode from collecting on the inner surface of said envelope adjacent said anode.

8. An electron discharge device comprising an evacuated envelope of insulating material, an electrode structure supported within said envelope and comprising, in the order named, a cathode, a control electrode, and a screen grid, an anode comprising a narrow band of conductive material extending around the inner surface of said envelope and surrounding said electrode structure, and means comprising a coating of conductive material extending around the inner surface of said envelope adjacent said anode for preventing electrons emitted from said cathode from collecting on the inner surface of said envelope adjacent said anode, said layer being connected to said anode and having electrical conductivity substantially lower than that of the material comprising said anode.

HAROLD A. WHEELER.

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