US2492643A - Electron discharge tube - Google Patents

Description

Dec. 27, 1949 FiledJan. 14, 1946 H. K. ISHLER ELECTRON DISCHARGE TUBE 2 Sheets-Sheet l `-OOOO 0 ll'l's A Ec 6 77- Lm .il mmm Eb Ea :J:- 8/

pag. 5

v INVENTOR.

HARRY KENNETH lsHLeR ATTORNEY Dec. 27, 1949 H. K. .SHLER 2,492,643

ELECTRON DISCHARGE TUBE Filed Jan. 14, 1946 2 Sheets-Sheet 2 INVENTOR. HARRY KENNETH tsr/LER AT TORNEY Patented Dec. 27, 1949 ELEc'rRoN DISCHARGE TUBE Harry Kenneth Ishler, Nassau, N.

Y., assigner to Sylvania Electric Products, Inc., a corporation of Massachusetts Application January 14, 1946, Serial No. 641,045

(Cl. Z50-27.5)

9 Claims.

This invention relates to electron discharge tubes and more particularly to electron discharge tubes employing grid structures effecting more eilicient utilization of lamentary cathodes.

The advantage of a lamentary cathode, Wherein the electron emissive material is coated directly on the heater Wire, is that a smaller input of power to the heater wire is required to produce the necessary electron emission than with the 'unipotential or indirectly heated cathode in which the emissive material is coated on a sleeve which contains an insulated heater wire. Electron tubes with lamentary cathodes are therefore much in demand for portable or battery service where the power consumed must be kept as small as possierating temperature very rapidly after the heater voltage is applied, and the small mass of iilamentary cathodes permits the use of filaments with high natural periods of vibration which are desirable in some applications.

A disadvantage of the fllamentary cathode, which will be more fully discussed in connection with the drawings, is that not all points on the iilamentary cathode are at the same potential with respect to the other electrodes of the tube.

When the heater power is supplied by a source of direct current voltage, the heater current nowing through the filament produces a direct current voltage drop along the filament. In particular, therefore, the bias, by which is here expressly meant the direct current potential difference between the cathode and the grid in the absence of alternating current signal, varies along the length of the filamentary cathode.

Since, with all other factors remaining constant, the plate current of the tube is controlled by the voltage of the inner grid nearest the cathode, the plate current varies along the length of the cathode in accordance with the changing bias caused by the direct current voltage drop in the heater wire. If the grid is biased negatively and the tube designer chooses the electrical and mechanical dimensions of the tube so that the proper plate current flows at the more positive end of the lamentary cathode, then the more negative end of the filament will have a small value of bias and consequently an abnormally large plate current, thus wasting in plate current fromthe plate voltage supply part of what is gained by using the filamentary cathode in preference to the indirectly heated cathode. If the designer arranges the tube so that-the proper plate current flows at the more negative end of the .,lament, then the more positive end of the filament will have too high a bias, the plate current will be smaller than desired, the total length of the iilament will thus not be effective and part of the emission will be lost and with it part of the advantage of employing a lamentary cathode.

The primary object of the invention is to compensate for the foregoing disadvantage and provide grid structures which permit the eiicient utilization of the electron emission from the entire length of a filamentary cathode.

In the design of electron tubes one of the factors which controls the value of the plate current, or the stream of electrons ilowing past the control grid, is the pitch or the number of grid wires per unit length of the grid. The designer ble; in addition, filamentary cathodes reach cpf may adjust this pitch in conjunction with the other parameters of the tube to secure a desired value of plate current for a given set of electrode voltages incl-uding'in particular the grid bias. As noted above, however, the designer may select his parameters including the grid pitch to Secure the desired plate current from oneend of the lamentary cathode corresponding to the bias thereon, but the plate current will be high or low at the other end because of the different bias. The invention accomplishes its principal object and compensates for this disadvantage by providing novel grid structuresin which the pitch of the grid Winding is changed to secure a better distribution of the electroncurrent along the length of the filamentary cathode. In one embodiment of the invention, a singlel grid with variable pitch is employed, while in another embodiment control of the electron stream from the filament is divided between two grids with diierent pitches.

Both of these embodiments together with the advantages and features of novelty which characterize the invention will be described in detail in conjunction with the accompanying drawings, in which Fig. 1 is a front elevation of a lamentary cathode and a variable pitch grid in accordance with one embodiment of the invention; Fig. 2 is a front elevation of a filamentary cathode and a pair of grids With diierent pitches according to a second embodiment of the invention; Figure 3 is a partial sectional top view along line 3-3 of Fig. 4 showing only the cathode and grids; Fig. 4 is a sectional side elevation of an electron discharge tube employing the variable pitch grid of Fig. 1; Fig. 5 is a schematic diagram of a simple amplifying circuit using a triode tube with a lamentary cathode; Fig'. A6 is a partial sectional top View along line 6 6 o f Fig. '7 showing only the cathodes and grids; and Fig. '7 is a sectional side elevation of an electron discharge tube employing the double grid arrangement of Fig. 2.

The principle of operation of the invention may be explained by referring to Fig. 5 in which the triode tube constructed according to the invention is schematically shown connected in a simple amplifying circuit. Let point be =a point of zero potential to which other voltage measurements are referred. Lower` end 4 of the cathode I is therefore at zero p'tential, whil'e upper end 5 is at the potential Ea of battery 6 which supplies heater current to the cathode. In operation asl an amplier, signal voltage Es would be applied between the bias battery 'I and grid 2. The present description, however, will be conducted assuming that the signal voltage isabsent so that the potential of the grid is E, the potential of the bias battery. It is now seen that the direct current potential difference between the cathode' and grid at the lower end 4 of the cathode isEc while the potential dilerence between the cathodeandthe grid at `the upper end 5 of the cathode S Ec-I-Em Electronsare emitted foilthe hot cathode and strearn pass the grid wires to plate 3,- which is supplied' with voltage from battery 8 through load impedance A9. The valterhating current component of the output voltage Eo appears across impedance 9 when signal voltage Es is supplied and the tube is operated as an amplifier. In the 'absence of signal, suppose that the biasbattery 'I has its negative terminal connected to the grid 2 and its positive terminal to the ground point U, and that the cathode heater supply battery 6 has its negativeterminal connected to the ground point and its positive terminal connected tothe upper end oi the lamentary cathode I. The wires of the grid z are shown with variable spaeing according to the invention,l Abut if the grid wires were equally spaced as in tubes previously known, the voltagejdrop `from y5 to 4 produced by the heater current through the llamentary cathode I would causeA the electron current which ilows from any point on the cathode to vary along the length of the'cathode. The electron current would be a maximum at the lower end 4 where the grid isA below the cathode by a potential of only En volts, and a minimum at the upper end 5 where the bias battery 1 and the heater battery 6 combines to place the grid below the cathode by a potential of E+Ea volts.

One of the most important factors which deter'- mines the magnitude of the'plate current is the spacing between the grid wires II. Given the other dimensions of the tube and the electrode voltages, the' designer can determine a pitch for the grid wire which will give the desirable plate current. In a practical case, of course, it is une der'stood that not all of the other factors can be chosen arbitrarily and the' grid pitch finally determined to match them. The Various lfactors must be chosen in con-'junction with each other so that all the 'dimensions are reasonable.

The preceding `description of Fig. 5, however, sh'ows that, using grids with equally spaced wires, the designer cannot 'ehos'e his parameters so as to secure uniform electron current from all 'points of the filament. explained, the potential diiL ference between the grid 'and cathode is not a single nxed value which lends itself to ei'cient design but varies along the length of the cathode. It has therefore previously been impossible to construct an electron discharge tube in which the full length of the lamnta-y 'cathode is eicientl-y utilized.

The diiculty was `aggra'vate'd in proportion to the magnitude f the heater supply voltage and therefore the magnitude of the supply voltage which could economically be employed to heat filamentary cathodes was sharply limited.

The invention makes possible the efficient utilization or the entire length of a lamentary cathode and thereby extends the range of filament voltages which may economically be employed.

The foregoing principles have been discussed in connection with Fig. 5 which shows a plane triode. l1-he embodiment of the invention shown in Figs. l, 3, and 4 omprises an electron discharge tube havingalamentary cathode surrounded by a variable pitchgrid of the wound elliptical helical type commonly employed in electron discharge tubes.

The electron emission current emanates from thelil'an'ieiitarg'z4 cathode I which passes vertically along the axis of grid 2. Heating power is supplied to the cathode in Fig. 1 by battery 6, which has its positive terminal connected to upper end 5 of the cathode I. The upper end 5 is therefore at a positivepotential of Ea volts with respect to ground point l). Lowerend 4 of the cathode is connected directly to the ground point and is therefore at 'a potential of zero' volts. The grid 2 is negatively biased to a potential of E@ Volts by th'e battery I connected between the ground point 0 and the grid. Thus, as we have seen in the previous discussion, the potential difference between the grid cathodein the absence f a signal voltage varies from Ec volts at the lower end el of the cathde to Ec-I-Ea volts at the upper end 5.

vThe grid 2 resembles the usual construction in that it is composed of helical wire winding I I supported by side rods Io. In accordance with the invention, however, the spacing between the turns of the grid winding I I increases successively from the bottoni of the grid opposite the lower end 4 of the cathode to the top of the grid opposite the upper end 5. Qualitatively it is apparent that at the top of the larnentary cathode 5 Where the potential difference between the grid and cathode is Ee-I-Ea diie to the coinbined effects of the voltagesof heater battery' is' and bias battery 1, it

will be necessary to space the turns of the grid lii/inding' further 'apart in order to allow the same electron current to flow from the cathode as at the lower end where the potential difference between the filament and lcathode is only Ec volts.

This particular embodiment shows a pentode tube with screen grid I4 and suppressor grid I5 surrounding the control grid 2, while the sections of the plate 3 are placed outside the suppressor grid. The entire electrode structure which is assembled with internal shield `2 I, is supported on pins I1 extending through header I8, which in turn is sealed at its outer periphery to bulb I9.

Voltages are supplied to the electrodes through the pins Il, the appropriate connections between pins and electrodes being made inside the tube. During the construction of the tube getter 22 is vaporize'd to absorb residual gases not removed by 'the exhaust pumps, External to the bulb is metal shielding base 2?), cemented to the header and the lower portion ofthe bulb.

In designing a tube with the turns of the grid wire spaced according to the invention, a practical 'designer might specify the spacing between turns at several points along the filament according to vthe potential 'diie're'ce between the grid and cathode at those points. Fuller theoretical analysis a'r'id experimental data, however, maybe used to determine 'the increase in spacing with each successive rturfn as the pitch is varied con"- tinuously from oneend o f the grid to theother;

The `electroncurrent which ows from the fila# mentary cathode and pastthe control grid will of course be determined by several considerations. The operating temperature and composition oi the lamentary cathode, the wire diameters and winding pitches of all the grids, the relative sizes and spacings of the grids and the plate, and all electrode voltages must be considered in deter-Y mining the plate current. Y

At this point we are not concerned with any of these determining factors except the pitch of the grid winding nearest the cathode. Given the dimensions of the tube, the temperature and the composition of the fllamentary cathode, the electrode voltages, the required mutual conductance, and all other necessary factors, the designer can juse the theoretical equations for the plate current, limited by space charge, of a multi-grid elec tron discharge tube to obtain a rst approximation to the required pitch of grid winding. 'Ihe theoretical equations are supplemented by for'- mulas and data taken from models and from experimental results, and modications may be made which to a considerable degree compensate for the divergence of the theoretical equations from the practical structure. By making this calculation for each turn of the grid winding, he can estimate the manner in which the pitch of the grid winding needs to be changed with each turn along the length of the grid in order to maintain the same value of space current from all points of the cathode.

Using these calculations and previous engineering experience and data, the designer can then experiment with various grids until he obtains one which will produce the maximum plate current for a given input to the filamentary cathode and the given mutual conductance. Since the mechanical problems involved in the windings of such a grid with a continuously variable pitch may be quite complicated, the practical designer, as previously mentioned, may consider that a suficient approximation to constant plate current along the length of the cathode may be obtained by changing the pitch at several specified points along the length of the grid rather than changing the pitch of every turn. The mechanical difficulties involved in even this simplied method may in many cases be uneconomical and I have conceived another embodiment of my invention which is easier to construct and which yet gives greatly improved results over previously known grids.

This embodiment, shown in Figs. 2, 6, and '7 of the drawings, comprises a pair of grids 2 and 2a mountedside by side with the lamentary cathode I running up the axis of grid 2 and down the axis of grid 2a and being supported at the top by tab I2 and tension spring I3. The ends Il and 5 of the lamentary cathode are shown in Fig. 2 connected to the heater supply battery 6, so that the lower voltage section of the cathode is surrounded by grid 2 while the higher voltage section is surrounded by grid 2a. As shown in the drawings, the turns of the grid winding IIa of grid 2.a are more widely spaced than the turns of the grid winding II of grid 2, so that the control of plate current approximates that shown with the variable pitch winding of Fig. 1. The pitches are chosen so that equal amounts of electron current flow from the two sections of the cathode, although it is not pos-sible to have uniform current along the length of each section.

In assigning the Values to the pitches of the 6 two grids, there is another factor which may be considered in some forms of this particular ern-f bodiment of the invention. The double grid structurewith the support II which also serves `as a center tap for the lamentary cathode permits the use oi the lamentary cathode in either a parallel or series connection. In a tubeV espe-f cially intended for such versatility of operation, the designer may compromise in the choice of grid pitches to insure good operation for either parallel or series connection, although such a compromise would not secure equal amounts of plate current from the two sections for either mode of operation.

Likel the device of Figs. 3 and 4, this embodiment shows a pentode tube with screen grid I4 and suppressor grid I5. In this construction each of these grids surrounds both of the control grids. The electrode structure is assembled with internal shield 2I and supported on pins I'I sealed through header I8. The periphery of the header is sealed to bulb I9, and external shielding metal base 20 is cemented to the header.

yAnother advantage of this double grid construction is that the interelectrode capacity be, tween grid and anode is materially reduced without reducing the length of the filament. Since the lengths of the grid and plate are reduced by half without making a large increase in the dimensions of the grid, the capacity is substantially reduced by half. .l

It is seen that the length of the lamentary cathode usable with a grid with equally spaced turns is severely limited because the ulllength of the cathode .cannot be efficiently employed.

With the structure of Fig. 2 it i-s clear that the 'usable length of lament is materially increased over that usable with a single uniformly spaced grid, while withthe continuously variable grid of Fig. 1, of course, the increase in usable length is even greater.

Modiiications of the particular arrangements which I have disclosed embodying my invention will occur to those skilled in the art, so that I do not desire my invention to be limited to the particular construction set forth and I intend in the appended claims to cover all modifications which do not depart from the spirit and scope of my invention.

What I claim is:

l. An electron discharge tube comprising an envelope containing a lamentary cathode having a rst portion and a second portion lying alongside one another, an anode, a first grid comprising a wire Winding surrounding said rst portion of said cathode and comprising a second grid surrounding a second portion of said cathode having a winding with a pitch different from the pitch of the winding of said first grid, and a plurality of additional grids surrounding said first grid, said second grid, and said cathode.

2. An electron discharge device comprising an envelope containing a filamentary cathode, a pair oi grids surrounding said cathode, and an anode, said grids comprising elliptical wire helices of different pitches wound on supporting rods, means supporting -said grids side by side with their longitudinal axes parallel, the minor axes of their ellipses parallel, and the major axes of their ellipses in the same line, an end portion of said lamentary cathode comprising less than half its length extending coaxially with one of said grids, the other end portion of said lamentary cathode comprising less than half its length extending coaxially with the other of said grids, and

9&99348 7 means for supporting said cathode attached 'to the remaining central portion extending between the grids.

3. An electron discharge device comprising an envelope containing a lamentary cathode, a pair of grids surrounding said cathode, and an anode, said grids comprising elliptical Wire helices of diierent pitches Wound on supporting rods,

means supporting said grids side by side with 'their longitudinal axes parallel, the minor axes 'of their ellipses parallel, and the major 'axes of their ellipses in the same line, an end portion of said i'llamentary vcathode comprising less than half its length extending coaxially With one of said grids, the other end portion of said filamentary cathode comprising less than half its length extending coaxially with the other of said grids, and means attached to the remaining central portion of said cathode extending between said grids for supporting said cathode and conducting current to said cathode.

4. An electron discharge device comprising an envelope containing a lamentary cathode, an anode, two grids comprising elliptical Wire helices of diierent pitches wound on supporting rods, means supporting said grids side by side With their longitudinal axes parallel, the minor axes of their ellipses parallel, and the major axes of their ellipses in thesame line, an end portion of said lamentary cathode comprising less than half its length extending coaxially with one of 'said grid-s, the other end portion of said cathode comprising less than half its .length extending coaxially with the other of said grids, means for supporting said cathode attached to the remaining central portion of said cathode, and a plurality of additional grids surrounding both of said first grids and said cathode.

5. An electron discharge device comprising an envelope containing a filamentary cathode, a pair of grids comprising elliptical Wire helices of different pitches Wound on supporting rods, 'means supporting said grids side by side with their longitudinal axes parallel, the minor axes of their ellipses parallel, and the major axes of their ellipses in the same line, an end portion of said iilamentary cathode comprising less than half its length extending coaxially with one of said grids, the other end portion of said lamentary cathode comprising less than half its length extending coaxially with the other of said grids, and means Afor supporting said cathode attached to the remaining central portion of said cathode and electron emissive coating only on said end portions of said cathode.

6. An electron discharge device comprising an envelope containing an anode., an approximately U-shaped iilamentary cathode, a helical grid sur- 3 rounding one leg of said cathode and a second helical grid With a diierent pitch of helical winding surrounding the other leg of said cathode.

7. An electron discharge device comprising an envelope containing an anode, an approximately U-shaped iilamentary cathode, a helical grid surrounding one leg of said cathode, a second helical grid With a diierent pitch of helical winding surrounding the other leg of said cathode, and a plurality of additional grids surrounding both the legs of the iilamentary cathode and the said grids which surround the individual legs.

8. An electron discharge tube comprising an envelope containing an anode, an approximately U-shaped filamentary cathode, conducting and supporting means attached to the center of said cathode, and means for securing a compromise between equal anode current from each leg of said cathode during operation with said legs con nected in parallel and equal anode current from each leg of said cathode during operation with said legs connected in series comprising two grids having different wire spacings, each of said grids surrounding one of said legs of said U-shaped cathode.

9. An electron discharge tube including an envelope containing a lamentary cathode having a first portion and a second portion lying alongside one another, a first grid including a wire-Winding surrounding the iirst portion of such cathode, a second grid surrounding the second portion of said cathode and including a, Wirewinding with a pitch different from'the pitch of the Winding of said rst grid, and an anode surrounding both of said grids.

HARRY KENNETH ISHLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,480,219 Nicolson Jan. 8, 1924 1,542,386 Hartley June 16, 1925 1,670,503 Greenbowe May 22, 1928 1,948,122 McCullough Feb. 20, 1934 2,015,327 Wheeler Sept. 24, 1935 2,038,360 Harries Apr. 21, 1936 2,048,224 Snow July 21, 1936 2,048,225 Snow July 21, 1936 2,048,230 Snow July 21, 1936 FOREIGN PATENTS Number Country Date 10,140 Australia Feb. 14, 1934 485,798 Great Britain May 25, 1938

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