US2435475A - Ageing tubes having space charge grids - Google Patents

Description

1948- c. F. STROMEYER AGEING TUBE HAVING SPACE CHARGE GRID Filed April 14, 1942 IN V EN TOR. Charles PM Simneyer BY M? TTOR NE Y5 Patented Feb. 3, 1948 OFFICE AGEING TUBES HAVINS SPACE CHARGE Charles Francis Stromeyer, Marblehead, Mass., or, by mesne assignments, to Remco Electronic, Inc., New York, N. Y., a corporation of New York Application April 14, 1942, Serial No. 438,885 6 Claims. (Cl. 316-17) This invention relates to a new method of ageing electron discharge tubes having space charge grids such, for example, as are shown in my Patent No. 2,256,177. Such tubes are hereinafter referred to as space charge tubes.

An object of this invention is to provide a novel method of ageing space charge tubes which will produce tubes with a better life performance than is obtained with previous methods.

Another object is to rid the #2 (control) grid of any coating of any rare earth element.

A further object is to produce a space charge tube in which the so-called contact potential of the control grid may be maintained approximately constant during the life of the tube.

In the drawings:

Figure 1 is a diagram for describing the so called contact grid potential of a vacuum tube;

Figure 2 is a basic circuit for ageing space charge tubes;

Figure 3 is a basic circiut for ageing dynamic coupled space charge tubes;

Figure 4 is a circuit for measuring the emission of an output section of a dynamic coupled space charge tube; and

Figure 5 is a circuit for measuring the emission of a driver section of a dynamic coupled space charge tube.

Figure 2 shows a tube which, as explained in my Patent No. 2,256,177, has a cathode It, a space charge grid 2, a special grid 3 as described in said patent, a screen grid 4, a suppressor grid 5, and an anode 6. In Figures 3, 4 and 5 similar elements are given the same reference numerals except that the cathode, or portion of the cathode, for the space charge tube elements is numbered 1. Those figures show a dynamically coupled space charge tube having a driver section consisting of a driver cathode H (which may of course be a separated section of the cathode 1), a driver grid l5, and a driver anode IS. The space charge section of the tube is the part having reference numerals for the anode and grids corresponding to those in Figure 2.

If space charge tubes having a usual cathode coating, such as strontium and barium oxide, for example, are processed in the usual manner their life performance is unsatisfactory. Their plate currents (Ip) will drop, perhaps 50%, without an accompanying change in emission. This drop in plate current is usually noticeable within a few hours and levels off in about 96 hours. If the grid bias is shifted to adjust the In to its original value, little or no shift in transz conductance (Sm) will be noted. After the tubes have undergone the lp shift, ordinary emission testing or prolonged heater burning will restore their Ip.

It is evident that the difilculty is linked with p a change in the so-called contact potential (E) of the control grid (#2). E measurements of the #1, #2, and #3 grid, taken during stages of the life span, show that the E value for the #2 shifts tremendously while the E value for the #1 and #3 only undergo the small shift customarily found with any properly processed tube. The shift in #2 is from the usual negative value (-0.8) up towards zero and sometimes actually going positive as much as 0.5 volt. This behavior is radically different from any ordinary cathode type tube.

E readings were taken in the usual waythat is, the value represents the voltage necessary on the particular grid to suppress its grid current to 0.2 mma. (only tubes having effectivelyzero leakage and zero gas current were considered). In the case of #1 grid, other grid and anode potentials were removed as indicated in Figure 1. For #2 grid, normal operating potentials were used. For #3 grid, normal #1 and anode potentials were used and the #2 grid placed at cathode potential. This manner of taking the various E readings is perfectly satisfactory because they are influenced only slightly by whether or not normal cathode current is flowing, and, furthermore, this problem deals with changes in E rather than absolute values.

I find it convenient to consider the significance of the so-called contact potential, E, as follows:

It represents a complex quantity. One factor is the fictitious potential required directly between the cathode and grid surfaces to suppress electrons whose initial velocities are sufficient to reach the grid in absence of any diiference in E. M. F. between the cathode and grid surface. This we will call 6, and it can be assumed e remains constant during the life span under investigation, since neither emission nor Sm varies. It is evident that e cannot be measured directly due to the thermal E. M. F. difference between the cathode and grid surfaces. This thermal E. M. F. is equivalent to a series potential and may be denoted by e as illustrated in Figure 1. The voltage in Fig. 1 may be further defined as the equivalent series voltage produced by the thermal E. M. F. difference existing between the cathode and grid surface. The polarity of e is always as shown, the cathode being positive with relation to the grid. This is so because the thermal E. M. F. is proportional to the work function and the cathode, of course, has a lower work function than any other element in the tube. (The element having the lower work function is always positive with respect to the other.) When e varies with a constant e, the applied voltage E must be shifted to maintain constant word barium" is used it is to signify any of the f alkaline earth elements which may be present within the tube. e is maximum when the grid surface is free of contamination. It should be; included here that certain alloy grid wires may produce initially somewhat diflerent es due to their surfaces inhibiting "meta contact to the barium deposit to various extents but the change in e is the important consideration here-not the initial values. Since usual cathode coating and usual exhaust processing consistently produce well activated cathodes, I decided not to investigate the possibilities of eliminating the life difiiculty bysome special untried cathode coating, but rather, obtain the necessary control by special ageing.

The initial E values for #1, #2 and #3 grids, when ordinary processing was used, were nor-, mal and usually all E values. were within a few tenths of each other. If ordinary forced ageing (D. C. applied to the respective grids to the extent of making the grids actually show color) were used, the initial E values could be affected as is usual with such ageing, but no condition of selected voltages would cause the E of #2 to go to zero or positive and remain substantially stationary during life. Furthermore, such treat? ment definitely puts the tube in an abnormal condition as the Sm would be considerably different than it would have been had the tube not been "forced aged. I I

Apparently, the barium deposit on the #2, which arrived there during the exhaust and ageing processes, evaporates during the early stages of the life span and is not replenished. The barium deposits on the #1 and #3 must tend to evaporate but are continuously replenished by the barium evaporating from the cathode since a state of equilibrium exists with regard to their respective E's. This indicatesthat the barium evaporating from the cathode becomes negatively charged (negative ions) and is attracted to positive elements. This theory is further supported by the fact that after a tube has been life tested so that the surface condition of its #2 grid has reached a state of equilibrium the original E value for the #2, grid can be partially restored (at least 50%) by merely taking an emission taining it during life would indeed be small. It can be assumed that the state of equilibrium depends upon the useful microscopic surface area of the grid being covered by barium of at least a molecular layer. A greater quantity than this probably does'not change the value of E. An interesting observation which supports this idea is the rate at which the E of the control grid of this space charge tube changes during life. It takes many hours for this E to g up to approximately zero and stabilize, as a result of life burning a. tube processed in the usual manner. If the E of this tube is now restored to the original value by simply taking a. few successive emission readings, the E can again be made to return to zero but now only a short burning time is required. However, if the E were restored to the original value by a vigorous, ordinary ageing schedule, rather than by reading emission, a considerable burning time would be necessary.

This theory is not exploded by the fact that ordinary tubes, those which have a negative #1 grid, do not exhibit this diflicultyn The close proximity of the negative #1 grid to the cathode accounts for considerable barium continuously distilling on the grid surface. Some ofthis barium might well be uncharged particles.

Since the evaporation of uncharged particles occurs in straight lines radiating from the source, some will not be intercepted by the first grid of a space charge tube and may travel on to be deposited upon the #2 grid. Undoubtedly, the rate at which these barium particles reach the #2 grid is insufiicient to maintain a molecular layer over the useful macroscopic area. It is also probable that only a negligible quantity could get by the positive #1 grid without acquiring a, charge.

. It can be assumed that these particles are negatively charged ions since the behavior of the positive #3 grid is essentially the same as the positive #1, in spite of 'the fact that it is considerably reading of the type where grids and anodes are made positive with respect to the cathode. This change takes place very rapidly. It occurs during the short interval required to rapidly read an emission meter. The emission voltage was purposely restricted to limit the emission current. Attempts to eliminate this effect by further lowering the emission voltage were not satisfactory unless it was lowered to such an extent'that the emission reading no longer had any significance.

If the condition necessary to reach a state of equilibrium for the E of a grid was a matter of having a constant amount of barium deposit, the probability of obtaining this condition and maindistant from the cathode.

Since it is impossible to maintain a molecular layer of barium on the control grid #2 during its life, the problem is to rid this grid of barium and maintain it free of barium.

After the foregoing theory was appreciated correction of the life dimculty required the solution of two problems: first, what type of ageing would produce grid surfaces essentially the same as that which results after considerable life burning; second, how can emission-be measured without altering the desired grid surfaces. The ageing schedule, of course, must also clean up residual gas and thoroughly activate the cathode.

The simple arrangement of Fig. 2 provides the solution of both problems. The tube is first heater flashed to clean the cathode surface, this being usual practice. Thetube is then subject to the A. C. step illustrated in Fig. 2. Two things occur to rid the barium from the #2 grid.

Since the #2 grid is at cathode potential, it will emit to the #1 providing its temperature is-suflicient. This causes the barium to evaporate from the #2. On the reverse cycle, when the cathode and #2 are positive with respect to #1, emission r the importance of having both actions. The A. C. voltage must be appreciable but not excessive. Specific examples are given later. Too great a voltage will cause excessive barium to arriv at the #1, and some may go on to the #2. A practical indication of the arrival of excessive barium at the #1 grid is obtained by noting whether a current reading of a D. C. meter in series with the lamp decreases after the voltage has been applied a short time, and whether the lamp grows brighter. If so, the voltage is too high. During the reverse cycle, there is far toomuch current flowing. The current which flows from the #1 grid to the cathode and #2 grid on one half of the A, C. cycle should be less than the current flowing in the reverse direction on the other half of the cycle.

Fortunately, an A. C. step is particularly useful in cleaning up residual gas. If activation is not complete, other steps can be used, but the final step should he like Fig. 2 or its equivalent.

After this final step, the tubes should never be allowed to burn with just a heater voltage for any appreciable tim for this will cause the E of #2 tobecome different from the stabilized life value. With some tubes, it may be desirable to make the A. 0. step more elaborate; for example, the screen grid #3 and plate may also be tied to the #1 and suitable series lamps provided.

This A. C. step was incorporated in the ageing schedule of combination tubes of the dynamic coupled" type as shown in Fig. 6 of my Patent No. 2,256,177. A simple schedule that has been used successfully is given in the following table in connection with the illustration of Fig. 3 of this application:

Step Time gig; s, Em s Min I 2 l4 =Onen... Open. [1 5 9 Closed Do. III 4 9 Open... 11 i Closed IV 9 .do. 60 Do where S1 is the position of switch S1 in Fig, 3, Bee the applied alternating voltage (EAc) in Fig. 3, and S the position of the switch S in Fig.3.

In this table steps customarily employed with radio tubes, while steps III and IV are new in accordance with the present invention.

A satisfactory emission test for space charge tubes can be made by simply not including the #2 grid in the emission circuit, It should remain at cathode potential. of course, a new emission test voltage may be desirable and current limits adjusted accordingly. With combination tubes of the type shown in Fig. 6 of my Patent No, 2,256,177, the method illustrated in Figs. 4 and 5 has proven satisfactory, Fig. 4 illustrating the emission test for the output section and Fig. 5 the test for the driver section, which is constituted by the cathode, grid, and anode shown at the left in the two figures. In these tests the current readings indicated as 75-80 milliamperes and 26-28 milliamperes, respectively, are for thoroughly activated tubes.

I prefer to read the output section emission substantially independent of the driver section emission-therefore, separate circuits are used. However, a combination circuit could be used, provided the grid #2 (which is connected to the driver cathode) is maintained substantially at ground potential. Incidentally, if a usual emission circuit is used in measuring the output, a

' to the space charge grid and applying I and II are ageing steps tube having an input section thoroughly activated cathode will produce an emission current of approximately 90 ma. when volts are applied.

I do not desire to be limited to the precise details shown but only as indicated by the scope of the. appended claims.

What is claimed is:

1. The method of ageing an electron discharge tube having a space charge grid, a control grid, a screen grid, a cathode, and an anode, which comprises connecting the control grid with the cathode, connecting the screen grid and anode an alternating current voltage in series with a resistor across the cathode and the connection to said screen grid, anode, and space charge grid, while independently heating the cathode.

2; The method of ageing an electron discharge tube having an input section comprising a cathode, grid, and anode, and an output section comprising a cathode, space charge grid, control grid, screen grid, suppressor grid, and anode, in which the cathode of the input section is connected for signals directly to the control grid of the output section which comprises connecting the control grid and the suppressor grid of the output section to the cathode, connecting the driver anode and the screen grid and space charge grid of the output section to a common connection, and applying an alternating current voltage in series with a resistor across the cathode and common connection, while independently heating the cathode.

3. The method of ridding the control grid of an electron discharge tube having a space charge grid, a control grid, and a cathode of rare earth elements deposited from the cathode coating which comprises applying an alternating current voltage in series with a resistance between the space charge grid on the one side and the cathode and control grid on the other side while independently heating said cathode. I

4. The method of ageing an electron discharge tube having a space charge grid, a cathode, and a control grid, which comprises applying an alternatlng current voltage through a resistance to the space charge grid on one side of said voltage and to the cathode and control grid on the other side of said voltage, while independently heating said cathode.

5. The method of ageing an electron discharge comprising a cathode and an output Section comprising a cathode, space charge grid, control grid, screen grid, suppressor grid, and anode, in which the cathode of the input section is adapted to be connected to the control grid of the cathode of the output section, connecting the anode and the screen grid of the output section and the space charge grid of the output section to one side of a resistance, applying an alternating current voltage between said cathode and the other side of said resistance through a switch which will hereinafter be referred to as the switch S, independently heating said cathode by means of a heater which is energized through an alternating current transformer, the primary of said transformer being connected to said cathode through a switch which is hereinafter referred to as switch S1, and as a first step applying a heater voltage of 14 volts fOr two minutes with the switches S and S1 open, as a second step applying a heater voltage of 9 volts for five minutes with the switch S1 closed and the switch S open, as ;a third step applying a, heater voltage of 9 volts for four minute with the switch S1 open and the'switch S closed and a voltage introduced into the' circuit through the switch S of 118 volts, and as a fourth step applying a heater voltage of 9 volts for ten minutes with the switch S1 open and the switch 8 closed with a voltage of 60 volts applied to the circuit through the switch S. 1

6. The method of ageing an electron discharge tube containing an input section and'an output section, the output section comprising a cathode,

necting said control grid and suppressor grid to said cathode, connecting said anode, screen grid,

and space charge grid to a common connection, n pplying an alternating current voltage in series with a resistor across said cathode and common connection, while independently heatin said cathode. I

CHARLES FRANCIS STROMEYER.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,987,998 Edwards et a1. Jan. 15, 1935 15 1,477,868 Donle Dec. 18, 1923 2,093,626 Ward Sept. 21, 1937 I 2,109,225 Ulrey Feb, 22, 1938

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