US3350653A

US3350653A - Thermionic electron tube circuit

Info

Publication number: US3350653A
Application number: US415917A
Authority: US
Inventors: Tenny D Lode
Original assignee: Rosemount Engineering Co Ltd
Current assignee: Emerson Process Management Ltd
Priority date: 1964-12-04
Filing date: 1964-12-04
Publication date: 1967-10-31
Anticipated expiration: 1984-10-31

Description

Oct. 31, 1967 THERMIONIC ELECTRON TUBE CIRCUIT Filed Dec. 4, 1964 4 Sheets-Sheet 1 FIG. I

INVENTOR TEN/VY D. LODE T. D. LODE 3,350,653

0a. 31, 1967 T. D. LODE 3,350,653

THERMIONIC ELECTRON TUBE CIRCUIT Filed Dec. 4. 1964 FIG. 2

I NVENTOR TENN) D. Z. 005

4 Sheets-Sheet 2 Oct. 31, 1967 T. D. LODE 3,350,653

THERMIONIC ELECTRON TUBE CIRCUIT Filed Dec. 4. 1964 4 Sheets-Sheet 4 INVENTOR TENNY 0. L005 United States Patent Ofiice 3,350,653 THERIVIIONI'C ELECTRQN TUBE CmCUIT Tenny D. Lode, Madison, Wis, assignor to Rosemount Engineering Company, Minneapolis, Minn., a corporation of Minnesota Filed Dec. 4, 1964, Ser. No. 415,917 2 Claims. (Cl. 328270) This invention relates to thermionic electron tubes. More particularly, it relates to the automatic regulation or control of the heater power so as to increase the usefuel life of a thenrnionic electron tube.

Thermionic electron tubes, in spite of their advantages and widespread use, have been among the less reliable electronic components. A particular reliability problem is that thermionic tubes are subject not only to gradual deterioration, which allows preventive maintenance and replacement before complete failure. They are also subject to sudden and unexpected failures due to such causes as heater burn-outs and internal short circuits. Such sudden and unexpected tube failures have remained a major source of unreliability in electronic equipment incorporating thermionic electron tubes.

The limited life and unreliability of thermionic electron tubes may be attributed largely to the high internal temperatures which are normally required for their operation. The desirability of lower operating temperatures is well recognized. Various studies have indicated that the probable life may be increased by a factor of two or more by a reduction of heater voltage of approximately 5 percent. Unfortunately, it is not practical to simply reduce the heater voltage to a low value. As an extreme example, it might be pointed out that reducing the heater voltage to zero would greatly extend the life of the tube at the penalty of its not performing a useful task. The heater of a thermionic electron tube is normally designed so as to maintain a cathode temperature which will allow emission of the maximum design cathode current after tube aging and at the minimum heater voltage limit. The penalty which is paid to meet these requirements is a shorter probable life than if the cathode were operated at a temperature just suflicient for the current actually required in the specific application and without a reserve for aging and heater voltage fluctuations.

An object of the present invention is to automatically regulate the heater power so as to maintain the cathode of an electron tube ata temperature just sufficient to provide the required emission current at a particular time. A further object is to increase the probable life of an electron tube and its associated equipment by reducing its average operating temperature.

In a particular form of the present invention, an electron tube is connected in a class A voltage amplifier circuit arranged to amplify alternating voltage signals. The plate current from the electron tube is filtered or smoothed and passed through the control winding of a magnetic amplifier. The output of the magnetic amplifier is connected to the electron tube heater. In operation, the magnetic amplifier will vary the heater voltage so as to maintain a desired average plate current. The required heater voltage will usually be significantly less than the nominal rated heater voltage of the electron tube. The electron tube operating temperatures will be correspondingly lower. Hence, the deteriorating effects of high temperatures and temperature cycling upon the electron tube and its surroundings are greatly reduced.

In the drawings:

FIGURE 1 is a schematic illustration of a first form of the invention, showing a class A amplifier circuit arranged for the amplification of alternating voltage signals;

FIGURE 2 is a schematic illustration of a second form of the invention, showing a circuit arranged for the am- 3,359,653 Patented Get. 31, 1967 plification of both unidirectional and alternating voltage signals;

FIGURE 3 is a schematic illustration of a third form of the invention, showing its application to the control of the average brightness of a cathode ray tube presentation; and

FIGURE 4 is a schematic illustration of a fourth form of the invention, showing its application to the control of the peak brightness of a cathode ray tube presentation.

In FIGURE 1, the cathode of electron tube 11 connects through resistor 12 to ground 14. Capacitor 13 is connected in parallel with resistor 12, and serves as an alternating current bypass. The control grid of electron tube 11 connects via line 15 and capacitor 16 to input terminal 17. Line 15 connects via resistor 18 to ground 19. The anode of electron tube 11 connects via capacitor 20 to output terminal 21. The anode of electron tube 11 also connects via resistor 22 to line 23. Line 23 connects to a first side of resistor 24. Magnetic amplifier 25 includes control winding 26 and output Winding 27, both wound upon magnetic core 28. The second side of resistor 24 connects to a first side of control winding 26, and the'second side of control winding 26 connects to terminal 29. Capacitor 30 is connected between line 23 and ground 31, and serves as a filter capacitor to isolate the electron tube circuit from alternating voltages developed across winding 26. Terminal 32 connects to a first side of output winding 27, whose second side connects to the anode of diode 33. The cathode of diode 33 connects to line 34. Capacitor 35 is connected between line 34 and ground 35. Line 37 connects from line 34 to a first side of the heater of electron tube 11. The second side of said heater connects to ground 38.

The circuit of FIGURE 1 is generally that of a conventional thermionic electron tube alternating voltage amplifier, except for the presence of magnetic amplifier 25 and the circuit elements associated therewith. In operation, a source of positive voltage is connected between terminal 29 and ground to provide power for the plate circuit of electron tube 11. A source of alternating voltage is connected between terminal 32 and ground to provide power for the heater circuit of electron tube 11. A source of alternating voltage signals which are to be amplified is connected between input terminal 17 and ground. Terminal 21 serves as the signal output terminal for the circuit, and output signals may be measured between terminal 21 and ground.

Magnetic amplifier 25 of FIGURE 1 is of the type 'known as a flux reset magnetic amplifier. When the circuit of FIGURE 1 is first energized, electron tube 11 will be cold. Except for the transient charging current of

capacitors

20 and 30, essentially no current will flow through control winding 26 of magnetic amplifier 25. With no current flowing through control winding 26, output winding 27 will offer little impedance to the flow of current from AC terminal-32 through control winding 27, diode 33, into filter capacitor 35 and the heater of electron tube 11. Thus, the nominal full voltage will be applied to the heater of electron tube 11. As electron tube 11 warms up, current will begin to flow in its plate circuit and through control winding 26. As the current through control winding 26 increases, the rectified output of magnetic amplifier 25' and the heater power supplied to electron tube 11 will be reduced.

In a properly designed circuit, the plate current and heater voltage will stabilize at equilibrium values. At equilibrium, the plate current through control winding 26 will generate a magnetic amplifier output, to the heater of electron tube 11, just sufiicient to maintain the desired average electron tube plate current. If the electron tube plate current increases, the heater voltage will be reduced so as to decrease the plate current to its nominal value. If the plate current decreases, the heater voltage will be increased so as to return the plate current to its nominal value. This regulation also provides a stabilization of the heater voltage with respect to variations of the AC power line voltage. Cathode resistor 12 maintains a small negative grid bias so that the cathode emission and plate current of electron tube 11 are partially limited by the negative potential of the grid with respect to the cathode, as well as being limited by the cathode temperature. A small alternating voltage signal applied to the control grid will cause corresponding fluctuations of the instantaneous plate current of electron tube 11. A corresponding output signal will appear across plate resistor 22 and at output terminal 21. In class A-1 operation, in which the tube is not driven to cut off and in which the grid is at all times negative with respect to the cathode, the instantaneous variations of the plate circuit current caused by the alternating signal Will not significantly change the average current through control winding 26. Hence, these variations will not significantly affect the electrical power input to the heater of electron tube 11 or the cathode temperature. Thus, the circuit of FIGURE 1 may be used as an alternating voltage amplifier or control circuit. In a particular experimental model, constructed along the lines of the circuit of FIG- URE 1, the magnetic amplifier employed a torroidal core wound from a .002 inch thick tape of a nickel-iron alloy known under the trade name of Deltamax. Such cores have a rectangular hysteresis loop and are particularly suitable for use in magnetic amplifiers. The control winding was wound with a suitable number of turns so that an electron tube plate current of the order of .001 ampere, the desired nominal plate circuit current, would generate a core magnetizing force approximately equal to the coercive force of the core material. The output winding was wound with a number of turns such that it would inductively support a 6.3 volt 60 c.p.s. sinusoidal waveform, with core magnetic flux excursions of approximately /2 to /3 of the saturation value. The B+ voltage at a terminal corresponding to terminal 29 of FIGURE 1 was of the order of 250 volts. 6.3 volt AC power, derived from a secondary winding of a transformer connected to a 60 c.p.s. AC power line, was connected between ground and a terminal corresponding to terminal 3.2.

Resistors

12, 18, 22, and 24 were 1000 ohms, 470,000 ohm, 100,000 ohms and 20,000 ohms respectively.

Capacitors

13, 16, 20, 30 and 35 were 50 microfarads, .05 microfarad, .05 microfarad, 8 microfarads, and 1000 microfarads respectively.

Capacitors

13, 30 and 35 were electrolytic types, and capacitors 16 and 20 were of the paper dielectric type. Diode 33 was a conventional semiconductor junction rectifier. The electron tube was a type 6AB4, a miniature triode with a medium amplification factor of approximately 20. The 6AB4 is electrically similar to each of the two triode units in commonly used 12AU7 dual triode, and is rated at a nominal heater voltage of 6.3. The equilibrium plate circuit current of the experimental model was approximately .001 ampere, with a 1 volt negative grid bias due to the voltage drop across resistor 12. The circuit functioned as a voltage amplifier in the manner described above.

When first connected and turned on, the equilibrium heater voltage for the .001 ampere plate current was 3.4 volts. During the first 100 hours of operation, the equilibrium heater voltage rose slowly to 3.6 volts. It then decreased slowly to 3.2 volts after 300 hours of operation. For a subsequent year and a half of continuous operation, the equilibrium heater voltage remained at 3.2 volts. The tube continued to function as an amplifier and displayed no ill effects from continued operation at a low heater voltage. It is estimated that the cathode temperature Was several hundred degrees cooler than when operated at the nominal 6.3 heater voltage. The tube envelope remained relatively cool to the touch.

FIGURE 2 illustrates a second form of the invention which may be used to amplify or control unidirectional or low frequency signals. Electron tube 41 of FIGURE 2 is a dual control pentode in which the third grid is used to divide the cathode current between the second or screen grid and the plate. Such tubes are used as variable gain amplifiers, and as multiple input gates or switches in vacuum tube digital computing and control circuits. The cathode of electron tube 41 connects through resistor 42 to ground 44. Capacitor 43 is connected across resistor 42, and serves as an alternating current bypass. The anode of electron tube 41 connects to output terminal 45 and to a first side of resistor 46. The second side of resistor 46 connects to line 47. The second or screen grid of electron tube 41 connects via line 48 to line 47. Magnetic amplifier 49 includes control winding 50 and output winding 51, both wound upon a magnetic core 52. Resistor 53 connects from line 47 to a first side of control winding 50. The second side of control winding 50 connects via line 54 to positive voltage supply terminal 55. Variable resistor 56 is connected between lines 47 and 54. Capacitor 57 is connected from line 47 to ground 58. AC power input terminal 59 is connected to a first side of output winding 51. The second side of output winding 51 connects to the anode of diode 60, the cathode of which connects to line 61. Capacitor 62 connects from line 61 to ground 63. Line 64 connects from line 61 to a first side of the heater of electron tube 41. The second side of said heater connects to ground 65. The third or suppressor grid of electron tube 41 connects to input terminal 66. The first or control grid of electron tube 41 connects through capacitor 67 to input terminal 68. This first grid is also connected via resistor 69 to ground 70.

The circuit of FIGURE 2 generally resembles the circuit of FIGURE 1, except for the substitution of three grid electron tube 41 and the addition of variable resistor 56 as an adjustable current bypass around control winding 50.

In the circuit of FIGURE 1, the average plate current is controlled by magnetic amplifier 25. Hence, the circuit of FIGURE 1 is useful primarily to amplify and/ or control signals of frequencies which are high with respect to the response time of magnetic amplifier 25 and the heater and cathode of electron tube 11. Electron tube 41 of FIGURE 2 is a dual control pentode, in which the third grid is used to divide cathode current between the second or screen grid and the plate. Since the average potential of the first grid of electron tube 41 will be near ground, the cathode current of electron tube 41 will be brought to an equilibrium value in the manner described for the circuit of FIGURE 1. If grid #3, connected to terminal 66, is strongly negative with respect to the cathode, little current will be received by the anode of electron tube 41. The majority of the cathode current will be collected by the screen grid and will flow through line 48. If a positive potential is applied to terminal 66 and grid #3, a greater fraction of the cathode current will be collected by the plate and a lesser fraction collected by the screen grid. Since both screen and plate currents pass through the control winding circuit of magnetic amplifier 49, the control of plate current by the #3 grid of electron tube 41 does not significantly vary the total cathode current as seen by magnetic amplifier 49. Hence, low frequency or DC signals may be amplified and/ or controlled. Variable resistor 56 is an adjustable current bypass around control winding 50. This adjustable bypass is one means of adjusting the sensitivity of magnetic amplifier 49 and, hence, the equilibrium plate current of electron tube 41.

If desired, alternating voltage signals may be applied to input terminal 68. The output voltage at terminal 45 will then be simultaneously aifected by signals applied to the first and third grids of electron tube 41.

FIGURE 3 illustrates a third form of the invention, showing its application to the control of the average brightness of a cathode ray tube presentation. In FIG- URE 3, the cathode of cathode ray tube 81 connects via resistor 82 to ground 84. Capacitor 83 is connected across resistor 82. The beam current collection electrode of cathode ray tube 81 connects to a first side of resistor 85 which connects in turn to line 86. Magnetic amplifier 87 includes first control winding 88, output winding 89 and second control winding 90, all wound upon magnetic core 91. Resistor 92 connects from line 86 to a first side of control winding 88. The second side of winding 88 connects to terminal 93. Capacitor 94 connects from line 86 to ground 95. AC input terminal 96 connects to a first side of output winding 89, which connects in turn to the anode of diode 97. The cathode of diode 97 connects to line 98, which connects through capacitor 99 to ground 100. Line 98 connects through line 101 to a first side of the heater of cathode ray tube 81. The second side of said heater connects to ground 102. Input terminal 103 connects via line 104 to a control grid of cathode ray tube 81. Line 104 also connects through resistor 105 to line 106. Line 106 connects through capacitor 109 to ground 110, and through resistor 107 to a first side of winding 90. The second side of winding 90 connects to ground 108.

The operation of the circuit of FIGURE 3 generally resembles the operation of the circuits of FIGURES 1 and 2. The major differences are the use of the invention for the control of the beam current of a cathode ray tube, and the addition of second control winding 90 to magnetic amplifier 87. A positive high voltage source is connected between terminal 93 and ground to provide the high voltage for the operation of cathode ray tube 81. An alternating voltage source is connected between terminal 96 and ground to provide power for the heater of cathode ray tube 81. A signal source, providing an electrical signal in accordance with the desired beam current and spot brightness, is connected between terminal 103 and ground. The beam current from cathode ray tube 81 is passed through control winding 88 of magnetic amplifier 87. Magnetic amplifier 87 controls the heater voltage applied to cathode ray tube 81 so as to maintain the desired average beam current and, hence, the desired average brightness.

In many instances, it will be desirable to vary the average brightness of a cathode ray display in accordance with a received signal. In the circuit of FIGURE 3, the received signal is applied to a control grid of cathode ray tube 81 so as to vary the instantaneous beam current in the normal manner. In addition, the signal from input terminal 103 is filtered and applied to a second control winding of magnetic amplifier 87. This will cause the equilibrium beam current and average display brightness to vary in accordance with the average received signal.

FIGURE 4 illustrates a fourth form of the invention, showing its application to the control of the peak beam current and display brightness of a cathode ray tube. In FIGURE 4, the cathode of cathode ray tube 121 connects via resistor 122 to ground 124. Capacitor 123 is connected across resistor 122. The beam current collection electrode of tube 121 connects via line 125 to a first side of variable resistor 126. The second side of variable resistor 126 connects to line 127, which connects to terminal 128. Line 125 connects to the cathode of diode 129, the anode of which connects to line 130. Capacitor 1-31 and resistor 132 are connected in parallel between

lines

127 and 130. Line 130 connects to the base of transistor 13-3. The emitter of transistor 133 connects to a first side of resistor 134, and through capacitor 135 to line 127. Magnetic amplifier 136 includes control winding 137 and output winding 138, both wound upon magnetic core 139. The negative terminal of battery 140 connects to the collector of transistor 133, and the positive terminal to line 127. Terminal 141 connects to a first side of output winding 138, the second side of which connects to the anode of diode 142. The cathode of diode 142 connects to line 143, which connects in turn through capacitor 144 to ground 145. Line 146 connects from line 143 to a first 6 v side of the heater o f cathode ray tube 121. The second side of said'heater is connected to ground 147.

The operation of the circuit of FIGURE 4 generally resembles the operation of the previously described circuits. The major difference in FIGURE 4 is the addition of circuitry between the beam collection electrode of cathode ray tube 121 and the control winding of magnetic amplifier 136. This additional circuitry causes magnetic amplifier 136 to respond to the peak beam current, rather than the average beam current. A positive high voltage source is connected from terminal 128 to ground to furnish the high voltage for the operation of cathode ray tube 121. A source of alternating voltage is connected between terminal 141 and ground to provide power for the heater of tube 121. An electrical signal to vary the cathode ray tube beam current and spot brightness as desired is connected between terminal 148 and ground.

Variable resistor 126, diode 129, capacitor 131 and resistor 132 form a peak holding rectifier circuit in which the stored voltage across capacitor 131 corresponds to the peak current through line 125. Transistor 133 is con nected as an emitter follower to couple the circuit of control winding 137 to capacitor 131 without drawing excessive current. For convenience, the collector supply voltage to transistor 133 is shown as battery 140. In practice, it may be more desirable to use a non-battery power supply for this circuit. With the addition of this circuitry between line and control winding 137, magnetic amplifier 136 will control the heater voltage of cathode ray tube 121 so as to maintain a desired peak beam current and peak spot brightness. Varying the magnitude of variable resistor 126 will change the sensitivity of the magnetic amplifier control circuit and the result-ant peak beam current of cathode ray tube 121.

The circuit of FIGURE 4 may be particularly useful for radar display applications in which it is desired to maintain a constant peak brightness, in spite of changes in the average brightness due to factors such as variations in the size and number of displayed targets.

The drawings and the preceding description have shown the control of the cathode temperature and cathode current of 1 and 3 grid thermionic electron tubes, and of grid controlled thermionic cathode ray tubes. Similar techniques may be employed to reduce the operating temperature and increase the reliability and probable life of other forms of thermionic electron tubes such as klystrons, traveling wave tubes, and other thermionic electron devices in which an electron beam may be controlled both by varying the cathode heater power and by using additional electrostatic and/ or electromagnetic means.

The preceding description has implied the use of indirectly heated cathodes, electrically separate from the cathode heater element. Similar techniques could be used with directly heated cathodes, such as filament type cathodes. The preceding description has shown the use of a magnetic amplifier to control the heater voltage or power. Other forms of amplifying and controlling means, for example, transistor amplifiers, may be similarly used with similar results.

What is claimed is:

1. An electron tube circuit comprising an electron tube having a thermionic electron emitting cathode, an anode and a control grid, heating means for varying the cathode temperature, means coupled to the heating means for controlling the magnitude of the heating thereby controlling the cathode temperature, power terminals for connecting a direct voltage supply in series with the anode and cathode so that a current may flow from anode to cathode, signal terminals for applying a voltage signal to the control grid, first means responsive to the anode current effecting a first control of the magnitude of the heating, and second means responsive to the voltage at the signal terminals effecting a second control of the magnitude of the heating, whereby the temperature of the cathode is controlled in part by the anode current and in part by the voltage at the signal terminals.

2. An electron tube circuit for controlling the average brightness of a cathode ray tube presentation in response to a voltage signal comprising, a cathode ray tube having a cathode heater, a cathode, an anode and a control grid, a magnetic amplifier having a first control winding connected to the anode, a second control winding connected to the grid and an output winding connected to the heater, means for passing a current through the output winding and the heater whereby the output winding current is effective in controlling the cathode temperature, means for passing a beam current through the first control winding and between the cathode and anode whereby the beam current affects the output winding current to cause the cathode temperature to vary inversely with beam current, and signal terminals for applying a signal voltage to the grid and a signal current through the second control winding whereby the signal current affects the output winding current to cause the cathode temperature to vary with signal current.

References Cited UNITED STATES PATENTS 2,236,195 3/1941 McKesson 3l5106 2,834,883 5/1958 Lukoft 328151 2,940,010 6/1960 Kenny 315l06 DAVID J. GALVIN, Primary Examiner.