US3218543A

US3218543A - Surge suppressor employing capacitor charging means

Info

Publication number: US3218543A
Application number: US145032A
Authority: US
Inventors: Nobis Wilhelm Heinrich
Original assignee: Fritz Hellige und Co GmbH
Current assignee: Fritz Hellige und Co GmbH
Priority date: 1960-10-13
Filing date: 1961-10-13
Publication date: 1965-11-16
Anticipated expiration: 1982-11-16
Also published as: DE1141350B

Description

Nov. 16, 1965 w. H. NOBIS 3,218,543

SURGE SUPPRESSOR EMPLOYING CAPACITOR CHARGING MEANS Filed Oct. 13. 1961 AT 1A 20 g 30 :E .1 56

i 556* 16 i; ji E: 35 zg/Hilyi 60 T E 50 54 E 46 w 5 k L I a :2 1 T 1:4: I 4 4E I United States Patent 3,218,543 SURGE SUPPRESSOR EMPLOYING CAPACITOR CHARGING MEANS Wilhelm Heinrich Nobis, Merzhausen, near Freiburg im Breisgau, Germany, assignor to Fritz Hellige & C0.,

G.m.b.H., Freiburg, Germany Filed Oct. 13, 1961, Ser. No. 145,032 Claims priority, application Germany, Oct. 13, 1960,

40,666 14 Claims. (Cl. 32341) The present invention relates to surge suppressing circuits, and more particularly to a surge suppressing circuit for inhibiting the response of an electronic circuit to power supply interruptions.

In circuits such as oscillators and amplifiers employing either vacuum tubes or transistors, current-responsive resistors (hereinafter referred to as thermistors) are frequently included to stabilize or control operation of the circuit.

Advantage is taken, in such applications, of the fact that the resistance of a thermistor is determined by the power dissipated in, or the current through the thermistor. By selecting a thermistor having an appropriate temperature coefiicient of resistance, it is possible to compensate for variations in the remaining elements of the circuit in operation.

However, such circuits as developed in the past have frequently suffered from the serious shortcoming that very short power supply interruptions may produce output current surges which are detrimental, either to components in the circuits themselves or to apparatus such as a meter or recorder to which the circuits supply signals. These voltage surges frequently reach values which are significantly above those appearing under normal operating conditions.

If, for example, in a normally operating oscillator, alternating current generator or amplifier, the power supply is interrupted for a short time, such as 15-25 seconds, a thermistor forming part of any of the stages of the circuit will have cooled off during this period of time and returned to its quiescent value of resistance. However, the cathodes of vacuum tubes will ordinarily still be at a temperature sufficiently high to permit electron emission. Under such circumstances restoration of the power supply will produce an output current surge because the vacuum tubes are conductive while the thermistor presents a resistance other than its normal operating value.

A number of circuits employing thermal or time delay relays have heretofore been proposed to eliminate current surges of the type described. However, such circuits suffer from the serious disadvantage that the minimum period of inhibition possible is generally from thirty to sixty seconds, which may be in excess of that actually required. At the same time, such circuits frequently fail to protect against voltage surges caused by very brief interruptions. Such circuits also frequently involve switching, which may itself produce transients or surges. Accordingly, there is a definite need for automatically responsive surge or transient suppressors which operate to inhibit a circuit for a short period no longer than necessary for the circuit elements to return to their normal thermal conditions.

It is, therefore, an object of the present invention to provide a circuit automatically responsive to power supply interruption to suppress output current surges or transients which would ordinarily occur upon restoration of the power supply within a short period.

A further object of the present invention is to provide a surge suppressor circuit for inhibiting the response of an electronic circuit following a brief interruption in 3,218,543 Patented Nov. 16, 1965 power supply to the circuit which operates automatically upon restoration of power to the circuit, yet requires no moving parts or switches.

A further object of the present invention is to provide a surge suppressing circuit which is automatically operative to protect a number of stages of a multistage circuit from current surges or transients due to power supply interruption.

A further object of the present invention is to provide a circuit for use with a vacuum tube or transistor stage to inhibit the response of the stage for a period of time following power supply interruption and restoration not in excess of that necessary to ensure normal operation of the circuit, regardless of how brief the interruption.

In accordance with the invention, a circuit using electron discharge devices includes circuit elements energized by the voltage supply when it is restored subsequent to interruption which operate to apply a negative cutoff biasing potential to one of the eletcrodes of one of the electron discharge devices, the cutoff biasing potential thereafter gradually decreasing over a time sufficiently long to permit the circuit components to return to their normal operating conditions. It will be understood that the circuit components permitted to return to their normal operating conditions are mainly the cathodes, in the case of vacuum tube circuits, and, furthermore, thermistors which start heating up again upon restoration of power supply until their resistance value reaches that of continued normal operation. It will be clear from the foregoing that an important feature of the present invention, residing in the application of a cutoff bias to an electrode which occurs substantially instantaneously, inhibits current surges in the output circuit of the entire system since the stage of the circuit of which the biased electrode forms a part is inhibited automatically and gradually returns to its normal state of operation.

In accordance with another important feature of this invention, the inhibited stage may be distinct from that which includes the thermistor. In other words, the thermistor may form part of the cathode branch of the plate circuit in one stage of a multistage oscillator, alternating current generator or amplifier, while the stage including the circuit of the present invention may be another stage of the same circuit system. Alternatively, the thermistor may form part of a coupling network between stages preceding the stage including the circuit of the present invention.

Further features of the present invention include a capacitor which is maintained under charge when normal circumstances prevail, and means for discharging the capacitor upon interruption in power supply, so that the capacitor constitutes a shunt for connecting a negative potential to an electrode which may be the suppressor grid or the control grid of a vacuum tube. It will be apparent that, upon restoration of the power supply, a strong negative potential appears at the electrode to inhibit the response of the circuit.

Further features of the present invention include a voltage divider connected between the power supply terminals, a capacitor connected across one resistor of the voltage divider and arranged to discharge therethrough upon interruption of power supply, and a diode forming an additional discharge path for the capacitor. It will readily be understood that optimum results are achieved if the capacitor is permitted to discharge in a very short time. Thus, in accordance with an important feature of the invention, the time constant of the discharge path is relatively short. When the power supply is restored, the capacitor is recharged through the voltage divider, the recharging circuit including a relatively high resistance, so that the recharging circuit possesses a time constant which is greater by several orders of magnitude than the dis charge time constant. Accordingly, as the capacitor charges at a comparatively low rate, the cutoff biasing potential applied to the electrode decreases at the same rate, so that a thermistor forming part either of the same stage or of another stage, as stated above, returns to its high value resistance before the full output current level is reached and no surges may occur.

It will be clear from the foregoing general description that the surge suppressor of the present invention operates automatically since it is energized by the power supply of the system. More specifically, a charge is maintained on the capacitor under normal operating circumstances, and upon power supply interruption the capacitor discharges automatically and substantially instantaneously. In the discharged state, the capacitor presents a short circuit between the negative power supply terminal and the grid of the stage to be inhibited, ready to act in the event power supply reappears regardless of the duration of the interruption. When power is restored, the capacitor immediately starts recharging and the cutoff biasing potential also immediately starts to decrease toward its normal value. It is to be noted that the entire operation is, therefore, controlled by the power supply interruption and not by the characteristic of a component outside and distinct from the circuit, as is the case with prior art surge suppressors using, for example, bimetal strips.

Further features, objects and advantages of the present invention will be more clearly understood by a consideration of the following detailed description and the accompanying drawings in which FIG. 1 is a circuit diagram of a typical prior art circuit utilized in describing the purposes of the present invention;

FIG. 2 is a circuit diagram illustrating one embodiment of the present invention; and

FIG. 3 and FIG. 4 are circuit diagrams illustrating other embodiments of the present invention.

The same or similar elements are designated by the same reference numerals throughout the several figures of the drawing.

Referring now to FIG. 1, one stage of a prior art circuit forming part, for example, of an oscillator or an amplifier, includes a pentode electron discharge device it) having an indirectly heated cathode 12, a plate 14, a control electrode or suppressor grid 16 and a control grid 18. Plate 14 is connected to a voltage source 24 through a load resistor 28, while cathode 12 is connected to the remaining terminal of voltage source 24 through a resistor 32. While voltage source 24 is depicted in FIG. 1 and the remaining figures as a battery, it will be understood that, in practice, a conventional power supply of relatively low internal impedance energized from a commercial source may be employed. A grid leak resistor 26 is connected between control grid 18 and the negative terminal of voltage source 24 which may be grounded. Input signals to the stage may be applied across a pair of input terminals 20, connected between control grid 18 and ground, while output signals may be taken from a pair of output terminals 22 connected across load 28 and voltage source 24.

Feedback energy taken from another stage (not shown) may be applied to the stage at terminal 30 connected to cathode 12, if the circuit of FIG. 1 forms one stage of a more complex circuit.

Considering now the operation of the circuit thus described, resistor 32 inserted in the cathode lead of the plate circuit will be assumed to be a thermistor. Upon interruption in plate current supply, thermistor 32 will cool off within a very short time period, and if power is supplied again after a period of time which is so short that cathode 12 is still hot, a voltage surge will appear across load resistor 28 and output terminals 22. This surge may exceed by far that applied to the load resistor under normal operating conditions when thermistor 32 has reached its usual operating temperature level. It is well known that such voltage surges cannot occur if the device is started after a longer period of non-operation, since the cathode 12 is cold and unable to emit electrons. The phenomenon described above is well known in the art and has led to the designs mentioned in the foregoing general description of surge suppressors using, for example, bimetal strip-relay combinations.

Referring now to FIG. 2, there is shown a circuit diagram of one embodiment of a surge suppressor according to the present invention. As shown in the figure, the circuit comprises input terminals 20, output terminals 22, grid leak resistor 26, load resistor 28, and feedback input terminal 30 and electron discharge device 10 corresponding to those in FIG. 1. Resistor 34 in the cathode branch of the plate circuit may be a thermistor having a characteristically low thermal inertia which causes the voltage output surges which must be suppressed. However, it is to be understood that resistor 34 need not necessarily be the current or temperature dependent element of the complete system. Rather, resistor 34 may be a substantially temperature constant circuit element and any other stage of the complete system may include the thermistor. In other words, the stage shown in FIG. 2 may be combined with other stages, such as that of FIG. 1 in which resistor 32 is temperature dependent.

In accordance with the invention, the circuit of FIG. 2 includes an arrangement for applying a suppressing biasing potential to a control electrode such as a suppressor gird 16 to suppress voltage surges upon power supply interruption and restoration. A voltage divider 36 connected across power input leads 38 and 40 comprises a comparatively high resistance resistor 42 and a relatively low value resistance resistor 44, serially connnected to each other at junction 46. Junction 46 is connected to suppressor grid 16 and

resistors

42 and 44 are selected to provide a normal operating or a slightly negative bias on grid 16 with respect to cathode 12. A capacitor 48 is connected across resistor 44, and the junction 50 of resistor 42, resistor 44 and capacitor 48 is connected by a diode 52 to a grounded busline 54. Power for the plate circuit, as well as the voltage drop across the voltage divider 36, is supplied from a potential source 56, having a tap 58 connected to busline 54. It will be clear that the upper portion 60 of voltage supply 56 provides the plate potential for pentode 10 while the entire source 56 energizes the voltage divider 36, so that, under normal operating conditions, capacitor 48 will be charged to a potential corresponding to the voltage drop appearing across resistor 44. The potential applied between busline 54 and lead 40 by source 56 should be sufiicient to bias tube 10 to cut-ofi when applied between control electrode 16 and busline 54 when the circuit operates in its suppression mode, as will be described hereinafter.

Considering now the operation of the circuit thus described, when power supply source 56 is interrupted, all of the potential differences in the circuit will disappear substantially instantaneously. Consequently, capacitor 43 will discharge through resistor 44 and, since the circuit elements involved exhibit a short time constant, discharge occurs substantially instantaneously. This eliect is enhanced by the presence of diode 52, since an additional discharge path for capacitor 48 is formed through diode 52, lead 54, the lower portion of power supply 56 and lead 40. At this point it should be noted that the circuit would be operative without employing the diode 52. However, it will readily be understood that a short time constant for the discharge of capacitor 44 is important inasmuch as the surge suppressor is ready to act when the capacitor is discharged but not before. With the diode and the additional discharge path formed by it, the charge on capacitor 48 disappears substantially instantaneously, so that, in practice, output surges caused even by extremely short supply interruptions are elTectively eliminated.

When the power supply is restored, capacitor 48, now discharged, effectively short circuits resistor 44 of voltage divider 36 so that the voltage supplied by the lower portion of source 56 is effectively applied directly between suppressor grid 16 and cathode 12. The potential applied as discussed constitutes a cutoff biasing potential which inhibits the conduction of pentode 10, so that, at the beginning of this period, practically no plate current may flow and output current surges are effectively eliminated.

Once the power supply is restored, the voltage drop reappearing on voltage divider 36 operates to recharge capacitor 48 through high value resistor 42. Considering the comparatively long time constant of the recharging circuit, it will be understood that recharging occurs at a comparatively slow rate. At the same time, the potential at junction 46 becomes gradually more positive so that the biasing potential applied to grid 16 decreases. As a result, plate current is gradually restored at the same rate until full plate current and output from the entire system are reached. It will be clear from the foregoing discussion that during this gradual change of conditions in the circuit, the thermistor which may be resistor 34 in FIG. 2 or which may be any other resistor in the output circuit associated with another stage of the same circuit, will heat up until normal operating conditions prevail.

FIG. 3 illustrates another embodiment of the invention which differs from that of FIG. 2 in that the stage comprises a triode electron discharge device 64 and, in the absence of a suppressor grid, the suppression of response contemplated according to the present invention is based on the utilization of control grid 62 as the control electrode. The circuit operates essentially in the same manner as that of FIG. 2, and identical components are designated by the same reference numerals. However, the junction 46 between

resistors

42 and 44 of voltage divider 36 is connected to control grid 62 through grid leak resistor 26. It will be seen that charging the capacitor 48 during normal operation of the circuit occurs in a manner identical to that of the circuit of FIG. 2. Similarly, both discharge paths for the capacitor 48 are the same in FIGS. 2 and 3 and include, on one hand, the resistor 44 and, on the other hand, diode 52, and the lower portion of tapped voltage source 56. In this embodiment, as well, the additional discharge path through diode 52 may be omitted, although the circuit preferably includes the diode, in order to prevent output current surges in the event of very short power supply interruptions.

The bias applied to the tube to inhibit its response should, of course, be that appropriate to the use of grid 62 as the control electrode for the circuit.

FIG. 4 illustrates another embodiment of the invention wherein the automatic control for eliminating voltage surges is arranged to remove a conductive bias from the tube when the power supply is interrupted. As shown in the figure, the circuit comprises a separate voltage source 66 for the voltage divider 36, which includes a high value resistor 68 and a low value resistor 70. Plate current is supplied from voltage source 72, the latter being entirely separate from the voltage divider. Under normal operating conditions, junction 74 is maintained at a potential which is slightly negative with respect to cathode 76, so that no current flows through diode 52, which should also be true with respect to diode 52 in the circuits of FIGS. 2 and 3.

It will be clear that capacitor 48 of the circuit of FIG. 4 is normally charged in accordance with voltage drop across resistor 70, which applies a sufiiciently positive bias to the grid-cathode circuit of the tube to achieve normal response. Upon interruption in power supply, and the removal of the voltages supplied by both source 72 and additional source 66, capacitor 48 discharges through resistor 70, and through diode 52 and cathode resistor 34.

It should be noted that since resistor 34 forms part of one of the discharge paths for the capacitor, the circuit of FIG. 4 is preferably used in a system in which the thermistor is associated with another stage of the circuit because if resistor 34 is a thermistor, it may in its cold state have a resistance value higher than suitable to achieve substantially instantaneous discharge of the capacitor. Upon restoration of power supply, control grid 78 of the pentode is maintained at ground potential by the discharged capacitor 48, which now shunts resistor 70, and through grid leak resistor 26 so that pentode 80 is practically inhibited and carries almost no plate current. Once the power supply is restored, the voltage drop reappears on voltage divider 36 so that capacitor 48 is recharged at a comparatively slow rate in accordance with the long time constant determined by high value resistor 68. Simultaneously, and at the same rate, junction 74 becomes relatively more positive and the negative bias potential on control grid 78 decreases. It can be seen that, in principle, the circuit employing the additional voltage source 66 operates in a manner which is analogous with that of the circuits shown in FIGS. 2 and 3.

For practical purposes it should be stated that it is desirable to inhibit one stage of a circuit for a period in the range between 2 and 30 seconds, since, with shorter interruptions in supply, thermistors are generally still at an elevated temperature, while after periods longer than 30 seconds the cathodes of conventional tubes are sufficiently cold to prevent output current surges.

It is to be understood that the above-described embodiments are illustrative of the application of the principles of the invention, and numerous other circuits may be devised by those skilled in the art without departing from the spirit and scope of the invention. Thus, by way of example and not of limitation, as stated above, the principle of the present invention may be applied to circuits using one single stage and also to multistage circuits. In the case of single stage circuits, the thermistor may form part of the suppressor circuit as symbolized by the resistor 34 in any one of FIGS. 2, 3, or 4. If, on the other hand, more than one tube or transistor is employed forming distinct stages of the circuitry, the thermistor may form part of the circuit of any other stage, or serve as a coupling element between stages in which case resistor 34 in any one of FIGS. 2, 3 and 4 may be a standard resistor. It has been pointed out that the power supply for circuits of this type is generally fed from a network; the symbols for batteries have been used in the drawing for the purpose of simplification, it being assumed that the internal impedance of such supplies is relatively low, where the supply itself forms part of the capacitor discharge path.

Accordingly, from the foregoing remarks, it is to be understood that the present invention is to be limited only by the spirit and scope of the appended claims.

What is claimed is:

1. An electronic circuit including at least one discharge device having at least one control electrode and a cathode, and a resistive device coupled to said cathode, said resistive device having a positive temperature characteristic, voltage supply means for said discharge device, said electronic circuit comprising means for eliminating output current surges which occur due to the characteristic of said resistive device having a positive temperature characteristic and are caused by transient voltage supply interruptions, said current surge eliminating means including a capacitance and a shunt resistance across it, both being connected in series with said voltage supply and also connected in the grid-cathode circuit of said discharge device, for supplying through said discharged capacitance a negative cut-off biasing potential to said control electrode substantially instantaneously upon restoration of the voltage supply, and means for gradually recharging said capacitance to thereby decrease the biasing potential over a period of time sufficiently long to permit said resistance having a positive temperature characteristic to return to its normal operating conditions.

2. In an electronic circuit, an electron discharge device having a control electrode and a second electrode, at least one resistor having a positive temperature characteristic being coupled to said second electrode, means for supplying plate current to the device, and means for inhibiting current surges upon restoration of plate current supply subsequent to short interruption, said surge inhibiting means including means for applying substantially instantaneously a negative cut-off potential to said control electrode of said device upon restoration of plat-e current supply, and means for decreasing the negative cut-off potential at a rate sufiiciently slow to prevent output current surges resulting from restoration of plate current supply by inhibiting said electron discharge device until said resistor returns to its resistance value at operating conditions.

3. A surge suppressor according to claim 2, in which said electrode is a control grid.

4. A surge suppressor according to claim 2, in which said electrode is a suppressor grid.

5. A system for inhibiting an electron discharge device upon power supply interruption, said electron discharge device having at least one control electrode and forming one stage of an electrical circuit, said electrical circuit including a resistive circuit element having a positive temperature characteristic, the system comprising means for storing an electric charge during normal operation conditions, means for substantially instantaneously discharging the storage means upon power supply interruption, means energized by the power supply, when it is restored, for applying a negative cut-off bias to said control electrode of said electronic discharge device through said discharged storage means, and circuit means for gradually decreasing the cut-off bias to restore full output current from the stage to permit the system to return to its normal operating conditions.

6. In a surge suppressor, at least two vacuum tubes, each tube forming part of a distinct stage of a circuit and having at least one control electrode, a resistor having a positive temperature characteristic coupled to one of said tubes, plate current supply means for said tubes, means for applying a negative cut-off potential to one of the control electrodes of the other of said tubes substantially instantaneously upon interruption in plate current supply, and means for decreasing the potential applied to said electrode of said other tube at a rate of the same order of magnitude as that of temperature decrease at the cathode of said one tube and as that of temperature increase of said resistor, whereby output current surges are suppressed.

7. In a surge suppressor for a circuit using vacuum tubes, means for supplying plate current to the tubes, a temperature-responsive resistor associated with the cathode branch of the plate circuit of one of the tubes, a capacitor, means for applying a relatively constant potential to the capacitor under normal operating conditions, means for instantaneously discharging the capacitor upon interruption of the plate current supply, means for applying a negative cut-off grid biasing potential to one of the tubes through the discharged capacitor, and means for comparatively slowly recharging the capacitor to decrease the biasing potential applied to the grid.

8. In a circuit, at least one vacuum tube, a resistor having a positive temperature characteristic controlling the output current of said circuit, means for supplying plate current to the tube from a network subject to short interruptions, means for eliminating output current surges, :said means comprising a capacitor, means for applying :a voltage which is controlled by the plate current supply to the capacitor, circuit means having a short time constant for substantially instantaneously discharging the capacitor upon plate current supply interruption, means for applying a negative cut-0E biasing potential through the discharged capacitor to the grid of the tube when plate current supply is reapplied, and high resistance circuit means for relatively slowly recharging the capacitor to gradually decrease the cut-off grid potential, whereby full plate current is restored when the positive temperature characteristic resistor and the tube cathodes have substantially reached their ranges of normal operating temperature conditions.

9. In a vacuum tube circuit including a resistor having a positive temperature characteristic for controlling the output current, means for eliminating output current surges caused by short power supply interruptions, said means comprising a capacitor, a source of supply voltage, means including a voltage divider for applying a portion of the supply voltage to the capacitor, means including a resistor having a relatively low resistance connected across said capacitor for substantially instantaneously discharging the capacitor upon interruption in supply voltage, means for applying a portion of the supply voltage, when restored, through the discharged capacitor to a grid of one of the vacuum tubes to prevent plate current flow by excessively biasing the grid, and long time constant circuit means including a relatively high resistor forming part of the voltage divider for recharging the capacitor to gradually restore a normally operating negative grid bias at a rate substantially corresponding to the rate of cooling of the cathode of the vacuum tube and of heating of said resistor.

10. In an operating circuit for a vacuum tube, having at least one control grid, the circuit including a resistor having a positive temperature characteristic, a power supply having positive and negative supply lines and an intermediate tap for supplying an intermediate potential, means for suppressing output surges caused by short power supply interruptions, said means comprising a voltage divider including a high-value and a low-value resistor and connected in series across the supply lines, a capacitor connected across the low value resistor and to the negative supply line, means connecting said intermediate power supply tap to the cathode of the tube to apply an appropriate plate-cathode voltage to the tube, a diode for connecting the voltage divider junction between the resistors and the power supply tap to form an additional discharge path for the capacitor through a portion of the power supply, the junction between the resistors being connected to said control grid of the tube, whereby a cutoff biasing potential is applied to said grid through said capacitor when power is restored following interruption, the capacitor thereafter being recharged at a comparatively slow rate through the high value resistor of the voltage divider, and the biasing potential being gradually decreased until normal temperature operating conditions of the tube cathodes and of said resistor are restored.

11. In an electronic circuit arrangement for eliminating output current surges caused by short supply intrruptions and including a resistor having a positive temperature characteristic, a vacuum tube having a plate, a cathode, a control grid for receiving signals and a suppressor grid, means including a positive and a negative lead for supplying power to the plate circuit of said tube, additional power supply means connected in series with said plate circuit supply means, a voltage divider connected across both power supplies and including a comparatively high value resistor at its positive end and a comparatively low value resistor at its negative end, the junction between said resistors being connected to said suppressor grid to impress a slightly negative potential on said suppressor grid under normal operating conditions, a capacitor connected across the low value resistor of the voltage divider for accumulating a charge in accordance with the voltage drop appearing across said low value resistor, a diode connected between said junction and the negative plate current supply lead for substantially instantaneously discharging the capacitor upon supply interruption, the capacitor forming a shunt across the low value resistor, whereby a negative cut-01f biasing potential is impressed on said suppressor grid and the capacitor recharges at a comparatively slow rate through said high value resistor of the voltage divider upon restoration of the power supply, said negative cut-01f biasing potential decreasing at the ysame rate until normal operating conditions of said cathode and said resistor having a positive temperature characteristic are restored.

12. In an electronic circuit arrangement for eliminating output current surges caused by short supply interruptions, a vacuum tube having a plate, a cathode and a control grid for receiving signals, means including a positive and a negative lead for supplying power to the plate circuit of said tube, additional power supply means connected in series with said plate circuit supply means, a voltage divider connected across both power supplies and including a comparatively high value resistor at its positive end and a comparatively low value resistor at its negative end, the junction between said resistors being connected to said control grid to impress a slightly negative potential on said control grid under normal operating conditions, a capacitor connected across the low value resistor of the voltage divider for accumulating a charge in accordance with the voltage drop appearing across the low value resistor, a diode connected between said junction and the negative plate current supply lead for substantially instantaneously discharging the capacitor upon supply interruption, the capacitor forming a shunt across said low value resistor, whereby a negative cut-otf biasing potential is impressed on said control grid, and the capacitor recharges at a comparatively slow rate through said high value resistor of the voltage divider upon restoration of the power supply, said negative cut-off biasing potential decreasing at the same rate until normal operating conditions are restored.

13. In a surge suppressor for a circuit using vacuum tubes, means for supplying plate current to the tubes, means including a resistance having a positive temperature characteristic for stabilizing operation of said circuit, whereby output voltage surges are caused upon transient voltage supply interruptions, a capacitor, means for applying a relatively constant potential to the capacitor under normal operating conditions, means for instantaneously discharging the capacitor upon interruption of the plate current supply, means for applying a negative cut-oft" grid biasing potential to one of the tubes through the discharged capacitor, and means for comparatively slowly recharging the capacitor to decrease the biasing potential applied to the grid.

14. A vacuum tube circuit comprising, means including a resistance having a positive temperature characteristic for stabilizing operation of said circuit, whereby output voltage surges are caused upon transient voltage supply interruptions; and means for eliminating said surges comprising a capacitor, means for applying a relatively constant potential to the capacitor under normal operating conditions, means for instantaneously discharging the capacitor, means for applying a relatively constant potential to the capacitor under normal operating conditions, means for instantaneously discharging the capacitor upon interruption of the plate current supply, means for applying a negative cut-off grid biasing potential to one of the tubes of the circuit through the discharged capacitor, and means for comparatively slowly recharging the capacitor to decrease the biasing potential applied to the grid.

Reierences Qitcd by the Examiner UNITED STATES PATENTS 2,468,082 4/ 1949 Chatterjea et al 330-143 2,758,273 8/1956 Martin 323-41 X 2,826,734 3/1958 Miller 323-41 2,831,130 4/1958 Obloy 317-149 2,902,548 9/1959 Moeller 330-143 LLOYD MCCOLLUM, Primary Examiner.