US2709768A - Cathode ray tube protection circuits - Google Patents

Description

May 31, 1955 G..w. KING CATHODE RAY TUBE PROTECTION CIRCUITS 2 Sheets-Sheet l Filed June 5, 1952 May 31, 1955 2 Sheets-Sheet 2 Filed June 5, 1952 -Ab I CLOSED @any ca/vmcr.:

" nited States Patent 2,709,768 Patented May 31, 1955 CATHGDE RAY TUBE PRTEC'HON CIRCUITS George W. King, Pleasantville, N. Y., assignor to General Precision Laboratory Incorporated, a corporation of New York Application June 5, 1952, Serial No. 292,011

13 Claims. (Cl. 315-20) This invention relates to a protective circuit for use in connection with cathode ray, or other discharge tubes. In particular, the invention relates to a circuit for preventing damage to a discharge tube consequent upon the failure of any portion of its circuit or associated circuits. The invention is particularly adapted to the prevention of beam burns7 on the fluorescent screen of a cathode ray tube having normally continuous horizontal and vertical scansion. Such beam burns are caused by the absence of, or the reduction of either the horizontal or vertical scanning velocity.

This invention is applicable to any magnetically detlected cathode ray tube, but is particularly adapted to the type having a fluorescent screen target for the cathode ray, although it is also applicable to the Lenard tube to prevent damage to the external target thereof. The invention is also applicable to any other type of discharge tube associated with a sweep circuit, to prevent darnage due to loss of the sweep voltage.

Although the instant invention has many applications in discharge tube circuits where protection is desirable, the mostimportant application is in circuits of such type that protection against any possible loss of scansion is necessary. In this class are high voltage kinescopes for use in large screen projection television receivers. For example, in order to secure enough light from a kinescope to project onto a theatre screen it is necessary to employ a very intense cathode ray beam. In the event of the cessation of either the vertical or horizontal scan a line will be burned on the uorescent screen in much less than a second, and the loss of both horizontal and vertical scan will burn a spot on the screen in a period measured in microseconds.

In order to protect the tube from any fault that is at all likely to occur that might result in excessive instantaneous beam current being applied to the screen, the protective circuit must protect the tube against damage in the event anything happens which seriously reduces the effective area over which the beam scans and also must protect against loss of bias potential. Protection against loss of scansion must protect against this loss from any cause, including opening, short-circuiting or grounding of the deflection yoke itself, and complete protection must include protection in turn against failure of any part of the protective circuit itself, ,and against failure of all power supplies to that circuit. Protection must also extend to the occurrence of any or all of these types of fault at the same time.

The instant invention provides this type of protective circuit, including appropriate components for protection against failures of both horizontal and vertical scanning circuits. These protective components are energized by special windings incorporated in the deilection yoke, one such winding being closely coupled magnetically to the horizontal deflection winding therein, and another such winding being similarly coupled to the vertical deflection winding therein. In addition to the usual `overvoltage relay cutout in the high voltage supply, and a control pulse.

grid positive current limiter, the power supply circuits include relays actuated by loss of any of the several voltages to open selected primary windings of the power transformers, with the effect of reducing the kinescope beam current to zero.

The portion of the protective circuit associated with the horizontal deiiection yoke winding comprises a detector or demodulator having circuits With appropriately short time constants. In the vertical scan protective circuit, however, such a circuit cannot be employed with success to sense the loss of the vertical scansion, because the sensing must be accomplished in a fraction of one vertical time period, for which the usual detection circuits are unsuitable. A triggered multivibrator. circuit is therefore employed in conjunction with an` electromagnetic relay to safeguard the circuit within a few milliseconds after the loss of one vertical scan sawtooth This speed is ample for protection because the time constants of the conventional vertical deflection circuit are great enough to prevent complete collapse of vertical scansion Within this time.

One object of the present invention, therefore, is to provide a circuit for the prevention of damage in a cathode ray tube having a moving cathode ray beam due to failure of the beam to move in its normal manner.

Another object is to provide a circuit for the prevention of beam burns in the fluorescent screen of a cathode ray tube.

Another object is to provide a circuit adaptable to the protection of any discharge tube.

Still another object is to provide a circuit for the reduction of the cathode ray beam of a cathode ray tube to a harmless intensity upon failure of either or both horizontal or vertical scanning circuits associated with the tube.

Still another object is to provide a circuit for protecting cathode ray tubes in the event of simultaneous failure of both the scanning circuit, and the protective circuit itself.

A further understanding of the invention may be secured from the detailed description and the accompanying drawings in which:

Figure l is a schematic diagram of the circuit of the invention.

Figure 2 graphically indicates the operation of the circuit of the invention.

Referring now to Fig. l, a cathode ray tube of the type employed for theatre television projection is indicated at 11. This tube may be, for example, of the type 7NP4 designed for the application of an 80 kv. supply to its accelerating anode 12. The maximum instantane ous beam current may be greater than 6 milliamperes, so that the maximum beam energy at the screen is more than 48() watts. At this rate the beam power concentrated at a small spot on the screen upon the failure of both scanning circuits attains the extremely high beam density of 6400 kw. per square inch of screen surface, which will cause the destruction of the screen in a time measured in microseconds. The beam power that is concentrated in a line on the screen by the failure of either scanning circuit is 8 kw. per square inch, suiiicient to ruin the tube within milliseconds.

At these power rates it is obvious that in order to preserve the tube from instantaneous destruction the continuity of both scans must be maintained, or in the event of interruption of either scan the tube beam current must be cut off almost instantaneously.

The tube 11 is provided with a cathode 13, control grid 14, No. 2 grid 16, and a focusing grid No. 3 indicated at 17. The tube neck is surrounded by a deilecting yoke containing a horizontal deflection winding 18 and a vertical detiection winding 19, these windings being drawn arcanes schematically and separate from the tube for clarity. The horizontal winding 13 is vconnected to a conventional horizontal deection or scanning circuit 21 which energizes it with a varying potential having a period of 631/2 microseconds. A conventional vertical deflection or scanning circuit 22 energizesthe winding .19 with a varying potential having a period of of a second. These circuits are timed, respectively, by horizontal and vertical synchronizing pulses from a composite television signal through a synchronization signal separator (not shown), this signal being received -by radio or cable from the television transmitter.

The video component of the television signal is amplified in an amplifier, -thenal stage of which comprises the tube 23 having an output biack level of +200 volts and an extreme whitelevel value of -l-75 volts which is applied from the plate 2t? of tube 23 to the cathode 13 of the cathoderay tube.

The control grid 14.is normally biased at +75 volts corresponding to a black level bias of -l25 volts relative to the cathode .13, which at picture black normally reduces the beam current to zero.

The voltage `on grid 14 is supplied through a l0() K. resistor 24 to preventcxcessive current flow if the grid should become positiv/e. The voltage is supplied through conductor 25, aconventional manual ori-off switch 26, contacts 27 of relay 23, conductor 29, cathode 31 of a triode section 32 and cathode resistor 58. The relay 23 is normally closed ,during picture reception, and the cathod resistor 58 is connected to the 105 volt point on a source of direct current potential 32.

This cathode 31 is maintained at +75 volts in the following manner. A pick-up winding 33, having a total of six turns, is wound around the horizontal detiection winding 18 of the deflection yoke so as to have very close magnetic coupling thereto while electrically insulated from it. A voltage is induced in this pick-up coil 33 by any change of current in the winding 18 and, being proportionalto the rate of change of current, is much greater during flyback than during the forward scan. The polarity is such-that the pulse lproduced during llyback is positive. This positive puise is supplied through a shielded conductor 34 .and a 200 ohm resistor 36, which consitutes the input for tube .42. The pulse potential at the junction 37 of resistor 36 is applied to an integrating circuit composed of va 100 Kresistor 3S and a 100 mmf. condenser 39, the resulting integrated voltage being of such reduced peak value and reduced rate of decay as to be suitable for energization of the control grid 41 of a pentode amplifier 42 within each horizontal line period. The outputis coupled from plate 43 through a condenser 44 toa rectifier comprising two diodes d6 and 47. The relatively continuous direct current output is applied between the anode connection 43 and the cathode i9 of a triode section 51 connected as a diode limiter, the anode connection 48 being at a positive potential. The cathode 49 of the limiter 51 is maintained at +65 volts by its connection to a voltage divider composed of series resistors 52 and 53 connected between the plus 400 voltage supply bus 54 and ground. The potential of the anode of this limiter may therefore drop below plus 65 volts but cannot rise above it. However, component magnitudes of the associated circuit are such that with normal horizontal scan current flowing the integrated anode voltage is fully maintained at .plus 65 volts at all times. This value of positive voltage is applied through conductorSG to the control grid.57 of a cathode follower section c32, and results in such current ow as to cause .the cathode 31. to set itself at a potential about l0 volts above the grid 57', or at plus 75 volts. This voltage may also be considered as the drop in resistor S due to the tube current, and is applied as described through conductor 29, closed relay contacts 27, closed switch 26,

conductor 25, andjresistor 24 to the No. l grid 14 of Vcathode: .ray tube 11.

The twin triode having sections 32 and 51, has its heater S0 energized from an insulating transformer 55 in order to prevent cathode ray tube failure consequent upon breakdown of the insulation between heater and cathode in this twin triode.

A negative bias potential of minus 180 volts is connected through 100 K. resistor 59 to the fixed Contact 60 of switch 25. This potential has no effect upon the cathode ray tube 11 when the plus 75 volt bias is applied, but if the plus .volt circuit is opened the voltage of the control grid -14 immediately drops from plus 75 volts to minus 180 volts. For example, opening of the on-off switch 26 causes the control .grid 14 to go to minus l volts, applying cut-off bias to the cathode ray tube and thereby interrupting its cathode ray beam. Opening of the relay contact 27 has the same effect.

The relay 28 is kept in its normal operating condition with its contacts 27 closedvin the following manner. A winding 61, having a total of l2 turnss urrounds the vertical defiection winding `1,9, Aand is closely coupled to the latter but insulated therefrom. This pick-up coilel therefore has a potentialinduced in its proportional to the rate of change of vertical deflection current, so that a sharp positivev pulse is produced atthe time of each vertical yback. This positive pulse isgraphically depicted at A in Fig. 2. It is transmitted through conductor 62 to the control grid y63a .of a triode amplilieni, from the plate 66 of which the pulse is further ampliiied in triode 67. If the succeeding tube 68 be removed from its socket the pulse at plate69 ot' triode V67 has the appearance shown at B, Fig. 2, the series capacitances in this two-stage arnplifying circuit being so large las to exert very little diiferentiating effect.

The tube 68 together with tube 71 comprises a monostable, or one-shot multivibrator, the tube 71 being normally conducting and tube 68 normally non-conducting. Upon application of the positive pulse shown in B, Fig. 2, to the grid72, the tube 68 .becomes conducting and the tube 71 non-conducting. This unstable state persists' for approximately 60% of one vertical scan period, or l0 milliseconds, when the multivibrator spontaneously returns to its normal state. This cycle is indicated by vthe curve C, indicating-the change of potential at the grid 72 with time. During thevtime that grid 72 is positive, plate current iows through tube 68, the current-time curve being indicated at D. .This current flows through resistor 73 and relay 28 from lthe plus 285 volt supply, the relay being shunted bya resistor .-74 to reduce its release time. This release time isdesigned so that the contacts 27 open about l0 milliseconds after the relay winding current has been, interrupted. This Arelease time therefore is greater than the no-current interval ingraph D, but is less than one vertical scan period, andwill occur vat the point R in curve E, lmillisecondsafter the .end of any ymultivibrator unstable state period `when the next lfollowing vertical pulse. isomitted. Accordingly, since` the period of the vertical sweep 'is -less than :the period of the multivibrator plus therelease interval, the winding 23 remains venergized and switch 27 closed as long as the vertical sweep pulses are not interrupted yor the frequency thereof is not appreciably decreased.

The power supply for the video tube 11 includes a conventional high voltage .supply -76 which is independent of the'delection circuits. Thepower supply furnishesv 8() kv. for the cathode ray tube accelerating anode 3.2 and 18 kv. for the focusing grid 17. `Conventional overload protec- .tion is included. v An 80G-volt lsupply 77 is directly conltnal video amplifier` tube ,23 fand also theiprotective circuits. The winding of relay 79 is connected across the minus S-volt output of the power supply unit 82, which supplies bias voltage to the final video tube 23 and to the protective circuits. This supply unit 82 also supplies minus 180 volts to the cathode ray tube 11 as a cut-oit voltage, and at the same time energizes the protective circuits. Thus any interruption of the plus 2S5-volt supply circuit 81 output opens the SOO-volt, 18 kv., and 80 kv. power supply circuits, and any interruption of the minus 180/ 10S-volt supply unit 82 does the same and in addition opens the plus 285-volt power supply circuit 81. The protective circuits described protect against probable faults occurring singly or in any possible combination as may be seen from the subsequent description.

The loss of, or material reduction in the magnitude of the horizontal scanning magnetic ield may be caused by a fault in the horizontal deflection circuit Z1, in loss of horizontal synchronizing pulses thereto, or by an open circuit, short circuit, ground or cross in the deflection yoke itself. Loss or serious reduction of horizontal scansion due to any cause is .sensed by the pick-up coil 33, and causes such loss or reduction of the signal pulse voltage in conductor 34 that the potential of junction 37 falls to ground, or nearly to ground. The integrated pulse output of amplifier 42 therefore disappears, and because of loss of input, the bias on the control grid 41 drops below cut-off negative potential. The rectified potential at the cathode of tube 47 disappears and the potential at junction 48 falls to minus'105, causing the triode 51 to become almost cut-off. The connection 56 therefore applies the minus 10S-volt potential to the control grid 57 of cathode follower 32, so that its cathode 31 assumes a potential of about minus 90 volts. This potential is applied through conductor 29, closed contacts 27 of relay 28, closed switch 26, conductor 25, and resistor 24 to the control grid 14 of the video tube 11, reducing the beam current of the latter to zero. This occurs for the reason that since the cathode 13 is at plus 200 volts, the grid bias relative to cathode becomes minus 300 volts, which is far below the cut-off value of minus 125 volts for this tube.

The circuit parameters are so chosen that this entire cut-off action, from the time of loss of the first horizontal signal pulse to reduction of beam current in the video tube 1l to zero, takes no more than the time of live or six horizontal periods, or not over 400 microseconds. Such speed is adequate for protection of the cathode ray tube 11 under all conditions involving loss of horizontal scanning.

A fault in any of these five tubes and tube sections causing zero tube current or even materially reduced tube current causes the cathode ray tube beam current to become zero. The same action occurs upon failure of the plus 40G-volt supply.

In a similar manner, loss of or substantial reduction in the magnitude of the vertical scanning magnetic field may be caused by a fault in the vertical deflection circuit or by loss of the vertical synchronizing pulses, or by an open-circuit, short-circuit, ground or cross in the deilection yoke itself. Loss or substantial reduction of vertical scansion due to any cause is sensed by pick-up coil 61, and results in such loss or `reduction of the pulses applied through conductor 62 to the amplifier 64 that the multivibrator energized therefrom is not triggered. The multivibrator tube 68 therefore remains continuously non- 'conducting, starting at the end of its last non-stable state. The relay 28 connected thereto opens about 4 milliseconds after the time of beginning of the tirst omitted or weak vertical deflection signal pulse, causing minus 180 volts to be applied through resistors 59 and 24 to the control grid 14 of the video tube 11 thus applying a relative negative bias of minus 380 volts to the tube, and reducing its beam current to zero. Any tube or other component fault which causes current through the winding of relay 28 to be interrupted causes the opening of contacts 27 thus applying a high negative bias on control grid 14 thus making video tube 11 inoperative and insuring against any injury to the latter.r

ply or with opening of switch 26, the control grid 14 would fall only to ground potential. This will ordinarily result merely in the production of a very dim picture on the luorescent screen of the video tube 11 and only in the event of a high resistance leak between the control grid 14 and a source of positive potential, either within or outside of the video tube will the beam current rise dangerously. To protect against this contingency the relay 79 is provided. It is also necessary to protect against loss of the minus 10S-volt supply because this supply is used to bias the final video tube 23. Bias loss would cause such high current in this tube as to reduce the potential of cathode 13 causing a dangerous rise in beam current. However, loss of the minus 105 bias voltage is protected against by the provision of the same protective relay 79, since the minus 10S-volt supply is derived from the minus lSO-volt supply. The contacts are connected to open the primary power supply to the plus 285-volt, plus SOO-volt, 18 kv. and 80 kv. supply circuits. Upon opening these circuits, no beam current whatever can ilow.

Loss of the plus 285-volt supply to the video tube 11 would cause the cathode 13 to go -to ground potential, causing an excessively high beam current. The relay 78 is therefore shunted across the output of this supply, so that failure causes opening of the plus SOO-volt 18 kv.

and kv. supplies, again preventing any beam current Therefore, this circuit, consisting of protective sections cooperating with the video supply and horizontal and vertical deection yoke supplies, is arranged to cut o the cathode ray beam current upon the occurrence of any probable type of failure, and fails safe upon the failure of any of its own components.

What is claimed is:

l. ln a circuit for energizing a cathode `ray tube having a plurality of electrodes including a cathode, an anode, a control electrode, electro-magnetic means for defiecting said electron beam in an oscillatory manner in response to a pulsing electrical potential, means magnetically coupled with said electro-magnetic deecting means for cooperatively controlling the energization of said respective electrodes of said tube in response to the energization of said deecting means.

2. In a circuit for energizing a cathode ray tube having a plurality of electrodes including a beam-generating cathode, an anode and a control electrode, deflecting means for oscillating an electron beam along orthogonal axes in response to respective pulsating electrical potentials, a source of biasing potential for said control electrode, means including conjointly acting protecting circuits for applying a normal biasing potential to said control electrode in response to predetermined normal ener-- gization of said deecting means and for immediately applying a cut-olf bias potential to said control electrode in the event of substantial deviation in the energization of the means for oscillating the electron beam along either orthogonal axis, one of said protecting circuits being responsive to a train of electrical pulses for applying the normal biasing potential to said control electrode, the other of said protecting circuits being responsive to a single pulse of the respective energization potential for applying the normal biasing potential to said control electrode.

3. The combination as set forth in claim 2 in which said deflecting means comprises an electromagnetic detiection yoke having two deflection coils the axes of which are disposed in orthogonal relation and in which said protective circuits include two pick-up coils respectively coupled magnetically to said deflection coils, one of said protection circuits including an integrating network which is responsive to a continuous train of impulses for establishing a normal biasing potential for said control electrode, the other of said protective circuits including a multivibrator having a periodicity which is less than the periodicity of the pulses normally supplying its associated deflection coil.

4. The combination as set forth in claim 2 in which said other protection circuit includes a multivibrator having a periodicity less than the periodicity oi the pulses normally supplying` its associated deflection coil, a relay, energized by said multivibrator, having a release time of such value thaty when added to the periodicity of said multivibrator the total period is greater than the period of the pulses normally energizing the associated deliection coil, said relay adapted to control the circuit through which the normal energizing potential is applied to said control electrode. Y Y

5. In a circuit for energizing a cathode ray tube having a plurality of electrodes' including a beam-generating cathode, an anode, a tirst control electrode and a second control electrode, an electromagnetic detiection yoke having a horizontal dedection coil arranged for energization by sawtooth current pulses and a vertical deilection coil adapted to be energized by sawtooth current pulses at a frequency less than that of the frequency of the pulses energizing said horizontal deflection coil, modulator amplitier means for applying video signals between said first control electrode and the cathode oi: said video tube, respective sources of power supply for supplying a normal biasing voltage between said electrodes, means including conjointly acting protecting circuits for establishing circuits for applying said normal operating potentials to said electrodes, said protective circuits including respective pick-up coils magnetically coupled to said deflection coils and means responsive to the normal energization of said deflection means for establishing and maintaining circuits for applying a normal biasing potential to said electrodes and for immediately applying a cut-off bias to said first control electrode in the event of substantial deviation in the energization of either of said deliecting coils and means responsive to the failure of either of said protective circuits for interrupting the bias potential on said second grid, said-protective circuits including electrical interlock means for causingl a protective bias to be established between the cathode of said video tube and said tirst control grid in the event of failure of the power supply to either of said protective circuits.

6. A cathode ray tube circuit comprising, a cathode ray tube including a fluorescent screen, a beam-generating cathode, a control grid', an anode and an electromagnetic deflection yoke having at least a horizontal deflection coil arranged for energization by sawtooth deflection voltage pulses, power supply means for supplying bias voltages to said cathode and said control grid, and a protective circuit including a pick-up coil magnetically coupled to said horizontal del'lection coil for reception or voltage pulses during horizontal tlyback time, integrating circuit means connected to said pick-up coil to integrate said voltage pulses, a discharge tube ampiitier connected to said integrating circuit rncans for energization thereby, a rectiiier in the output of said amplifier, a positive voltage limiter' in said rectifier output, output discharge tube means for applying said positive limited voltage to said control grid to maintain it at a selected negative voltage level relative to the zero modulation cathode voltage level, whereby the failure or substantial reduction of said horizontal voltage pulses causes the bias voltage on said control grid to go below cut-off voltage level cutting off said beam, and automatic means for cutting ofi said beam upon failure of any part of said protective circuit.

7. A cathode ray tube circuit comprising, a cathode rag/*tube including a fluorescent screen, a beam-generating cathode, a control grid, an anode and an electromagnetic deflection yoke having at least a vertical detiection coil arranged for energization by sawtooth deflection voltage pulses, bias power supply means for supplying bias voltages to said cathode and control grid, and a protective circuit includuing a pick-up coil magnetically coupled to said vertical deliection coil for reception of voltage pulses during vertical flyback time, a discharge tube amplitier connected to said pick-up coil for actuation thereby to produce amplified voltage pulses, a mul tube monostable multivibrator having a stable and an unstable state, one of the tubes of said multivibrator being non-conductive in the stable state, electrical connection means from the output of said amplifier to said multivibrator for causing conduction through a selected tube by said amplitied voltage pulses whereby the multivibrator is maintained in its unstable state at least 40% of the time, an electromagnetic relay, the winding thereof being connected for energization by the plate currentbf said selected tube and the normally-open contacts thereof being connected to apply normally operative voltage to said control grid, an electrical connection from said bias power supply means to said control grid whereby upon the omission of a single vertical pulse said electromagnetic relay contacts are released and cut-off biasing voltage from said bias supply means effects cut-oft of said beam, and means for cutting oli said beam upon failure of any part of said protective circuit.

8. A cathode ray tube circuit comprising, a cathode ray tube having a plurality of electrodes including a beamgenerating cathode, a control electrode, an anode and a deflection yoke including a horizontal deiiection coil and a vertical deiiection coil, means for energizing said vertical detlection coil with periodic voltage pulses to move said cathode ray vertically, means for energizing said horizontal deflection coil with periodic voltage pulses to move said cathode ray horizontally, modulating amplifier means for applying amplified modulating video signals between said control electrode and said cathode, power supply means for applying bias voltages to said electrodes, a pick-up coil magnetically associated with said vertical deflection coil, a second pick-up coil magnetically associated with said horizontal deflection coil, and a protection circuit normally continuously actuated from said horizontal and vertical pick-up coils, said protection circuit being connected to said power supply and to said cathode ray tube for control of the application of said bias voltages, whereby cessation or reduction of the energization of either said horizontal or vertical coils or of any bias potential or failure of any part of said protection circuit reduces the cathode ray beam to a harmless magnitude.

9. A protective circuit for a cathode ray tube comprising, a deflection circuit adapted to operate cyclically, means for developing a direct voltage pulse once during each cycle of operation of said deliection circuit, a monostable multivibrator which when triggered remains in its unstable state for a period of time less than the duration of one cycle of said deflection circuit, means for applying each developed pulse to said multivibrator, a relay including an operating winding and a set of normally open contacts, said relay being one which releases a short time after deenergization of its operating winding, means for energizing said winding while said multivibrator is in its unstable state, and means for supplying an electrode of said cathode ray tube with operating potential through said contacts.

10. In a circuit for energizing a cathode ray tube having a plurality of electrodes including a cathode, an anode and a control electrode, a protective circuit comprising a source of negative bias voltage connected to said control electrode, a source of anode voltage, and means responsive to failure of said source of bias voltage for disabling said source of anode voltage.

ll. In a circuit for energizing a cathode'ray tube having a plurality of electrodes including a cathode, an anode, and a control electrode and having deflecting means for causing the electron beam to scan the face of the tube, a protective circuit comprising a source of negative bias voltage connected to said control electrode, a source of anode voltage, means responsive to the energization of said deflecting means for overcoming said bias voltage, and means responsive to failure of said source of bias voltage for disabling said source of anode voltage.

12. In a circuit for energizing a cathode ray tube having a plurality of electrodes including a cathode, an anode, and a control electrode, a protective circuit comprising, a source of low voltage for applying a positive potential to said cathode, a source of high voltage for applying a higher positive potential to said anode, and means responsive to the failure of said low voltage source for disabling said high voltage source.

13- In a circuit for energizing a cathode ray tube having a plurality of electrodes including a cathode, an anode, and a control electrode and having deecting means for References Cited in the tile of this patent UNITED STATES PATENTS 2,098,384 Goodrich Nov. 9, 1937 2,119,372 Wendt May 31, 1938 2,202,171 Stocker May 28, 1940 2,261,645 Delvaux Nov. 4, 1941 2,261,776 Poch Nov. 4, 1941 2,577,848 Greenleaf et al Dec. 11, 1951 2,584,932 Snyder et al Feb. 5, 1952 2,635,208 Cage Apr. 14, 1953

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