US2839704A

US2839704A - Switching circuit

Info

Publication number: US2839704A
Application number: US525853A
Authority: US
Inventors: Schlang Arthur
Original assignee: Chromatic Television Laboratories Inc
Current assignee: Chromatic Television Laboratories Inc
Priority date: 1955-08-02
Filing date: 1955-08-02
Publication date: 1958-06-17
Anticipated expiration: 1975-06-17
Also published as: DE1025935B; GB830085A

Description

June 17, 1958 A. SCHLANG SWITCHING CIRCUIT 3 Sheets-Sheet 1 Filed Aug. 2, 1955 R O T N E V m Ari'hur Schlong BY Bww%.

ATTORNEYS June 17, 1958 A. SCHLANG SWITCHING CIRCUIT 5 Sheets-Sheet 2 Filed Aug. 2, 1955 INVENTOR Arihur Schlong ATTORNEYS June 17, 1958 A. SCHLANG 2,839,704

SWITCHING cmcun Filed Aug. 2, 1955 s Sheets-Sheet a I INVENTOR 0 Arthur Schlong ATTORN EYS SWITCG CIRCUET Arthur Schlang, Valley Stream, N. Y., assignor to Chromatic Television Laboratories, Inc., New York, N. Y., a corporation of California Application August 2, 15755, Serial No. 525,853

' 6 Claims. (Cl. 315-21) This invention relates to a method and means for successively charging a capacity in opposite polarities in order to produce across the capacity of a voltage having, with respect to time, the shape of a square Wave or of a combination of square waves which may cyclically include time intervals of Zero voltage. The invention finds particular application in the operation of cathode-ray tubes of the type disclosed in Patent No. 2,692,532. Cathode-ray tubes of this type include, positioned between the electron gun and phosphor screen or target area, one or more grids generally coextensive in area with the screen. Each grid (if more than one are provided) comprises two sets of interlaced mutually insulated conductors, usually wires, disposed on a generally plane or cylindrical surface. As described in Patent No. 2,692,532 alternate conductors of the grid belong to the two insulated sets of conductors which make up the grid. By the application of potential differences between the two sets, the electron beam, whatever its point of impact on the area of the grid, will in its passage between the grid and screen be deflected in a direction generally transverse of the grid conductors. In one form of tube of this type the screen comprises a plurality of strips of phosphors luminescent on electron impact in a plurality of colors. These phosphor strips, which may for simplicity be referred to as strips of differently colored phosphors, are laid down in a repeating cycle of colors, the strips extending generally parallel to the conductors of the grid. The arrangement is such that with a potential difference of one polarity and appropriate magnitude between the two sets of grid conductors the electron beam will excite phosphor areas of one color whereas with the opposite potential difference the phosphor areas of another color will be excited, and with no potential difference between the two sets of conductors phosphor areas of a third color will be excited. The grid may therefore be called a color switching grid. In these tubes the grid is employed in conjunction with a conducting layer on the side of the phosphor pattern adjacent the electron gun in order to provide an accelerating and focusing voltage between the grid and screen.

For presentation on the screen of data in different colors, switching voltages must be applied to the grid i. e. a succession of potential differences must be applied between the two sets of grid wires. For certain types of display this is advantageously done with square voltage waves, and the present invention relates to such use of these cathode-ray tubes.

At frequencies which, in presently preferred cathode-' ray tubes of the type above referred to, are below 100 kc. or thereabouts, the impedance of the grid structure,

presented at terminals to which the two sets of conduc- Patented June 17, 1958 have a substantially rectangular wave form. The invention provides a method and means whereby such a rectangular wave form, including potential differences of opposite polarity and intervals of zero potential difference, may be developed between the two sets of grid conductors from a unidirectional potential source.

The invention will now be described in detail with reference to the accompanying drawings in which:

Figs. 1-4 are schematic diagrams of various embodiments of the invention; and

Fig. 5 is a diagram of exemplary wave forms useful in explaining the operation of the invention.

In Fig. 1 the capacity with respect to each other of the two interlacted sets of grid conductors in a cathoderay tube of the type disclosed in Patent No. 2,692,532 is schematically indicated at C. The circuit of Fig. 1 is arranged to produce across C from a unidirectional voltage source 2 a voltage of rectangular wave shape. For this purpose C is connected between two diagonally opposite terminals or arm junction points 4 and 6 of the bridge circuit whose other diagonally opposite

terminals

8 and 10 are conductively connected with the source 2. For charging of the capacity C in successively opposite polarities the bridge circuit includes four switches in the form of vacuum tubes V V one in each of the arms of the bridge. Each of these tubes includes an anode, a cathode, and a control grid. Tubes V and V have their anodes adjacent junction 8, which is connected to the positive terminal of source 2, while tubes V and V, have their cathodes adjacent junction 10, which is connected to the negative terminal of the source 2. In the embodiment of Fig. 1 the tubes by themselves make up the four arms of the bridge, but as will be seen from other embodiments of the invention presently to be described the bridge arms may contain other impedance elements.

If tubes V and V in one pair of oppositely disposed arms of the bridge are caused to conduct while tubes V and V in the other pair of opposite bridge arms are held cut-oil on their control grids, the capacity C will be charged in one polarity to a potential approaching with time the voltage of the source 2. If instead the tubes V 1 and V are made conducting while V and V, are held out off, the voltage from C will in the same manner appreach the voltage of source 2 but in the opposite polarity. In either case the capacity charges according to an ap proximately exponential function. If all four tubes conduct simultaneously, the potential across C will go to zero.

The conduction times required of either pair of tubes for the voltage across C to reach the voltage of source 2 or to return to zero value is a function of the plate resistance of the tubes and of the internal resistance of the source 2, and also of the shape of the voltages applied to the tube grids. These times can be made very short-of the order of a few microseconds.

In the embodiment of Fig. 1 the tubes V V are held cut off on their control grids by separate bias sources, and are caused to conduct in opposite pairs by means of externally generated signals. The bias sources are shown at 12 for V 14 for V and 16 for V and V A transformer 13 is provided for the application of an unbiasing signal voltage to V and V together while a transformer 20 similarly permits the application of signal voltage to tubes V and V Each transformer includes two secondary windings, each of which is connected in the grid-cathode circuit of one of the tubes. The secondaries of transformer 18 are connected in the grid-cathode circuits of tubes V and V while those of transformer 20 are connected in the grid-cathode circuits of V and V These transformers are energized on their primary windings by means of voltages of appropriate shape, in

narrow pulses, which will be discussed with reference to Fig. 5. The polarities must of course be so arranged that 'the signals applied to transformers 18 and 24} resultin the application of positive voltages to the control grids of the tubes.

If V and V, are brought. into conduction for a sufficient length of time (which may in practice be very short),

the voltage across C will be limited to some fraction of the supply voltage. It may be advantageous, for speed in charging, to provide a source 2 of voltage higher than ,that desired across C, and to limit the voltage to which C is charged by limiting the time during which it is allowed to charge.

With C charged in one polarity by conduction in one pair of tubes, return of those tubes to cut off condition,

as by removal of the signal from transformer 18, will leave C with a charge which it will retain, except as discharged through the leakage resistances of the circuit, until tubes V and V are rendered conducting. If, 'at'or after the instant when V and V are restored to cut-off condition, V and V are brought into conduction by the application of a signal to transformer 29, C will be rapidly discharged and will thereafter be charged in the opposite polarity. In the discharge phase of the process the operative potential will in fact be the sum 'of the instantaneous potential across C added'in series.

aiding relation to the voltage of the source 2. If, as is commonly desired, zero potential difference is desired across C during one phase of the complete switching cycle, this may be achieved by simultaneously driving both of the

transformers

18 and 20 so as to render all of the tubes V -V conductive.

While the circuit shown in Fig. 1 is fully operative, and has been shown for the sake of simplicity and generality, the

separate bias sources

12, 14 and 16 are inconvenient, and it is usually more advantageous to provide some or all of the tubes With self-bias, in the. form of parallel combinations of resistance and capacity in the grid-cathode circuits thereof. To protect the tubes against excessive conduction in the event of failure of the driving signals the separate bias source 16 may be retained for tubes V and V self-bias being employed for the others.

It is usually desirable that the transition times for C between a desired voltage of one polarity and a desired voltage of the opposite polarity and/or zero voltage be achieved in a very short time. This may be achieved by driving the grids of the tubes with wave forms having very steep leading edges, by the provision of small internal plate resistances for the tubes, by minimizing the internal resistance of the source 2, and by providing a terminal voltage at the source 2 higher than the maximu voltage to which C is to be charged.

Various circuits are known for the development of square wave pulses having steep leading and trailing edges which may be used to drive the grids of tubes V -V Low plate resistance may be achieved for the tubesby a proper selection of their characteristics and particularly by driving them with strong signals, including signals which will raise the grids thereof into the region of grid current. driving pulses must be narrow in order to avoid excessive grid dissipation in the tubes.

To obtain the desired heavy drive of the switch tube grids, applicant has found it advantageous in some applications to construct the

transformers

18 and 20 of Fig. 1

as the transformers of two blocking oscillator circuits.

.For this purpose, the plate-to-grid feedback transformer If the grids are driven positive however, the

of each circuit is provided with two additional windings,

, these w w nd s arecon c betwee t e r and cathode of two opposite switch tubes in the bridge circuit of the invention.

Alternatively, as indicated in Figs. 2 and 3, the switching tubes V V may be arranged to function as blocking oscillators externally controlled. In Fig. 2 the tubes V 'V,, have associated therewith transformers TR TR., respectively. Each of these transformers includes a winding 21 for the input to the transformer of a triggering voltage and two additional inductively coupled

windings

22 and 23, one in the plate-cathode and'the other in the gridcathode circuit of the associated tube. The connection of the

windings

22 and 23 is such that they provide regenerative feedback from plate to grid as in a blocking oscillator. A signal of proper polarity applied to the winding 21 appears across the winding 23 and causes the tube to conduct, and the regenerative action between

windings

22 and 23 results in a single short pulse of heavy plate current after which the tube returns to a cutoff condition and so remains until a new signal is applied to its associated winding 21. The tubes have separate cut-off bias sources 12,1 5, 15 and 17 applied thereto so that they will conduct only when triggered through their transformers. The primary windings of transformers TR TR TR and TR may be connected together in pairs in series, in parallel, or otherwise to excite tubes V and V to conduction together and to excite tubes V and'V to conduction together. t may be noted that in Fig. 2 the capacity C may be connected either between the cathodes of V and V or between the plates of V and V and the same is true of the embodiment of Fig. 3. In both of Figs. 2 and 3 the'plates of tubes V and V;

are separated from the bridge junction 8 by'the winding for the tubes and without exposing the tubes to the risk of destruction in the event of failure of the driving signals by means of the arrangement illustrated in Fig. 4.

Fig. 4 illustrates an embodiment of the invention which includes a number of features not shown in the embodinents of Figs. 1-3. In Fig. 4 tubes V V are connected ,III a bridge circuit as in the embodiments previously described. V and V are shown as pentodes and V and V as two triode sections in a common envelope. All four tubes are provided with self-bias bymeans of resistors 30 and parallel capacitors '32 connected in their grid-cathode circuits.

The capacity C to be successively charged in opposite polarities is indicated as existing between the two sets of grid wires diagrammatically indicatedat 114 and 116 in a cathode-ray tube of the type disclosed in Patent No. 2,692,532 generally indicated at 1420. This tube includes a cathode 102, heated by a filament 104, a control grid M6, first and second anodes 108 and 110 and a switching grid generally indicatedat 112. The grid 112 includes the two sets of insulatedinterlaced conductors 114 and 116. Beyond the switching grid is located a fluorescent screen 120. The second anodeis connected to a conducting coating 118 on the inside wall of the conical part of the tube. The electrode 118 may take the form of a 'metallic cone in tubes of metallic rather than glass con- 48. Each of these transformers has a primary Winding 50 and two secondary windings 52 and 54. In transformer 46 the windings 52 and 54 operate to cause conduction in tubes V and V while in transformer 48 windings 52 and 54 cause conduction in tubes V and V respectively.

The unidirectional voltage source is shown at a dash line box 2 including a rectifier 34 having an output filtering circuit of known type including series resistors 33 and 35, shunt capacitors 37, and a cascade of voltage regulator tubes 36. The positive and negative terminals of the source 2 are indicated at 38 and 40 respectively. These power supply terminals are conductively connected to the bridge junction points 8 and through two equal resistors 42 and 44. In the event of a failure of the driving signal to the grids of the tubes from transformers 46 and 48 these resistors limit to a safe value the current drawn by the tubes.

Equal resistors are provided, one in each lead from the 2 source 2, in order to prevent changes in the average potential at the surface of the grid 112 with respect to the remaining electrodes of the cathode-ray tube during the switching cycle, i. e. cycle of voltages between the conductor sets 114 and 116. While a single resistor such as either of the resistors 42 and 44 would protect the tubes of the bridge circuit against destruction in the event of a failure of the switching signal at the inputs to transformers 46 and 48, the provision of unequal resistance between the

current terminals

8 and 10 of the bridge and the terminals of the power supply would result in a variation over the switching cycle of the average potential at the surface of the gridhereinafter referred to simply as the grid plane. Thus for example if the resistor 42 were present and the resistor 44 absent, charge of the grid 112 in either polarity to the full output voltage of the source 2 would leave the average potential of the grid plane halfway between the extremes of voltage available at the terminals 38 and 40. This would moreover be true even if the grid capacity C were limited to a lesser charge by the times during which the tubes are allowed to conduct by transformers 46 and 48. Consequently even with unequal resistances in the two leads from the bridge to the source of charging current, the average potential at the grid plane will be uniform with respect to the remaining electrodes of the cathode-ray tube for both charged phases of the switching cycle, assuming, as will generally be the case, a fixed voltage for some point in the source 2 with respect to the other electrodes intube 160. However when it is desired to have no voltage across the two halves of the grid, all four tubes V -V conduct simultaneously to discharge the grid. During such conduction the average voltage of the grid plane will depart from one-half the value supplied by source 2 and hence will shift with respect to the other elements of tube 100 if the resistances between junctions S and 38 and between junctions 10 and are unequal. Such inequality in resistance will therefore result in a modulation of the average potential at the plane of grid 112 with respect to the electron gun in tube 190. This varies slightly the deflection sensitivity of the tube, i. e. the ability of voltage differences between the two grid halves 114 and 115 to deflect the electrons of the cathode-ray beam to areas of one or another color on the screen 12%, and false colors may result therefrom. Accordingly when resistance is inserted in series with the bridge circuit and power supply it should be inserted equally in the two leads between the bridge circuit and the positive and negative terminals of the power supply as illustrated in Fig. 4.

The current limiting resistors 42 and 4 tend to affect adversely, i. e. to increase, the rise times required for the voltage across the capacity C of the grid 112 to reach a desired value, but this may be compensated for as previously suggested by increasing the source voltage and/ or by driving the tubes V V to greater plate currents.

'bodiment with square wave voltages.

In the embodiment of Fig. 4 the source 2 is employed to provide a desired voltage bias between the average potential at the plane of grid 112 and the voltage of the tube shell 118. The shell is made positive with respect to the average potential of the grid in order to remove secondary electrons generated by impact of the cathoderay beam on the grid conductors as the beam is scanned over the grid in tracing a raster on the screen. These secondary electrons if not removed would otherwise be drawn to the screen 120 by the accelerating voltage applied between the grid as a whole and the screen, and in impacting the screen they would desaturate the colors displayed in view of their random distribution over the screen, To provide such a bias resistors 70 and 72 are connected across the output of rectifier 34, and a tap 74 connects between a point on resistor 70 and the shell 118. This connection establishes a reference between the average potential of the plane of grid 112 and the shell, to which the other electrodes of the tube are related in voltage by the high voltage power supply or supplies for the tube, not shown. The average voltage atthe plane of grid 112 should be limited to values negative with respect to that of the shell. To this end resistors 70 and 72 are so proportioned that, taking into account the voltage drop in resistors 33 and 35, the voltage at the low potential end of resistor 70 which can be reached by tap 74 is above the voltage of terminal 40 by at least one-half the potential difference existing between terminals 33 and 40. The voltage difference between the shell and grid plane, termed a seeker voltage is in practice of the order of from 200 to 600 volts D. C.

The embodiment of Fig. 4 illustrates circuits appropriate to driving the switch tubes l -V4 of that em- These driving circuits will be explained with reference to Fig. 5. In Fig. 5 wave form B is a square wave of voltage generated by means not shown, and assumed to be available at an input terminal 200 to the driving wave form generator shown enclosed within the dash line box 202. Throughout Fig. 5 only the A. C. components of the voltage waves are shown, and no attempt is made to show'correct relative amplitudes among the various wave forms. Moreover the rise and decay times of the grid voltages on the tube control grids, and of the resulting voltages across the capacity C of the cathode-ray tube switching grid have been neglected as small in comparison to the time intervals or phases of the switching cycle shown, over which intervals the capacity is successively charged in one polarity, discharged, and charged in the opposite polarity. In these wave forms, accordingly, it may be assumed that the capacity C is charged in either polarity to the full voltage available from the unidirectional voltage source. Wave form B has a fundamental frequency of 2t wherein it is the time interval between adacent vertical lines in Fig. 5. Wave form B may be derived by known means, for example a bistable multivibrator, from a pulse periodic in the time interval t such as that shown at wave form A.

Wave form B is differentiated in an RC combination comprising capacitor 294 and resistor 206, and the differentiated voltage (wave form C) is applied to a bistable multivibrator consisting of triodes V201A and V201B. The resulting voltages on the plates of these tubes are shown at wave forms D and B respectively. These voltages are applied to the control grids of triodes 1292A and V2il3A respectively, each forming part of an adding circuit comprising in the one case tubes V202A-B and in the other case tubes V203A-B. Wave form B is applied to the control grids of the other tubes V2023 and V2038 of these adders. The sum of voltages B and D developed in the adder V202A-B is shown at wave form F and the corresponding sum voltage at the plates of VZQSA-B is shown at wave form G. Wave forms F and G are seen to be rectangular voltage wave forms Whose fundamental frequency component has a period At, the fundamental component in wave form G being 180'? displaced i nase from the corresponding component ih wave form F.

' V 2 04A andV204B are clippers which clip elf the low voltagepportion of the sum voltages appearing on the paralleled plates OfVZGZA B and V293A B in order to 1 the amplitude of the rectangular waves F and G which are passed by them to the grids of V205A and V 2616A respectively.

' Double triode tubes V205A-B and V206A-B shown in Fig. 4, one in the driving signalchannel leading to transformer 46 and the other in the driving signal channel tube transformer 48, are provided to eliminate the D. C.

component from the current flowing through the transformer primary windings 50. While other methods may be employed to prevent transformer saturation by D. C. current, that illustrated is desirable in embodiments of the invention having a low frequency switching cycle, where capacitors for a similar purpose might be inconveniently large. An embodiment of the invention similar to that illustrated in Fig. 4 has been successfully operated at switching cycles of the order of 60 C. P. S.

Toeliminate the D. C. component of wave forms F and G from the transformer primary windings 50, wave forms F and Gare applied in unfiltered form to tubes V205A and V206A respectively. Before being applied to tubes V205B and V2063 the wave forms are reduced to their average or D. C. values in resistancecapacity filtering circuits generally indicated at 208 and 216.

'Tubes V205A-B and V206A-B are cathode loaded, and

the primary winding of transformer 46 is connected across the cathodes of tubes V205A-B while the primary of transformer 48 is similarly connected across the cathodes of V206A-B. Since only the D. C. component of wave form F appears across the cathode of V205B, the A. C. component only is applied to the primary winding of transformer 46, and similarly in the case of Wave form G and transformer 48. By proper selection of leads wave forms F and G may be taken from the secondary windings of transformers 46 and 48 to appear between grid and cathode of the switch tubes associated with those secondary windings respectively in inverted polarity, as indicated at wave forms H and I. These voltages, applied respectively to V V and to V V produce across the capacity C of grid 112 in the cathoderay tube 100 a voltage as shown in wave form J. The

polarity indications of wave form I are entirely arbitrary.

The bridge circuits of the invention, including the embodiment' of Fig.4, may however be driven with pulse rather than square wave driving voltages. There is in general no necessity for maintaining conduction in either pair of switch tubes throughout the charged phases of the switching cycle, nor for maintaining conduction in all fourswitch tubes throughout the discharged phase of the switching cycle. The driving pulses need have only sufficient time duratiomin relation to the plate currents which they produce in theswitch tubes, to permit charge of the .the two pairs of tubes of the bridge circuit of the invention in order to produce an output wave form 1 identical with that produced by the rectangular wave forms H and I.

Of course other rectangular wave forms can be generated on the grid capacity C. Wave forms M and N illustrate two driving wave formsresulting in arectangu- 'lar wave form 0.

The invention contemplates controlirr a varietyof ways over the amplitude of the, voltage differences .developed across the capacity C to be charged. Some of these have already been mentioned. Thus, thefinite rate of chargeof the capacity C may be employed, in conjunction with driving pulses of appropriately. selected short duration, to control the charge to levels lower than that of the source, effective control being maintained by of the duration of the pulses with a given source or vice versa.

The amplitude of the rectangular voltages across the capacity C may also be varied according to the invention by control of the bias on the bridge tubes. These tubes need not be biased to cut off. Except possibly in the case of bridge tubes connected as blocking oscillators one or bothpairs may be allowed to conduct'at low levels between the driving signals applied thereto. In the case of fixed bias, bias control is of course effected by variation .of the'value of the bias sources. In embodiments employing self-generated bias, the bias levels may be controlled by variation in the amplitude of the driving signals.

All of these methods may be employed to produce asymmetrical switching voltages across the capacity C, i. e. a voltage difference in one polarity of greater amplitude than the voltage difference in the opposite polarity. Thus the pulses used to drive one pair of tubes may be made of greater duration than those applied to the other pair. Alternatively, the bias levels in the two pairs of tubes may be made unequal by either of the methods above described for control of bias level. For example in the embodiment of Fig. 4, an asymmetric wave form for the voltage across the two halves of the cathoderay tube grid 112 may be obtained by making unequal the amplitude of the driving signals applied torthe two pairs of tubes V V and V V to develop unequal biases on these two pairs of bridge tubes. Control over amplitude of the driving signals is available at the potentiometers 212 and 214 in the plate circuits of the clipper tubes VZllA and V204B. In these plate circuits the

additional potentiometers

216 and 218 connected as variable resistances are provided to insure a minimum clipping level which will clip off variations in output voltage from the adder tube pairs V2tt2A-B and V203A-B associated with conduction in one versus both halves of those adders. Within wide limits, whatever the bias level developed in the bridge tubes according to the amplitude of the driving signals applied thereto, those signals will bring the tubes to substantially zero bias-and to full conduction while the driving signals are on. According to the bias levels thereof however, the tubes may be fully'cut off or semiconducting during the intervals between driving signals. Evidently, low conduction in one pair of tubes as contrasted with cut-olf will reduce the rate at which full conduction to the other pair of tubes will charge the capacity C.

It is also within thescope of the invention to make the effective source voltage greater for charge of the capacityv C in one polarity than for charge in the opposite polarity. Thus for example a pair of series-connected potentiometers may be connected across the voltage source, with thetap on one potentiometer leading to the plate of one bridge tube V and the tap on the other potentiometer leading to the cathode of the opposite bridge tube V; which conducts simultaneously with V to form one charging path for the capacity C. If the taps on these two potentiometers are moved through equal voltage changes in opposite direction, for example by means of a suitable coupling or gauging control, one toward higher voltages and the other toward lower voltages, the effective source potential for the tubes V and V may be increased or decreased, for example from a median value at the mid-points of these otentiometers. With this arrangement the available voltage at the grid plane resulting from a charge through V and V, will still be halfway between the limits of potential dilference available from the source. If desired, the other pair of tubes may be similarly connected in series with the ultimate voltage source through a similar pair of potentiometers with the taps of potentiometers associated with tubes V and V ganged together while those associated with V and V, are similarly ganged together. With four potentiometers so interlinked a reduction in the efiective source potential applied to the series connection of V C and V, will be accompanied by an increase in the effgiive source potential applied to the series connection of V C and V I claim:

1. The method of applying voltage differences of successively changing polarity between the halves of a color switching grid in a cathode-ray tube which comprises connecting said halves one to each of two opposite junctions of a four-arm bridge circuit each of whose arms includes a vacuum tube, connecting a unidirectional potential source across the other two junctions of said bridge circuit, and causing the tubes in opposite pairs of said arms to conduct together according to a common cyclical function of time with the tubes of said two pairs in different phases of said function.

2. A switching circuit adapted to charge a capacity successively in opposite polarities from a source of unidirectional potential diiference, said circuit comprising four vacuum tubes each including an anode, cathode and control grid, means connecting said tubes into a circuit including two parallel-connected pairs of two seriesconnected tubes each, two of said anodes being conductively connected together at one end of said parallel connection and two of said cathodes being conductively connected together at the other end of said parallel connection, means connecting said source across the ends of said parallel connection, means connecting said capacity between the junctions of the tubes in each of said pairs, a parallel combination of resistance and capacity connected between the grid of each of said tubes and its cathode, and a resistor connected between each end of said parallel connection and one terminal of said source, said resistors being substantially equal.

3. A switching circuit adapted to charge a capacity in successively o'pposite polarities from a unidirectional potential source, said circuit comprising four vacuum tubes each having a plate, grid and cathode, means connecting said tubes into a bridge circuit with one tube in each arm thereof and with like electrodes of said tubes adjacent a first diagonally opposite pair of junctions of said bridge arms, a transformer associated with each of said tubes for coupling the plate circuit thereof to the grid circuit thereof for positive feedback from plate to grid, a parallel combination of resistance and capacity connected between the grid and cathode of each of said tubes, and means to initiate conduction simultaneously in pairs of said tubes oppositely disposed in said bridge circuit.

4. A switching circuit adapted to charge a capacity successively in opposite polarities from a unidirectional potential source, said circuit comprising four vacuum tubes each including a plate, cathode and control electrode, means connecting said tubes into a bridge circuit having four arms connected at four junctions into a loop with one of said tubes in each of said bridge arms, with the plates of two of said tubes adjacent the junction of two of said bridge arms and with the cathodes of the other two of said tubes adjacent the junction of the other two of said bridge arms, means adapted to bias the control grid of each of said tubes negatively with respect to its cathode, means connecting said source across said last-mentioned two junctions, means connecting said capacity across the other two of said four junctions, and two transformers having each a primary winding and two secondary windings, each of said secondary windings being connected in the grid-cathode circuit of one of said tubes, the two secondary windings of one of said transformers being connected in the grid-cathode circuits of the tubes in two non-contiguous arms of said bridge circuit and the secondary windings of the other of said transformers being connected in the grid-cathode circuits of the other two of said tubes, whereby upon the application of unbiasing signals to said tubes in diagonally opposite pairs successively said capacity will be charged in successively opposite polarities and upon application of unbiasing signals to all of said tubes simultaneously said capacity will be substantially discharged.

5. A circuit for the application of switching voltages to the color switching grid of a cathode-ray tube, said grid including two sets of mutually insulated interlaced conductors, said circuit comprising four electron discharge tubes each having an anode, cathode and control grid, means conductively connecting together the anodes of two of said tubes, means conductively connecting together the cathodes of the other two of said tubes, junction means conductively connecting together in pairs of unlike electrodes the remaining anode and cathode electrodes of said tubes, and means connecting one set of said conductors to each of said junction means.

6. A switching circuit adapted to charge in successively opposite polarities the switching grid of a cathoderay tube, said circuit comprising four vacuum tubes each including a plate, cathode and control grid, means conductively connecting said tubes into two pairs of seriesconnected tubes with unlike electrodes at the junctions of the two tubes of each pair, means conductively connecting said tube pairs in parallel with like electrodes at the junctions of said tube pairs, means connecting a source of potential across said last-named junctions, means connecting said switching grid between said firstnamed junctions, means adapted to bias the control grids of said tubes negatively with respect to their cathodes, and means to apply unbiasing signals to said tubes in pairs the members of which are spaced from each other by another one of said four tubes.

References Cited in the file of this patent UNITED STATES PATENTS 2,478,527 Dawson Aug. 7, 1949 2,487,510 Baker Nov. 8, 1949 2,502,887 Rava Apr. 4, 1950 2,590,104 King Mar. 25, 1952 2,691,116 Allwine Oct. 5, 1954 2,728,021 Banks Dec. 20, 1955 UNITED STATES PATENT OFFICE CERTIFICATE OF -CORECTION June- 17, 1958 Arthur Schlang Column 1, line 17 after "capacity" strike out "of"; column 2, line 20, for "the bridge" read 8. bridg 6 column v4, line- I, for "each circuit" read Each such circuit Signed and sealed this 23rd v(flay of September 1958.

SEAL) ttest:

KARL H. AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No- 2,.839,704 June 17, 1958 Arthur Schlang It is hereby certified that error appears in the-printed specification of the above "numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1,. line 17, after "capacity" strike out "of"; column 2,, line 20, for "the bridge" read a bridge a; column v4 line 1, for -."each circuit" read Each such circuit Signed and seal-ed this 23rd .day of September 1958.

i SEAL) ttest:

KARL WINE ROBERT c. WATSON Attesting Officer Commissioner of Patents