US1986599A

US1986599A - Frequency stabilizing means

Info

Publication number: US1986599A
Application number: US700855A
Authority: US
Inventors: Francis R O'leary
Original assignee: Pratt Read and Co
Current assignee: Pratt Read and Co
Priority date: 1933-12-04
Filing date: 1933-12-04
Publication date: 1935-01-01
Anticipated expiration: 1952-01-01

Description

Jan. 1, 1935. F. R. OLEARY FREQUENCY STABILIZING MEANS Filed Dec. 4, 1933 4 Sheets-Sheet 2 IIAQVENTOR 444.6(2) 6 0'4 ATTORNEYS Jan. 1, 1935. I R -O'LEARY 1,986,599

FREQUENCY STABILIZING MEANS Filed Dec. 4, 1933 4 Sheets-Sheet 4 W k 3 n k N k Q 3 T NVENTOR V ATTORNEYS Patented Jan. 1, 1935 UNITED STATES PATENT OFFECE FREQUENCY STABILIZING MEANS Francis R. OLeary, West Roxbury, Mass, as-

signor to Pratt Read & Company, Deep River,

Conm, a corporation of Connecticut Application December 4, 1933, Serial? No. 700,855

20 Claims. (on. 24.)

My present invention has for its object to pro- 1 to a large number of additional note-producing vide an oscillatory circuit especially adapted for circuits (only one being shown). producing electrical oscillations in the audible Fig. 3 is a schematic representation of an arrange, which may be amplified and used for musirangement of parts useful with a keyboard of five 5 cal purposes. The invention further comprehends successive octave intervals, particularly illustrat- 5 a novel control means forstabilizing the freing the various fork-drives and gaseous oscilquency of an oscillatory circuit employing a twolatory circuits necessary for sounding its sixty element gaseous discharge tube as one of its elenotes when utilizing the embodiment shown in merits, so that the circuit will not, by reason of Fig. 1.

changes due to tube aging or other causes, produce Fig. 4 is a similar diagram showing the man- 10 frequencies different from that to which it is inner in which the range of notes may be extended itially adjusted or tuned. by the use of intervening repeater circuits.

My invention has for its further object to pro- Fig. 5 is an illustration of still another arvide a system of two-element tube oscillatory cirrangement of parts by means of which the varicuits for producing a plurality of musical notes, ous notes of a keyboard comprising a large num- 1 each of which circuits is stabilized in frequency. ber of Successive tfi y be e I efl'ect this frequency stabilization with the Fig. 6 is a view showing the shielding of a gas assistance of electromechanical devices and, as filled tube. a further feature of the invention, the means Similar reference numerals in the several fig- I employ for imparting frequency control from ures indicate similar parts. 20 these devices to the two-element tube oscillatory In the production of a musical instrument in circuits is such that a single electromechanical which the notes produced are originated by an device may be .used as a frequency controlling oscillatory tube circuit, a large part of the initial element for several of such oscillatory circuits. cost can be eliminated by using the simpler and In addition to the foregoing my invention also more easily constructed type of two-element 25 comprehends the use of a controlled two-element gaseous discharge tubes. This type of tube also tube oscillatory circuit for controlling other ospossesses the further advantage that it lends itself cillatory circuits of the same nature, by the into a simple type of oscillatory circuit so that the terposition between the first and second menapparatus required for a keyboard instrument tioned circuits of a thermionic type of amplifier. comprising several octaves may be assembled in a 30 In this way with the same electromechanical decomparatively small compass. In my investigavices the frequency control can be extended to tions of this subject in a practical way I have much greater number of note-producing circuits. learned that the chief drawback to the use of a An extension of this idea includes provision for gaseous discharge tube is its tendency to change also making this repeater an oscillator tuned to its internal resistance and other operating char 35 the frequency of the oscillatory circuit feeding acteristics for one cause or another, such as into it. In this manner, high amplification and changes in internal pressure, gas absorption by frequency stabilization are combined. the electrodes or like effects, generally included in To these and other ends my invention covers the expression aging of a tube. The sensitiveness certain improvements and arrangements of parts of this yp of tube to these factors and Others, 40 n as will be further described, the novel m such as temperature changes, makes it diflicult tures thereof being set f th in the appended to maintain its associated circuit in tune over any clams considerable period of time. Besides, it is un- In the drawings. reliable during operation in that its frequency of Fig 1 is an illustration of an oscillatory on discharge in response to its tuned circuit is not steady.

cuit for producing a note of given frequency and Oe fh 'et of i t" mone embodiment of my invention for effecting n 0 t e c S my nven 10m Is to co its stabilization as well as that of others (not ai s- 12 52:? g figfiz fif irg g zi 22 2 3 222? shown)- V which will act upon the tube itself and exert such 50 2 15 an lnustratlonof one of the gaseous influence upon the discharge or breakdown of the tube oscillatory circuits which may be controlled tu created by t tuned circuit, that t in f q y s Shown in 1 d Of a Su equency of the oscillations generated will remain quent repeater circuit, also acting as an oscillator, constant. 55 for extending the control features shown in Fig. In illustrating my invention I have shown in 55 Fig. 1a gaseous discharge tube A1, having the twointerior electrodes a and b connected in a circuit including the

wires

1 and 2 leading to opposite poles of a source ofpotential such as the battery 13. i In the wire 2 is a tuned circuit 3 comprising a resistance R1 and a condenser C1,-the relative values of which are selected to create the desired frequency of the oscillations which are to-be produced by the alternate charging and discharging of the condenser, as will be understood. l

The condenser C1 charges instantaneously each time the potential across the electrodes a and b reaches a value causing a dischargeto occur through the conducting gas, and thereafter discharges progressively through the. resistance R1. For a gaseous tube having given operating characteristics, the frequency of the oscillations would be solely determined by the respective values of the condensers C1 and resistance R1, provided these characteristics did not change during the operation of the device. However, as already stated, changes-in the operating characteristics of gaseous discharge tubes are very likely to occur, due to various factors, and I therefore deem it essential that some means be provided to counteract the effects of these changes and thereby stabilize the frequency of the oscillations produced by the alternate charging and discharging of the condenser C1. A method which I have found very effective for accomplishing such frequency stabilization is illustrated in Fig. 1. Broadly stated, this method consists in'creating in the vicinityof the electrodes a and b an external potential at all times positive, but varying at a predetermined frequency, which may be the same as that to which the oscillatory circuit 3 is tuned. The selected frequency of this control potential I render stable by an electromechanical device, which, as will hereinafter appear, may be utilized to control several other oscillatory circuits related to one another and to that of the circuit 3 by frequency multiples, Fig. 1 showing the possibility of employing five different tubes, A1 to A5, in association with the same control circuit in this manner, To this end I provide each of the tubes with a cap or shield 4, preferably of, lead, which closely surrounds the exterior of the tube envelope and encloses the interior electrodes, as shown in Fig. 6. The area covered by the cap is suflicient to create a shielding or capacitance effect around the electrodes and, as I have discovered, the desired frequency stabilization is produced when a potential varying at the proper frequency is applied to the cap or shield. That is, the voltage value at which discharge takes place through the conducting gas is maintained substantially constant.

I have found in practice that the potential sup-' plied to the shield on a given oscillator tube should have a frequency equal to the desired frequency to be established by the oscillatory circuit associated with said tube or related thereto by a multiple thereof. The external shield then apparently acts as a starting electrode for-the gaseous discharge tube. Theoretically, breakdown of the tube or the discarge therethrough should occur each time the space charge in the gap between the interior electrodes is neutralized, which corresponds to the condition the tuned circuit pulsating potential supplied to the shield neutralizes at the proper frequency part of the space charge, which causes the breakdown of the tube to occur at the frequency to which the oscillatory circuit of the tube is tuned.

I have used successfully for the controlling potential impulses a separate oscillatory circuit controlled in its periodicity by a mechanical vibrator such as a tuning fork 5, capable of vibrating at a constant frequency. As will hereinafter appear, the frequency of the potential impulses supplied to the shield 4 is determined by the natural period of vibration of the fork 5 and is selected to cause the above mentioned relation between the controlled oscillations and the controlling potential impulses. One means by which my invention may be carried out is illustrated in Fig. 1 in which the output of a three-element thermionic vacuum tube D supplies the periodic potential impulses above mentioned for stabilizing purposes and also for maintaining the tuning fork in continuous vibration for any desired length of time.

The tuning fork 5 is mounted to permit its prongs 5a and 5b to vibrate freely. In proximity tothe prong 5a electromagnets 5c are arranged so that, when a current impulse passes through their windings a magnetic attraction will be created on this prong and by its recurrence serves to keep the fork in a state of constant vibration. Adjacent the other prong 5b are similar electromagnets 5d. Associated with both sets of magnets and connected in any suitable manner to their yokes are permanent magnets 6 and 7. One terminal'of the windings 5c is connected by wire 8 to the plate of the vacuum tube D and the other terminal is connected by wire 9 to the positive pole of the plate-supply source or battery B. The plate-cathode circuit of the tube is completed via wire 10 leading from the negative pole of the battery and includes a switch -11 and a by-pass condenser 12 shunted by a high resistance 13. The grid of the tube is connected through the secondary winding 102 of a transformer T to the wire 10. The resistance 13 is chosen to furnish the desired grid bias for the particular type of vacuum tube D employed. The output circuit of the tube comprises the wire 14 connecting the plate and cathode and containing the voltage divider 15 having a slider 15a connected by the conductors 16 with the shields 4 on the various tubes A1 to A5.

Changes in potential on the grid of the tube D in accordance with the set frequency of the tuning fork are obtained by. connecting the terminals of the primary winding 101 of transformer T by

wires

17 and 18 to the terminals of the windings of magnets 5d. Each to and fro movement of the prong 51; will set up an induced current in the circuit thus formed, which causes a corresponding potential impulse to be impressed through the transformer T upon the grid of the tube D.

It will be seen from this arrangement that, with each change in potential of the grid, there is a corresponding change in the flow of current in the plate circuit, which varies the energization of the electromagnets 50 to impart movement to the prong 5a. It is apparent therefore that, with proper values of grid bias for a given tube, energy will be supplied to the magnet assembly at the determined frequency of the tuning fork. The latter will thus be maintained in continuous vibration and the tube D will act as an amplifier of the potential impulses impressed on the grid.

The output wave supplied through the resistance 15 provides a potential at the shields 4 fluctuating at the natural period of vibration of the fork 5 and the intensity of which may be adjusted by the slider 15a, this potential stabilizing the frequency of discharge in the tubes A1 to A5, provided their circuits are related to one another by frequency multiples, as already explained.

When the disclosure herein is used in a musical instrument, the gaseous oscillatory circuit may feed through a transformer T1, the primary winding 16 of which may be in the wire 1. The secondary winding 17 of this transformer is then connected to an amplifier 18 leading to a sound producing device, such as the loud-speaker 19. A switch S1 is provided for sounding the note derived from the electrical wave supplied from the gaseous oscillatory circuit and may be arranged as a musical instrument key, as shown. This switch in its normal position short-circuits the secondary 17 of the output transformer, this preventing impulses passing to the amplifier until the key is depressed to open the switch contacts and cause the note to be sounded. If desired, the note-sounding key may be connected, as shown in dotted lines, in the tuned circuit 3 in series with the resistance R1.

It is understood that the four additional tubes A2 to A5, shown in Fig. 1 as also being controlled from the same fork-oscillator, may each be connected in a separate oscillatory circuit similar to that in which Al is connected and feed through a separate transformer provided with a 5111111211. note-controlling key into the same amplifier 18. The five circuits may all be tuned to different frequencies all related to the controlling frequency at the shields 4 in the manner described. In this manner a given note and four successive octaves of that note may be sounded at unvarying or accurate pitches.

In Fig. 3, I have shown in simplified manner the circuit lay-out of Fig. 1 extended to include such additional circuits as are necessary for the playing of the various notes composing a keyboard. It will be observed from this figure that this simple arrangement affords a keyboard of sixty notes or of five successive octaves of the lowest note thereon. The keyboard is represented by the symbol K and the various fork-oscillators required by the symbols F1 to F12. The symbols A1 to A512 represent the various gaseous tubes, some of which are not shown for purpose of clarity. It is understood that each of these tubes is associated with the other elements comprising its respective note-producing circuit. A single master vibrator, or tuning fork and its associated vacuum tube circuit, is required for each set of five note-producing circuits related by octaves. For the particular keyboard illustrated a totalof twelve-master vibrators and a total of 60 note-producing circuits are required.

In order to provide for the sounding of a large number of notes, for example to permit rendi-' tion of musical effects comparable with an organ having three or four manuals and a large variety of stops, I have provided means whereby greater power can be obtained so that an increased number of oscillatory note-producing circuits can be controlled from the same fork-oscillator.

Fig. 2 shows an extension circuit which may be used in association with each or at least some of the various fork-oscillators. In the embodiment illustrated, this extension circuit not only operates as an amplifier, but is designed to independently produce oscillations of the same frequency as that of the oscillations which are supplied. It is controlled in operation by the stabilized frequency of the input oscillations and in turn controls the frequency of the potential impulses supplied to the shields controlling the note-producing circuits. It comprises a thermionic vacuum tube E, preferably of the multi-element type, wherein I am able to utilize a particular element combination providing a repeater tube effect and at the same time employ another causing it to act as a generator.

In the arrangement disclosed the tube E comprises a cathode is, adapted to be raised to electron-emitting temperature by the heater h, a grid or input element g, a suppressor grid g and a plate 10. Current may be supplied to the heater h from any suitable source, such as the power mains through a suitable transformer. Located between the two grids is a screen .9. The grid-cathode circuit of the tube comprises the secondary 30 of an input transformer T5, one terminal of this secondary being connected by a wire 31 to the grid g and the other terminal being connected by a wire 32 to the cathode k. Means are provided for transferring energy from the plate p to the suppressor grid g and in the embodiment disclosed this means comprises a mutual inductance M, one winding of which is connected by a wire 35 to said plate, a condenser 36 being provided in the wire 37 connecting the other winding m to said suppressor grid. The plate-cathode circuit is established by connecting the intermediate terminal of the mutual inductance by a wire 38- to the positive terminal of a potentialsupply 39, the negative terminal of which is in electrical connection through a biasing resistance 33 and condenser 34 in parallel therewith with the cathode k. This potential-supply 39, it is understood, may be derived from any convenient source, such as the power mains through a suitable rectifier, and for convenience here is indicated as a battery. A resistance 40, through which the condenser 36 periodically discharges, is connected at one end to the suppressor grid g and at the other end through a biasing source of potential, herein shown as a battery 41, to the cathode k. Operating potential at the screen s is supplied from the source 39 by a wire 42.

The oscillations supplied through the transformer T5 to the tube E may be derived from any stabilized frequency source and in the present instance I have shown the latter as comprising one of the five oscillatory circuits the two-element tubes of which are shown as being controlled by the shields 4 in Fig. 1. In practice I utilize one of the series of tubes that is strongly stabilized by the circuit of Fig. 1, usually the one providing the fourth octave below fork frequency. In the embodiment illustrated in Fig. 1, the tube A5, designated 4th octave, would thus be employed. Itscircuit is shown in Fig. 2 and includes the wire 43 connecting one of its electrodes a to the primary 44 of the transformer T5 and the wire 45 joining its other electrode b to the condenser C5 across which the resistance R5 is connected. Said condenser and resistance are connected by a wire 46 to the negative terminal of .the battery 39, the other terminal of the primary 44 being connected by a wire 47 to a point of, positive potential on said battery.

It will be seen from the above description that the stabilized frequency impulses supplied from the circuit of tube A5 through the transformer T5 across the grid g and cathode is of the tube E will be repeated in amplified form because of the energy resulting from the electron stream from the cathode k to the plate 1). A fluctuating potential of increased amplitude is produced at the plate p, which may be used to impart frequency control to a large number of gaseous tube oscillatory circuits. To this end, the plate conductor 35 is connected by wires 49 to the shields 4 fitted on the various tubes of such circuits, in this instance seven tubes A6 to A12, being shown. These circuits, one of which is illustrated, may derive their energy from the battery 39 and may each feed, in the manner as already described, through a separate transformer provided by a note-sounding key into the amplifier 18 having the loud speaker 19 connected thereto.

Increased amplification is effected as a result of the self-oscillations created by the tube E and parts associated therewith, and, as I have found, this method of insuring high amplification is particularly advantageous in that frequency stability is in no way impaired. As will be hereinafter more clearly understood, by proper adjustment of certain parts these self-oscillations are caused to take place at the same frequency and in phase with the input oscillations supplied to its grid from the oscillatory circuit of tube A5. The manner in which the self-oscillations are established will be presently described and for convenience in description it will first be assumed that such input oscillations are not present.

Observing that the space current in the platecathode circuit of the tube E flows through branch 'm. of the inductance M, its fluctuations cause an induced voltage to be established across branch m due to the inductive coupling of the two branches. Assuming a time when the space current is rising, the induced voltage across branch m acts around the loop 36, g, k, 39 and m in such a direction as to change the potential at the suppressor grid 9' in a positive direction, 1. e., it renders the suppressor grid potential less negative (for the particular connections of the battery 41) and finally positive with respect to the cathode k. A current is thus caused to flow in the suppressor gridcathode circuit, which causes the condenser 36 to become charged. With increase of potential at the suppressor grid the space current approaches a maximum. Since induced voltage depends on varying current and since the space current now approaches a steady state, the induced voltage in branch m diminishes and the suppressor grid g rapidly becomes more negative, the induced voltage in branch m finally ceasing to act. As this occurs, the plate-cathode resistance increases reducing the space current and an oppos'ite induced voltage appears in the loop 36, g, k, 39 and m, further increasing the negativeness of the suppressor grid 9' until the point is reached corresponding to which space current is practically cut off. The suppressor grid is now maintained negative by the charge accumulated by the condenser 36 during the rectifying phase. This charge is dissipated through the resistance 40 over a period of time determined by the relative magnitudes of the condenser 36 and resistance 40 until the suppressor grid g becomes sufficiently less negative to permit a sensible rise in space current, after which the succeeding cycle is initiated.

The actual current fluctuations in the platecathode circuit of the tube E are not, however, the self-oscillations described, but fluctuations produced by reason of the fact that the tube acts simultaneously as an oscillator and as an amplifier. Such repeated or amplified fluctuations derived from the oscillatory circuit of tube A5 also set up induced voltages in the inductance branch m and therefore have a controlling effect on the operation of the tube E as an oscillator. In other words the self-oscillations are forced to keep in step with the repeated oscillations, this, nevertheless, depending on the requirement that the selfoscillations be tuned to or at least tend to take place at the frequency of the repeated oscillations. Such tuning may be effected by proper adjustment of the relative values of the capacity 36 and resistance 40, which values govern the time rate of discharge of the capacity through the resistance. Adjustment of the natural frequency of the selfoscillations may also be effected by proper selection of the suppressor grid supply-potential 41, which determines the time at which each cycle is initiated upon discharge of the condenser, further by proper design of the mutual inductance M, on which depends the amount of energy transferred from the plate-cathode circuit to the condenser as a charge.

From the above description it is seen that the electron stream fluctuating at frequencies controlled by the grid g is used as a coupling means between a source of stabilized frequency and a generator of oscillations utilizing the suppressor grid g as its own controlling grid. The staticcontrolled oscillatory circuit of gaseous discharge tube A5 provides the stabilized frequency oscilla- I tions, which through the medium of the transformer T5 are supplied to the grid-cathode circuit of tube E. The screen s, having a positive poten tial, functions as a plate with respect to the grid 9.

a certain amount of space current flowing through 5 the screen-cathode circuit. It actsin effect to isolate the two grids g and g from one another, preventing the potential impulses at one affecting those at the other. Due to the porosity of the screen, most of the electrons from the cathode pass on to the plate p, causing a modification in the self-oscillatory action of the tube E. This electronic control, when circuit constants are properly chosen to have the suppressorgrid controlled self-oscillatory circuit in tune or substantially in tune with the stabilized frequency oscillations in the grid-cathode circuit, tends to greatly stabilize the frequency of the resultant oscillations in the plate-cathode circuit. The feeble energy of the oscillatory circuit of tube A5, which has been stabilized in frequency, is therefore considerably amplified without loss of stability, the amplified energy appearing in the form of poten-' tial impulses at the plate 1), which impulses may be used, as already explained, to stabilize a large number of other gaseous discharge tube circuits.

In Fig. 4 I have shown the arrangement of Fig. 3 extended to provide additional octaves by the use of a repeater circuit as shown in Fig. 2 for each of the fork-oscillators. As observed, each of the repeater circuits, represented by Q1 to Q12, is disposed between one of the gaseous tube oscillatory circuits directly controlled bya fork-oscillator and a plurality of additional noteproducing circuits, the possibility of seven others being illustrated. The various gaseous discharge tubes necessary are represented by the symbols A1 to A12-12. For a keyboard of 11 successive octave intervals, 12 fork-oscillators, 12 repeater circuits and 144 gaseous discharge tube oscillatory circuits would be required.

In the arrangement shown in Fig. 5, the forkoscillators, F1 to F12, do not directly control any of the note-producing circuits, but control gaseous oscillatory circuits, A1 to A12, supplying their oscillations to the repeater circuits Q1 to Q12. The various note-producing circuits, the gaseous tubesof which are represented by A2 to A812, are directly controlled by the repeater circuits. This arrangement of parts, for a keyboard of 7 successive octave intervals, necessitates 12 forkoscillators, 12 repeater circuits and 9 6 gaseous oscillatory circuits.

What is claimed is:

1. The combination with an oscillatory circuit employing a gaseous discharge tube, of means for stabilizing the frequency of the oscillations in said circuit comprising a metallic shield disposed in the vicinity of the electrodes of said tube and a device for supplying to said shield a potential fluctuating at a predetermined stable frequency.

2. The combination with an oscillatory circuit employing a gaseous discharge tube, of means for stabilizing the frequency of the oscillations in said circuit comprising .a metallic shield surrounding the interior electrodes of said tube and a device for supplying to said shield a positive potential fluctuating at a predetermined stable frequency.

3. The combination with an oscillatory circuit employing a gaseous discharge tube, of means for stabilizing the frequency of the oscillations in said circuit comprising a metallic shield closely surrounding the envelope of said tube and enclosing its interior electrodes and a device for supplyingto said shield a positive potential fluctuating at a predetermined stable frequency.

4. The combination with an oscillatory-circuit employing a gaseous discharge tube, of means for stabilizing the frequency of the oscillations in said circuit comprising ametallic shield disposed in the vicinity of the electrodes of said tube and a device for supplying to said shield a varying potential of stabilized frequency equal to the desired frequency to be established by said circuit or related thereto by a multiple thereof.

5. The combination with-an oscillatory circuit employing a gaseous discharge tube, of means for stabilizing the frequency of the oscillations in said circuit comprising a metallic shield disposed in the vicinity of the electrodes of said tube and a fork-oscillator for supplying a varying potential of predetermined stable frequency to said shield, said fork-oscillator comprising a fork having a natural period of vibration corresponding to said predetermined frequency and a thermionic repeater tube circuit having input and output branches respectively electromagnetical- 'ly cooperating with the two prongs of said fork.

6. The combination with an oscillatory circuit employing a gaseous discharge tube, of a metallic shield disposed in the vicinity of the electrodes of said tube, a thermionic tube amplifier, and means for supplying oscillations of predetermined stabilized frequency to the input of said amplifier, the potential oscillations at the output of said amplifier being communicated to said shield at said stabilized frequency.

7. The combination with an oscillatory circuit employing a gaseous discharge tube, of a metallic shield disposed in the vicinity of the electrodes of said tube, a combined repeater and self-oscillatory thermionic tube circuit, and means for supplying oscillations of predetermined stabilized frequency to the repeater input of said thermionic tube circuit, the self-oscillations created by said thermionic tube circuit being caused to take place in step with the repeated oscillations, the resultant potential oscillations at the output of said thermionic tube circuit being communicated to said shield at said stabilized frequency.

8. The combination with an oscillatory circuit employing a gaseous discharge tube, of a metallic shield disposed in the vicinity of the electrodes of said tube, a repeater and self-oscillatory circuit comprising a thermionic tube having a hot cathode, a plate and two grids, said second circuit also comprising a resistance in electrical connection at its ends with said cathode and one of said grids, said second circuit also comprising a condenser and coupling means for transferring potential impulses from said plate through said condenser to said one grid, said condenser alternately accumulating a charge and discharging through said resistance, and means for supplying oscillations of predetermined stabilized frequency across said cathode and the other of said grids, the self-oscillatory fluctuations in the plate current being caused to take place in step with the fluctuations derived from said input oscillations, the resultant potential oscillations at said plate being communicated to said shield at said stabilized frequency.

9. The combination with a plurality of gaseous discharge tube oscillatory circuits tuned to frequencies related by multiples to one another, of means for stabilizing said frequencies comprising a plurality of metallic shields one for each circuit in the vicinity of the electrodes of its tube and a device for supplying to said shields a varying potential of stabilized frequency equal to the bilized frequency equal to the frequency to which one of said circuits is tuned or related thereto by a multiple thereof,the potential oscillations being communicated to said shields at said stabilized frequency.

11. The combination with a plurality of gaseous discharge tube oscillatory circuits tuned to fre-" quencies related by multiples to one another, of a plurality of metallic shields one for each of said gaseous discharge tube oscillatory circuits in the vicinity of the electrodes of its tube, a combined repeater and self-oscillatory thermionic tube circuit, and means for supplying to the repeater input of said thermionic tube circuit oscillations of stabilized frequency equal to the frequency to which one of said gaseous discharge tube oscillatory circuits is tuned or related thereto by a multiple thereof, the self-oscillations in said thermionic tube circuit being caused to take place in step with the repeated oscillations. the resultant potential oscillations at the output of said thermionic tube circuit being supplies to said shields at said stabilized frequency.

12. In a musical instrument, a plurality of groups of note-producing gaseous discharge tube oscillatory circuits, all of such circuits being tuned being one of' said' device for each of said circuit groups and each of said devices supplying potential'inlpulses of frequency equal to the frequency to which one of the circuits of its associated group is tuned orrelated thereto by a multiple thereof. 1 1'3. The combination with a gaseous discharge tfibe oscillatory circuit, of a metallic shield disposed-in the vicinity of the electrodes of the gaseous discharge tube of said circuit, another gaseo'u's discharge tube oscillatory circuit, another shield" disposed in the vicinity of the electrodes of the gaseous discharge tube of the second-mentioned circuit, means for supplying to the secondmentioned shield potential oscillations of stabilized freq'uency'equal to the frequency to which the'secohd-mentioned circuit is tuned or related thereto by a multiple thereof, the frequency to which the second-mentioned circuit is tuned being equal-'to'th'at to which the first-mentioned circuit is tuned or related thereto by a multiple thereof, andamplifying means to which potential oscillations from the second-mentioned circuit are supplied and supplying its output potential oscillations 'to 'the first-mentioned shield at the stabilized frequencyto which the second-mentioned circuit is tuned.

'14-.-'The combination with a gaseous discharge tube oscillatory circuit of a metallic shield disposed in the vicinity of the electrodes of the gaseous discharge tube of said circuit, another gaseous discharge tube oscillatory circuit, another shield disposed in the vicinity of the electrodes of the gaseous discharge tube of the second-mentioned circuit, a fork-oscillator for supplying to'the second-mentioned shield potential osciliations of stable frequency, equal to the frequency to which the second-mentioned circuit is tuned or related thereto by a multiple thereof, said forkos'cillat'or comprising a fork having a natural frequency of vibration equal to the frequency of the oscillations to be supplied to' the second-mentioned shield and a thermionic repeater tube circuit-having input and output branches respectively electromagnetically cooperating with the two prongs of said fork, the frequency to which the second-mentioned circuit is tuned being equal to that to which the first-mentioned circuit is tuned or related thereto by a multiple thereof, and amplifying means to which potential oscillations from the second-mentioned circuit-"are supplied and supplying its output potential oscillations to the first-mentioned shield at the stabilized frequency to which the secondmentioned circuit is tuned.

The combination with a gaseous discharge tube oscillatory circuit, of another gaseous discharge tube oscillatory circuit, a metallic shield for each of said circuits disposed in the vicinity of the electrodes of its gaseous discharge tube, means for supplying to the shield associated with the' second-mentioned circuit potential oscillations of stabilized frequency equal to the frequency to which the second-mentioned circuit is tuned or related thereto by a multiple thereof, the frequency to which the second-mentioned circuit is tuned being equal to that to which the first-mentioned circuit is tuned or related thereto by a multiple thereof, and a combined repeater and self -oscillatory thermionic tube device to the repeater input of which potential oscillations from the second-mentioned circuit are supplied, the self-oscillations created by said device being caused to take place in step with the repeated oscillations, said device supplying its resultant output potential oscillations to the shield associated with the first-mentioned circuit at the stabilized frequency to which the second-mentioned circuit is tuned.

16. In a musical instrument, a' plurality of groups of note-producing gaseous discharge tube oscillatory circuits, all of such circuits being tuned. to frequencies corresponding to the respective notes of a keyboard and each of said groups having the frequencies to which its circuits are tuned related to one another by successive octave intervals, a plurality of metallic shields one for each of said circuits in the vicinity of the electrodes of its gaseous discharge tube, and a plurality of fork-oscillators one for each of said groups for supplying potential impulses of predetermined stabilized frequency to its shields, each of said fork-oscillators comprising a fork having a natural frequency of vibration equal to the frequency to which one of the circuits of its associated group is tuned or related thereto by a multiple thereof and a thermionic repeater tube circuit having input and output branches respectively electromagnetically cooperating with the two prongs of its fork.

17. In a musical instrument, a plurality of groups of note-producing gaseous discharge tube oscillatory circuits, all of such circuits being tuned to frequencies corresponding to the respective notes of a keyboard and each of said groups having the frequencies to which its circuits are tuned related to one another by successive octave intervals, a plurality of metallic shields one for each of said circuits in the vicinity of the electrodes of its gaseous discharge tube, and a plurality of devices one for each of said groups for supplying potential oscillations of predetermined stabilized frequency to its shields, each of said devices comprising an auxiliary gaseous discharge tube os' cillatory circuit tuned to a frequency equal to that to which one of its associated group of noteproducing circuits is tuned or related thereto by a multiple thereof, each of said devices also comprising an auxiliary metallic shield disposed in the vicinity of the electrodes of its gaseous discharge tube, each of said devices also comprising means for supplying to its shield potential oscillations of stabilized frequency equal to the frequency to which its gaseous discharge tube oscil latory circuit is tunedor related thereto by a multiple thereof, each of said devices also compris ing amplifying means to which oscillations from its gaseous discharge tube oscillatory circuit are supplied and the output potential oscillations of which are supplied to the shields cooperating with its associated group of note-producing circuits.

18. In a musical instrument, a plurality of groups of note-producing gaseous discharge tube oscillatory circuits, all of such circuits being tuned to frequencies corresponding to the respective notes of a keyboard and each of said groups having the frequencies to which its circuits are tuned related to one another by successive octave intervals, a plurality of metallic shields one for each of said. circuits in the vicinity of the electrodes of its gaseous discharge tube, a plurality of combined repeater and self-oscillatory thermionic circuits one for each of said groups, and means for supplying to the repeater input of each of saidthermionic tube circuit oscillations of stabilized frequency equal to the frequency to which one of its associated group of note-producing circuits is tuned or related thereto lty a multiple thereof, the self-oscillations in each of said thermionic tube circuits being caused to take place in step with the repeated oscillations therein, each of said thermionic tube circuits supplying its resultant output potential oscillations to the shields cooperating with its associated group of noteproducing circuits at the stabilized frequency of the oscillations supplied to its repeater input.

19. In a musical instrument, a plurality of groups of note-producing gaseous discharge tube oscillatory circuits, all of such circuits being tuned to frequencies corresponding to the respective notes of a keyboard and each of said groups having the frequencies to which its circuits are tuned related to one another by successive octave intervals, a plurality of metallic shields one for each of said circuits in the vicinity of the electrodes of its gaseous discharge tube, and a plurality of devices one for each of said groups for supplying potential oscillations of predetermined stabilized frequency to its shields, eachof said devices comprising an auxiliary gaseous discharge tube oscillatory circuit tuned to a frequency equal to that to which one of its associated group of note-producing circuits is tuned or related thereto by a multiple thereof, each of said devices also comprising an auxiliary metallic shield disposed in the vicinity of the electrodes of its gaseous discharge tube, each of said devices also comprising a fork-oscillator for supplying to its shield potential oscillations of stabilized frequency, each of said devices also comprising amplifying means towhich oscillations from its gaseous discharge tube oscillatory circuit are supplied and the output potential oscillations of which are supplied to the shields cooperating with its associated group of note-producing circuits, each of said fork-oscillators comprising a fork having a natural frequency of vibration equal to the frequency to which its associated auxiliary gaseous discharge tube oscillatory cir-' cuit is tuned or related thereto by a multiple thereof and a thermionic repeater tube circuit having its input and output branches respectively electromagnetically cooperating with the two prongs of its fork.

20. In a musical instrument, a plurality of groups of note-producing gaseous discharge tube oscillatory circuits, all of such circuits being tuned to frequencies corresponding to the respective notes of a keyboard and each of said groups having the frequencies to which its circuits are tuned related to one another by successive octave intervals, a plurality of metallic shields one for each of said circuits in the vicinity of the electrodes of its gaseous discharge tube, and a plurality of devices one for each of said groups for supplying potential oscillations of predetermined stabilized frequency to its shields, each of said devices comprising an auxiliary gaseous discharge tube oscillatory circuit tuned to a frequency equal to that to which one of its associated group of note-producing circuits is tuned or related there to by a multiple thereof, each of said devices also comprising an auxiliary metallic shield disposed in the vicinity of the electrodes of its gaseous discharge tube, each of said devices also comprising means for supplying to its shield potential oscillations of stabilized frequency equal to the frequency to which its gaseous discharge tube oscillatory circuit is tuned or related thereto by a multiple thereof, each of said devices also comprising a combined repeater and self-oscillatory thermionic tube repeater tube circuit to which oscillations from its gaseous discharge tube oscillatory circuit are supplied and the output potential oscillations of which are supplied to the shields cooperating with its associated group of note-producing circuits.

FRANCIS R. OLEARY I