US2408589A

US2408589A - Apparatus for producing electrical oscillations

Info

Publication number: US2408589A
Application number: US442804A
Authority: US
Inventors: Alvin W Wells
Original assignee: JEWEL W WELLS
Current assignee: JEWEL W WELLS
Priority date: 1942-05-13
Filing date: 1942-05-13
Publication date: 1946-10-01
Anticipated expiration: 1963-10-01

Description

Oct. 1, 1946. w, WELLS I 2,408,589

APPARATUS FOR PRODUCING ELECTRICAL OSCILLATIONS Filed May 15. 1942 3 Sheets-Sheet 2 RESISTOR PHu'ru ELECTRIC csu.

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Oct; 1946- A; w. WELLS 2,408,589

APPARATUS FOR PRODUCING ELECTRICAL dscIL ATIoNs Filed May 13. 1942 3 Sheets-Sheet 5 C 3mm 7 fllviiz/ 14 Weds.

Patented Oct. 1, 1946 APPARATUS FOR PRODUCING ELECTRICAL OSCILLATIONS Alvin W. Wells, Chicago, Ill., assignor of one-half to Jewel W. Wells, Decatur, Ill.

Application May 13, 1942, Serial No. 442,804

3 Claims. 1 The present invention relates to a system or method and apparatus for the production of'clectrical oscillations, and relates more particularly to an electrical system which uses as a source of electrical oscillations, a plurality of glow discharge oscillator circuits.

It is an important object of my invention to provide a method and apparatus for'producing electrical oscillations as obtained from one or more glow discharge oscillators which may be advantageously and economically used in the manufacture of an electric musical instrument, such as an electric organ.

While the invention is hereinafter described in connection with a musical instrument, it will be understood that the invention contemplates providing a method and apparatus for the production of electrical oscillations as obtained from one or more glow discharge oscillators which may be capable of general application.

It is well known that a glow discharge oscillator may be employed to produce electrical oscillations varying in frequency from less than 1 cycle per second to more than 30,000 cycles per second. It has been attempted to employ such a glow discharge oscillator to produce electrical oscillations of audio frequencies capable of being used in a musical instrument. Great difliculty has been encountered, however, when attempting to use a plurality of glow discharge oscillators to produce a complex audio oscillating signal, such as would be obtained when playing two or more notes simultaneously in the production of a harmonic passage. To produce a harmonic 'pa'ssage, it was necessary to electrically connect two or more oscillator circuits to an output amplifier and speaker. The difiiculty encountered was apparently due to the fact that the electrical coupling or connection between two or more glow discharge oscillators would usually cause the oscillators to tie in together and behave as a single oscillator. That is to say, the individual frequencies of the individual glow discharge oscillators, when electrically coupled together, would be degenerated to a single newfrequency and would not be a,

complex frequency resulting from the simultaneously superposi'ng of the two individual frequencies upon each other. Therefore, it'was apparent that two or mor glow discharge oscillators could not be electrically coupled together at the same time to produce a harmonic passage because of the fact that the specific frequencies of the individual oscillators were changed.

Attempts have been made to overcome this difiiculty by employing separate oscillation or buffer amplifiers for each glow discharge oscillator to thereby separate the electrical circuits of each oscillator from one another when two or more of the oscillators are simultaneously connected or keyed to the output. Since the electrical instrument may contain from sixty-one to several hundred oscillators, in order to afford a satisfactory tonal range, the practice of using separate oscillation or buffer amplifiers for each oscillator would render the musical instrument very expensive and complicated to manufacture.

By my invention, I have provided a method and apparatus for coupling two or more glow discharge oscillators simultaneously to an output circuit without affecting the individual oscillation frequency of the separate glow discharge oscillators. I have provided a photo-electric system which will be responsive to the glow or fluctuating light emitted by the glow discharge tube of the oscillator circuit. The light from the glow discharge tube fluctuates at the exact frequency of the oscillating circuit. By optically coupling the glow discharge oscillator to the photo-electric system and associated amplifier for reproducing the electrical signal into audible sound, I have provided a system whereby two or more glow discharge oscillators may be connected at the same time to the output without affecting the individual frequencie of th oscillator circuits. This photo-electric system of coupling the glow discharge oscillators to an output circuit is very economical to manufacture.

It is, therefore, a further object of my invention to provide a method and apparatus of optically coupling two or more glow discharge oscillators to a photo-electric reproducing circuit for translating the oscillation frequencies generated in the oscillator circuits to audible frequencies.

It is a further object of my invention to provide a method of producing a complex electrical oscillation resulting from the effect of a simultanecus exposure of two or more fluctuating light sources as obtained from two or more glow discharge oscillators upon a photo-electric reproducing system.

It is still a further object of my invention to provide a musical instrument employing the optical system of coupling the glow discharge oscillators to the photo-electric reproducing system having means associated with a key board for selectively exposing each of the glow discharge oscillators to' the photo-electric reproducing system.

It is another object of my invention to provide' a musical instrument employing the optical- 3 system of coupling the glow discharge oscillators to the photo-electric reproducing system having means associated with a key board for selectively exposing each of the glow discharge oscillator tubes to the photo-electric element and so constructed that the amount of optical exposure of the glow discharge tube to the photo-electric element will be proportional to the amount of pressure applied to the clavier key of the key board, thus offering a means of varying the dynamics of the individual electrical signals and enabling the production of percussion effects, or the like.

It is still a further object of my invention to provide a musical instrument employing the optical system of coupling the glow discharge oscillators to the photo-electric reproducing system in which the glow discharge oscillator circuits are provided with a means for periodically raising and lowering the oscillation frequencies to thereby produce a vibrato effect.

Further objects and advantages of my invention will be apparent by reference to the follow ing specification and drawings, in which:

Figure 1 is a schematic diagram of a conventional glow discharge oscillator using a neon glow tube;

Figure 2 is a schematic diagram of a glow discharge oscillator in association or optically cou pled to a photo-electric reproducing system comprising a photo-electric cell and a photo-electric amplifier of standard design; Figure 3 is a schematic diagram of a number of glow discharge oscillators provided with a con:- mon source of applied voltage and showing a means for periodically raising and lowering the applied voltage to provide a tremolo or vibrato effect;

Figure 4 is a diagrammatic showing of one form of apparatus for selectively exposing the glow 4 discharge tube of an oscillator circuit to a photoelectric cell by means of an optical shutter in connection with a clavier key;

Figure 5 is a schematic diagram showing a plurality of glow discharge oscilators and diagrammatically showing a single photo-voltaic type photo-electric element responsive to the glow of any of the glow discharge tubes in the circuit;

Figure 6 is a cross-sectional view or the photovoltaic type cell diagrammatically shown in Figure 5;

Figure 7 is a diagrammatic showing of a modified form of key arrangement for selectively exposing the glow discharge tube of an oscillator circuit to a photo-electric cell in which the degree of exposure will be proportional to the amount of pressure on the key or the relative depressed position of the key;

Figure 8 is a diagrammatic showing of still another modified form of key arrangement to i provide pressure responsive dynamic control of the individual tones produced; and

Figure 9 is a front detail view of the optical shutters shown in Figure 8.

Referring to the drawings, Figure 1 is a scheage, cause the circuit to oscillate, and the frequency of the oscillation may be varied by adjusting the constants of the circuit, such as the resistor 2 and the condenser 4. The frequency of the oscillation produced is aiTected by the capacity, resistance, and applied voltage in the circuit. Therefore, the glow discharge oscillator circuit may be tuned to different oscillation frequencies by varying the value of any of these constants, such as the capacity, resistance, and applied voltage. If the voltage is maintained constant, the condenser or resistor values may be varied to change the oscillation frequency. I have found, by experimentation, that the best practice for tuning the oscillator to desired frequencies is to adjust the values of the resistor 2 for the lower frequencies included in the range up to 1,000 cycles per second and to adjust the value of the condenser 4 for the higher frequencies included in the range of 1,080 cycles per second up to the highest frequency desired.

In Figure 2 of the drawings, I have shown a schematic diagram for a glow discharge oscillator that is optically coupled to a photo-electric reproducing system. The glow discharge tube 5 is connected in series with the resistor 6 and the applied voltage as obtained from the voltage di vider l. A condenser 8 is shunted in parallel with the resistance 6. By adjusting the resistance 6 or the condenser 8 as described above in connection with Figure l, the frequency of 0*- cillation in the oscillator circuit may be changed. It is a function of the glow discharge oscillator tube 5 that the light or glow emitted by the tube when oscillating in an oscillating circuit will flue-- tuate at a frequency which is identical to the frequency of the oscillating circuit. Optically conected to the glow discharge tube 5 is a photoelectric cell or other photo-electric responsive element 9. This photo-electric cell is provided with a suitable photo-electric amplifier which I have shown in Figure 2 of the drawings to be of the resistance coupled type. Plate, grid, and bias resistors R, together with coupling and by-pass condensers C, triode tube Hi, and output transformer T, constitute a schematic showing of a standard type of resistance coupled amplifier. It should be understood that any other desired form of amplifier, such as a transformer-coupled or a direct-coupled amplifier, may be used in place of the resistance-coupled amplifier which I have shown. It will be apparent that any desired number of photo-electric cells may be connected to a single amplifier.

The operationof the electrical system shown in Figure 2 of the drawings and described above is as follows: When the oscillating circuit is energized, th glow discharge tube 5 will emit a fluctuating light corresponding in frequency to the frequency of the oscillating circuit in which the glow discharge tube is connected. The fluctuating light emitted by the glow discharge tube 5 is directed upon the photo-electric element 9 and will cause the photo-electric element 9 to translate the light impulses into a fluctuating or oscillating current which will be coupled to the photo-electric circuit by the resistance-coupled preamplifier and transformed by the output transformer T to the proper impedance desired for a power amplifier or th voice coil of a loudspeaker (not shown). Thus, an electrical system is provided for coupling a glow discharge oscillator to a reproducing system in which the circuit for the glow discharge oscillator is electrically independent of the circuit for the reproducing c resses r system. With such an arrangement of optical coupling, it is possible to connect two. or more glow. discharg oscillators simultaneously to a photo-electric reproducing system, and. because the circuits of the oscillators and the circuit of. the reproducing system are independent, there will be no electricalcoupling or connection between the. individual oscillator circuits, and the difliculties previously encountered, wher electrical coupling or connection between the oscillators was used,.are avoided.

Figure 3 of the drawings shows a plurality of glow. discharge oscillators provided with a common source of applied voltage. The appliedvoltage for the circuit. shown in Figure 3 is, periodicallyvaried about a mid-point to obtain a period: ie rise and fall in the frequency of oscillations produced in the glow discharge oscillator circuits. A voltage divider generally indicated by D. is divided into resistance segments ll, [2, l3, and M. An appliedvoltage of 350 volts is supplied to the voltage divider D. At the point between the resistances. H and 12, the switch contact I5 is connected, and at the point between the resistances l2 and IS, the glow discharge oscillator are connected by means of the line l6. At. the point between resistances l3 and M, the switch contact H is electrically connected. The switch blade l8is pivoted at 19 and is connected by means of the pitman arm 20 to a crank 2| on the shaft ofv a small electric. motor 22,. The small electric motor is preferably of the synchronous type which will run at constantspeed, or. it may, if desired, .be providedwith suitable means to vary the speed, and it will be apparentthattheswitch blade l9. willbe periodically connectedat regular intervals with the switch contacts l5. and I! when the electric motor is inoperation. Theswitch blade I8 i electrically connectedthrough an on or off switch 23. and resistor 24t0, the line l6 supplyingapplied voltage to. the glow discharge oscillator circuits. The glow discharge oscillator circuits which are connected to the common source ofapplied voltage, as supplied to the line I 6 .and the ground connection 25, are provided witha number of glow discharge tubes 26,. It will benoted that. some of the glow discharge tubes. 26 are provided with variable resistances 21 to adjust the. frequency of the oscillating circuit and thereby the frequency of the fluctuating light emitted by the discharge tube, while others of the glow discharge tubes are provided with variable. condensers 2B for adjusting the frequency of. the oscillations and the fluctuating light. As previously mentioned, the variable resistor. 21 is adjusted in the oscillating circuit when the oscillator is tobe tuned to lower frequencies, whereas the variable condenser 28 is adjusted when tuning the oscillating circuit to higher frequencies.

As the oscillation frequency of the glow dis.- charge oscillator is a function of the resistance capacity and applied voltage, it will be apparent that varying the applied voltage will vary the frequency of oscillations. As shown in Figure 3, a plurality of glow discharg oscillators are provided with a common source of applied voltage. An electrical system is also shown in Figure 3 and described above for periodically raising and lowering the applied voltage about a mid-point, so asto periodically raise and lower the oscillation frequencies of the oscillator circuits, and thereby produce a vibrato effect which will be audible when the fluctuating light from the glow discharge tubes is interpreted by the -photoeelec- 6 tric reproducing system; and speaker. In operation; the electrical. system for periodically raising and-lowering the applied voltage to the oscillator circuit is as-follows: The; voltage of 350 volts appliedacross the voltage divider D which is comprised of. the resistance segments ll, l2, l3, and I4. When the vibrating switch blade I8 is in the mid-position shown in Figure 3 of the drawings andis not electrically contacting the switch contactslfisor H; the applied voltage supplied to the line H5 in the glow discharge oscillators is obtained from the voltage divider D at the point between. resistance segments l2 and I3. The oscillators will then be supplied with a normal amount of applied voltage. When the vibrating switch blade I8 is electrically contacting the switch contact I5, the resistance 24 will be placed in parallel with the resistance segment, l2, thus effectively reducing the total resistance of the resistors I l and I2. and increasing the applied voltage tothe line l6 and the glow discharge oscillators. The frequency of the oscillators will then be raised above, the normal. When the vibrating switch blade I8 is in electrical contact with the switch contact [1, the resistance 24 will be connected in parallel with the resistance l3, thus effectively reducing the amount of resistances l3 and I4. As the resistances I3 and I4 are part of the voltag divider and are connected across the lines Hiv and 25 of the applied voltage for the glow discharge oscillators, the applied voltage to the line 16 and the oscillators will be reduced. Therefore, the frequency of the oscillators will then be lowered below the normal. Thus, it will be seen that the vibrating switch blade 18, periodically contacting the switch contacts l5 and I1, will cause the appliedvoltage to the line I S and the glow discharge oscillators to be periodically raised and lowered about a normal. When the switch blade i8 is in the position shown in Figure 3 and not contacting the electrical contacts 15 and I1, the applied voltage will be at a midpoint because of the fact that the resistance 24 is not electrically connected-into the circuit. It is important that the applied voltage common to all of th oscillator circuits be varied above and below a normal or mid-point as this prevents th illusion of the musical instrument'going out of tune when the vibrato is turned on or oif. A fixed condenser 29 is shown across the output of the voltage varying device to eliminate any undesirable oscillations in the glow discharge oscillators, due to the sparking of the switch contacts I5 and H with the vibrating switch blade l8.

In Figure 4, I have diagrammatically shown oneform of apparatus for selectively exposing the fluctuating light from the glow tubes to a photoelectric element. Mounted within the lightproof box 30 is the photo-electric cell 31 and neon glow tube or its equivalent 32. The condenser 33 and resistor 34 for the glow discharge oscillator circuit are also mounted within the lightproof box. A thimble 35 encloses the glow tube 32 and is provided with an aperture or window 36. Positioned'in front of the window 36 i a shutter element 31 pivoted on the spring arm 38 bolted to the lightproof box at 39. A connecting rod in is connected to the spring arm 38 of the shutter 31 and passes through the lightproof box to connect with the'clavier key 4| mounted upon the spring arm 42. When the clavier key 4| is depressed, as shown in the dotted lines, the shutter 31 will be removed from in front of the window 36, allowing the fluctuating light from the glow discharge tube 3.2 toimpinge upon the elements of the photo-electric cell 3|. With this form of apparatus, it is possible to maintain the glow discharge oscillators in operation at all times. Whenever it is desired to optically couple a glow discharge oscillation circuit to the photo-electric reproducing system, the clavier key is depressed, thus removing the shutter from in front of the aperture and permitting the fluctuating light from the glow discharge tube to affect the photoelectric element. Any number of lightproof boxes containing the photo-electric tube, together with the glow discharge tube and shutter, may be provided and simultaneously operated, if desired, without affecting the individual oscillation frequencies of the individual glow discharge oscillators, since the electrical circuits for the photoelectric reproducing system and the glow discharge oscillators are independent of one another.

In Figure 5 of the drawings, I have shown a number of glow discharge oscillator circuits provided with glOW tubes 43 and supplied with a common source of applied voltage of 350 volts. Variable resistors 44 and variable condensers 45 are used to adjust the frequency of the fluctuating light given oil by each of the glow discharge tubes 43. A photo-voltaic type of photo-electric element 46 is provided to receive the fluctuating light of the glow tubes 43. The photo-voltaic type of photo-electric cell is an elongated tube which can be made of sufiicient length to simultaneously respond to a large number of glow discharge tubes. If the glow discharge oscillator circuits and tubes are to be used in the construction of a musical instrument, such a an electric organ, it will be necessary to have at least ninetysix glow discharge tubes and associated oscillator circuits. These glow disharge tubes may be placed in a line within the cabinet of the musical instrument, and the elongated photo-voltaic type of photo-electric cell may be optically aligned with the line of glow discharge tubes. If de sired, however, more than one photo-voltaic type of photo-electric cells may be used to take care of all of the oscillator circuits and glow discharge tubes necessary to produce the ordinary range of musical notes on a keyboard instrument.

A cross-section of the photo-voltaic type of photo-electric cell is shown in Figure 6. An elongated casing 41 is provided with a glass window 48. Positioned within the casing is a surface of light-sensitive substance 49 deposited on a metal plate 59. An electrical contact or binding post 5| is connected to the metal plate 50, while another electrical contact or binding post (not shown) is connected to the light-sensitive surface 49. The electrical.,contacts of the photovoltaic type of photo-electric cell are connected in the usual manner to a suitable amplifier.

In Figures 7, 8, and 9 of the drawings I have shown two modified forms of keying arrangements for selectively exposing the light of a glow discharge tube to a, photo-electric element. The use of an optical keying system such as my invention employs offers a means for varying the dynamic volume of the individual electrical oscillations produced by the photo-electric reproducing system in accordance with the manner in which the keys are struck. That is to say, I am enabled to proportionally expose the glow discharge tube to the photo-electric element in accordance with the pressure applied and the position of the manually depressible key to thus give a greater dynamic volume to the electrical oscillation or tone produced in response to a greater pressure applied to depress the clavier key.

Referring to Figure 7, a clavier key 52 mounted upon the spring arm 53 is connected by the connecting rod 54 to the shutter 55 pivoted on the spring arm 53. A thimble 5i encloses the glow discharge tube 58 and has an aperture 59 through which the light from the glow discharge tube may pass to the photo-electric cell 69. When the clavier key 52 is depressed the shutter 55 will be removed from in front of the aperture 59, thus eyposing the glow discharge tube 58 to the photoelectric cell 60. It will be apparent that the amount of exposure or uncovering of the aperture 59 will be proportional to the pressure applied to depress the clavier key 52. To indicate the relative depressed position of the clavier key 52, I have provided stops El, 62, and 63 cooperating, respectively, with

spring

54, 65, and 66. If the key 52 is depressed slightly, stop 6i will contact sprin 64, thus giving an indication of the position of the key 52 and the relative exposure of the aperture 59. If greater pressure is applied to depress the key 52 the stop 62 will engage spring 65, thu giving a further indication of the position of the clavier key and the relative exposure of the aperture 59. A rest 57 is provided to indicate to the player when the clavier key has been depressed all of the way to completely expose the aperture 59, thus giving the greatest dynamic volume to the electrical oscillation reproduced through the photo-electric system.

By skillfully playing, it is possible, with the form of my invention just described above, to accent the individual notes in a passage to any degree desired, producing various musical effects, such as percussion or pizzicato effects.

Referring to Figures 8 and 9, a slightly modified key arrangement is shown for the application of my invention to an instrument primarily designed for producing percussion effects by proportionally exposing the glow discharge tube to the photo-electric cell in response to the pressure applied to the key. The clavier key 68 is pivoted at 69 and is provided with a stepped arrangement of stop positions 10, ll, and i2. Springs l3, l4, and 15 are adapted to cooperate, respectively, with the stop positions Iii, II, and 12. A rest 11 is provided for indicating the fully depressed position of the key 68. A shutter element [8 having an aperture or window 19 is slidably positioned for vertical movement within the light proof box and in front of the aperture or window 8| of the thimble 82 enclosing the glow discharge tube 83. A detailed front view of the shutter element 18 is shown in Figure 9 of the drawings. A weight 84 is positioned at the top of the shutter element 18. When the key 68 is struck the lever end 85 of the key will cooperate with the bottom end of the shutter element 18 to thrust said element upward, and the intensity of the impact will determine the upward distance which the shutter 78 is moved. If the key 68 is struck sharply the shutter element 18 will be thrust upward to its full extent, thus completely exposing the glow discharge tube to the photo-electric cell. When the key is fully depressed, as shown by the dotted lines A, the shutter element 18 will be moved so that the center line C of the aperture 19 will be in the position shown at A to completely register the shutter 19 with the window or aperture 8| of the thimble 82. If the key 68 is depressed to the position shown in dotted lines at B the center line of the aperture 19 will be in the position shown at B, thus partially exposing the light of the glow tube to the photo-electric element. By skillfully playing with this arrangement the key 68 may be struck sharply tomomentarily completely expose the glow discharge tube, but due to the weight 84 and as soon as the momentum of the shutter element I8 is overcome the shutter will fall to again shield the light of the glow discharge tube from the photo-electric cell, thus producing a percussion tone of great initial dynamic volume, but decaying to a tone of low volume or of no volume at all. If the player desires, the key 68 may be struck sharply to the position shown in the dotted lines at A, and then be released to the position shown in dotted lines at B, thus momentarily completely exposing the glow discharge tube, but subsequently allowing the glow discharge tube to be but partially exposed. This is of great importance when the player desires to cause the volume of the tone to be of great initial intensity but to decay gradually to a lower volume level and thus prevent an abrupt ending of the tone.

I have described above a system for producing electrical oscillations in which a plurality of glow discharge oscillator circuits may be used. By optically coupling the individual glow discharge oscillator circuits to the reproducing system, I am enabled to couple two or more of the individual oscillator circuits to a single reproducing system without electrically connecting the individual oscillator circuits to each other. The fluctuating light emitted by the glow discharge tube of the glow discharge oscillator will fluctuate at the exact frequency of the oscillation in the oscillating circuit, and as the photo-electric element is responsive to this fluctuating light, the electrical signal generated in the photo-electric cell will be identical in frequency to the frequency of the oscillating circuit.

While I have described above, in connection with Figures 4 and 7 through 9 of the drawings, mechanical methods of selectively exposing a glow discharge oscillator to a photo-electric reproducing system, it should be understood that other means of selectively coupling the oscillator circuits to the reproducing system may be employed. Instead of maintaining the discharge tube and oscillator circuit energized at all times and using an optical shutter to control the coupling of the glow discharge tube to the photoelectric cell, I may provide a switch, such as is shown in Figure 1 of the drawings, in series with the glow tube and its oscillator circuit. This switch S, shown in Figure 1, may be connected to 10 the clavier key M to thus selectively energize the desired glow tube and oscillator circuit to cause the glow tube to emit the desired frequency of fluctuating light for afiecting the photo-electric reproducing system.

It is to be understood that the system shown and described is merely an illustration of the preferred embodiment, and that such changes may be made as fall Within the purview of one skilled in the art without departing from the spirit of the invention and the scope of the appended claims.

I claim:

1. A light sensitive system comprising a plurality of glow discharge tubes, oscillating circuits continuingly energizing said tubes, means for tuning each of said circuits to a certain frequency, electric means outside of said oscillating circuits adapted to respond to the fluctuating light emitted by said tubes, means normally shielding said photo-electric means from said light, selectively operable means whereby said shielding means is rendered ineffective as regards the light emitted by one or more of said tubes so that said photo-electric means is correspondingly activated, and an amplifier fed by said photo-electric means.

2. A light sensitive system comprising a plurality of glow discharge tubes, oscillating circuits continuingly energizing said tubes, means for tuning each of said circuits to a certain frequency, photoelectric means outside of said oscillating circuits adapted to respond to the fluctuating light emitted by said tubes, a movable shutter associated with each of said tubes normally shielding said photo-electric means from said light, means for selectively moving said shutters to permit activation of said photo-electric means by the light emitted by one or more of said tubes, and an amplifier fed by said photo-electric means,

3. A light sensitive system comprising a plurality of glow discharge tubes, oscillating circuits adapted to continuingly energize said tubes, a common normal voltage supply for said circuits, means for tuning each of said circuits to a certain normal frequency, a photo-electric system inde pendent of said circuits disposed for energization by light emitted b said tubes, means for selectively controlling such light emission, an amplifier fed by said system and means for rapidly cyclically varying the voltage supply between above and below normal whereby to cause corresponding variations in the frequency of said tubes and thus produce a vibrato effect.

ALVIN w. WELLS.