US2506679A

US2506679A - Vibrato system for electrical musical instruments

Info

Publication number: US2506679A
Application number: US725848A
Authority: US
Inventors: James A Oswald
Original assignee: Central Commercial Co
Current assignee: Central Commercial Co
Priority date: 1947-02-01
Filing date: 1947-02-01
Publication date: 1950-05-09
Anticipated expiration: 1967-05-09

Description

May 9, 195o VIBRATO SYSTEM FOR ELECTRICAL MUSICAL INSTRUMENTS Filed Feb. l, 1947 y.1. A. oswALD `2,506,679

2 Sheets-Sheet l TGENEA T01? HSK/70E Kga/LL May 9, 1950 J. A. OSWALD 2,506,679

VIBRATO SYSTEM FOR ELECTRICAL MUSICAL INSTRUMENTS Filed Feb. 1, 1947 2 Sheets-Sheet 2 Patented May 9, 1950 'VIBRATO SYSTEM FOR ELECTRICAL MUSICAL INSTRUMENTS James A. Oswald, Chicago, Ill., assignor to Central Commercial Company, Chicago, Ill., a corporation of Illinois Application February 1, 1947, Serial No. 725,848

Claims.

This invention relates tothe art of electrical musical instruments, and, more particularly, to a novel and improved vibrato system for instruments of the described character.

Conventional electrical musical instruments generally comprise a plurality of -oscillation generators of the space discharge tube type of which one is provided for each note of the entire range of the instrument. The most common practice at the present time to obtain good frequency stability is to provide twelve so-called master oscillators which are of high quality and possess excellent frequency stability. Each of these master oscillators is tuned to a diiferent note of a single octave of the equally tempered scale and controls the frequencies of a cascaded set of octavely related slave oscillators. As the frequency relationship of these so-called slave oscillators is determined by that of the respective master oscillators, they may be relatively simple and inexpensive in character whereby substantial savings may be accomplished in the cost of the entire instrument. An electrical musical instrument of this general type is disclosed in the Larsen Patent #2,403,090, wherein each master oscillator stabilizes a set of octavely related slave oscillators of the multi-vibrator type.

As an interesting modification of this principle embodied in most electronic organs mention is made of the use of cascaded aperiodic frequency dividing circuits, such as Eccles-Jordan circuits, a cascaded set of which is coordinated to each master oscillator. are incapable of independent oscillation, they are capable of deriving octavely related output signals for any input signal frequency. Electrical musical instruments of this type are disclosed in the copending U. S. patent of Nicholas Langer, No. 2,486,039, dated October 25, 1949 and entitled Electrical musical instrument.

In all electrical musical instruments, it is desirable to provide for slight frequency modulation of the oscillations produced. This is carried out at a very slow rate in the range of about 6 cycles per second and gives rise to a very attractive musical eiect, commonly called vibrato. As in electrical musical instruments of the master.`

controlled slave oscillator type the frequencies of the slave oscillators directly follow those of the master oscillators a good vibrato eiect may be obtained by frequency-modulating the master oscillators. Various diiculties were, however, experienced in carrying this principle into practice. A master oscillator in order to provide its characteristic stabilizing function upon a set of.

While these circuits cascaded slave oscillators has to be of extremely high stability. On the other hand, an oscillator which may readily be frequency-modulated is characterised by relatively low frequency stability. Obviously, these two conditions are mutually exclusive. Thus, While frequency modulation of some oscillators may be accomplished by electrode voltage shifts, for example, these circuits are inherently not sufficiently stable to permit their satisfactory use in musical instruments which have to maintain their tuning for long periods of time. That is to say, if a frequency modulation or vibrato effect may be obtained by intentionally varying electrode voltages of the oscillator tube, then any random variation in electrode voltages will likewise produce a shift in frequency. This would necessitate the use of voltage regulated power supply sources 4and would also mean that any change in tube characteristics would detrimentally iniiuence the tuning of the instrument.

In the Larsen patent referred to in the foregoing, there is disclosed a vibrato system which provides a practical solution of the outstanding` problem. In this system, each master oscillator comprises a triple grid super control ampliiier tube which is electrically associated With a frequency determining LC or tank circuit to consti-y tute an electron coupled tuned grid oscillator.-

Frequency modulation of each master oscillator is accomplished by means of an individual triodey tube, the plate circuit oi which is in series with a capacitor shunted across the tank circuit. A vibrato signal derived from an oscillator of very 10W frequency, for example 6 cycles per second, is simultaneously impressed upon the grids of all of the said triodes in such a manner that the resulting variations in the plate resistance of the said triodes will produce variations in the capacitive reactance in the tank circuits, which, of course, will cause frequency modulation of all master oscillators at the desired vibrato rate. It will be noted that in this vibrato system the individual triode tubes coordinated to each master oscillator function not only as amplifiers of the vibrato signal but also serve to isolate the master oscillators from the common vibrato oscillator.

While the Larsen vibrato system provides excellent results, it has the disadvantage that it requires a relatively complicated circuit includ-v ing 12 triode tubes. The necessary Yaddition of such a large number of tubes, with their sockets and circuit elements and power requirements have materially increased the cost of the entire instru-l ment. Although various other suggestions and accompanyingldrawifngsi- Y' proposals were Vmade to solve the outstanding problem, none, as far as I am aware, of these suggestions and proposals have been completely tioned problem. mayrbe fsolvedin :a A'remarkablyY simple manner.

It is an object of the present invention torprovide a vibrato system for electrical musical in,-I

struments Which avoids the difcultiesand rinlcon-Y r Y A:coenilileif-oscillators, this second triode unit funcveniences connected with prior vibratosystems. Y It is Vanother object of the present invention to provide a vibrato system oj-novelandfimproved l character which permits thef--requencyflmoelula;

tion of a set or group of master oscillators by acommon vibrato oscillator mithoutfnesnitting to separate and individual isolating and amplifying v tubes, while retaining the normal and 'desirable l high frequency stability characteristics of the master'ioscklatora.

It iisna' furthersubject'off-the.inventionV tto lerovidefa greatly vsirnpliried vibrato system for electricaJlf .fxnu'sicail'` instivumentsV which. the 'vibrato eiectisAproducedfbya.novelficircuitorganization. ot thezmasterfosciiflatcr where'by frequency .inodw virtuali threeelement tubes; .one `of which is i utilized:as:fanueieetronecoupledJosciiiatorfoffgreat control grid and the screen performing the function oiV anode for thetriode oscillator thus formed. Most of the electrons of the electron stream emitted by the cathode and moving towards the screen grid of small physicalY dimensions are going to miss the screengrid, and proceed towards theanodefoiitneftube. This maires the screen grid infeiect'fa yi-rtilal-fcathode for the second triode unit constituted by the screen grid, suppressor grid and plate.

In-,the normalpractice of designing electron tions as an amplifier for the oscillations gener- -jated -infth'efoscill-atory circuit and alsoserves to islolaiteitlie seii'diosciilatory circuit from load variation-sin thev 'output circuit. Y In contrast to this,

in1accordanoei withlthe invention, the second triodeV unit is utilized not only as an amplifier but also as airequency modulating device acting upon the oscillatory,7 circuit associated with the first triodeunit. Y

In..tleoregoingfthe :system formed by the screengrtid, suppressorV rgrid,;'-and :plate was: :likened toa triode.Y tlisspossibl'e,thereforegtosvay the effectiveV ,plate'-resistance of'thi'ssv-irtualzt-rioiie by impressing. :a signal'V upon its ".icontrol grid;

frequencyfsta litylwhilerltlie .other functions as:

2i 'f-lqunyimodilatingftube and iis-instrumentai inprodueingtiieivibratoeect.

Y'Ifriie invention also contemplates -Aa y-viEr-ato' sys-- scillator type which is extremely simple in con#V l struction, cloni-pact in character andi Whichmay be'fincorporatedflfirito fan organ ait 1eracticallyv ino extrawcostexcept dorf-that' of ay vibrato signal l Y generator. Y

` :Other and`r @wither objects: and 'advantages .of Vtlie inventionw'fiiIi-beeome #apparent fiumi-the folm loWi-iigsjd'escription, -tekfen in-conjuncti'onwithf itlfie.I Y which: Y Y' Fig-ure l is the :circuit-ua rrxaster oscillator,

including means lfor 'freguencyemodailatiigg the vou'tputl'oithesame atwibrato Lrate; embodying ticeprinciples-of-'jhepresent invention;`

Yeligure'iZ@isabloicl: diagram slrowingltl'ie basic circuit forganiaationvu of' a group .of master os cilL i laters, tregnencymndulated'byfa cominonfvibr-ato the.v asentado'. type` electricaily associated Vwitla circaitl to; constitutei an. electron 'coupled'.- osciilalion,y 73E-he. :electron coupled* oscillator has:4 the Vadvant-age`thatinfan oscillator -of this; type. the.A screen',andpplatefsupply voltages; can be so arrangedathat ashifit-,in Ythe'lprimary"supplyvoltagepmay loev compensated. inl sucia, a: way-that V there will be; nozresulting shift in operating,

reduency.

' nzan electron'coupled oscillator, thezioscillatoryf circuit;isi'electrically associatedf with the rstz tlhree elementsaof thetubeimamelyb.tnecatl1nde.;,.

the pentoie .type hayingf'ae/cahode iU., a controls grid SH', la `screensgrid 12; .3,fsuppressor 4'3" andan anodeforzplate |54". .For'thefsake of clarity,VV

wlfiiclr isY the suppressonagrid: of the gpentode. By

Vconneet-ing a capacitor from-'the hotend-of they tank. circuit' to the :plate oT the: tube, the spiatey resistance ol the said-virtual ftriodefa'ndfthe saidy capacitor in series therewith Vwill .beshunted acrossfthetank Since the-virtual V'cathode of the system, :tl'i'atis theesereen grid, V'istat' ground potentialfwithf'respect to2 A. C. sig-nails', by supply4= ing the: suppressor; .grid With-ra 'vibrator isigna'l ofl very IDW frequency, suela :asf'fapproximately 6 cy,iA

cles per;v second", a corresponding irequency shift will fbe obtained, so'thatlthe `*output signal taken f-norrrtlueplate of :thegtube will be :frequencymodulated.A andr alsoA will be.- :amplitude modulated to' a; sliglitrexte'nt'. This .frequency modulated signal.

is fed. to a cascaded*groupffofoctavelyrelated slave r'oscillators upon which :theiamplitude vari.- ations present.. in the,- fontput.. signalv 'Will :have noi eiiect,r tirait a. pureA fnequency modulation 'Orf vibmtoieffect will- 1-r'errrain. Y

' In order to give thoseffskilled'i-nfftheart afbet-v ter understanding fof the invention, yreferencefis made to'iligiire 1f 'ot theidrawi-ng which isv a. preferred zformfrof fcircuift-yorganization'oi av frequency' modulatedmaster oscillator'embodyingthe prin.-

ciplesofthe presentuinventionz. Reerenceharlacten 'Vi-'4 dental-,esl a .space discharge: tube oi illustration of.' .theY conventional heater* 'of the cathode has beenv omitted.. Platei voltage oifsuable. value isi appliedtoplate. i eromavsource lei. connected to termin'alr.A,through.fai dropping .refs-ist'anceY Rf-'lg and screen: voltagefz :appriziprrate value :is appliedf1to1screenl grid f''firomva tap 'B oir Vav voltagev dividerI resistance connected. between; A. and therfgrolundf. -liihilefpoint;Bis-

substantially aboverground potential-nasiiar as:

D. C. potentialy vis concerned, itis"V effectively at! Vgroundpotential for A. C: component: .prese ent ldimiento thev connectionuof. a.bypass-condenser A tank :circuitcomprisinginductancezLfl .andy capacitorrCe-thaso'n'eiof its: terminals Liteon--VV small-grid capaciton Whilesitscotherr termi nalv fll" is; connected', to. ground. Awconventionalg: magridi-leak isiconnected-'across'capeicitorG24;

grid, the screen grid Willi be readily understood. byv those. skilled.' inv the art. Referring first to' the firstl triode section of. tube V-l `constituted'fby cathode l0, control grid Il and screen grid l2, it will be noted thatthisi electrode together with tank circuit L--l, C-l and the other circuit elements electrically associatedYV therewith form a tuned gridV oscillator, feedback between the grid. and the plate circuits beingv prcvidedby the coupling. between inductances L-I and L-2. Electrical oscillations will be produced in this' circuit in a manner well-understood, the frequencyofsaid oscillations being. determined by the value'of inductance'L-l andr capacitance C-I. As most of the electrons emitted by cathode l Si miss screen grid l2, a substantial portion of'suc'li electrons will proceed to plate IQ. An oscillatory signal of the produced frequency may be withdrawn atl the plate and may be appliedl to the octavely related cascaded oscillators as this will'be set forth more fully in the following.

Screen grid l2, suppressor grid I3 and plate lf3 of tube lT-l operate as a second virtual triode, the screen grid functioning as the cathode, the suppressor as the control gridV and the plate as the anode. Therefore, the vibrato signal of vibrato oscillator I3 will vary the effective plate resistance of the second triode at the vibrato rate and will' act as. a conducting path of variable resistance elect'ively connecting capacitor C--l` to capacitor C-I of the tank circuit. This will cause frequency modulation and to a minor extent also amplitude modulation of the oscillations withdrawn;

It will be noted thatthe vibrato system of the invention embodying the frequency modulated master oscillator justdescribedl provides various important advantages. The most important of these is that no individual triode or modulator tube is required'for each master oscillator since the same pentode tube serves both as the usual electron coupled oscillator and the frequency modulator tube. Another important advantage ofthe circuit is that the inherent frequency stability of the electron coupled oscillator is retainedsince changes in both the screen current and anode current are reilected to the oscillatory circuit as a frequency shift so that by proper manipulation of the electrode voltages the shift due to the one may be caused to exactly cancel the shift due to thevother. This cancellation, however, does not affect the frequency modulating or vibrato circuit, since there only the plate currentis varied. In order to obtain maximum vfrequency stability, it is necessary, however, that the anode Hl andthe screen grid l 2v be rsupplied from the same source, using a voltagedivider network as shown in the form ofv tapped resistance R-Z in Figure 1. It is perfectly satisfactory to connect the screens of all master oscillator tubes together and-tosupply them from a-singlevoltage-v divider networkprovided' that the annees. of auf. tubes are supplied from-1 the same-source.. It will'.

further be noted thatthe`- suppressor' grids ofi all. master oscillator tubesmay be.` tied. together and supplied from a` single'. vibrato oscillator' I8, generating a vibrato signal having. a frequency of. approximately 6 cyeles. per second.. In" other words, terminals AI,-B`, and @maybe common for all master' oscillators ofthe?electrical'musical instrument.-

In a practical elctricali musical instrument. generally 12 masterr oscillators arev employed.,- each of which" is tuned'V to a differentnote ofi a' single octave of the tempered scale, which in most cases is therhig'hest octavelo'fi the rangeof.y the instrument. 'Ifl'leoutput of` each master oscillator is'appliedto a-cascaded group of. octavely related slave oscillators froml which4 the oscillations of mlsic'al.v frequency' may.l bev selectively withdraw-n in accordance Withthef requirements' of' aA musical composition bymeans of' switches under the control of a manually. operableV keyboard.

A system` ofthe described character is diagrammatically illustrated in tile-block` diagram of Figure 2, in whichreferenc'e numerals l"2.1., 22 andi 23 designate three ofthe twelve: master oscillators, each of whichA has the circuitiorganizationV shown in Figure 1f. Plate andfscre'en voltages are applied toall master-oscillators through a common conductor 214 from power supply 2b? The suppressor grids of all master oscillators are likewise connected'to a common. conductorl 25;' supplied with the vibrato signal fromY an oscillator 2l. Thus, the outputfrequenciesof' all master oscillators will be frequency modulatedV at the same vibrato rate and will be also amplitude modulated. to a minor extent.

The output of eachi of 'master oscillators 2l, 22, 23, ete. is. connected to a cascaded group of octavely related slave oscillaters' 2B, 219, 30, etc. respectively. As a. result of these'v slave oscillators being locked together with eachother and with their corresponding master oscillator for synchronous operation, the output ofthe slave oscillators will be likewise frequency-'modulated'at the same vibrato rate while the slight amplitude modulation presentin the out-putof the master oscillator willbe removed. A plurality of groupsV of switching keys diagrammatically indicated by

reference numerals

3l, 32, 33, etc. is provided to selectively Withdraw musical oscillations of the desired frequencies into a common output bus bar se from which the signalsmay be introduced into an amplifier and may beA converted into sound by means of a loudspeaker;A Of course, in a practical musical instrument various additional rennements may be employed to 'modify the wave form of the output signal and thereby to obtain various musically desirable tone colors. For full details of such arrangements and also of preferred forms of octavely relatedV cascaded slave oscillaters reference maybe had to the aforesaid Larsen patent, #2,403,090, also the disclosure in copending application Ser'. No. 702,742.

The vibrato oscillator employed in the vibrato system of the invention may be of anyy suitable type capable of producing'vosc'illations of very low frequency, in the order of 6 cycles per second' with a good waveform ofgenerall'y sinusoidal character. An oscillator of' the so-called phaseshift type which is particularly advantageous for the purposes of. the invention is'illustrated in Figure 3 of the drawing; For details of the Iconstruction and..A operation.v of? this oscillator Yreference'may be had to the textbook Theory and Applications of Electron Tubes by H. J. Reich, 2nd edition, page 398. j i p Referring now more particularly to Figure 3 of the drawing, there is provided a triode V 2, having a cathode 35, a control grid 36 and an anode or plate 31. Plate voltage is supplied to anode 31 through a resistance Eil-4. A resistance-capacity network comprising capacitors C 5, C-B and C l and resistances 3 5, R- 6 and R-l is connected in the anode circuit of the said triode and is calculated to impart a phase shift of 180 degrees to any signal of appropriate frequency impressed upon the anode. As there is in any vacuum tube an inherent phase shift of 180 degrees Vbetween the grid and the anode, the signal at the junction of C 5, R. 5 will be 360 degrees out-of-phase with respect to the initial signal at the grid; As in any recurrent wave, a phase shift of 360 degrees is the same as zero phase shift, any signal applied to grid 36 of tube V 2 will be returned to said grid after amplification in the tube in such a manner as to cause the production of sustained oscillations. The frequency of said oscillations is determined by the relative values of 0 5, 0 6, C-l' and R 5, Rf-, B since for any particular set of values of the circuit constants there will be only one frequency at which the phase shift will be exactly 180 degrees. In actual practice this circuit will start oscillating due to any slight fluctuation in electrode voltages or tube characteristics and, once the tube started to oscillate, it will continue to produce oscillations of ystable frequency in the manner described in the foregoing. Because of the losses in the phase shift network, a certain minimum gain is necessaryfin the amplifier circuit in order to sustain oscillation. It can be shown that this gain has to be in the` order of 29.

For best results C B should equal C and C 'I and R-5 should equal B S and R-'i in the phase shift network. However, small variations in frequency due to standard tolerances in the components may be compensated for by making corresponding adjustments on any one of the above elements.

In the phase-shift oscillator just described, the frequency of the oscillations and their stability will directly depend upon some particular phase shift. Therefore, any loading of the oscillator will tend to change the frequency or to stop oscillation altogether. For this reason, it is necessary to interpose an amplifier between the output of the oscillator circuit and the load circuit. This amplifier may be a triode, such as tube V 3 shown in Figure 3, having a cathode 3B, a control grid 39 and a plate d. Cathode 38 of tube V 3 is directly connected to cathode 35 of tube V Z and grid 38 of tube V 3 is directly connected to grid 35 of tube V 2. Grid bias is obtained by means Vof a common cathode resistor R- connected between the cathode and ground and having a by-pass capacitor 0 8 connected across the same. Plate voltage is applied to anode @D of tube V 3 through a resistance B Si and the amplified oscillations of low frequency are applied to the load, that is to the suppressor grids of the master oscillators, through a Vcoupling condenser C 9.

Since the cathode and grid circuits of the oscillator and amplifier tubes V 2 and V may be connected in parallel, the circuit readily lends itself to the use of a high-mu dual triode, such as the tubes type 6SL7 or 6SC7.

The circuit just described has important advantages over the' Van der Pol oscillator and other negative resistance oscillators used for generating' very low frequencies. Of greatest impartance among these advantages are the excellent waveform and the small physical size of the components necessary to generate oscillations having a frequency of Iapproximately 6 cycles per second as is normally required in a vibrato system for musical instruments. Other advantages are the great stability of oscillation and the fact that the oscillation frequency is in no way a function of the tube characteristics, such as the plate resistance, transconductance, etc.

The electrical values of the elements as numbered and lettered on theV drawings and several circuits and networks and preferred types of vacuum tubes employed are, approximately, as

follows:

'Oondensers I have described use of my invention in a system employing 12 master oscillators. Resistance R2 which is common to all master oscillators has an entire resistance of 15 K., and is tapped at 10 K. as shown at Figure l.

The coefficient of coupling, coils Ll and L2 is approximately .77, being the average position of the iron core.

Although the present invention has been described in connection with a preferred embodiment thereof, variations and modifications may be resorted to by those skilled in the art without departing from the principles of the invention. I consider all of these variations and modifications to be Within the true spirit and scope of the present invention, as disclosed in the foregoing description and dened by the appended claims.

What is claimed as my invention is:

l. In an electrical musical instrument, a frequency-modulated generator of audio frequency electrical oscillations which comprises a spacedischarge tube having at least a cathode, a plate, a control grid, a screen grid and a suppressor grid; means including an oscillatory circuit electrically associated with said cathode, said control grid and said screen grid to produce electrical oscillations of audio-frequency in said circuit; a high resistive load circuit; connections from said plate and screen grid to spaced points in said high resistive load circuit, and means for impressing a signal having vibrato rate upon said suppressor grid whereby the frequency of the oscillations produced in said oscillatory circuit will be modulated at said vibrato rate.

2. In an electrical musical instrument, a frequency-modulated audio frequency oscillation generator which comprises a space discharge tube having a cathode, a plate, and control, screen and suppressor grids; said cathode, control grid, and screen grid constituting a rst triode unit and said screen grid, suppressor grid and plate constituting a second triode unit, the first and second triode units being electron coupled to eachV other; means including `an oscillatory circuit electrically associated with said rst triode unit to produce audio-frequency oscillations therein and to make such oscillations effective in said second triode unit; a high resistance connected between said plate and cathode, a tap on said high resistance connected with said screen grid; and means for varying the effective plate resistance of said second triode unit at a vibrato rate thereby to frequency-modulate at such rate the oscillations generated in said rst triode unit.

3. In a frequency-modulated oscillation generator for electrical musical instruments, the combination which comprises a space discharge tube having at least a cathode, a plate and control, screen and suppressor grids; said cathode, control grid and screen grid constituting a first triode unit of which the screen grid is the plate and said screen grid, suppressor grid and plate constituting a second triode unit of which said screen grid is the cathode; said rst and second triode units being eifectively coupled to each other by the transfer of electrons from one unit into the other; means electrically associated with said first triode unit to cause a flow of electrons varying at a musical frequency rate through both triode units; a high resistance circuit connected between said plate and cathode, a tap in said high resistance circuit leading to said screen grid means including a vibrato oscillator for varying the effective plate resistance of and the iiow of electrons through said second triode unit at vibrato rate thereby to frequency-modulate said electron flow at such rate; and coupling means for withdrawing frequency-modulated oscillations of musical frequency from at least one of said triode units.

4. In an electrical musical instrument, the combination with a plurality of frequency-modulated generators; each of said generators comprising a pentode tube, having a cathode, control grid, screen grid, suppressor grid and a plate and in Which the cathode, control grid and screen grid constitutes a first triode unit and the screen grid, suppressor grid and plate constitutes a second triode unit electron coupled to the rst unit; connections supplying plate and screen voltages to all of said pentode tubes from a common source of current; a set of reactive circuit elements for the first triode unit of each pentode thereby to produce oscillations of musical frequency therein; a vibrato oscillator having its output directly connected to the suppressor grid of each pentode to simultaneously apply oscillations of vibrato frequency to the second triode unit o f each pentode thereby to frequency modulate said oscillations of musical frequency at a common vibrato rate; a high resistance circuit connected between said plate and cathode, a tap in said high resistance circuit leading to said screen grid and coupling elements individual to said pentodes to withdraw frequency-modulated oscillations therefrom, the frequency of said oscillations andthe rate of their modulation being substantially independent from variations in the voltage of said common source of current.

5. In an electrical musical instrument, the combination which comprises a group of twelve electron-coupled master oscillators, each ineluding a pentode tube including a cathode, control grid, screen grid, suppressor grid and a plate, a tank circuit electrically associated with the cathode and control grid and tuned to a musical frequency corresponding to a different note of a single octave of the tempered scale, a common source of plate and screen voltages for said tubes to cause the generation of oscillations of described frequencies by said master oscillators and to render such oscillations effective in the respective plate circuits of said tubes, a high resistance circuit connected between said plate and cathode, a tap in said high resistance circuit leading to said screen grid, a common vibrato frequency oscillator having its output directly connected to the suppressor grids of all of said tubes thereby to frequency modulate said musical oscillations at vibrato rate, a cascaded set of octavely related slave oscillators for each of the said master oscillators locked in with but effectively isolated therefrom, and key-controlled switching means to selectively withdraw oscillations from said slave oscillators, each of said withdrawn oscillations being octavely related to one of the master oscillators and all of the withdrawn oscillations being frequency modulated at the same vibrato rate.

JAMES A. OSWALD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,254,284 Hanert Sept. 2, 1941 2,321,354 Bell et al June 8, 1943 2,328,282 Kock Aug. 31, 1943 2,341,040 Hathaway Feb. 8, 1944 2,347,458 Brown Apr. 25. 1944