US2905905A - Electric musical instrument - Google Patents

Description

Sept. 22, 1959 T. J. GEoRGE ELECTRICAL MUSICAL xNsTRUMENT 2 Sheets-Sheet 1 Filed Feb. l. 1954 fun" Ti/0,1445 J EORQE,

' 1N VEN TOR.

/ rme/Ey Sept. 22, 1959 T. .1. GEORGE ELECTRICAL MUSICAL INSTRUMENT 2 Sheets-Sheet 2 Filed Feb. 1, 1954 filo/144s J, GEORGE l N V EN TOR.

E s TM C United States Patent Office ELECTRIC MUSICAL INSTRUMENT Thomas J. George, North Hollywood, Calif. Application February 1, 1954, Serial No. 407,223

29 Claims. (Cl. 331-49) This invention relates to electrical musical instruments and more particularly to improvements in electrical switching systems employed to generate various notes in such instruments.

In electrical musical instruments, such as electronic organs, a keyboard which is provided with a plurality of manually or pedally operated keys, is employed lo operate switches that control circuits that generate electrical waves of various frequencies and these waves are reproduced acoustically to produce musical sounds. In certain types of such instruments, oscillators are employed to generate the sounds of the various notes. In certain electrical musical instruments the electrical tone generated by each oscillator includes not only a cornponent of fundamental frequency that represents the pitch, but also overtones, or harmonics of the second order and higher. With instruments of this type, formant circuits are employed to modify the tones produced by the oscillators so as to simulate the sounds of various mechanical musical instruments. In some instruments a separate oscillator is controlled by each key.

In any electrical musical instrument employing a keyboard, it is necessary to provide a switch for each key. lIn some electricall musical instruments, the key switches employ many pairs of contacts that are connected o corresponding circuits which control the harmonic content, or spectrum, of sound produced when the respective keys are pressed. In others, one pair of contacts is employed to tune the oscillator and another pair to energize the oscillator. Such switches are frequently a source of trouble, due particularly to the failure of the contacts to close properly and thereby operate the circuits when the corresponding keys are pressed. Such failure of contacts often occurs because tarnish develops on the contact surfaces. This is especially true when the contacts are employed in low-voltage circuits. To overcome this difiiculty, precious metals such as palladium and platinum have been used to minimize contact failure. This of course is very expensive especially when each key switch is of the multiple contact type.

Various systems have been proposed to eliminate the need to provide an individual oscillator having its Own amplifying tube for each of the keys. For example, circuits have been devised which permit one oscillator to be employed to generate any one of two or more consecutive notes, as for example, C, Cit and D. With such a system, only one of these notes can be played at a time even though more than one key is pressed. The fact that two of such notes can not be played simultaneously is not a serious limitation since it is rare that any two notes differing by only one or two semitones would be played in the same chord. In the past, the switching systems that have been employed with such a multiplenote oscillator have employed switches that utilized at least two pairs of switch contacts per switch, the pairs of contacts of each switch being closed simultaneously to create the musical note corresponding to each key, one pair of contacts being employed to tune the oscillator Patented Sept. 22, 1959 to' the desired frequency and the other pair being ernployed to energize the oscillator.

According to the present invention in its simplest form, a single pair of contacts operated by pressing each key is employed to tune the oscillator to a predetermined frequency and to simultaneously initiate oscillation. Furthermore, in accordance with this invention, a plurality of such switches that are operated by different keys are connected to tune and operate a common oscillator and the switches are so arranged that a frequency corresponding to only one of the keys is produced and is of the correct value, even though some other key associated with the same oscillator is pressed simultaneously. Also, in accordance with the present invention, the switch contacts are included in high voltage circuits, thereby permitting the employment of relatively inexpensive contact material such as silver. Also, in accordance with the present invention, as illustrated in the forms thereof that are described herein, the various switches corresponding to different keys in a group associated with a single oscillator are arranged to connect resonant circuit means, such as reactive elements, of different values, to an oscillator that contains other resonant circuit means, when the respective keys are pressed, in order to selectively tune the oscillator to a fundamental frequency of the value that corresponds to the note being played. l In the forms of the invention described herein in detail, 'the resonant circuit means associated with the respective switches are reactive, and upon operation of lthe switches, they are selectively connected in parallel with'other reactive means that is permanently connected in the oscillator to effect the desired tuning. In one form of the invention, the resonant circuit means associated `with the switches are capacitive while the resonant circuit means connected permanently in the oscillator includes an inductive element; in another form of the invention, they are inductive while the resonant circuit means connected permanently in the oscillator includes a capacitive element. As will become apparent hereinafter under some circumstances, all of the tuning for one note of a group of notes may be accomplished by means of a resonant circuit permanently connected to the oscillator amplifier, and this frequency may be altered by some of the switching keys to produce the other notes of the group. Also, under some conditions, no reactive element at all need be connected permanently to the amplifier of the oscillator.

Inv both forms of the invention, the oscillator includes an amplifier in the form of a triode that is inoperative when biased beyond cutoff and is operative when biased below cutoff, and the same contacts that are employed to determine the tuning of the oscillator also shift the bias to initiate the oscillation.

' Accordingly, to understand the principles of the invention, reference is made to the following description and to the drawings, wherein like reference characters in the different figures indicate like elements and in which:

Fig. l is a wiring diagram of a portion of an electronic organ to which my invention is applied;

Fig. la is a fragmentary wiring diagram of an alternative form of the invention; and

Fig. 2 is a wiring diagram of an alternative form of oscillator.

manual keyboard 10, a plurality of multiple-note oscillators 20, a keying potential source 50, a formant circuit 70, a power amplifier 80, and a loudspeaker 90.

This electronic organ operates on the chromatic scale, each octave containing four groups of three notes each.

namely, the first group, A, Aft, B, the second group C, Cit, D, the third group Dit', E, F, and the fourth group Fi?, G, Git. The keys in each group are employed to control-the generation of electricwaves by a corresponding oscillator 26. While the organ mayfcontain three, or even five octaves, only two oscillators are shown.

,For convenience in describing the invention, the superscripts etc., are employed to designate identical or corresponding `parts of the circuits that are located in the respective oscillators 2t). However, the superscripts are sometimes omitted when. the description applies equally toditferent groups of notes. v l, Y

All ofthe oscillators corresponding tothe respective groups arefof the same design excepthowever, that some ofthe 4components have different values l,as explained more fully hereinafter. The oscillatorl' is operated by vthe keys KA, 'KAL and KB in the "r'st group, while oscillator 20" is operated by the keys KC, KC?, and KD, in the `second group, etc. Outputs from all the oscillators 20226, etc., are transmitted through the common formant circuit 70 to the power amplifier St), 4which supplies the loudspeaker 9G in which thevarious musical l.tones are audibly reproduced.

,Though the oscillators 20 may be of other types, as Vshown here, they employ Hartley circuits that utilize ampliertubes in the form of vacuum triodes T, hereinafter for siniplicitymerely called ampliers. Three switches S1, S2, and S3 are arranged in sequence in each oscillatorl() to tune and operate each oscillator 20 ata different frequency laccording to the key that is pressed. The rst switch S1 has a pair of normally-closed upper contacts U1 and a pair of normally-open lower contacts L1;V the secondswitch S2 has a pair of normally-closed upper contacts U2 and a pair of normallyfopen lower contacts L2; and the third switch S3 has onlya pair of normally-open lower contactsLg. The designations upper and lower are employed here not `to designate physicial positions infthevorgan itself, but merely to assist'in distinguishing between the pairs of contacts.

The keys KA,4 KAit, KB, KC, KCit, and KD, .are connected through suitable linkages or levers of conventional typeto the corresponding switches S3", S2', S1', S3,QS2", and S1", respectively, so that each of 'the switches is operated when the corresponding key is pressed and restores when the key is released. Sometimeshereinafter, when the description is equally applicable tofany key, itis rererredtoV simply by the symbol K. Likewise the symbolsL and U are employed to refer to contacts of'a typicalswitch S.

When no note is being played, Vall ofthe switchesrS1', S2', S3', S1", S2", S3", etc., are intheir normal, or restored positions in which the'u'pper Vpairs of contacts U1', U2', U1,YU2", etc., are closed, andthelower pairs'of contacts L1', L2', L3', L1, L2, L3", 610-; areopen. But

when a note is to be played the corresponding key K is depressed thereby opening the vnormally-'closed u'ppker contact U andclosing the normally-open lower contact L of the corresponding switch S.

Oscillator design and switching circuit Each of the oscillators employs a triode and a tuned `parallel-resonant lcircuit O that includes a capacitor-"C0 and an inductor L0. The inductor L0 is provided with end terminals T1 and T3 and is divided into two parts by means of a grounded center tap T2. One terminallTa .of the inductor L0 is connected to the grid g of ,the corresponding triode T through a blocking capacitor C1.and also through a grid resistor R1 that is connected in parallel with the capacitor C1. each inductor L0 is connected to the anode of thecorresponding triode T through a feedback resistor R2 and` a blocking condenser c2 in series with the feedback resistor R2. Positive voltage from a suitable power supplyv hav- The other terminal T1 of i ing a negative point grounded is applied to the B+ terminal, this power being applied to the anode a through 'through a potential dividing network including resistors R4 and R5. The latter resistor R5 of the potential divider is connected between the cathode k and ground and is shunted by a by-pass condenser c3. The output of each oscillator 20 appearing at the anode a is applied through the correspondingA blocking condenser c2 and a corresponding plate resistor 'Rfto the input of the formant circuit '7,0.l

The yalues oftheresistoirs R3, R.1,'a`nd R5,'a're`so chosen that when no bias is 'applied't'o the Ytriode T through any of theswitches S1, S2,vv and S3, thegrid `g is biased a few Volts beyond'cutol. YIn this`cond'iti`onftle 'trio'deis inoperative and the oscillatorl` does not 'scillate.

The stationary members of the pairs of lower contacts L2 and L3 of the two switches S2 and S3 are connected through condensers C2 and C3 to the terminal T1 of the oscillator circuit O remote from the connections to the grid g. vThestatit'nary` members' of the pairs of lower cont'actsrL1, L2,faii`d L3of` all'ltliree itcliesare4 connected' to the'keying'p'tential source`50 th: 'ufghirdiyidual isolation resistors R1(,4 R11, and R12"respectively,'and"a comriinvariabl'ent'uning 'resistor R14. V

` The movable arm offthe rstfs'witch S1 is connected directlyto theg'rlidg. The movable Vrm of theV second s`wit'AchfS2 irs, iconn'eclt'edwto thestatio' ryrhember of lthe gripper vpair l`offc'oiitactslU1 of the 'first-switch 'S1, andthe lr'noyatileiarm ofthe 'third Switchs, "is conneefed'to' the stationary'rnembfer'zof"tlie'fupper contactUz Vof' thef secoiid'switch'52. ",Thuafjwhen thesl' itchesare in 'their Aorf restoredcnditionf,Ilo Avoltage from 'tlekey- `ing"poteri*tial ,source r'is'suppliedfto Vtlie'grid g' and 'itfs itiaseijfo cutoff, A1510, 'in this' nrralor'restoredee `t `n'"the; 'moval5le'farm of 4the`thi'rd switch S3 'isf con- 1 giiiibe ppzrrepr tint-a capacitor, C1 @indue Cencted 'betw enI 4 e' Lst ationaiyjriilemblerf'of the lowerfpair ,.Of, centactsjrmf 'the'vr'srswifeh s1 and tneaermiaarr, 'VordenY to'` tune the 'oscillator `to the dfesiedf`Y frequency 'whenfcontacts' Lfarerclos'ed. Furthermore, it will be enoted that'ify such capacitor C1i`mp1oyedgthe aparcvtodCodrri'aybe ornitted'xfas'fshown Vin Fig. l'afprovided 1only th'atthe' value Sof the 'capacitors C1,C2C1,'a're` so n astofprodce"the'desired tuning. YlH'w'ever, 'con- `sfidrfable-4ecnomy ,isfeffected'vby omitting ondens'er C1, fand., employing themain'tuning condenser C0 asshown in Fig; l.

Operation L u The/operation of a typical'os'cillato 20" for generatir'lg the notes C, Cit, Vand D" is'describedindetailwbelow.

playlthe'note D, the'corresponfding keyKDl is de* pressed, thereby operating the corresp'onding'rst keying 'switch S1", opening 4 'the upper contacts U1""V and ,clo'sing ithe llower;contacts L1". "Whent'heV `first switch" S1" "is thusoperated', the'secfo'nd and thirdfswit'ches S2" and S3 are rendered inoperative, being totallydisco'nriected fr'o'm 'the grjidizgj, wheajtiie starters," is' prated; 'pesittve `voltage from the keying potential'source*Silpis applied through afresis'torV Rw'pfand jthe llower* contacts L1",

voscillator and terminatingV the note the saine time attestata@ "attenere ps2," .ad S31" genaamd foroperationby beingconnectedagain Vto the grid g" lower contacts 1.2"., When the second switclrS2 is thus operated, the third switch S3 is rendered inoperative, being totally disconnected from the grid g". Also, when the switch S2 is thus operated, the condenser C2 is effectively connected in parallel with the main tuning capacitor C0, thus altering the resonant frequency of the oscillator circuit O. At the same time, positive voltage is applied from the keying potential source 50 through the resistor R11" to the grid g. As a result, the oscillator 20" is tuned to the fundamental frequency of the note C# and oscillation is simultaneously initiated. When the key KC# is released, the switch S2 restores thereby terminating the generation of the Cit note. At the same time the third switch is prepared for operation by being connected again to the grid g.

To play the note C, the corresponding key KC is depressed, thus operating switch S3", closing the lower contact L3. When the switch S3 is thus operated, the capacitor C3" is connected in parallel with the main tuning condensers C0, thus altering the resonant frequency of the oscillator circuit O. At the same time, positive voltage is applied from the keying potential source through the resistor R12 to the grid g". As a result, the oscillator 20 is tuned to the fundamental frequency of the note C and oscillation is simultaneously initiated. When the pressure on the key KC is released, the switch S3" restores, thereby terminating the generation of the C note.

In operation then, when any one of the keys KC, KCit, or KD is depressed, the corresponding note C, C# or D is produced. It will be noted however, that if two of the keys are depressed simultaneously, only the sound corresponding to the higher pitch note is produced. Thus, if the key KD and either the key KC# or the key KC are depressed, the note D is played because the operation of the switch S1 disconnects the moving arms of the switches S2 and S3 from the grid g. Similarly, if the keys KC# and KC are both depressed, the note C# is played, because the operation of the switch S2 disconnects the moving arm of the switch S3" from the grid g.

It is thus seen, that, although three switches associated with each oscillator are employed to produce three different notes, only one note can be produced at a time and the operation of more than one switch does not interfere with the correct and accurate production of that note. In spite of this, many or almost all chords that appear frequently in the usual musical composition can be played because it is seldom that chords employ notes that are only a tone or semitone apart. Even such chords as these may often be played if the close notes appear in groups of notes associated with different oscillators.

By employing a relatively high keying potential, such as a voltage of about 70 volts, across the contacts when open, low cost contacts, such as silver contacts, may be employed.

Frequency shift In practice, the frequency of oscillation is determined primarily by the values of the inductor L0 and the capacitor C0, and to some small extent also upon the value of the voltage applied to the grid g. For this reason, if the keying potential source 50 consists only a regulated voltage supply, the frequencies of the sounds produced upon the closure of any of the keys have some predetermined fixed constant values.

But, if the keying potential source 50 includes some means to cause its output to vary, then when any key is depressed, the voltage applied to the grid g varies, thereby changing the frequency of the note produced. In practice, the voltage from the keying potential source may include a relatively large DC. component and a relatively small A.C. component of 6 c.p.s. as indicated by the waveform W at the outputof vthe keying potential source,

When such a source of such fluctuating voltage is employed, a vibrato effect is produced.

Furthermore, the frequencies of the signals generated when the various keys associated with the oscillator are depressed may be varied somewhat by moving the sliding contact of the potentiometer p.

Provision is made for adjusting the frequencies of the individual notes by employing variable resistors as isolation resistors R10, R11, and R12.

Circuit constants In order to select the values of the tuning capacitors C0, C2", and C3, so as to generate middle C and the neighboring notes C@ and D, some suitable value of the inductor L0 is first chosen, and the sliding Contact s is set in midposition or at some other intermediate position, on the potentiometer p. The value of the main tuning capacitor C0 is then so chosen that the frequency of oscillation of the oscillator 20 under these conditions with the first switch S1" closed is 293.66 c.p.s., which is the fundamental frequency of note D. Next, the value of the capacitor C2 is so chosen that with the second switch S2 depressed, the oscillator oscillates at a frequency of 277.18 c.p.s., which is the fundamental frequency of note Cit. Then with only the third switch S3" depressed, the value of the capacitor C3 is so selected that a frequency of 261.63 c.p.s., which is the fundamental frequency of middle C, is generated by the oscillator.

When a 615 is employed as amplifier T and the value of the voltage supplied by the keying potential source 50 is 70 volts and the voltage applied to the B}- terminal is volts, suitable values for the various components of the oscillator 20 for generating the notes middle C, middle Cf, and middle D are as follows:

In this application, the following abbreviations have been used:

c.p.s.=cycles per second h=henries ,uf.=microfarad Similarly, the circuit constants of the other oscillators are so chosen that notes of the correct pitches are obtained.

It will be noted that once the circuit constants have been chosen, the frequencies produced may be varied somewhat by manipulation of the moving contact in the potentiometer p. Furthermore, if desired, the resistors R10, Ru, and R12 may be made variable so that the pitches of different notes produced by an oscillator may be varied somewhat as previously mentioned.

If desired, the amplifier tubes of various oscillators may be combined in a common envelope. Thus, instead of employing a 615 triode in each oscillator, a dual triode such as a 6SN7 may be employed for two oscillators. In this case it is apparent that it is only necessary to employ two dual triodes in the manufacture of the oscillators required to generate any note in an entire octave of the chromatic scale. Only twelve oscillators and hence only six dual triodes are required for a three-octaveorgan.

Y7 Tapped Transformer Type In an alternative embodiment of the invention lillustrated inFig; 2,'there is'show'n an arrangementin which theA keying switches tunethe Oscillator to the desiredA frequenciesby connecting different'inductances inparal'lel with a main tuning capacitor, the-value ofthe inductances beingso chosen asto tune the oscillator to the desired frequencies. This oscillator is very Asimilar to those described above in that the platecircuits andthe cathode circuits are of the same form. However, the tuning elements and the grid circuits are different as explained more fully hereinafter. l p

VIn the oscillator of Fig. 2, a tuning condenser C10 and tapped inductor L10V are employed to tune the oscillator 20'to the various frequencies desired, upon the closure of'thekeying switches S1, S2, and S3. In this case, however, though the switches S1, S2, and S3 are of the same construction as'th'ose previously described, they are connected to the grid g through the grid leak capacitor c1 and grid leali resistor R1 and they are employed to generate the lnotes C, Ci', and D, respectively, instead of D, Cit and C respectively, as in the former case. The tuning'inductor L10 is provided with tive terminals including end terminals T1 and T3, and taps T2, T.1,and T5. The tap T2-which is near the center of the inductor L10 is connected to ground. The terminals T3, T4, and T 5 are connected to the' stationary members of the lower contacts L1, L2, and L3 of the switches S1, S2, and S3, respectively.

The main tuning capacitor C is connected between the movable'arm of the first switch S1 and the terminal T1. This condenser is shunted by a small trimmer condenser C11 and a variable resistor R14. l

With this arrangement when thekey KCis depressed, the entire inductor L10 is connected across the condenser C10 and the switches S2 and S3 are rendered inoperative. When the key KCit is depressed, the portion of the inductor L10 between taps T1 and T4 is connected across the condenser C10, and switch S3 is rendered inoperative, and when the key KD-'is depressed, the portion of the inductor 4t? between the terminals T1 and T5 is connected across the condenser C10. In this oscillator, as in the oscillator previously described, even if more than one key is depressed, only one note is produced because when a switch is inoperate, it cannot effect the connections to the -grid g when the corresponding key is depressed.

In order that notes 'of desired fundamental frequencies may be produced, thevalue of the inductances connected across the capacitor C10 upon depression of the respective switches S1, S2, and S3 are so chosen that the resonant circuit 10 thereby formed has the desired characteristic. Thus, for example, if the note C is produced by operation of a switch S1 and the connection vof the entire inductor L10 across the condenser C10, then 'for operatio-n of switches S2 and S3 to produce the notes Ct and D respectively, the inductance of the portion of the inductor between the taps T4 and T1 should be 87.6%, and the inductance ofthe portion of the inductor between the taps T5 and T1 should be 73.8%, of the inductance of the entire inductor L10.

The system of Fig. 2 has the advantage over that of Fig. 1 in that aging of the inductor or capacitor of any oscillator does not change the ratio of frequencies of the three tones that may be ygenerated by that oscillator.

In the oscillator of Fig. 2, an adjustable negative voltage from a keying potential source 50 is connected to the 'grid circuit through a potential dividing network including resistors R15 Aand R10 which may have values of 500,000'w and 1,000,000 w, respectively. When all the switches S1, S2, vand S3 are'in their normal or restored conditions, the full voltage supplied from this source S0 is impressed uponl theV grid thereby biasing the tube T beyond cutotf. 'Buti when any of the'fswitches S1, S2, or S3,are closed,the grid'geis-c'onnected through the grid leak capacitor c1 and resistor R15 and through a. portion of the inductor L10 to ground, thereby changing the bias on thegri'd to below cutoff andv initiating the oscillation of the circuit.

It is thus seen that 1n this oscillator' -as"in the foregoing,

the `operation of each key switch,*botl1 tunes the circuit and changes the bias simultaneouslyjthereby initiating oscillation of the circuit at the desiredfrequency.

Conclusion it is to'be noted that in both types of"oscillators described herein, the switches perform three functions. First of all, each ofthe switches, except the switchS1 when capacitor C1 Vis omitted in the first form of oscillafor as shown inFig. 1,` connects a tuning element to a i xed tuned circuit or element to determine the frequency of oscillation. Secondly, the loperation of each switch changes the bias in a 'grid circuitto initiate oscillation. And thirdly, each v'of the switches except the last in the sequence, acts to'render each of 'the following switches in the sequenceinoperative, so that if morel than one key is depressed, nevertheless, -only one note'is produced. Furthermore, all 'of these functions are accomplished simultaneously in the most rudimentary fashion, 'namely one in which only one'pair of contacts closes, and no more than one pair of contacts `opens'when any one switch is operated.

`In the first form `of the invention described above, the fixed tuning element may be in the form of an inductive reactance provided by the inductor L0 or it may be in' the form of a reactive 'impedance' provided by the inductor L0 andthe capacitor C0. yIn this casel as described, the additional reactive elements are C1, C2 and C3 that'ar'e connected inthe circuit to chn'geits tuning are in the form'of capacitors. However, it is to be understood that at least from a theoretical standpoint they could bein the form of inductors. In the second form'of'the invention', however, the fixed tuning element is in the form of a capacitivereactance supplied by ythe capacitor C10 Vand theV addedi'tuning elements are in the form 'of Yinductive reactances provided by thevarious portions of lthe'in'd'u'ctor L10 that are connected to the lower contacts ofthe key switches. lt is to be Anoted that in the oscillators of this invention, a fixed tuning element `or` tuning circuit is permanently connected in the grid circuit and that theifrequencyof oscillation is altered by connecting a selected additional tuning element `to the fixed or main tuning element when one yof the key switches is depressed.

While only a limited number of forms of the yinvention have been'desc'ribed herein, it will 4be understood that the invention may be embodied in many other forms without departing from the underlying principles. it is therefore obvious that the invention is not limited to the'forms described herein, but is capable of a wide variety of embodiments. Various changes which will now suggest themselves to those skilled inthe art may be made in the form, details of construction, and arrangement land con# nection of the elements'without departing from the invention. Vlt is therefore to be understood that the invention encompasses all forms thereof that come within the scope within the appended claims.

The invention claimed is:

l. in an electrical musical instrument: an oscillator including an amplifier connected between coupledV input and output circuits, said amplifier being inoperative when biased beyond cutoff but operative when biased below cutoff; means for normally biasing said amplifier beyond cutoff; a plurality of sequentially arranged resonant circuit means, each being adapted to tune said oscillator to a different frequency when connected thereto;V a plurality of corresponding sequentially arranged normally restored key switches; a plurality of corresponding sequentially arranged keys, each being'connectedV to operate the corresponding key"switch`w hren Vsaid each key is pressed 'and toresto're the corresponding key Vswitch when'sid'each key isfrele'asedgineans 'controlled by theopeation 'of Aeach 9 ly switch eitcpt the last in the sequence for rendering inoperative the remaining key switches following said key in the sequence; means controlled by the operation of each operative key switch for connecting the corresponding resonant circuit means to the oscillator; and means controlled by the operation of each operative key switch for changing the bias on said oscillator from beyond cutoff to below cutoff.

2. `In an electrical musical instrument: an oscillator including an amplifier connected between coupled input and output circuits, said amplifier being inoperative when biased beyond cutoff but operative when biased below cutoff; means for normally biasing said amplifier beyond cutoff; a potential source; a plurality of sequentially arranged resonant circuit means, each being adapted to tune said oscillator to a different frequency when connected thereto; a plurality of corresponding sequentially arranged normally restored key switches, each including a pair of normally-closed contacts and a pair of normallyopen contacts, a plurality of corresponding sequentially arranged keys, each being connected to operate the corresponding key switch when said each key is depressed and to restore fthe corresponding key switch when said each key is released; the normally closed contacts of each key switch except the last in the sequence being so connected that upon operation of each key switch the remaining key switches following said each key in the sequence are rendered inoperative; and the normally open contacts of each operative key switch being so connected to the respective resonant circuit means and to said potential source that upon operation it connects the corresponding resonant circuit means to the oscillator and changes the bias on said oscillator from beyond cutoff to below cutoff.

3. In an electrical musical instrument: an oscillator including an amplifier connected between coupled input and output circuits, said amplifier being inoperative when biased beyond cutoff btu being operative when biased below cutoff; means for normally biasing said amplifier beyond cutoff; a resonant circuit connected to said input circuit; a resonant circuit means that is normally disconnected from said input circuit; first switching means adapted when operated to bias said amplifier below cutoff and when restored to bias said amplifier beyond cutoff; second switching means adapted, when operated while said first switching means is restored, to connect said resonant circuit means to said resonant circuit and to simultaneously bias said amplier below cutoff and when restored to disconnect said resonant circuit means from said resonant circuit and to simultaneously bias said amplifier beyond cutoff; first means including a first key for selectively operating and restoring said first switching means whereby oscillation of said oscillator is initiated and discontinued at one fundamental frequency; and second means including a second key for selectively operating and restoring said second switching means whereby when said first switching means is restored, oscillation of said oscillator is initiated and terminated at a different fundamental frequency.

4. In an electrical musical instrument: an oscillator including an amplifier connected between coupled input and output circuits, said amplifier being inoperative when biased beyond cutoff but operative when biased below cutoff; means for normally biasing said amplifier beyond cutoff; a first resonant circuit means connected to said input circuit; second and third resonant circuit means that are normally disconnected from said input circuit, said latter means having different values and being adapted when connected to said input circuit to tune said oscillator to different fundamental frequencies; first switching means adapted when operated to connect said second resonant circuit means to said first resonant circuit means and to simultaneously bias said amplifier below cutoff and when restored to disconnect said first and second resonant circuit parts and to simultaneously/bias said amplifier beyond cutoff; second switching means adapted when operated, while said first switching means is restored, to connect said third resonant circuit means to said first resonant circuit means and to simultaneously bias said amplifier below cutoff and when restored to disconnect said first and third resonant circuit means and to simultaneously bias said amplifier beyond cutoff; first means including a rst key for operating and then restoring said first switching means, whereby oscillation of said oscillator is initiated at one fundamental frequency and then discontinued; and second means including a second key for operating and restoring said second switching means, whereby when said first switching means is restored, oscillation of said oscillator is initiated at a different fundamental frequency and then discontinued.

5. In an electrical musical instrument: an oscillator including an amplifier connected between coupled input and output circuits, said amplifier being inoperative when biased beyond cutoff but operative when biased below cutoff; means for normally biasing said amplier beyond cutoff; first and second resonant circuit means that are normally disconnected from said input circuit, said resonant circuit means being adapted when connected to said input circuit to tune said oscillator to different fundamental frequencies; first switching means adapted when operated to connect said first resonant circuit means to said input circuit and to simultaneously bias said amplifier below cutoff and when restored to disconnect said first resonant circuit means and to simultaneously bias said amplifier beyond cutoff; second switching means adapted when operated, while said first switching means is restored, to simultaneously connect said second resonant circuit means to said input circuit and to simultaneously bias said amplifier below cutoff and when restored to disconnect said second resonant circuit means from said input and to simultaneously bias said amplifier beyond cutoff; first means including a first key for operating and then restoring said first switching means, whereby oscillation of said oscillator is initiated at one fundamental frequency and then discontinued; and second means including a second key for operating and then restoring said second switching means, whereby when said first switching means is restored, oscillation of said oscillator is initiated at a different fundamental frequency and then discontinued.

6. In an electrical musical instrument: an oscillator including an amplifier connected between coupled input and output circuits, said amplifier being inoperative when biased beyond cutoff but operative when biased below cutoff; means for normally biasing said amplifier beyond cutoff; first resonant circuit means connected to said input circuit; second and third resonant circuit means that are normally disconnected from said input circuit; first switching means adapted when operated to bias said amplifier below cutoff and when restored to bias said amplifier beyond cutoff; second switching means adapted when operated while said first switching means is restored, to connect said second resonant circuit means to said first resonant circuit means in Said input circuit and to simultaneously bias said amplifier below cutoff and when restored to disconnect said first and second resonant circuit means and to simultaneously bias said amplifier beyond cutoff, third switching means adapted when operated, while said first and second switching means are restored, to connect said third resonant circuit means to said first resonant circuit means and to simultaneously bias said amplifier below cutoff and when restored to disconnect said first and third resonant circuit means and to simultaneously bias said amplifier beyond cutoff; first means including a first key for operating and then restoring said first switching means whereby oscillation of said oscillator is initiated at one fundamental frequency and then discontinued; second means including a second key for operating and then restoring said second switching means only when said first switching means is restored, whereby oscillation of-said oscillator is initiated at a second fundamental frequency and then discontinued; andthird means vincluding a third 'key for operating and then restoringsaid Vthirdswitching means only when said first and second switchingmeans are restored, whereby oscillation of said oscillator is initiated at a third fundamental frequency and ythen discontinued. g n n y 7. in an electronic organ, a plurality o'f'sharednot'e oscillators, each of which is adapted to selectivelyproduce signals formingla sequence cfnotes of a musical scale, each oscillator having a 'predetermined quiescent output state, each of said oscillators comprising aii'rst keyoperated frequency control p means for Vselectively changing the output state of said oscillatorufromisaid quiescent state to a first predetermined lstate of Voscillation, a second ykey-operated frequency control 'means for selectively changing the output state of said oscillator from said Hquiescent state to a second predeterminedstate of'o'scill'ation, anda third key-operatedfrequency control means for selectively changing the output ystate of said oscillator from said quiescent state 't0 a -third predeternined output state, each of Ysaid first, second and Ythird frequency control means including circuit 'means for predetermining the output frequency of said oscillator, whereby said first, second and; third states of oscillation are characterized by three different output `frequencies respectively, means interconnecting said iir'st, second and third frequency control means of'each oscillator in an interlocked circuit whereby operationr of said ii'rst frequency control means renders said 'second and third frequency control means inactive, and'ope'r'ation'ofsaid second frequency control ymeans renders said 'third `fre quency control means inactive, and bias means'selectively coupled to each of said oscillators through said first, second and third frequency control'me'an's'whe'reby 'loperation of any of said frequency control means siiniiltaneously predetermines the frequency of theoutpilt'and alters the bias of said oscillator to cause 'said'oscillator to 'o-scillate at one of said predetermined frequencies.

8. In an 'electronic musical instrument,"'a m1'Jli1r'ality"c'af shared-note oscillatorsV adapted to apply to a coniinon output a sequence of 'notes of a musical scale, s'ucce'ssiye notes of said scale being spaced a half toneapart; each of Vsaid oscillators being adapted to oscillate at afrequency of any one of a corresponding 'group of'nomore than three consecutive notes, each oscillator 'having a predetermined quiescent state in "which no 'signal 'is applied to said output; each oscillator comprising a plurality of sequentially arranged 'individually 'operable 'frequency control means, each of said 'frequency 'control means being normally active, Vand each of'said frequency control means except the last having a normallyl closed circuit control means 'and a normallyopen'circit control means; circuit'means includingsaid'nbrmally closed "circuit controlmeans of each oscillator,'s`aid circuit'mens beingopened by the 'operation of 'each'frequency control means except the last 'in 'the sequericy for Vrendenng inactive the -remaining yfrequency contro-l 'means A"following said eachfrequencycontrol means'in' 'the'sfequ'nceg means controlled by closure of each s'aid'no'r'nially open circuit control means when active for tuning :the corresponding oscillator to a predetermined 'frequency corresponding to said each frequency contro'nieans irrespective of whether 'any "of the inactive 'frequency control means is operated, and'biasrr'ne'ans"copledto a preselected circuit control'mea'us 'of A'each'of 'said "frequency control means v'whereby clos'u'r'e' of 'e'achwfsaid normallyjopen circuit control "means `siniul'tarieu'sly tunes said oscillator' to a corresponding predetermined frequency and changes l meansto said'oscillator sothati'said Voscillatorp'r'odi'i'c'z'es 'an output at said predetermined frequency.

9. In an electronic"organ,` a 'plurality''of 11Asliaired-'note oscillators vatlapted to lapply tota common lout'pi'it""circuit la sequence of notes of a musical :scale consisting of at least one octave, successive notes of saidscalebeiig spaced ahalf tone apart; Veach of said oscillators 'being adapted to oscillate at a frequency of lany :one of'a corresponding group of no morethan'three consecutive n e's, each 'oscillator having a predetermined state inv which no signal is applied yto said output; each oscillator cornprisi-ng Ya Vplurality of sequentially arranged freqn' ncy control means, each said frequency control 'r'ieans being normally active, each of said frequency control ne'ans except the last having ra normally closed circuit control means and a normally open circuit control 'mean-s; means' for operating each of the frequency 'control 'means vidually; circuit means including said normallyv closed circuit control means of each oscillator, said circuit means being vopened by the operation of eachfrequencycontrol means `except the last in the 'sequence for rendering'inactive the remaining frequency control means Afoll'o g said each frequency control means 'in the sequence; and means controlled by closure of each said 'normally open circuit `control means when active for tuning the 'corresponding oscillator to a predetermined frequency/'corresponding to'said each frequency control 'mean-sii'respecive of Whether `any of 'the inactive frequency control means .is operated, whereby said oscillator applies asig'hal at said 'predetermined frequency to said output circuit.

l0. 'In an velectronic organ, a plurality of 4sharedno'te oscillators adapted to apply to a corri'rnon 'output signals forming a sequence of'no-tes consisting "of at least *one octave of a musical scale, successive notes of said scale being spaced la half tone apart; eachfoscillator having a predetermined state in which no sign'al'is"ap'jgxli'ell to said output'and each of said oscillators being'a'dapte'd to oscillate 'atla 'frequency of any one 'of a corresponding group of only three consecutive notes `separated by halftone intervals, independently of the oscillation of 'any of the other oscillators, each of said oscillators comprising a first frequency control means including a 'normally open first circuit control means and la normally closed second circuit control means, a second frequency control means including a normally open third circuit control means vand a normally closed fourth' circuit control means, and a third frequency control means including a'no'r# mally open fifth circuit control means; means including "a first note key for operating said iirst'frequency'conirol means lto close said first circuit control means'and to' o'pn s'aid second circuit control means, means including "a second note key for roperating'said second frequency c'on- Vtrol 'means to close said third circuit control means' and to open said fourth circuit control means, means'includin'g a third note key for operating said third'frequency'conthe-interconnection of's'aid 'bias trol'r'neans to close said fifth circuit ycontrol means; first means controlled by closure of said first circuitcont'r'ol means 'for 'causing said oscillator to oscill-ate =at a fref quency of a first note of said corresponding group of notes'irirespective of whether said lthird or fifth circuit control' means is closed and to apply a'si'gnal at the Ifrequency of said first note to said common output; second means controlled by closure ofsaid third circuit control means for causing said oscillator to 'oscillate at `a frequency of Ia second note of said corresponding "group" of notes provided said second circuit controlmeansis closed but irrespective of whether said fifth circuit control means is closed and to apply a signal at thefrequency of sa'id second note to said commonroutput; andthird means controlled by closure of said nfth circuit control means for Icausing said oscillator to oscillate at afre- 'quency of a third note of said group of'notes provided said'second and `fourth circui-tcontrol meansare both closed" and to apply a'signal at the frequency ofsaid' third note'to said common output; the characteristics" of said Vfirst, second and third means of the `oscillators"being such that the successive groups of'notes produced'by theoscillators areseparated by half-tone intervals."

I115111 an electricalv musical instrument: Tan'"oscillator havingfan amplifier that is normally biased beyond cutoff;v\a first tuning means including normally open first control means and normally closed second control means; a second tuning means including normally open third control means; means including a first key for operating first tuning means to close said first control means and to open said second control means; means including a second key for operating said second tuning means to close said third control means; biasing means connectable to said amplifier through either said first or said third control means when closed :to bias lsaid Iamplifier below cutoff; said first tuning means including means for causing said oscillator to oscillate at `one predetermined frequency when said first control means is closed irrespective of whether said lthird control means is closed; and said second tuning means including means connected to said oscillator through said second control means when closed but not through said rst control means when closed for causing said oscillator to oscillate at a second predetermined frequency.

12. In an electrical musical instrument: an oscillator that is normally biased beyond cutoff; a first switch including a normally open first pair of contacts and a normally closed second pair of contacts; a second switch including a normally open third pair of contacts; means including a first key for operating said first switch to close said first pair of contacts and to open said second pair of contacts; means including a second key for operating said second switch to close said third pair of contacts; means controlled by operation of either key for biasing said oscillator below cutoff; means including a first impedance means connected to said amplifier when said first pair of contacts is closed for causing said oscillator to oscillate at one predetermined frequency irrespective of whether said third pair of contacts is closed; and means including a second impedance means connected to said oscillator through said second pair of contacts when closed but not through said first pair of contacts when closed for causing said oscillator to oscillate at a second predetermined frequency.

13. In an electrical musical instrument: an oscillator including an amplifying device having a cathode element, an anode element, and a control element; voltage supply means having first and second terminals that are respectively positive and negative relative to one another, the first terminal being connected to apply a positive voltage to said anode element; means including a resistor connected between said cathode element and said second terminal for applying a further positive voltage to said cathode element; a first switch including a normally open first circuit control means and a normally closed second circuit control means; a second switch including a normally open third circuit control means, bias control potential supply means having one end connected to said second terminal and having the other end connectable to said second terminal through resistor means either through said first pair of contacts when closed or through said third pair of contacts when closed to bias said amplifying device below cutoff; first resonant frequency determining means, a first key for operating said first switch to close said first pair of contacts and to open said second pair of contacts whereby said oscillator oscillates at one predetermined frequency in accordance with a characteristic of said first resonant frequency `determining means when said first key is operated irrespective of whether said second key is operated; second resonant frequency determining means; and means including a second key for operating said second switch to close said third pair of contacts whereby said oscillator oscillates at a second predetermined frequency in accordance with a characteristic of said second resonant frequency determining means if said first switch is not operated.

14. In an electrical musical instrument: an oscillator having an amplifier that is normally biased beyond cutoff; a first tuning means including a normally open `first control means kand a normaliy closed second contrdl means; a second tuning means including a normally open third` co'ntrol means and a normally closed fourth control means; a third tuning means including a normally open fifth control means; means including a first key for operating said first tuning means t'o close said first control ineans and to open said second control means; means including a second key for operating said second tuning means to close said third control means and td open said fourth control means; means including a third key for operating said third tunnig means to close said fifth control means; biasing means connectable to said amplifier through either said first, third, or fifth control means when closed to bias said amplifier below cutoff; means for causing said oscillator to oscillate at a first predetermined frequency when said first control means' is closed irrespective to whether third or fifth control means is closed; means for causing said oscillator to oscillate at a second predetermined frequency when said third control means is closed provided said second control means is closed but irrespective of whether said fifth Vcontrol means is closed; and means for causing said oscillator to oscillate at a third predetermined frequency when said fifth control means is closed provided said second and fourth control means are both closed.

15. In an electrical musical instrument: an oscillator having an amplifier and a first impedance means connected therein, said oscillator being operative only when said amplifier is biased below cutoff and being normally biased beyond cutoff; a first switch including first and second stationary contacts and a movable contact arm; a second switch including first and second contacts, one of which is stationary and the other of which is movable, said first movable contact arm being connected to said first impedance means, said second stationary contact being connected to one of the contacts of said second switch, said first movable contact arm and said second stationary contact of said first switch being normally closed, said contacts of said second switch being normally open; a second impedance means connected to one of the contacts of said second switch, said oscillator being tuned to different frequencies according to whether or not said second impedance means is connected to said first impedance means; a first bias means for normally biasing said amplifier beyond cutoff; a second bias means; a first resistor connected between said second bias means and said first contact of said first switch; a second resistor connected between said second bias means and the Contact of said second switch that is not connected to said second contact; and means including two keys for operating the respective switches to alter a connection between said second bias means and said oscillator whereby when either key is operated the bias on said amplifier is changed from beyond cutoff to below cutoff and said oscillator oscillates at one predetermined frequency when said first switch is closed irrespective of whether said second switch is closed and at a different predetermined frequency when only said second switch is closed.

16. In an electrical musical instrument: an oscillator including an amplifying device having a cathode element, and anode element, and a control element; Voltage supply means having first and second terminals that are respectively positive and negative relative to one another, `the first terminal being connected to apply a high positive voltage to said anode element; means including a resistor connected between said cathode element and said second terminal for applying a low positive voltage to said cathode element; a first circuit means connected in one path between said second terminal and said control element; a first normally restored tuning means including a first normally open control means having a pair of control terminals, one of said control terminals being connected to a point between said first resonant circuit means and said control element; a second normally restored tuning means including a'second normally open control means having a pair of control terminals; a second circuit means connected in another path between said second vterminal of said voltage supply means and one of the control terminals of said second tuning means; bias control potential supply means having one end connected to said voltage supply means and having the other end normally connected to said tuning means, whereby when both said tuning means are restored said oscillator is biased beyond cutoff, and whereby when one of said tuning means is operated said oscillator is biased below cutoff; means including a first key for operating said 'first tuning means to close said first control means thereby to initiate oscillation of said oscillator at onel frequenc and means including a second key for operating said second tuning means thereby to initiate oscillation of said oscillator at a different frequency, said second lcircuit means being connected to said first circuit means when one of said control means is closed but not when only the other control means is closed, whereby said oscillator oscillates at one predetermined frequency when only a certain one of said tuning means is operated and at another predetermined frequency when the other of said tuning means is operated regardless of whether said certain tuning means is operated.

l7. In an electrical musical instrument: an oscillator including an amplifier that is inoperative when biased beyond cutoff` and is operative when biased below cutoff; means for normally biasing said amplifier beyond cutoff; a plurality of normally 'active sequentially arranged frequency control means, each said frequency control`means'being"normally restored; means for operating each of the frequency control means individually; means controlled bythe operation of each 'frequency control means except the last in'the'sequence for `rendering inactive the remaining frequency control means following said Aeach frequency control means in the sequence; and means 'f'nso controlled by operation of each frequency Ycontrol means when active for changing the bias on said amplifier from beyond cutoff to below cutoff and for tuningvthe oscillator to a predetermined frequency corresponding to said each frequency control means irrespective of whether any of the'inactive frequency control means is operated.

18. In an velectrical'musicalinstrument: an oscillator that is inoperative when biased beyond cutoff and 'is operative when biased below cutoff; first bias'means 'for normally biasing said oscillator beyond cutoff; a plurality of normally active sequentially arranged switches, each switch being normallyrestored,l each switchhaving a corresponding pair of normally open contacts; means for yoperating each of the switches individually; means controlled by the operation of each switch except the last in the sequence for rendering inactive the remaining switches following said each switch in the sequence; and 'circuit means completed by the closure of the-pair of Vcontacts of -an active switch when operated, said circuit means including second bias means for changing the bias on said oscillator from beyond cutoff to below cutoff, said circuit means further including means for tuning the'oscillator to a predetermined frequency corresponding to said each switch irrespective of whether Vany of the inactive switches is operated.

119, A musical instrument as defined in claim 1l in whichsaid biasing means comprises a source of biasing potential, a first-adjustable resistor providing the connection between ysaid lbiasingsource and said first control means, "and a second adjustable resistorprovidingthe connection between said biasing source and said second control means. Y

20. A musical instrument as defined in claim 19 -in which each of said tuning means comprises a different condenser.

2l. In an electrical 'musical instrument, a'shared-note Voscillatc'n''including an amplifier that'is normally biased `to` a 'quiescent state; first 'tuning 'means `including -first control means and a first condenser for causing said oscillator to oscillate at a first predetermined audio frequencywhen 'said first control means is operated; second tuning means including second `control Ameans anda second condenser for causing said oscillator 'to oscillate at asecondpredetermined audio frequency vwhen said second control means is operated; 'first and second adjustable resistors connected to saidiirst and second Vcontrol means; and a 'source of bias potential connected to saidv amplifier through said first resistor bysaid first control means when operated and through said second 'resistor by said second control means '.when operatedfor changing the bias on said amplifier to bias saidampliiier to an oscillatory condition the bias potentials applied to said implifier when either of said control means is operatedbeingindependently adjustable.

22. In an electrical musicalinstrument: a plurality of oscillators that are tunable to different audio frequencies, each oscillator comprising an amplifying device that is normally biased to a quiescent state; a commonsource of bias voltage for said oscillators; and controlmeans for selectively 'biasing each said amplifying device to change it from a quiescent state to an oscillatory state andto ytune each oscillator to a predetermined audio frequency, said control means comprising a normally open Ikey-operated switching means for each oscillator for changing the bias of each said oscillator to an oscillatory state, tuning means .including a parallel resonant circuit comprising an inductive elementand a capacitive element connected to tune each said oscillator to a different predetermined audio frequency only zwhensaid switchingV means is operated, and a separate adjustable -resistance means connected to said bias source and to each of said oscillators, whereby the operating 'biases' on said oscillators may be independently adjusted.

23. 'In' an electrical musical instrument: a plurality of oscillators that are tunable to difterent'audio frequencies, each oscillator comprising an amplifying device having a cathode element, an anode element, anda control element, each said amplifying device normally being biased to a quiescent state; means connected `to said elements of each amplifying device for rendering each said oscillator operative to oscillate when said each Vamplifying device is biased to lan oscillatory state, said means including a separate inductive element connected between the cathode element and the control `element of each saidamplifying device; a common source of bias voltage for said oscillators; and control means for selectivelychanging the bias on each said amplifying device to change it from a quiescent state to an oscillating condition and' to tune each said oscillator to a predetermined frequency, said control means comprising a ynormally open key-operated switching means forV each'oscillator, each Vswitching means having two terminals, one of-said terminals being connected to the control element of `each oscillator, tuning means including a condenser connected betweenA each said other terminal and the cathode element ofsaid each oscillatonand a separate -adj'ustableresistance' means connected'between said bias source and said otherterminal of each said switching means, whereby the operating biases on said oscillators may be independently adjusted. t

v24. In an electronicl organ, a Vplurality of shared-note .oscillators adapted to apply notes of a musical scale` to a Icommon output circuit,reach of said-oscillators-being -adapted'to oscillate atV a frequency of any one of a corresponding group of notes,each oscillatorl comprising means 'for establishing avpredetermined state in which no oscillating signal is applied to said output circuit, any two Aofthe notes produced-by each oscillator rarely occurring simultaneously in musical compositions, thenotes producedby the loscillators forming a-sequence of notes of amusical scale :eonsisting'of at least one octave, successive notes of' said scale being spaced a half tone apart, Cach' oscillatorcomiprisiug' a pluralityV` of sequentially `a1- 17 ranged frequency control means, each said frequency control means being normally active, each of said frequency controy means except the last having a normally closed circuit control means and a normally open circuit control means; means for operating each of the frequency control means individually; circuit means including said normally closed circuit control means of each oscillator, said circuit means being opened by the operation of each frequency control means except the last in the sequence for rendering inactive the remaining frequency control means following said each frequency control means in the sequence; and means including tuning means controlled by closure of each said normally open circuit control means when active for tuning the corresponding oscillator to a predetermined frequency corresponding to said each frequency control means irrespective of whether any of the inactive frequency control means is operated and for altering the state of said oscillator to an oscillating state n which a signal of said predetermined frequency is applied to said output circuit.

25. In a musical instrument, oscillating means normally biased to a quiescent state, a keying potential supply providing a keying voltage characterized by a relatively large direct current component and a relatively small alternating current component of low frequency, a plurality of note producing means including key-operated switching means for applying said keying voltage to said oscillating means to variable bias said oscillating means to an oscillatory condition whereby a selected note of vibrato character is produced upon operation of the corresponding note producing means.

26 In a musical instrument as defined in claim 11 in which said amplifier comprises an amplifying device having a cathode element, an anode element, and a control element, the combination therewith of means including a resistor connected to said control element and to one of said other elements to provide a conductive path from said control element irrespective of whether said control means are opened or closed.

27. In an electrical musical instrument, a shared-note oscillator including an amplifier that is normally biased to a non-oscillatory condition; first tuning means including first control means and a first condenser for causing said oscillator to oscillate at a first predetermined audio frequency when said first control means is operated; second tuning means including second control means and a second condenser for causing said oscillator to oscillate at a second predetermined audio frequency when said second control means is operated; first and second adjustable resistors connected to said first and second control means; and a source of bias potential connected to said amplifier through said first resistor by said first control means when operated and through said second resistor by said second control means when operated for biasing said amplifier to an oscillatory condition whereby the bias potentials applied to said amplifier when either of said control means is operated is independently adjustable.

28. In an electrical musical instrument; a plurality of oscillators that are tunable to different audio frequencies, each oscillator comprising an amplifying device that is normally biased to a non-oscillating condition; a common source of bias voltage for said oscillators; and control means for selectively biasing each said amplifying device to change it from a non-oscillating condition to an oscillating condition and to tune each oscillator to a predetermined frequency of oscillation, said control means comprising a normally open key-operated switching means for each oscillator for changing the bias of each said oscillator to an oscillating condition, tuning means including a parallel resonant circuit comprising an inductive element and a capacitive element connected to tune each said oscillator to a different predetermined audio frequency of oscillation only when said switching means is operated, and a separate adjustable resistance means connected to said bias source and to each of said oscillators, whereby the operating biases on said oscillators may be independently adjusted.

29. In an electrical musical instrument: a plurality of oscillators that are tunable to different audio frequencies, each oscillator comprising an amplifying device having a cathode element, an anode element, and a control element, each said amplifying device normally being biased to a non-oscillating condition; means connected to said elements of each amplifying device for rendering each said oscillator operative to oscillate when said each amplifying device is biased to an oscillating condition, said means including a separate inductive element connected between the cathode element and the control element of each said amplifying device; a common source of bias voltage for said oscillators; and control means for selectively biasing each said amplifying device to change it from a non-oscillating condition to an oscillating condition and to tune each said oscillator to a predetermined audio frequency of oscillation, said control means comprising a normally open key-operated switching means for each oscillator, each switching means having two terminals, one of said terminals being connected to the control element of each oscillator, tuning means including a condenser connected between each said other terminal and the cathode element of said each oscillator, and a separate adjustable resistance means connected between said bias source and said other terminal of each said switching means, whereby the operating biases on said oscillators may be independently adjusted.

References Cited in the file of this patent UNITED STATES PATENTS 1,749,739 Flaharty Mar. 4, 1930 2,020,950 Lee Nov. 12, 1935 2,045,172 Youngblut June 23, 1936 2,083,246 Smiley June 8, l937 2,211,540 George Aug. 13, 1940 2,274,199 Hammond Feb. 24, 1942 2,292,757 Hathaway Aug. 11, 1942 2,328,282 Koch Aug. 31, 1943 2,340,002 McKellip et al Jan. 25, 1944 2,492,320 Riggen Dec. 27, 1949 2,497,661 Dome Feb. 14, 1950 2,505,182 Haller et al Apr. 25, 1950 2,539,826 George Jan. 30, 1951 2,691,318 Hanert Oct. 12, 1954 FOREIGN PATENTS 613,812 Great Britain Dec. 3, 1948

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