US2083246A - Electronic musical instrument - Google Patents

Description

June 8, 1937. f G. SMILEY 2,083,246

ELECTRONIC MUS ICAL INSTRUMENT Filed Dec. 4, 1955 es sheets-sneet 1 @fyi ATTORNEYS June 8, 1937. G. SMILEY I ELECTRONIC MUSICAL INSTRUMENT f Filed Dec. 4. V19155 6 Sheets-Sheet 2 Inti ]V INVENTQR WW @u @ZAM ATTORNEYS June 8, 1937. G. SMILEY ELECTRONIC MUSICAL INSTRUMENT Filed Dec. 4, 1955 6 Sheets-Sheet 3 -Mii INVENTOR ml QJ @7M ATTORNEYS June 8, 1937. G, sMlLEY 2,083,246

ELECTRONIC MUSICAL INSTRUMENT Filed Deo. 4, 1935 6 Sheets-Sheet 4 @1d-Cathode Voltage Plate 0u rren o d Plate Cathode Voltage Eb IX u @914 @mi mi 19| f\ .A

U d J @15d v A I VA o I I INVENTOR /LWLW/ @ai ATTORNEYS June 8, 1937. y G, SMILEY ELECTRONIC MUSICAL INSTRUMENT e shee-sheet 5 Filed Deo. 4, 1933 Ec positive S Y E N R O w A INVENT. M7

lBY W June 8, 1937. G. sMlLEY ELECTRONIC MUSICAL INSTRUMENT 6 Sheets-Sheet 6 Filed Deo. 4, 1933 Okt Pa t Out Put NVENTOR Zig? ATTORNEYS Patented June 8, 1937 UNITED ELECTRONIC MUSICAL INSTRUMENT Gubert smiley, Brighton, assigner, by mesne assignments, toG. Willard Rich Application December 4.1933. serial No. '200,838

12 Claims.

The present invention relates to the production of musical sound by electrical means and has for an object to provide an oscillatory thermionic vacuuxntube circuit which will produce an electric wave form which is extremely rich in harmonics, the circuit preferably including suitable filter means by which those harmonics desired can be retained. Stated otherwise, the circuit of the present invention is one producing oscillations of a wave form having the harmonics necessary to adapt it to imitate or reproduce sound contained in the playing of any given instrument by inclusion of appropriate iilter means acting to suppress or attenuate certain of the harmonics, as well as to create new tone relationships not necessarily found in existing musical instruments, but which may be found desirable.

Another object of the invention is to provide an oscillatory vcircuit which is both s".nple in construction and reliable in operation. the irequency of its oscillations being characterized by extreme stability and capable of beingk determined or computed with ease and accuracy.

Still another object of the invention is to provide an oscillatory circuit which may be readily adjusted to cause it to oscillate at the proper frequency.

A further object of the invention is to provide 30 an oscillatorycircuit capable of having the frelil quency of its oscillations readily'altered during operation to predetermined values so that it may be effectively employed in the playing of different musical notes.

35 A still `further object of the invention comprehends the grouping of a plurality of such oscillatory circuits each'adjusted to a predetermined frequency, whereby various chords may be struck.

40 The invention comprehends in its broad as-V pect the provision of the necessary number of A oscillatory circuits above mentioned,ltering devices and switching arrangements and appurtenances for duplicating the effects obtained by the various pipes, reeds or other elements of musical instruments, including the effects obtainable by the use of various combinations of stops and key manuals, as well as for the origination of tonal effects, not necessarily characteristics of existing musical instruments, but nevertheless desirable.

As a further comprehension and in the nature of a refinement to the apparatus vby which the above objects are accomplished, the invention includes a controlling oscillator or master conlil-:ISSUED Fig. i is a diagrammatic illustration oi' an oscillatory circuit embodying the invention, illustrated in a simple form to facilitate the description of the fundamental principle of the invention.

Fig. 2 is a similar view showing another way in which the plate and grid potentials may be applied to the tube circuit. f

Fig. 3 is a modiiication showing the use of separate transformer windings in lieu of the-tapped mutual inductance of Figs. l and 2.

Fig. i is a diagrammatic layout oi a single tone instrument embodying the invention.

Fig. 51s a diagrammatic layout of a multitone instrument embodying theinvention.

VFig. 6 is a view similar to." Fig. 5, but in addition showing the master oscillator and coupling means through which the control may be effected from one oscillatory circuit to another.

Fig. il is a modification of Fig. 6 showing an intermediate repeater tube included in the coupling between oscillatory circuits.

.Fig 8 shows the possibility of associating with each oscillatory note-producing circuit a means for automatically attenuating the intensity of its note upon key depression.

Fig. 9 is a modification of Figs. 6 and '1, showing a multi-element tube substituted for the tubes V2 and V3 of thesegures.

Fig. l is a view showing the possibility of associating with each oscillatory note-producing circuit a means whereby the intensity of its note may be varied'at will by the distance its controlling irey is depressed.

Figs. il to 14 are curves showing corresponding changes occurring with time in the gridcathode voltage, plate current, plate-cathode voltage and grid current of the tube of the oscillatory circuit of the invention, respectively.

Fig. l shows two consecutive cycles of the output wave as derived from the grid-cathode branch of the oscillatory circuit, from the inetant the operation of the device is initiated.

ll'lg. 15a shows two consecutive cycles of the output wave as derived `from the plate-cathode 5 branch of the oscillatory circuit, from the instant the operation oi the device is initiated.

l'ig. 16 shows three grid-cathode voltage waves oef diii'erent frqnency,l illustrating frequency control by grid supplyppotential variation. lo Fig. 17 illustrates two sinusoidal wave forms of different. but harmonically related frequencies.

Fig. i8 illustrates the grid-cathode voltage wave ofthetubeoftheoscillatorycircuit oftheinvention and the trigg' action of a similar wave l5 superposed thereon by a master or control oscillator.

Figs. 19 and 20 show embodiments of the invention wherein the output wave is derived from the grid-cathode branch of the oscillatory circuit.

20 Similar reference characters in the several figures indicate similar parts.

In carrying out the invention the fundamental object of which is to provide a new apparatus for the production of musical sounds, there is a vacuum tube oscillator which will produce audible frequency oscillations the wave form of which lends itself to musical use. In other words an oscillatory circuit constructed in accordance with the invention will produce a wave which possesses at any of the frequencies to which the circuit may be tuned the harmonics required to give the desired musical color or timbre, so that the circuit may be adapted to match or reproduce the notes of any given instrument. as well as to geneme its own variations of tone, by proper inclusion of adjustable filtering elements. The circuit which is provided includes the further advantage that it gives an immediate full response upon the closing of a key without time lag incident to building up of the energy required for its operation and produces a sustained note of the predetermined frequency during the period of hey depression. 'lhsse factors are considered important characteristics of the invention and enable the production of musical sounds with great ndelity. Furthermore. it is possible to duplicate the eifects produced by a complicated instrument, such as an organ. by means of apparatua capable of assembly in a comparatively small space.

The oscillatory circuit of the present invention establishes audio-frequency oscillations by the time constant characteristic of a condenser discharging through a resistor. Generally stated. such circuit employs a thermionic vacuum tube in association with a potential feed-back means closely coupling the plate-cathode circuit through a condenser to the grid, the condenser being in electrical connection through the feed-back means with the cathode and therefore in a closed-circuit relation through the cathode with a resistor included in the grid-cathode circuit. The condenser alternately receives a charge and discharges the accumulated charge through the resistor and. because of the close coupling of the plate-cathode circuit to the grid. rapidly accumulates a charge such as to bias the grid beybnd cut-off with the result that the period of discharge substantially determines the frequency of tlm established oscillations. the feed-back means being properly poled for this purpose. Proper choice should obviously also be made of the supply-potential at the grid. It will be apparent from the following description that no precise construction of the interior elements of the vacuum tube is essential and that, although the conventional type of three element thermionic device is illustrated in the various figures. other types of multi-element tubes as distinguished from a triode tube may be used with equally satisfactory results.

'Ihe oscillatory circuit illustrated in Fig. 1 comprises a thermionic tube V having a cathode K, a grid G and an anode P. A source of potential, such as a battery b, has, its negative terminal connected to the cathode K and its adjustable terminal connected through portion Lp of an inductance M to the plate P, the other portion La of said inductance being connected at its outer end through a condenser C to the grid G. Also connected to the grid G is a resistor R., a second source of potential c being connected between this resistor and the cathode K. In practice .the potential values Eb and Ec may be adjusted by using a voltage divider D (Fig. 2), Ec being so determined with reference to the capacity of the condenser C that, when the latter is charged to its operating point. the grid is biased beyond the cut-ofi' point of the tube and no substantial current flows in the plate circuit.

'Ihe elements R, C, M and the two sources of potential are each variable at will. By their adjustment the fundamental oscillatory frequency of the circuit can be established in the first instance and may subsequently be changed as desired. The chief methods of controlling the frequency generated `by the circuit are by adjustment of the values of resistance R. and condenser C and by adjustment of the supplypotential Ec. By using fixed values of R. and C of approximately the proper relative values. final tuning may be accomplished by the variation of Ec. After this adjustment of the potential Ec. the potentials Eb and Ec should bear a fixed relation to each other. In order that they may maintain such relation, the potentials Eb and Ec should be derived from the source. as in Fig. 2.

In explaining the oscillatory action of the circuit and the production of audible frequencies determined by the above mentioned elements. it will be assumed that at a given instant such charge has been accumulated in the condenser C that the grid G of the tube V is sufliciently negative to prevent passage of plate current. Obviously the condenser will not retain this charge as there is a path through the resistor R., the instrumentalities producing the potentials Ec and Eb and the winding La, around which it can discharge at a rate determined by the resistance of this circuit.

'Ihe condenser C discharges according to the mathematical relationship that V,=V. s f-C, where V=the terminal voltage of the condenser at any instant, Vs=the terminal voltage of the condenser at time zero, e=the natural logarithmic base, t=the time in seconds from time zero, and R' and C' are the resistance and capacity respectively of the condenser-discharge circuit.

Designating the period of discharge of the con denser C during the operation of the circuit by the symbol T, the ratio Vg/Vgo at the end of such period is "nel and, for given values of R.' and C' and regardless of the initial voltage value Vgn, is a factor of the period T and of the period T alone.

As time progresses, therefore, the condenser C will continue to discharge through the resistor R and the effect of this discharge will be to render the grid-cathode voltage (Fig. 11) progressively less negative until, depending upon the parameters of the tube V, the time Twill be reached when the grid-cathode voltage is such that plate current (Fig. 12) can begin to pass, provided that the grid supply-potential Ec is insufficiently negative of itself to produce cut-olf conditions. As soon as plate current starts, and depending 15 upon its rate of change, a reactive voltage is set up in the winding Lp of inductance M, the latter winding opposing the change in the status quo. Through the medium of the mutual inductance M, however, induced voltage is transferred to the winding Lg, which causes the grid-cathode voltage to increase in a positive direction, which in turn, through the action of the triode, tends still further to increase the plate current. Thus a self-accelerating or regenerative action is created, wherein increasing positiveness of the grid sets up increasing plate current, which still further increases the grid potential in a positive direction. As the plate current increases, however, the effective plate-cathode voltage (Fig. 13) decreases owing to the opposing action of the reactive voltage set up by the rise of plate current through the winding Lp. Similarly, theincreasing positiveness of the grid causes the gridcathode voltage eventually to pass through aero from the negative to the positive region, and the grid-cathode circuit becomes conducting, at which point the grid starts to permit current to pass from the grid to the cathode (Fig. 14), which current causes a charge to accumulate in the condenser C. corresponding to which the condenser C no longer accumulates a 'charge and the grid, for this and other reasons, ceases to become further positive.

At this moment plate current has risen to amaximum and then a reverse regenerative action takes place. As soon as the rate of change of plate current ceases to increase, both the induced voltage in the winding Lg and the grid potential depending upon it .start to decrease.

This decrease .or return toward negativeness of the grid potential causes the plate current to decrease and the grid-plate regenerative action again takes place, though now in the opposite direction. As soon as the grid-cathode voltage 55 passes outI of the conducting region, the charge accumulated in the grid condenser C by the passage of current is locked in, as it were, and can only escape by way of the path offered by the resistance R. The plate current rapidly falls to substantially zero and the grid is now negative beyond the cut-off point because of the charge present in the condenser C. This charge includes the normal charge .present from the 'combined potentials of Eb and Ec, and is caused by a transfer of energy from the plate circuit to the grid circuit of the tube, a portion of which energy goes to make up the condenser charge, which may be represented by the symbol Q. From the time the grid-cathode voltage passes out of the conducting region to the time that the plate current ceases, such a small interval elapses that the discharge of condenser C through resistor R during this interval is, for all practical purposes, negligible. However, such discharge does take Shortly a limitation is reachedA place, and so cannot be overlooked. Thus, when the plate current cycle of operation is completed, the grid-cathode voltage is Q/C minus a small amount, where Q represents the maximum charge (over and above that charge induced by the potentials Eb and Ec), which was acquired during the action just described. From this point on resistor R and condenser C take charge of the operation of the circuit.

Referring to Fig. l1, the condenser C discharges from the voltage Vgn, equal in value to Q/C; to the voltage Vn causing cut-off potential at 4the grid G, at which instant the cycle of operations is repeated. Depending upon the constants of the circuit to some extent, the charge Q and the voltage Vga depending upon it are determined by the magnitude of the plate supply-potential Eb. Likewise the cut-off voltage Vn is determined by Eb through the medium of the ampliflcation constant of the tube. supposing Ec to be zero, it is obvious that, other things remaining constant, if Eb be increased or decreased by some factor, Vm and V3i will be similarly changed by the same factor. And, since the ratio 'changing both terms of that ratio by the same determined by the period of condenser dischargeI almost to the exclusion of all other factors.

Under the circumstances when Ec is not zero,

'it will be found that Ec must be changed in proportion to Eb if the same frequency of established oscillations is to be maintained. Variation in Ec acts to change the point of time on the discharge curve at which cut-oil potential is reached. The cut-off potential for zero grid supply'potential, or the condenser discharge voltage Vgl, may be represented by the ratio Eb/, where ais a constant of the tube. If now the grid supply-potential is made negative (by connecting the negative terminal of the instrumentality c to the resistor R), the condenser C must discharge to a voltage Vu equal to Eb/a minus Ec, rather than simply Eb/,r as is the case when Ec=0, the voltage Vga being therefore smaller than the voltage Vir. Referring toFig. '16, the condenser discharge period T when Ec=0 is indicated by TI. Since Vn is smaller than Vn, substituting Vgn in the exponential equation must of necessity increase, the period T to a definite value T2, all other factors being constant. Similarly, if Ec is made positive, the period T will decrease to a definite value smaller than TI, as the ultimate condenser discharge voltage Vga will be larger than Vn.

It will be apparent from the foregoing discussion that to keep the frequency of the established oscillations constant, it is necessary to change Ec in proportion to Eb, but that, regardless of the value of Ec, such frequency is substantially exclusively determined by the period of condenser discharge. The periods of time between condenser discharges are indeed extremely short and are purposely exaggerated in Figs. 1l to 16 for clarity of illustration, it being also apparent that the wave form of the oscillations established in either grid-cathode or plate-cathode circuit (see Figs. 15 and 15a) is extremely rich in harmonics.

So far the discussion has concerned itself with changes in operating potentials and the actual cycle of operation. Suppose either of the factors R and C or both of these factors be changed in the exponential equation by correspondingly altering the values of the resistance R and capacity C, it is seen from this equation that T increases with increases in either C or R' and that the increase in T is proportional to the increase in C or R'. This must be so if the exponent is to be kept constant to keep the ratio of full charge voltage to cut-off voltage constant, which is, of course, xed by other considerations, within wide variations of resistance R and capacity C. It should also be observed that the presence oi' leakage lnductance in the windings La and Lp or of resistance in these windings or in series with the supply-potentials or with the grid itself, will all aiect the initial charge Q, by limiting it to a greater or less extent, and to a corresponding extent also affect the frequency of the established oscillations.

The operation of the circuits of Figs.` 1 and 2 may be considered as typical of other arrangements which may be utilized to produce audiblefrequency oscillations of a wave form suitable for musical instrument purposes, wherein the coupling between the plate-cathode circuit and the grid is sufllciently close to permit the accumulation of a sumcient charge in a grid condenser to produce grid potentials in excess of cut-ofi' potential and wherein the condenser in discharging through a resistance determines the time period or frequency of the established oscillations. The oscillatory circuit, for example, may taire the form shown in Fig.` 3, where a transformer M 1 having its windings closely coupled to one another is substituted for the auto-transformer M of Fig. 1. The action of the circuit of Fig. 3 is practically identical to that of Figs. 1 and 2. but here the supply-potential Eb is separated from the condenser-discharge circuit. Similarly, any existing means of securing feed-back may be employed if suitable provision is made to assure that the charge accumulated by the grid condenser is sufficient to carry the grid potential beyond the cut-off point of the particular vacuum tube employed, so that the discharge characteristic of the condenser is the major factor in frequency determination.

The type of oscillator described is especially adapted for the purpose intended, as it maintains a substantially constant ratio of the limit voltages between which condenser discharge takes place, regardless of wide fluctuations in its supply potentials Eb and Ec, provided the instrumentalities producing these potentials are so arranged that, if any variation occurs in one, a corresponding or proportional variation will simultaneously occur in the other. This can be easily accomplished by proper derivation of the two potentials from the same source.

In further elaboration of this invention and in order to illustrate its embodiment in an instrument suitable for the creation of formal music, there are shown in Fig. 4 certain circuit arrangements by means of which single notes may be successively produced in the playing of melodies. The parts heretofore described bear the same reference characters in Fig. 4 and it is, therefore, only necessary to mention the additional elements introduced to provide the control features which permit such rendition of music. In Fig. 4 it will be observed that there have been substituted for the single resistance R oi Figs. 1 to 3 a plurality oi' resistances which, in practice, correspond in number to the number of notes desired,three being provided in the embodiment illustrated, namely RI, R2, and R3. These resistances ere all connected in series with one another and with the grid G of the tube V and are respectively associated with the switches i, 2, and 3, which may constitute the keys of the musical instrument. Potentiometera pl, p2, and p3, also corresponding in number to the number of notes desired, are bridged across the common source c of biasing potential, the switches I, 2, and l being respectively connected between the outer terminals of the resistances Ri, R2, and R3 and the sliders ia, 2a and 3a of the potentiometers pl, p2, and pl. Tuning for each of the separate notes within the range of the instrument may be effected by merely adjusting the sliders la, 2a, Ia, which determine the individual grid-supply potentials Ecl, Ec! and Ec3. Obviously, the frequency of the note produced by depressing key I is determined by the discharge of the condenser C through the resistance RI, the frequency of the note produced by depressing key 2 is determined by the discharge of the condenser C through the resistances RI and R2, and the frequency of the note produced by depressing key l is determined by the discharge of the condenser C through the resistances RI, R2, and RJ.

In the apparatus shown in Fig. 4 there is included in the circuit of cathode K an adjustable tuning resistor Rt by means of which the charge accumulated in the condenser C may be varied to alter the pitch of the instrument as a whole. 'I'hat is, the resistance Rt is used to tune all notes simultaneously and therefore provides a convenient means of tuning the instnxment with others or changing its tuning to conform with recognized pitch, such as international, orchestra or concert. In addition to the foregoing there is included in series with the several key resistances another resistance R1 having its outer terminal connected to the cathode K by a conductor 5 containing a source c' of predetermined voltage Ecl. Owing to the difficulty of constructing circuits in which leakage resistances are infinite, it may be considered commercially irnpractical to prevent the condenser C from discharging even with all keys open and R1 omitted. The result of this discharge is to produce a long term cycle upon key release and corresponding noise like spaced pops, if some means of preventing this is not employed. Thus the potential of c is chosen, together with the value of resistor R1, to apply a cut-olf or slightly greater potential to the grid when all keys are released, R1 being made sufficiently large to prevent the potential Ecl from having an appreciable effect on the tuning when any one of the keys is depressed.

Inserted in and across the plate-cathode circuit are a resistance R4 and a by-pass condenser C5, respectively. The condenser C5 acts as a reservoir condenser to by-pass the impedance of the instrumentality b and to insure a relatively steady plate supply-potential Eb during the passage of plate current within the tube. The resistance R4 serves to set the effective value of the supply-potential at the plate P and to isolate the oscillatory1 platecathode circuit of the tube from the instrumentality b, thereby preventing the creation of opposing voltages that might otherwise introduce undesirable coupling effects.

output wave of thc osthermionic amplifier I It is understood that the ciilator is amplified bya of suitable construction and thereafter reproduced in the desired volume by means of a suitable loud speaker 1. This wave is preferably fed to the ampliiier through an adjustable filter to permit the tone of the instrument to be controlled by cutting out those harmonics or component frequencies which it may be desired to suppress. Indicative of such a filter there is shown in the output circuit, comprising the Wire l, a resistance R5 with which two condensers C3 and Gi of different capacities are associated. These condensers are connected at one side to a conductor 8 leading to the resistance R5 and at the other side to terminals l and il, with which cooperates a switch arm I2 connected `(through the resistance Rt) to the cathode K. Between the resistance R5 and the plate P a blocking condenser C6 is introduced to prevent the plate supply-potential Eb from acting through the output vnet work, which condenser C6 iscf sufiicient capacity to permit the free passage of thealternating potentials developed. The resistance R5 functions as a lter resistor and isolating resistor and is of such a value that, even if a short circuit occurred at one of the contacts of the filter switch I2, no appreciable change in the frequency of the established oscillations would occur.

Also arranged in the wire lis a second resistance R8 and a potentiometer P4, the latter being connected to the amplifier 8. The resistance R8 limits the maximum volume of sound obtainable, while the potentiometer P4 acts as a volume control, its slider being adjustable from zero to the maximum voltage obtainable.

In practice the plate supply-potential Eb and the potential Ecl, as well as the potential for the cathode-heating element H (where a heater type i of tube is used) are derived from the power supply already available in the audio-frequency amplifier 6 used with the oscillator. The amplifier may be of conventional design, with the addition, if desired, of other filtering means for the further control of tone quality. By using the power Asupply circuits of a commercial amplifier,

as a source of supply potentials, the amplifier, in turn, being operated from the power means, and by supplying the potential required across the potentiometers PI, P2, and Pl either from the power mains with a separate power supply device or from the power supply circuits of the amplifier7 the various supply-potentials will in a properly designed oscillatory circuit be proportional to each other, or nearly so, at all times.

That is, the operation of the device will be substantially stable with relation to supply fluctuations.

The principles thus far disclosed in their relation to a single vtone instrument are equally applicable to an instrument capable of producing.

harmonies. Fundamentally there would be provided for such a purpose a plurality of oscillatorsy such as that of Fig. 1, each tuned by means of the proper choice of the resistance R, capacity C and biasing potential Ec to a predetermined fundamental frequency. Appropriate switching means, such as keys, would be provided, each oscillator being so arranged that, when its key is depressed, a proper proportion of itsoutput is caused to be conveyed to the input of an amplifier feeding int'o a loud speaker. Such an apparatus when properly assembled and tuned can be made to play chords and in its essence is the basis oi what may be termed an electric organ. By the addition o filter devices co-ordinated with stop-type controls, any desired change of timbre or tone shading can be accomplished, so that the apparatus will duplicate various instruments, or may be used for the purpose of originating its own discrete tonal values, as distinguished from existing musical instruments. likewise, all notes can be made subject to either manual ory automatic vibrato, and to volume changes accomplished by either knee or manually operated controls, such as are now used in organs fox` producing such effects.

While it is possible to evolve circuits by which one oscillator can be controlled from a variety of keys, so chosen that no two keys controlling the vsame oscillator would need to be simultaneously depressed in the rendering of formal music, it is considered best to assign an oscillator for each desired note in order that any desired combination of keys can be simultaneously depressed to produce the related chord sounding in each case.

Fig. 5 illustrates a circuit arrangement which may be used in constructing such an apparatus,

, two` oscillators being shown for the purpose of simplicity. 'I'he two oscillators are identical, except for the choice of the values which should be used furthe resistances RI and R2. condensers CI and C2 and grid lsupply-potentials, Ecl and Ec! in order that the two notes desired may be produced. In this arrangement a parallel feed may be used to connect the plates of the several tubes to the source b, resistors Rfl and RIZ of high value being employed through which the potential Eb supplied by said source is directly communicated to the plates Pi and P2 of the twov tubes VI and V! illustrated. The potentiometers pi and p2, ofcourse, serve for final tuning after the latter has been approximated by choice of the resistances RI and R2 and condensers CI and C2. The condensers Cbl and CU2, respectivelyA connected in the output circuits of the tubes VI and V2, isolate the steady potential Eb from these circuits and prevent its transfer across the plate-circuit windings of the inductances MI and M2, but are'y sodesigned as to freely pass the alternating potentials developed. Besides the condensersCbl and Cb! the output circuits ofthe tubes bl and b2 respectively comprise the resistances Rkl and Rk and the keys :ci and x2 associated therewith, the potentiometer or volume control p4 being common to both circuits with both keys depressed. The resistances Rkl and Rk2 are so chosen that the voltage appearing across the impedance of p4 when one of the note-producing oscillators is being sounded through its associated key, say the circuit of tube VI, will not be coupled back to any appreciable extent through this impedance into the other note-producing oscillator, the oscillator comprising the tube V2 under the assumed condition. Likewise the impedances of the instrumentalities supplying the operating potentials at the plates and grids of the tubes should be kept low in order to prevent undesirable coupling eftects.

As is shown in Fig. 5, a capacity Cf is connected across the potentiometer p4 as indicative of a simple means which may be employed. By making this Acapacity variable between the desired limits, various amounts of the higher harmonics may be retained or suppressed at will.

The arrangement of Fig. 5 enables excellent stability of tuning to be obtained. However, there are variable factors suchas the effects of humidity on leakage paths, the change in tube parameters with age, etc., which, over a period of time. may tend to throw the apparatus out of time. If this occursfthejpotentiometers pl and pl permit the instrument to be easily retuned dwing the useful lii'elof the tubes, which is considerable in the applatus of this invention owiM to the relatively low emission demands upon tlibm. (,'lianses in tuning must inevitably occimlbweveathoushusualiytheyareofsmall multitude. and for this reason it is deemed highly dedrable to provide a means for automatically wntaining the tuning of each note relative to ad the others.

Preliminary to explaining how this automatic control ia,eiiected. it is to be understood that the invention contemplates the use of a master oscillator. This may be one of the note-producinl oscillatory circuits. or a separate oscillatory circuit. preferably one oscillating at a frequency corresponding to one ci the higher frequencies of the keyboard. The master oscillator is tied in tothe other oscillators in such a manner that anydecadence in the operation of any one of the controlled oscillators normally causing it to lack in harmony u checked by controlling impulses from the output of the mutex' oscillator or from the output of a controlled oscillator, whereby all controlled oscillators are caied to oscillate with W rolularity in step with the master oscil- The practical or tempered (chromatic) musical scale as employed in a hey-controlled instrument is divided into twelve intervals to the octare. lach octave interval is a two to one frequency ratio and the twelve intervals between there is thus no harmonic relation between two notes of a ley-controlled instrument. other words, the tempered scale does not perthe playing of those frequencies which nademands in the rendition of music. it havingnbeen adopted for practical reasons due to the extremely large number of keys which would have to be provided as a result of the frequent chaines in the keynotes with reference to which is written. While the notes of the tem- True or scala lx o' C) Practical or tampered scala l'raotioml Decimal Ex nentlal Decimal l 1.o 2 M l. 0 WB l. llo 2 'm l. luid 514 l. 2B 2 il l. m2 4/3 l. lull 2 M l. 1MM N2 l. 6 2 'Il' L mi bla l. M 2 "Il l. Il?! It will be observed that the correspondence between the mathematically derived intervals and the actual musical intervals is relatively close, variations being limited to a few per cent, so slillit la to make lt dlllicult to detect by the trained musical ear. The notes of the tempered scale can therefore be stated to be naturally re- 75 lated for all practical purposes. If, now, the reaosaaae maining sernitone intervals be added to the intervals already tabulated, it will be possible, because of the almost true natural relationships of all notes of the tempered scale, to express any given note of said scale in a whole number fraction of any other.

As an example, let us consider the relationship between the note G and the note C next above it on the tempered scale. This relationship very nearly meets the condition that. for every four complete cycles of the C note, there will be very nearly three complete cycles of the lower note G. Thus at each fourth cycle of the note C and each third cycle of the note G, the two notes may be considered to be in phase. provided they are initially sounded together.

In Fig. 17 are shown two sinusoidal waves il and il having the above-mentioned frequency relationship of four to three, and it will be noted that at the third cycle of the lower frequency wave il and the fourth cycle of the higher frequency wave Il, the two waves pass through the .same value (aero) in the same direction (from plus to minus). Now Fig. 18 shows graphically the grid-cathode voltage wave llc of an oscillator constructed and controlled in accordance with the invention. This oscillator is assumed to operate at the frequency corresponding to the lower pitch note G of the tempered scale, while the controlling oscillator, from which oscillations are supplied to the controlled oscillator in a manner to be hereinafter described, is assumed to operate at the frequency corresponding to the higher pitch note C of the tempered scale. 'I'he oscillations produced by the controlling oscillator are similar in wave form to the oscillations produced by the controlled oscillator and under the assumed condition are very nearly related to the oscillations produced by the controlled oscillator by the ratio four to three, the edect of the controlling oscillator on the gridcathode wave of the controlled oscillator being to produce certain irregularities Ila in its condenser-discharge portions, corresponding to which the grid approaches cut-oi! potential in a positive direction. 'I'hese irregularities Ilo are only slight because the controlling wave is suitably attentuated prior to superposition. It will be observed that. during condenser discharge in the oscillator of the present invention, there is a considerable period when the grid-cathode voltage is very close to the cut-olf level, but has not quite reached that level, because of which impulses of comparatively small magnitude need be supplied to the grid during such period to cause a change in the point of time at which plate current will again be initiated. Therefore,

should the controlled oscillator tend to oscillate at any other than the desired frequency, such an impulse supplied at the proper instant to correct its grid potential to the cut-oil' value will cause lt to be triggered oil' into its true cycle of operation.

Assuming, for example, that the controlled oscillator producing the wave lla illustrated in Fig. 18 has a tendency to run a slightly lengthened period. then on each successive cycle this effect would normally become cumulative and overshooting would occur as indicated by the dotted lines at the iight in Fig. 18. However. on the third cycle the impulse from the controlling oscillator corrects such overshooting and starts the controlled oscillator olf in phase again. Obviously, the impulse from the controlling oscillator must be small enough so that it will not trigger the controlled oscillator at any other than the desired point of time. The line Vgn in Fig. i8 represents the cut-off potential of the controlled tube, and, while three consecutive impulses are superposed during the first, second and third cycles of the controlled wave, it is not until the fourth impulse that the trigger action takes place, since this fourth impulse occurs at 10 an instant when the grid potential of the controlled tube is sufficiently close to the cut-off level to permit the trigger action to take place. From these considerations it is obvious that, for maximum control, the controlling impulse must be as large as possible in order to trigger over the widest possible discrepancy, but not large enough to give rise to a triggering action before the proper cycle.

From the foregoing discussion Vit isI apparent that control vmay be effected between any two oscillators the frequencies of which are musically related to one another. Consequently, with all oscillators tuned to produce the various notes of the conventional keyboard, they can all, in effect, be tied together, either directly or through the medium of intervening circuits, until al1 are made subject to the control of a master oscillator. The latter may be an-y note-producing oscillator, such as the one producing the note middle C. It is possible to use one representing the highest pitch note on the keyboard, or a separate oscillator of still higher frequency may be employed, which is not used for sounding notes. In practice particular care is taken to render the master oscillator exceptionally stable. Some care should also be taken to insure that no controlled oscillator becomes the controller, which may be avoided, by proper coupling between oscillators and proper selection of circuit constants. The controlling l effect is sumciently marked to offset the normal detuning incident to tube aging within operating life limits and to small changes occurring in the values of the resistances and capacities used. Also, if the frequency to which the master oscillator is tuned be changed-by a small amount, the whole keyboard is shifted by that same amount, which provides a means of securing tuning to pitch. As another advantage, independent tuning of the various note-producing oscillators is greatly facilitated. That is, if any one of these oscillators is out of tune, such an oscillator will give indication of this fact by a slipping or beating effect, which only needs to be eliminated by adjustment of its circuit constants, to indicate its proper tuning, there being no possibility of doubt when a true tie-in is achieved. Still another advantage resides in the fact that tremolo or vibrato effects may be introduced into all of the controlled oscillators from a single point, namely, the master oscillator.

The frequency relationships between controlled and controlling oscillators must be chosen with a thought as to the simplest fractions, as the smaller the numerator and denominator of the fraction are, the wider the region of natural period change over which the trigger action may take place and the more readily control may be effected. A careful analysis of the tempered scale shows that tie-in sequences between certain notes may be so selected as to give a maximum of control with correct musical tuning, as the frequency relationships practically take the form of simple fractions, as already explained. In practice, a single octave may be completely tied in,

directly or through a sequence, to the master note, which may well be the upper C of that octave, and then other octaves may be tied in progressively below the master octave, thus completing a tie-in of the entire keyboard of the instrument. In this manner I have found that, by selectively grouping the various note-producing oscillators so as to cause the oscillators of each selected group to feed through a common output and by connecting appropriate filters one in each of the common outputs of these selected groups, any desired predetermined wave form for the keyboard as a whole may be attained.

Fig. 6 illustrates a means which has proved effective in practice for securing a tie-in of a plurality of note-producing oscillators with the master oscillator. The arrangement shown is for a multi-tone instrument and, except for the introduction of the master oscillator and the coupling elements by means of which the tie-in is effected, is the same as that of Fig. 5. 'Ihe master oscillator is shown for simplicity as a rectangle il, it being understood that its oscillations are established like those of the noteproduclng oscillators in accordance with the fundamental circuit of the invention. The master oscillator is coupled to the oscillator comprising the tube VI by. means of a condenser Cg and resistances Rc and Rd and the latter oscillator is coupled to the oscillator comprising the tube V2 by means of a condenser Cgi and resistances Rcl and Rdl, the resistances Rc and Rcl being respectively connected `in the grid-cathode circuits of the two tubes Vi and V2. Thus the oscillator comprising the tub VI is directly controlled by the master oscillator, while the oscillator comprising the tube V2 is indirectly controlled by the master oscillator through the intermediary of the controlled oscillator comprising the tube Vi. In this manner it is possible to tie-in any number of note-producing oscillators with the master oscillator, the condenser Cg2 and resistances Rd2 being shown connected to the output of the tube V2 for the v,purpose of illustrating this possibility. For a complete understanding of the apparatus shown, the function of the condenser Cgi and resistances Rci and Rdl, providing the coupling between the oscillators comprising the tubes Vl and V2, only need be described, it being understood that each of the couplings provided between oscillators operates in identical manner. The condenser Cgi acts as a blocking condenser by means of which the grid supply-potential of the tube V2 is isolated from the oscillatory plate-cathode circuits of the tube VI. but is so designed as to freely pass the controlling impulses supplied from the tube Vi. The resistances Rel and Rdi function as an ordinary voltage-divider, their values being so chosen that they`will attenuate the controlling impulses introduced into the grid-cathode circuit of the tube V2 to an extent determined by the considerations for maximum trigger action for the harmonic relationship peculiar to the respective notes produced by the oscillators comprising the two tubes VI and V2. The resistances Rel and Rdf also serve to attenuate impulses from the controlled tube V2 to the controlling tube VI thus eliminating to some extent the possibility of reverse control.

Another means by which assurance can be had that the controlled oscillators are always governed by the master oscillator is illustrated in Fig. "I, which shows the use of an isolating thermionie repeater tube V3 inserted between the controlling tube Vl and the controlled tube V2. In this instance the condenser Cyl acts as a blocking condenser between the tubes VI and Vl and is employed in association with a resistance Il to couple the tube Vi to the tube V), while the resistsnces Rcl and Rdi are employed together with a condenser It to couple the tube V8 to the tube V2, the values of the resistance Rol and MI being chosen to attenuate the controlling impulses supplied from the output of the repeater tube Vt. By thus interposing a vacumn tube repeater, the eifect oi reverse control can be entirely overcome where troiblesome. It should also be observed that by introducing lter networks (not shown) in the coupling means employed between oscillators, unwanted harmonics present in the output oi' a controlling oscillator can be excluded from the input circuit oi' the oscillator which it controls, the efiect being to purify or clarify the tone of the note produced by the controlled oscillator in certain instances, particularly where the controlled oscillator is at the end of a long controlled sequence. Moreover, by such filter networks improvement in the control may be obtained.

In an instrument of the character described various additional renements may be added for the purpose oi obtaining desired musical effects. For instance soms or all oi' the keys, instead of merely closing switches, may be caused to vary the intensity of the notes as produced, a 'means to this end being illustrated in Fig. 10. Referring to this ngure. the oscillator comprising the tube VI is shown associated with a key 2l adapted to cooperate with a variable resistance 2i arranged as an output coupling so as to supply in varying amount the outgoing oscillations from said oscillator to the input of a repeater tube VII. theresistanceRki being inthisinstance connected in the output circuit oi the tube Vit. The slider lia of the resistance is at all times in electrical connection with the grid G40 of the repeater tube, but is normally in contact with the zero potential and oi, the resistance 2l so that oscillations from tbe tube Vi are supplied through the repeater tube VII only upon depression o( tbe key Il, the note during its production being obviously capable of having its intensuy regulated at win by the distance me key Il is depressed. As shown, a blocking condenser Il is connected with the resistance 2i in the output circuit of the tube Vi, another blocking condenser 2| being connected with the resistance RM in the output circuit of the tube VII.

As a further elaboration oi the invention, some or all of the notes may be automatically attenuated in intensity during the period of key depression and thus produce an eilect similar to that oi a plucked string, this concept being iilustrated in Fig. 8. Here the oscillator comprislng the tube VI directly supplies its oscillations through s blocking condenser Iii to a repeater tube Vil having a condenser Ciii and a resistance RII connected in parallel with one another in its grid-cathode circuit. the note produced by l said oscillator being controlled by a key 3| adapted upon its depression to cause the bias at the grid GII oi the repeater tube to be progressively increased by discharge of the condenser CII through the resistance RIU. The key Ii is operatively associated with the contact Si movable therewith and adapted to normally engage a ilxed contact S2. still another contact SJ being provided to establish connection with the contact Si during key depression. To satisfy the requirements of the apparatus under discussion, the connection between the contacts Si and SI should be etiected momentarily and only during the downward stroke oi the key Il, for which purpose the contact S3 in the embodiment illustrated is made of a flexible strip oi' conducting material nxed at one end and provided with insulation i beneath its free end portion, this con.- struction obviously permitting the contact SI to wipe past the end portion of the strip during its downward movement and to wipe past the insulation i during its upward movement. The condenser Cil is adapted to be normally short-circuited by the contacts Si and S2, the movable contact SI being connected to the grid side of the condenser CIU and the nxed contact S2 to the cathode side. For reasons which will shortly appear, two sources oi' opposite polarity are operatively associated with the grid-cathode circuit of the tube Vill, these sources being indicated in the embodiment illustrated by batteries ci and c2. I'he battery ci is permanently connected in the grid-cathode circuit so that potential therefrom is at all time: supplied to the grid Gi.. while the battery c2 is so connected that its voltage is impressed across the condenser CHI and potential therefrom is SuDDDlied to the grid Gil during the short interval of time that the contacts Si and SI are caused to engage with one another, the flexible contact SI being connected through the battery c2 to the cathode side oi the condenser Cil. The battery ci determines the normal bias at the grid GII, its voltage being so chosen as to supply a potential beyond cutoff when the key Il is in its normal position 4of release. Both the batteries ci and c2 determine the initial" operating potential at the grid GII, the battery c2 having a voltage value such as to increase the potential at the grid Gill to an operating value by its opposition to the battery ci when rendered eifective by engagement of the contacts Si and SJ. It is thus apparent that oscillations from the tube Vi are supplied through the repeater tube Vil only upon key depression, The note is initiated at the instant connection is made between the contacts Si and S3 and, because of the momentary character of this connection, the condenser CIU ls almost instantly charged by the battery c2. Thereafter this charge is progressively dissipated through the resistance RIB, the effect of which is to progressively increase the bias at the grid Gili. This changing bias alters the amplification of the repeater tube, which amplification is gradually diminished with the result that the note is correspondingly gradually attenuated in intensity during its production. The action is comparable to that obtained in a piano wherein the hammer corresponding to the key depressed strikes the string and bounds clear until the key is again depressed. Moreover, when the key 3i is released. the contact Si makes permanent connection with the contact S1, thereby short-circuiting the condenser ciil and abruptly terminating the sounding of the note. This embodiment of the invention thus duplicates the effect obtained by the striking of a piano key and subsequent release thereof, or by the plucking oi' a string and subsequent iinger contact thereof. As shown, the

resistance Rki and a blocking condenser 34 are connected in the output circuit of the tube Vlil.

In Fig. 9 there is shown a modified circuit wherein a multielement tube V20 performs the separate functions of the tubes V2 and V3 of einsame Fig, 7. The single plate P20 performs both the function of the element P2 of the tube V2 and the function of the element P3 of the tube V3, by operating in conjunction with the two grids 5 GV2 and GV3, which perform the functions of the grids G2 and G3 of the tubes V2 and V3, respectively. The grids GV2 and GV3 are interposed in the electronic path between the plate P20 and the cathode K20, and one of these grids is surrounded by a perforated shield which receives a supply-potential from an external source cs. The controlling impulses are here attenuated by the resistances 40 and 4l, which serve together with the blocking condenser Cyl 15 to couple the controlling tube Vi to the grid In the embodiments which have been described, the output wave is derived from the plate-cathode side of the oscillator and is therefore repre- 20 sented by the curve shown in Fig. 15a. It is also possible, however, to derive the output wave from the grid-cathode circuit, inwhich case it 1 will then take the form shown in Fig. 15. It is understood, of course, that either form is rich 25 in harmonics and adaptable to musical purposes.

In Figs. 19 and 20, there are shown embodiments of the invention wherein the output wave is derived from the grid-cathode circuit. Thus, in either of these figures, the simple oscillatory 30 circuit of Fig. l is shown supplying oscillations from its grid-cathode circuit to a thermionic repeater tube V30, which merely serves to amplify and communicate such oscillations to the output terminals o. The latter, as is obvious from pre- 35 viou's description, may be connected through additional amplifying means, filter means or other accessories described to the loud speaker. In

Fig. 19 the intensity of the note produced is varied by a potentiometer p30, one end and the slider of which are connected to the terminals o.

In Fig. 20, the grid G30 of the repeater tube is connected to a slider cooperating with the resistance R of the oscillatory circuit, the adjustment oi the slider effecting a variation in the intensity of the note. Except for these differences, the two arrangements shown in Figs. 19 and 20 are simf ilar in operation. In either case, the grid and plate supply-potentials ECR and EBR are so selected that, during operation, the potential at the grid G30 does not pass beyond cut-off or into the positive region, In this way, the voltage wave across the grid-cathode branch of the oscillatory circuit may be extracted without sub- -stantially affecting the operation of the latter 55 circuit, the repeater tube circuit serving to effectively isolate the oscillatory circuit from the accessories in electrical connection with the output terminals o.

What is claimed is:

1. In a device for the production of musical sound, an oscillator comprising a thermionic tube having a plate, a grid, and a heated cathode, a plate-cathode circuit and a grid-cathode circuit, a resistance circuit, feed-back means having two closely coupled portions/one of which is connected in said plate-cathode circuit, a condenser connected at one side to said grid and at the other side to the other of said portions, said condenser being in electrical connection through said other portion with said cathode, and potential-supplying means for operating said tube, said feed-back means being poled to cause said condenserto periodically rapidly accumulate a charge such as to bias said 75 grid beyond cut-off whereby its period of disconnected in said grid-cathode charge through said resistance substantially determines the frequency of the oscillations.

2. In a device for the production of musical sound, an oscillator comprising a thermionic tube having a plate, a grid, and a heated cathode, ,a plate-cathode circuit and a grid-cathode circuit, a resistance connected in said grid-cathode circuit, a mutual inductance having two closely coupled windings one of which is connected in said plate-cathode circuit, a condenser connected at one side to said gridand at the other side to the other of said windings, said condenser being in electrical connection through said other wind- 4 ing with said cathode, and potential-supplying means for operating said tube, said inductance being poled to cause said condenser to periodically rapidly accumulate a charge such as to bias said grid beyond cut-oit whereby its period of discharge through said resistance substantially determines the freque cy of the oscillations.

3. In a device for be production of musical sound, an oscillator comprising a thermionic tube having a plate, a grid, and a heated cathode, a grid-cathode circuit and a plate-cathode circuit, a resistance connected in said grid-cathode circuit, an inductance having a portion connected in said plate-cathode circuit, a condenser connected at one side to said grid and at the other side to the other portion of said inductance, said condenser being in electrical connection through said other portion with said cathode, and potential-supplying means for operating said tube, said inductance being poled to cause said condenser to periodically rapidly accumulate a charge such as to bias said grid beyond cut-of! whereby its period of discharge through said resistance substantially determines the frequency of the oscillations.

4. In a device for the production of musical sound, an oscillator comprising a thermionic tube having a plate, a grid, and a heated cathode, a plate-cathode circuit and a grid-cathode circuit, a resistance connected in said grid-cathode circuit, feed-back means having two closely coupled portions one of which is connected in said plate-l cathode circuit, a condenser connected at one side to said grid and at the other side to the other of said portions, said condenser being in electrical connection through said other portion with said cathode, and means for supplying operating potentials from a single source to said plate and grid, said feed-back means being poled to cause said condenser to periodically rapidly accumulate a charge such as to bias said grid beyond cuteoff whereby its period of discharge through said resistance substantially determines the frequency of the oscillations. f

5. In a device for the production of musical sound, an oscillator comprising a thermionic tube having a plate, a grid, and a heated cathode, a. plate-cathode circuit and a grid-cathode circuit, a resistance connected in said grid-cathode circuit, feed-back means having two closely coupled portions one of which is connected in said platecathode circuit, a condenser connected at one side to said grid and at the other side to the other of said portions, said condenser being in electrical connection through said other portion with said cathode, means for supplying operating potentials to said plate and grid, said feed-back means being poled to cause said condenser to periodically rapidly accumulate a charge such as to bias said grid beyond cut-off whereby its period of discharge through said resistance substantially determines the frequency of the oscillations, and

means for adiusting the grid-operating potential to effect frequency adjustment.

8. In a device for the production of musical sound, an oscillator comprising a thermionic tube 5 having a plate. a grid, and a heated cathode, a plate-cathode circuit and a grid-cathode circuit, resistance means connected in said grid-cathode circuit, feed-back means having two closely coupled portions one of which is connected in said lo plate-cathode circuit, capacity means connected at one side to said grid and at the other side to the other of said portions, said capacity means being in electrical connection through said other portion with said cathode, and potential-supplying means for operating said tube, said feed-back means being poled to cause said capacity means to periodically rapidly accumulate a charge such as to bias said grid beyond cut-off whereby its period o! discharge through said resistance means substantially determines the frequency of the oscillations. at least one of said resistance and capacity means being variable to effect frequency variation.

7. In a device for the production of musical sound, a thermionic tube having a plate, a grid, and a heated cathode, a plate-cathode circuit and a grid-cathode circuit, a resistance and a switch connected in series with one another in laid grid-cathode circuit, feed-back means hav- 3o ing two closely coupled portions one of which is connected in said plate-cathode circuit,va condenser connected at one side to said grid and at the other side to the other of said portions, said condenser being in electrical connection through said other portion with said cathode, potentialsupplying means for operating said tube, said feed-back means being poied so that upon closing of said switch said condenser is caused to periodically rapidly accumulate a charge such o as to bias said grid beyond cut-oi! whereby its period of discharge through said resistance substantially determines the frequency ci' the oscillations, and means for normally supplying cut-oil! potential to said grid.

8. In a device i'or theiproduction of musical sound, a thermionic tube having a plate, a grid, and a heated cathode, a plate-cathode circuit and a grid-cathode circuit, a plurality of resistances, a plurality of operating switches respectively 50 connecting said resistances in said grid-cathode circuit, feed-back means having two closely coupled portions one of which is connected in said plate-cathode circuit, a condenser connected at one side to said grid and at the other side to the 55 other of said portions, said condenser being in electrical connection through said other portion with said cathode, and potential-supplying means for operating said tube, said feed-back means being poled so that upon actuation of any of said 60 switches said condenser is caused to periodically rapidly accumulate a charge such as to bias said grid beyond cut-off whereby its period of discharge through any of said resistances substantially determines the frequency of the oscilla- 65 tions, said resistances being of different values to eect the establishment of different frequencies.

9. In a device for the production of musical sound, a thermionic tube having a plate, a grid, and a heated cathode, a plate-cathode circuit and 70 a grid-cathode circuit, a plurality of resistances, a plurality of switches respectively connecting said resistances in said grid-cathode circuit, feedback means having two closely coupled portions one o! which is connected in said plate-cathode u circuit, a condenser connected at one side to said grid and at the other side to the other of said portions, said condenser being in electrical connection through said other portion with said cathode, and means for supplying operating potentias to said plate and grid, said feed-back means being poled so that upon actuation of any of said switches said condenser is caused to periodically rapidly accumulate a charge such as to bias said grid beyond cut-off whereby its period of discharge through any of said resistances substantially determines the frequency of the oscillations, said resistances being of different values to effect the establishment of different frequencies, said potential-supplying means including a plurality of tuning potentiometers respectively connected in said grid-cathode circuit by said switches.

10. In a device for the production of musical sound, two oscillators each comprising a thermionic tube having a plate, a grid, and a heated cathode, a plate-cathode circuit and a gridcathode circuit, a resistance connected in said grid-cathode circuit, feed-back means having two closely coupled portions one of which is connected in said plate-cathode circuit, and a condenser connected at one side to said grid and at the other side to the other of said portions, said condenser being in electrical connection through said other portion with said cathode, potentialsupplying means for operating both tubes, in each of said oscillators said feed-back means being poled to cause said condenser to periodically rapidly accumulate a charge such as to bias said grid beyond cut-ofi whereby its period o f discharge through said resistance substantially determines the frequency of the oscillations, said oscillators being adapted to produce musically related frequencies, and coupling means between said oscillators communicating attenuated impulses frorn one to the grid-cathode circuit of the other to cause said one to control the frequency of said other.

11. In a device for the production of musical sound, two oscillators each comprising a thermionic tube having a plate, a grid, and a heated cathode, a plate-cathode circuit and a gridcathode circuit, a resistance connected in said grid-cathode circuit, feed-back means having two closely coupled portions one of which is connected 'in said plate cathode circuit, and a condenser connected at one side to said grid and at the other side to the other of said portions, said condenser being in electrical connection through said other portion with said'cathode, potential-supplying means for operating both tubes, in each of said oscillators said feed-back means being poled to cause said condenser to periodically rapidly accumulate a charge such as to bias said grid beyond cut-off whereby its period oi' discharge through said resistance substantially determines the frequency of the oscillations, said oscillators` being adapted to produce musically related frequencies, and coupling means between said oscillators communicating attenuated impulses from one to the grid-cathode circuit of the other to cause said one to control the frequency of said other. said coupling means isolating said oscillators to prevent reverse control.

12. In a device for the production of musical sound, an oscillator comprising a thermionic tube having a plate, a grid, and a heated cathode, a plate-cathode circuit and a grid-cathode circuit, a resistance connected in said grid-cathode circuit, iced-back means having two closely coupled portions one of which is connectcdin said platecathode circuit, a condenser connected at one side to said grid and at the other side to the other of said portions, said condenser being in electrical connection through said other portion with said cathode, and potential-supplying means for operating said tube, said feed-back means being poled to cause said condenser to periodically rapidly accumulate a charge such as to bias said grid beyond cut-off whereby its period of discharge through said resistance substantially determines the frequency of the oscillations, and a repeater tube circuit through which the oscillations are supplied comprising a second condenser CERTIFICATE OF CORRECTION.

Patent No. 2,085,246.

as follows:

June 8, 1937.

GILBERT SMILEY.

ilt is hereby certified that error appears in the heading to the printed specification of the above numbered patent requiring Correction Line 5. serial. number,

for "700,838" read 700,856; same page, printed specification, first column, line 12, for the word "oontained" read obtained; line 49-50, for "characteristics" read characteristic; page 5, first column, line 64, strike out "of"; page 7, second column, line 35, for "tub" read tube; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case inl the Patent Office.

Signed and sealed this 24th day of August, A. D. 1937.

(Seal) i Leslie Frazer Acting Commissioner of Patents.

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