US2230429A - Means for generating electric oscillations - Google Patents

Description

Feb. 4, 1941. w. KOCK ET AL 2,230,429

MEANS FOR GENERATING ELECTRIC OSCILLATIONS gig ed April 11, 19:59 3 Sheets-Sheet l //v VENT'ORS. WINSTON E ffac/r JoH/v fr J'oHzM/v BY i W ATTORNEY Feb. 4, 1941. w. E. KOCK ETAL 2,230,429

MEANS FOR GENERATING ELECTRIC OSCILLATIONS Filed April 11, 1939 3 Sheets-Sheet 2 FLA Y/NG .SW/TGHEs //v VENTO as JoH/v E JEWDAN 1. J l 5) Q In "3 03 Arron/v57.- a

Feb. 4, 1941. w. E. KOCK ETAL 2,230,429

MEANS FOR GENERATING ELECTRIC OSCILLATIONS Patented Feb. 4, 1941 PATENT OFFICE MEANS FOR GENERATING ELECTRIC OSCILLATIONS Winston E. Rock and John F. Jordan, Cincinnati,

Ohio, assignors to The Baldwin Company, Gincinnati, Ohio Application April 11, 1939, Serial No. 267,312

17 Claims.

Our invention relates to the production of electric oscillations. As will be shown below, the invention is applicable in the electrical production of music, particularly as an assembly for generating complex electric oscillations to be used in polyphonic music production in which various qualities of tones are to be obtained independently and in various combinations; and while not necessarily limited thereto, we have in mind employing the invention as an oscillation generating assembly to be used in connection with the keypending United States patent application Serial No. 196,484, filed on March 17, 19-38, by Winston E. Kock, one of the present applicants, for Improvements in electrical organs.

As was shown in the Kock application No. 196,484, many novel advantageous results can be obtained when complex electric oscillations generated according to a multioctave musical scale, have the characteristics that waveforms are sub stantially similar, octave relationships are exact and octavely related oscillations are substantially in phase. It is an object of the present invention to provide an assembly developing complex electric oscillations with these characteristics and comprising oscillation sources for all the notes of a multioctave musical scale, in which assembly the oscillation generating means are of an electronic nature employing the conventional vacuum tubes and associated electrical parts such as are in current use in the radio manufacturing industrywith consequent economic advantages.

In part, the invention contemplates, as sources of complex electric oscillations, electronic oscillators of the spill-over type, wherein an oscillator generates high frequency oscillations (which may be in the radio range) in a self-intermittent manner, the intermittence being periodic so as to provide complex oscillations of a lower frequency which may be in the audio range; and it is an object of the invention to provide a device of this character, having advantages which will be set forth below, including a capability of having its intermittence controlled by electrical pulses from external means, so that the lower frequency of the device becomes a harmonic fraction of that of the pulses. We have found however, that by providing in a spill-over oscillator means as will be described below, the device can be made incapable of sustained oscillatory action While still being capable of producing its lower frequency complex oscillations at controlled frequency when pulsed as aforesaid by the external means. Accordingly it is an object of the invention to provide an electronic spill-over device which will perform as a non-oscillator frequency divider.

As an extension of the last two objects the invention seeks to provide, as oscillation sources for each octavely related series of notes in a multioctave musical scale, a cascaded series of spill-over devices headed by a suitable source of stable electric oscillations or master oscillator, of the frequency corresponding to the note of the series in a high octave register of the scale; the first spill-over device, of an intermittence approximately an octave below the master oscillator, being pulsed by the master to an exact octave below it; the second spill-over device, of an intermittence approximately two octaves below the master oscillator, being pulsed by the first to exactly two octaves below the master; and so on, to provide the respective sources of oscillations for the several notes of the series.

By providing twelve of the above cascaded series, the master or control oscillators thereof being tuned respectively to the twelve notes, say, of the diatonic scale in the highest octave register of a musical instrument (except where the scale range of the instrument is extended upwardly, for which a particular means will be described below) we produce the oscillations for all the notes of a multioctave musical scale, these oscillations comprising those from the master oscillator and the lower frequency oscillations of the spill-over devices. By tuning only the twelve master oscillators an entire instrument, such as an electrical organ, is tuned. While the master oscillators we employ are useful, as will be shown below, as sources of complex oscillations for tone production and for extending the pitch range of an electrical musical instrument, in addition to their primary function as frequency controls, it will be understood that we are not necessarily limited to the particular sources of stable oscillations disclosed. Others may be employed; if desired we may employ the master oscillators solely as frequency controls, using onlythe spill-over devices as the means for tone productive oscillations.

We may employ spill-over devices in this invention in either the above-mentioned oscillatory or non-oscillatory states, as the frequency dividers and oscillation producing elements in the cascaded series above outlined. The use, in these cascaded series, of spill-over devices when in the non-oscillatory condition is especially advantageous in the electrical production of music, since under this condition the failure of a device results only in silence from the notes corresponding to the elements below it in the series rather than the production of off-pitch tones. We have recognized the advantages of employing nonoscillatory frequency dividers in electrical musical instruments in our previous United States patent application Serial No. 76,709, filed April 27, 1936, entitled Frequency divider, and copending herewith, now matured as Patent No. 2,185,635, issued January 2, 1940.

We have outlined above that the oscillation producing means in this invention can be related by exact octaves and that the oscillations are of complex waveform. The manner of obtaining these features, together with the attainment of a similar waveform throughout the oscillations from the several sources and the attainment of an in-phase relationship throughout octavely related oscillations, will be described in detail below. For convenience, the invention will be set forth in the following detailed description in an exemplary embodiment of an oscillation generating assembly directed toward the electrical production of music; and as such, in addition to the foregoing objects, the invention seeks to provide means for eliminating cross-talk and hybrid coupling in the assembly, to provide a particular means for extending the pitch range of an electrical musical instrument, to provide a preferred oscillation waveform, to provide a master oscillator suitable both as a frequency control and as a source of oscillations, and to provide an improved tremolo. The invention however, is not limited by necessity to the music art, since it is employable (potentially at least) in such arts as television, radio transmission, electronic measuring and control equipment, or the like.

The manner in which the above objects are attained will now be described in detail, reference being made to the accompanying drawings where- Figure 1 is a wiring diagram of a hithertoknown spill-over type of electronic oscillator, shown for purposes of comparison with the spillover oscillator which we have found to be advantageous;

Figure 2 is: a wiring diagram of the spill-over oscillator found to be advantageous, including a lead connected thereto for frequency control;

Figures 3, 4, 5, 6 show various means for making a spill-over device incapable of sustained oscillatory action while being capable of producing lower frequency oscillations when under control, these figures each also including leads for control and output and means for modifying output oscillations;

Figure 7 is a wiring diagram of a group of spillover devices and a preferred form of master oscillator, forming a cascade frequency system;

Figure 8 shows the waveform of the output voltage from a spill-over device, when unmodified;

Figure 9 shows the output voltage waveform as modified to a preferred form;

Figure 10 is a wiring diagram of a group of spill-over devices and a preferred form of master oscillator, forming a cascade frequency system in an arrangement we prefer commercially, the figure also showing a tremolo means and means for minimizing cross-talk and hybrid coupling;

Figure 11 is an end view of an assembly of anode circuit-to-grid circuit coupling coils, advantageous in minimizing cross-talk and hybrid coupling;

Figure 12 is a sectional View taken on the plane |2!2 of Figure 11; and

Figure 13 shows a means for extending the pitch range of an electrical musical instrument.

As will be shown in the following description, the electronic tubes we employ are of the high vacuum type and may comprise, for example, the vacuum tubes in current use in the radio manufacturing industry. Each of these tubes includes at least one thermionic cathode, heated either by direct passage of current through the cathode or by radiation-conduction from an adjacent electrical heating element through which current is passed. The heating currents may be of low voltage derived by transformation from alternating current mains, as is usual, and any sources of electric potential We employ (as indiciated in the following by C, C+, B+, B, B'-|, B) may also be derived from commercial alternating current mains through (transformation if required) rectification, and filtering, according to usual practices.

In the accompanying drawings we employ like indicia to represent like parts and parts having similar functions. With reference now to Figure l, a known type of spill-over oscillator includes a vacuum tube T containing an anode A, a thermionic cathode K heated by a heater H and a control grid G between these two electrodes A and K. The oscillator also comprises an anodeto-cathode circuit containing a source of electric potential, the positive terminal of which, 13+, is connected to the anode through a coil P, the cathode being connected to the negative terminal B- of the source. The oscillator further includes a grid-to-cathode circuit comprising the grid, a condenser C1, a coil S, and the cathode, in

series in the order given. Connected in parallel with the series combination of the condenser C1 and the coil S is a resistance R1. The coil S is inductively coupled with the coil P and so phased therewith that an increase in anode-to-cathode current tends to increase the potential on the grid positively, and vice versa.

The action of the oscillator is as follows: the condenser C1 being assumed to be initially uncharged, any slight ambient change in anode current, or in voltage on the grid thereby causing anode current change, results in rapid electric oscillations in the circuits, through the regenerative action between the coil P and the coil S. The inductances and distributed capacities of the coils P, S may be small and the oscillations consequently may be at high frequency which may be in the radio frequency range. The oscillations in the grid-to-cathode circuit are of sufficient intensity to cause grid current to flow, and as known, this current flows only from grid to cath ode; consequently through this rectifying action the condenser C1 is rapidly charged, placing an increasingly negative voltage on the grid until the transconductance of the tube becomes very small and the oscillations suddenly cease. The whole of the foregoing action may occur in a very small amount of time. The condenser C1, through suitable selection of its value and that of the leak R1, then discharges relatively slowly through the resistance R1, the negative voltage on the grid decreasing until the transconductance of the tube is sufficient to permit of a second group of high frequency oscillations, again charging G1 and so on, the action being of a self-intermittent character according to a definite periodicity. This periodicity is determined principally by the values of the condenser C1 and resistance R1 since any one period is comprised of a discharge of the condenser C1 through R1, occupying nearly all of the period, and a group of high frequency oscillations, occupying a small part thereof. Through suitable selection of values for R1 and C1 the periodicity may correspond to any desired audio frequency and the device, as is known, may thus be productive of audio frequency electric oscillations.

The spill-over oscillator which we have found to be advantageous is shown in Figure 2. In this oscillator the same general principle of operation is established as in the oscillator of Figure 1, and electric oscillations of any desired audio frequency may be produced as before by appropriate selection of C1 and R1 values. In the Figure 2 oscillator however, we have placed the condenser C1 and the resistance R1 as a parallel combination between the cathode K and the negative terminal B- of the source of potential in the anode-tocathode circuit, the remote-from-grid lead of the coil S being connected to the end of the parallel R1-C1 combination remote from the cathode; and in the Figure 2 oscillator, it is primarily the anode-to-cathode, rather than the grid-to-cathode, current that charges the condenser 01 under the influence of the high frequency oscillations to quickly raise the cathode potential positively with respect to the grid. This, as before, rapidly lowers the tube transconductance, oscillations cease, the condenser C1 discharges relatively slowly, oscillations begin again, and so on. We have found however that our oscillator has increased frequency stability over that of Figure 1. This is advantageous, for example in the electrical production of music.

While we desire a spill-over oscillator which is capable of maintaining its audio frequency oscillations within certain frequency limits, we do not desire an oscillator of extremely rigid frequency, since we seek to control its frequency, for the advantages outlined above. Thus we have found that the oscillator of Figure 2 will when unattended maintain its frequency within a two semitone interval but is also susceptible of having its frequency raised to a controlled value when appropriately driven. In this respect we have indicated in Figure 2 a source of electric pulsations D connected between the grid G, through a lead I, and the junction of the anode-to-cathode and grid-to-cathode circuits. If the oscillator be set so as to have an unattended frequency a few semitones below 1046.5 cycles per second (0. p. s.) (1046.5 0. p. s. corresponds to a C note in the equitempered scale based upon A=440 c. p. s.) and if positive pulses at a frequency of 2093 c. p. s. (corresponding to the C note an octave above the 1046.5 C) be delivered to its grid G, the oscillator audio frequency will be raised to exactly 1046.5 0. p. 5. under the control of the 2093 pulses, thus establishing an exact 1:2 octave relationship. We explain this as follows: the first pulse, say, from the controlling source D triggers the oscillator into high frequency action; at the instant of the second pulse, however, the condenser 01 has not discharged sufficiently and the transconductance of the oscillator tube T is still very low, hence the second pulse has no effect; at the instant of the third pulse however, the transconductance has reached a sumcient value that the oscillator is again triggered, and so on, synchronization thus resulting. In a similar manner we can establish 1:3, 1:4, etc., frequency relationships between driver and oscillator, though we prefer in the present exemplary embodiment to employ only the harmonic fraction 1:2.

As outlined above we can make a spill-over device incapable of sustained oscillatory action while still productive of oscillations of controlled frequency when pulsed, and the various ways in which we have made a non-oscillatory frequency divider of this character are shown in Figures 3, 4, and 6. In these figures there is the same arrangement of parts as in Figure 2, to which however other parts have been added, as will be explained.

In Figure 3 we have placed a resistance R3 across the coil S. The inductance of the coil S and its distributed capacity must, of course, be thought of as a local resonant circuit, sometimes called a tank circuit; and it is the building up of sufficient oscillatory energy in this tank circuit that enables the oscillator to continuously repeat the intermittent oscillatory action above described. This building up of energy is directly associated with the ratio of reactance to resistance, commonly termed Q, of this circuit; the resistance R3 lowers the Q and by employing a sufficient value for R3, the oscillatory potential given to the grid G-from oscillations set up in the grid-to-cathode circuit as the result of induction through the coupling P, S from oscillations in the anode-to-cathode circuitis insufficient to promote enough intensity of oscillation in the anode-to-cathode circuit to make the device operate as an oscillator according to the above. However if electrical pulses are conducted to the grid G, through the lead I as before, and the non-oscillatory device is set so that if the resistance R3 were removed it would oscillate a few semitones below a desired harmonic fractional frequency of the pulses, the device is influenced by the pulses, if sufficiently intense, to generate electric oscillations at the desired harmonic fraction. These oscillations are of complex waveform; they may be derived across a resistance R2 placed in series with the condenser C1 in the parallel R1, C1 combination in Figure 3 and their waveform as thus derived in the output lead 2 connected to the top of R2in the absence of the condenser C2 (to be explained shortly) is shown in Figure 8 as sharp peaked pulses. These output pulses may be modified in being derived, to the saw tooth form, of lesser complexity, as shown in Figure 9, by connecting the smoothing condenser C2 across R2. The advantages of the saw tooth form of electric oscillaton in electrical production of music have been explained in the aforementioned Kock application No. 196,484.

Thus we provide a non-oscillatory frequency divider in an electronic spill-over device and other ways in which we also accomplish this are shown in Figures 4, 5, and 6. In Figure 4 a resistance R4 is placed in series with the coil S. While as explained above, our spill-over device operates, as an oscillator, through the condenser C1 being intermittently charged, primarily by anode-to-cathode current, there is also grid current flow with anode current flow charging this condenser. The potential drop across the resistance R4, owing to the grid current, lowers the oscillatory grid potential to produce a similar effect as in Figure 3 and to provide a desired frequency divider of non-oscillating character. In Figures 5 and 6 we bias the grid sufficiently negative so as to reduce the transconductance of the tube enough to prevent self-oscillation. In Figure 5 this is done by direct placement of a source of electric potential C, 0+ in the grid-tocathode circuit and negative toward the grid,

while in Figure 6 the same result is effected by= raising the potential of the cathode. K positive with respect to its grid G, from the source of anode potential B+ by connecting an appropriate resistance R5 from 13+ to the cathode. As a general consideration, the four above methods of making an electronic spill-over device function as a non-oscillatory frequency divider comprise various. ways of reducing the action of its gridto-cathode circuit upon its anode-to-cathode circuit, and the apparatus involved in these various ways, or their equivalent, we term oscillation impeding means.

In Figure 7 is shown a cascade system of octave relationships comprising a master oscillator as a source of stable electric pulsations, and a series of the frequency dividers above explained. Such a system, as aforementioned, is useful in the electrical production of music, and while we are not limited to a particular form of source of master electric oscillations, we prefer as a master oscillator of high audio frequency to employ a vacuum tube oscillator. This is shown as the left hand tube and its associated parts in Figure 7. The tube anode-to-cathode circuit contains a source of electric potential B+, B and connected to B+ the positive terminal thereof, is a resistance Rs. Connected between the other end of this resistance and the anode A is an inductance L1. Regeneratively coupled to the anode-to-cathode circuit is the grid-to-cathode or frequency control circuit (as an alternative, the frequency control circuit may comprise the anode-to-cathode circuit). In this control circuit a tuned tank impedance comprising an inductance L2 and a condenser C3 in parallel is connected between the grid G and cathode K. A trimmer condenser C'3 is in parallel with C3 for critical tuning of the oscillator. The coupling between the two circuits is, as shown, between the inductances L1 and L2, which may have a common ferromagnetic core M1 as illustrated. Also contained in the grid-to-cathode circuit are a condenser C4 and resistance R7 in parallel. These serve to provide grid bias for the oscillator and to insure that it starts oscillating when the system is turned on, by setting up a transient potential of the grid under initial condition. -As above outlined we employ in an electrical musical instrument twelve cascade systems such as considered and in the anode-to-cathode circuit of each of the master oscillators of these is a resistance R as described. One purpose of these resistances is to decouple the various master oscillators, i. e., to prevent cross-modulation between them.

The other two tubes and their respective associated parts in Figure '7 constitute non-oscillatory frequency dividers as aforedescribed. In the first divider of these, central in the figure, the non-oscillatory means constitutes the resistance R5 as placed and described in Figure 6. We find that this form of non-oscillatory means in the first divider is advantageous in inhibiting noise in a cascade system. In the other dividers-see for example the second divider, in the right side of Figure '7-we employ the resistance R3 as non-oscillatory means, according to the method of Figure 3. It will be understood that by removing the resistances R3 and R5 from these dividers respectively they become oscillatory dividers. Moreover, while we have shownonly two dividers in the series of Figure '7, others of lower frequencies may be added, as indicatedby various of the leads extending to the left of the figure. Thus if the master oscillator operatesat divider.

a-high C; of 2093 c. p. s. the first divider will be influenced thereby through the lead 3 to operate at the exact octave below, 1046 c. p. s.; the second divider, similarly, will be influenced by thefirst divider through the lead I to operate exactly two octaves below the master, and so on throughout the system to provide all the C notes required in a. musical instrument. To accomplish this we employ appropriate values for L2,-C3 and C's .in the master oscillator frequency determining circuit, and likewise proper values for R2 and C1 in the respective dividers. As a matter of preference, we give such values to R2 and C1 in a divider so that as an oscillator it would oscillate unattended about six semitones below the frequency to which it is raised by influence.

In the influence lead 3 from the master oscillator -to the first divider is shown a high-pass filter C5, Re, C6. This is to sharpen up the pulses from the master oscillator so as to furnish betterv driving pulses for the first divider influenced thereby. In the various leads I are condensers C7 for isolating 13+ from the grids of the dividers. Ground, as illustrated, is employed as a common return in the system.

In this way we provide the cascade systems of this description and it remains to describe the characteristics of the output oscillations derived therefrom. We take output oscillations from the various dividers through the leads 2 as above described'these oscillations being of the saw tooth form of Figure 9. Likewise We take oscillations for tone production from the master oscillator through the lead. 2 connected to the impedance combination R9, R10, R11, Ca, Ca interposed between the cathode K of the master and ground. While the oscillations in the grid-to-cathode control circuit of the master are substantially sinusoidal in waveform (and useful for a purpose to be set forth below) its anode-to-cathode pulsations are complex. These pulsations we employ for tone production, modifying them in deriving them, through the impedance R9C9, to a waveform approximating that of Figure 9.

Thus as shown, the various oscillations derived from a cascade system are at exact octaves and similar in waveform; by appropriate design of the apparatus they can be given similar amplitudes. We shall now show that the various oscillations occur in phase:

l/Vhen current is increasing in the anode-tocathode circuit of the master oscillator a positive pulse is delivered into its output lead 2 for tone production- Likewise as this current increases, a u

operation to rapidly charge its condenser C1, and

concurrent with this charging a positive pulse is delivered into the tone output lead 2 of the first Now the operation of the divider to charge its condenser C1 goes on at a very high frequency (we employ radio frequency at about 400 k. c.) and-when the divider is arranged for.

the non-oscillatory condition as aforedescribed a single double-pulse occurs in its lead I at the .R12 and the coil P inthe-an0de=-t0-cath0de cir- Y cuit of the divider to the grid of the next divider. Consequently the positive component of the double pulse in the lead I from the first divider fires the second divider into operation and concurrently a positive pulse occurs in the tone lead 2 of the second dividerand similarly for the remainder of the series. In this Way it will be seen that the complex output oscillations for tone production from a system comprising a master oscillator and its associated dividers, occur in phase. It will be understood, of course, that since the various members of the system are arranged in octave relationship, every other pulse from an influencing member fires an influenced member, as was previously explained. An explanation similar to the above applies to the system when its dividing members are arranged as oscillatory dividers.

We employ a slightly lower value for the source of anode potential B+, B- for the dividers than for the master oscillators, as is indicated in Figures 7 and 10.

In Figure 10, wherein the foregoing described parts bear similar indicia, is shown a cascade system similar in principle and essentialities to that of Figure 7 but better adapted for commercial production, in an electrical musical instrument. In the system of Figure 10 we have illustrated a complete cascade for the note generators for example, indicating the inclusion of intermediate dividers by dashed lines in the central part of the figure and also illustrating leads to the other eleven cascade series.

As shown in Figure 10 we employ a double type of tube T having therein the necessary elements for two of the tubes T of the foregoing figures. This of course effects an economy in tubes and in size of the generating assembly. Now in a musical instrument such as an electrical organ having for example a pitch range extending from low 0 at 32.7c. p. s. to a high G note at 2093 c. p. s. there will be seventy three notes, thus requiring thirty-seven of the double tubes T with half of one of the tubes unused. We employ this spare tube section to advantage however, to provide a frequency tremolo or vibrato in the musical tones produced in the instrument. This we have illustrated at the left of the figure as a vacuum tube oscillator similar to the master oscillators of the cascade series but adapted to oscillate at a sub-audio frequency, 6 c. p. s. or thereabout. This oscillator consequently has an inductance L3 in its anode-tocathode circuit and an inductance L4 and capacity C10 in parallel in its grid-to-cathode or control circuit, the inductance L4 being regeneratively coupled to L3 through a common ferromagnetic core M2 as shown. It also has the parallel R7, 04 combination in its grid-to-cathode circuit, as in the master oscillators. A lead extends as shown from the grid side of the L4, C10 combination to the side-remote-from-grid of the L2, C3 combination in each of the master oscillators. Consequently the sub-audio oscillations of the tremolo oscillator (which are substantially sinusoidal in its grid-to-cathode control circuit) vary the grid potential of the master oscillators, thus to vary the frequencies of these oscillators slightly, to produce the desired vibrato efiect in the entire generator assembly. A variable resistance R13 is incorporated in the anode-tocathode circuit of the tremolo oscillator for adjusting the tremolo rate and an off-on switch 4 for the tremolo is also included in this circuit.

In a commercial arrangement of an oscillation generating assembly as above described, we mount the oscillators, dividers and associated parts on a chassis similar to that for radio receivers. For economy of space, the various members of the equipment will be in proximity thereon, and since as explained, we employ very high frequency oscillations as a basis for operation, cross-talk or cross-modulation are likely in the arrangement. To obviate such effects the coils P and S aforedescribed are comprised each of a pair of similar coils in series, P, P and S, S as shown in Figure 10. These coils are of course inductively coupled, P, S and P, S, so that the coupling effects are additive for the regenerative purpose above set forth; however the members of each pair of coils are so phased with respect to each other in space that the effects of external electromagnetic fields upon the pair are substantially cancelled therein. In this way we eliminate cross-talk or cross-modulation, for all practical purposes. A mounting for the two pairs of coils of a divider, suitable for installation on the chassis, is shown in Figures 11 and 12, in which each coil P and its coupled coil S are mounted on a wooden dowel 6. The two dowels 6 are attached by screws 1 to a wooden member 5 having a hole in one end thereof whereby it is attached to the chassis.

A means for extending the pitch range of an electrical musical instrument is shown in Figure 13. This figure is a fragmentary view of the left end of Figure 7 (or of Figure 10) showing the master oscillator thereof, to which the means has been added. As was shown, the master oscillators we choose to employ are basically of the conventional audio frequency regenerative type employing grid circuit tuning. While complex oscillations may be derived from this oscillator as was also shown, the oscillations within the tank circuit L2, C3 in its grid circuit, are sub stantially sinusoidal in character and relatively intense so as to control the oscillator frequency. Hence by placing a third winding L5 on the transformer formed of the coils L1 and L2 we can derive sinusoidal oscillations from the oscillator by inducing them into this winding.

We then connect the terminals of the winding L5 to the input terminals of a bridge type, full-wave rectifier r and derive oscillations for tone production across the rectifier output terminals, attaching an output lead 8 to one of these output terminals, grounding the other and connecting a resistance R14 across the terminals. While the input oscillations to the rectifier are sinusoidal, the output oscillations comprise inverted sine waves, thus are of complex waveform and double the frequency of the input oscillations. By providing twelve of the above frequency-doubling means connected respectively to the twelve master oscillators, the pitch range of the musical instrument can be extended upwardly a complete octave. We find that copper-copper oxide rectifying elements are very satisfactory in rectifiers as above.

It is not essential that we apply sinusoidal oscillations to the input terminals of the above frequency doubling means. A sine wave is a specific case of a symmetrical wave, i. e., a wave in which the negative half thereof, if 'moved along the neutral axis to a position opposite the positive half, becomes the mirror image of the positive half of the wave; and We have found that we can effect frequency doubling, as above, with input oscillations of any symmetrical waveform.

It will be understood that the various parts which have been given above in the exemplary embodiment employed in the present description of our invention, may be varied tosuit various specific conditions, and that modifications may be made in the invention without departing from its spirit. Having thus described the present invention, we claim:

1. In an electrical musical instrument, apparatus for generating harmonically related electric oscillations, comprising a source of audio frequency electric oscillations, a device comprising a vacuum tube having an anode, a thermionic cathode and a control grid, an anode-to-cathode circuit containing a source of steady electric potential positive toward said anode, a grid-tocathode circuit, means regeneratively coupling said two circuits sufficiently to produce electric oscillations of a relatively high frequency in said device, and a capacity and a resistance connected in parallel between said cathode and said grid, said capacity having a period of discharge through said resistance, in said device, corresponding to a frequency less than one half the oscillation frequency of said source, a connection from said source of oscillations to said gridto-cathode circuit in said device for impressing oscillations from said source between said cathode and said grid, and oscillation impeding means in said device, whereby said device operates as a non-self-oscillatory frequency divider to generate complex oscillations of a frequency a harmonic fraction of the oscillation frequency of said source.

2. Apparatus as set forth in claim 1, wherein said coupling means comprises a coil contained in said anode-to-cathode circuit and a coil contained in said grid-to-cathode circuit so coupled inductively to said first mentioned coil that said grid-to-cathode circuit is coupled to said anodeto-cathode circuit as set forth, and wherein said oscillation impeding means comprises a resistance connected across at least a part of said second mentioned coil, said resistance being of a value that changes in electric potential of said grid as caused, through said coupling, by electric oscillations in said anode-to-cathode circuit, are lowered sufiiciently to impede oscillations set forth.

3. Apparatus as set forth in claim 1, wherein said coupling means comprises a coil contained in said anode-to-cathode circuit and a coil con.- tained in said grid-to-cathode circuit so coupled inductively to said first mentioned coil that said grid-to-cathode circuit is coupled to said anodeto-cathode circuit as set forth, and wherein said oscillation impeding means comprises a resistance in said grid-to-cathode circuit, said resistance being of a value that changes in electric potential of said grid as caused, through said coupling, by electric oscillations in said anode-tocathode circuit, are lowered sufficiently to impede oscillations as set forth.

4; Apparatus as set forth in claim 1, wherein said coupling means comprises a source of steady electric potential in said grid-to-cathode circuit and negative toward said grid, said source in said grid-to-cathode circuit being of an intensity that said grid is given a sufficiently negative electric potential to impede oscillations as set forth.

5. Apparatus as set forth in claim 1, wherein said capacity and resistance in parallel are connected between the junction of said two circuits and said cathode, and wherein said oscillation impeding means comprises a conductive connec tion, including a resistance, from the positive terminal of said source of potential to said cathode, of a value that said cathode is maintained at a sufficiently positive electric potential with respect to said grid to impede oscillations as set forth.

6. Apparatus as set forth in claim 1, wherein said capacity and resistance in parallel are connected between the junction of said two circuits and said cathode, and wherein a resistance is ineluded in said parallel arrangement in series with said capacity, and including means for deriving electric oscillations across said series resistance. '7. In an electrical musical instrument, a cascade system comprising a source of stable electric oscillations of audio frequency, a series of devices each comprising a vacuum tube having an anode, a thermionic cathode and a control grid, an anode-to-cathode circuit including a source of steady electric potential positive toward said anode, a grid-to-cathode circuit, means regeneratively coupling said two circuits suficiently to produce electric oscillations of a relatively high frequency in said device, and a capacity and a resistance connected in parallel between said cathode and said grid, said capacity having a period of discharge through said resistance, in said device, corresponding in the first device of said series to a frequency less than one half the oscillation frequency of said source, in the second device to a frequency less than one fourth said oscillation frequency, and so on throughout said series, a connection from said source of oscillations to said grid-to-cathode circuit of said first device for impressing oscillations from said source between said cathode and said grid thereof, and a a connection from each preceding device to said grid-to-cathode circuit of the succeeding device adjacent thereto in said series for impressing electric oscillations from said preceding device between said cathode and said grid of said succeeding device, and oscillation impeding means in. each of said devices, whereby said first device operates as a non-self-oscillatory frequency divider to generate complex electric oscillations of a frequency a harmonic fraction of the oscillation frequency of said source, said second device operates as a non-self-oscillatory frequency divider to generate complex electric oscillations of a frequency a harmonic fraction of that of said complex electric oscillations generated by said first r device, and so on throughout said series.

8. Apparatus as set forth in claim 7, wherein the oscillations of said source are of audio frequency corresponding to the frequency of a note in a high register of a mutioctave musical scale, and wherein said capacities have such periods of discharge that said first device generates complex electric oscillations of a frequency one half the oscillation frequency of said source, said second device generates complex electric oscillations of a frequency one half that of said complex electric oscillations generated by said first device, and

so on throughout said series, whereby complex electric oscillations are generated at a succession of octavely related frequencies suitable for the production of musical tones.

9. Apparatus as set forth in claim 7, wherein the oscillations of said source are of audio frequency corresponding to the frequency of a note in a high register of a multioctave musicale scale, wherein said capacities have such periods of discharge that said first device generates complex electric oscillations of a frequency one half the oscillation frequency of said source, said second device generates complex electric oscillations of a frequency one half that of said" complex electric oscillations generated by said first device, and so on throughout said series, whereby complex electric oscillations are generated at a succession 'of octavely related frequencies suitable for the production of musical tones, and wherein in said first device said capacity and resistance in parallel are connected between the junction of said two circuits and said cathode and said oscillation impeding means comprises a conductive connection, including a resistance, from the positive terminal of said source of potential to said cathode, of a value that said cathode is maintained at a sufficiently positive electric potential with respect to said grid to impede oscillations as set forth.

10. Apparatus as set forth in claim '7, wherein said source consists of an oscillator susceptible of having its frequency varied within the limits of a few musical semitones by means non-essential to its operation, said oscillator comprising a vacuum tube having an anode, a thermionic cathode and a control grid, an anode-to-cathode circuit containing an inductance and a source of electric potential positive toward said anode, and a grid-to-cathode circuit containing in parallel a capacity and an inductance inductively coupled to said first mentioned inductance for operation of said oscillator, and said means for varying the frequency of said oscillator comprising a source of sub-audio frequency electric oscillations connected in said grid-to-cathode circuit of said oscillator between said cathode and said capacity and inductance in parallel, whereby a sub-audio cyclic variation in frequency is produced in the electric oscillations of said oscillator and hence in the complex electric oscillations of said devices in said series influenced thereby.

11. Apparatus as set forth in claim '7, wherein said coupling means in at least some of said devices comprises in each an inductance contained in said anode-to-cathode circuit and an inductance contained in said grid-to-cathode circuit so coupled inductively to said first mentioned inductance that said grid-to-cathode circuit is coupled to said anode-to-cathode circuit as set forth, said inductances each comprising at least two coils connected in series and inductively related to the corresponding coils respectively of the other inductance so as to constitute pairs of coils, which pairs of coils are so related in space as to minimize the effect of external electromagnetic fields upon said divider.

12. In an electrical musical instrument, a cascade system comprising a source of stable electric oscillations of audio frequency, a series of devices each comprising a vacuum tube having an anode, a thermionic cathode and a control grid, an anode-to-cathode circuit including a source of steady electric potential positive toward said anode, a grid-to-cathode circuit, means regeneratively coupling said two circuits sufficiently to produce electric oscillations of a relatively high frequency in said device, and a capacity and a resistance connected in parallel between the junction of said two circuits and said cathode, said capacity having a period of discharge through said resistance, in said device, corresponding in the first device of said series to a frequency less than one half the oscillation frequency of said source, in the second device to a frequency less than one fourth said oscillation frequency, and so on throughout said series, a connection from said source of oscillations to said grid-to-cathode circuit of said first device for impressing oscillations from said source between said cathode and said grid thereof, and a connection from each preceding device to said grid-to-cathode circuit of the succeeding device adjacent thereto in said series for impressing electric oscillations from said preceding device between said cathode and said grid of said succeeding device, whereby said first device generates complex electric oscillations of a frequency a harmonic fraction of the oscillation frequency of said source, said second device generates complex electric oscillations of a frequency a harmonic fraction of that of said complex electric oscillations generated by said first device, and so on throughout said series.

13. Apparatus as set forth in claim 12, wherein the oscillations of said source are of audio frequency corresponding to the frequency of a note in a high register of a multioctave musical scale, and wherein said capacities have such periods of discharge that said first device generates complex electric oscillations of a frequency one half the oscillation frequency of said source, said second device generates complex electric oscillations of a frequency one half that of said complex electric oscillations generated by said first device, and so on throughout said series, whereby complex electric oscillations are generated at a succession of octavely related frequencies suitable for the production of musical tones.

14. Apparatus as set forth in claim 12, wherein said source consists of an oscillator susceptible of having its frequency varied within the limits of a few musical semitones by means non-essential to its operation, said oscillator comprising a vacuum tube having an anode, a thermionic cathode and a control grid, an anode-to-cathode circuit containing an inductance and a source of electric potential positive toward said anode, and a grid-to-cathode circuit containing in parallel a capacity and an inductance inductively coupled to said first mentioned inductance for operation of said oscillator, and said means for varying the frequency of said oscillator comprising a source of sub-audio frequency electric oscillations connected in said grid-to-cathode circuit of said oscillator between said cathode and said capacity and inductance in parallel, whereby a sub-audio cyclic variation in frequency is produced in the electric oscillations of said oscillator and hence in the complex electric oscillations of said devices in said series controlled thereby.

15. Apparatus as set forth in claim 12, wherein said coupling means in at least some of said devices comprises in each an inductance contained in said anode-to-cathode circuit and an inductance contained in said grid-to-cathode circuit so coupled inductively tosaid first mentioned inductance that said grid-to-cathode circuit is coupled to said anode-to-cathode circuit as set forth, said inductances each comprising at least two coils connected in series and inductively related to the corresponding coils respectively of the other inductance so as to constitute pairs of coils, Which pairs of coils are so related in space as to minimize the effect of external electromagnetic fields upon said divider.

16. In an electrical musical instrument, a source of electric oscillations of audio frequency corresponding to the frequency of a note in a high octave register of a multioctave musical scale, said oscillations including a series of electric oscillations of substantially symmetrical waveform, means for deriving electric oscillations for musical tone production at the frequency of said source, a series of frequency dividing devices operatively associated with said source and arranged under influence of said source to operate at frequencies which bear successively lower octave relationships to the frequency of said source, means for deriving electric oscillations for musical tone production at said frequencies bearing lower octave relationships, and means for extending the pitch range of said musical instrument, comprising an arrangement for deriving said oscillations of substantially symmetrical waveform from said source, a full-Wave rectifier of electric oscillations, connections from said arrangement to the input terminals of said rectifier, and means for

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