US2294178A - Electrical musical instrument - Google Patents

Description

'll- 25, 1942- I .J. M. HANERT v 2,294,178

ELECTRICAL MUSICAL INSTRUMENT Filed ubv. 10. 1939 s Sheets-Sheet 1 T [/2 yen for; John 114. A i/29x" 1942- J. M. HANERT 2,294,173

ELECTRICAL MUSICAL INSTRUIENT Filed Nov. 10, 1939 3 Sheets-Sheet 3 gkre M Bane/"2 Patented Aug. 25, 1942 ELECTRICAL MUSICAL INSTRUMENT John M. Hanert, Wilmette, 111., assignor to Hammond Instrument Company, Chicago, 111., a corporation of Delaware Application November 10, 1939, Serial No. 303,728

14 Claims.

My invention relates generally to electrical musical instruments and more particularly to improvements in instruments of the melody type wherein changes in pitch of the tone produced are obtained by changing the frequency of oscillation of an electron discharge device oscillator.

It is the primary object of my invention to provide a very simple form of electrical musical instrument which may be manufactured at relatively low cost, but which will nevertheless be capable of providing a large variety of interesting and entertaining musical effects.

A further object is to provide an improved electrical instrument ofthe melody type having a single oscillatory generator for providing the tone frequencies, with simplified key controlled means for tuning said generator.

A further object is to provide an improved keying circuit for an electrical musical instrument in which the note frequencies are generated by an adjustable frequency oscillator, the keying circuit including means for paralyzing the amplifier of the instrument when two or more keys are simultaneously depressed.

A further object is to provide an improved electrical musical instrument of the melody type, in which the potential upon the suppressor grids of a pair of tubes in a push-pull amplifier stage are brought to a potential beyond cutoff whenever two or more keys are simultaneously depressed.

A further object is to provide an improved melody type electrical musical instrument having means to suppress undesirable transients irrespective of whether the instrument is played.

with a legato or non-legato touch.

A further object is to provide an improved electrical musical instrument of the melody type having means-for providing a smooth and con- "trolled rate of tonal attack and decay.

General description In general, the instrument of my invention will comprise a keyboard consisting of one or keys of a piano keyboard, or of any other suitable construction, the instrument herein described being assumed to comprise three octaves of keys designated herein as keys C2 to B4, in accordance with a common system of musical note designation as set forth in Hammond Patent No. 1,956,350, granted April 24, 1934. Keys C2, C3#, D4, C2#, D3 and D4# are shown in the drawings as representative of the three octaves of keys. Each of the keys successively operates three switches, the first of which controls the potential impressed on the suppressor grids of a pair of tubes of a push-pull stage of the amplifier of the instrument; the second of which operates to tune the oscillator to the frequency corresponding to the key depressed; and the third of which renders the amplifying system operative to transmit a signal from the oscillator to the speaker or other electroacoustic translating system.

The instrument comprises a simple form of audio frequency oscillator which, in one form of the invention, is tuned by changing the capacity be controlled.

Frequency generating oscillator The frequency generating oscillator as shown in Fig. 1, comprises an electron discharge device 30 which may be a pentode of the 6J'7G type, the input and output circuits of which are coupled by means of an oscillation transformer 32 having a primary 34 and a secondary 36. The pentode 30 comprises a cathode "K, a control grid IOGI, a screen grid 30G2, a suppressor "G3, and a plate "P. The frequency of oscillation is ,determined principally by the inductive reactance of the oscillation transformer 32 and the capacitative reactance of one of the condensers CO2, CC3#, CD4, CC2#, CB3, or CD4#, depending upon which of the keys C2, C3#, D4, C2#, D3 or D4# is depressed. The keys when depressed connect the enumerated condensers to ground.

It will be understood that the last enumerated keys and condensers are representative of three or more octaves of keys, each having associated more octaves of keys, similar in general to the therewith a condenser. The values of these condensers will be so chosen that the tube 38 will oscillate at the frequency of the key associated with that condenser whenever such key is depressed.

Bias to the tube 38 is supplied by means of a grid leak RI, which is in parallel with a grid condenser C6. Screen bias to tube 38 is supplied through a voltage dropping resistor R2, and filter condenser 01. The plate load on tube 38 comprises a resistor R3 in series with winding 36 of oscillation transformer 32, as well as resistances R4 and R5. Filter condenser C8 connects the plate circuit to ground and filter resistance R6 connects it to terminal +388V of the power supply. The power supply system is not disclosed herein, but it may be of any suitable construction designed to supply the designated voltages. A power supply of the type illustrated in the copending application of Laurens Hammond and John M. Hanert, Serial No. 293,444, filed Septemher 5, 1939, would be suitable for this purpose. The suppressor grid of tube 30 is connected to the cathode thereof.

The resistors R4 and R constitute a high impedance voltage divider for the output of the oscillator, which is taken from a point 38 intermediate the resistors R4 and R5, being coupled to the control grid of an amplifying triode 48 (Fig. 2), through a blocking condenser C9.

The inductance of the primary winding is preferably made variable in any well known man- -ner, as by adjustment of the amount of iron in Means are provided to shift the frequency of oscillation of the oscillator tube 30 through a. limited range (in the order of 3%.of the fre-,

' justableresonant and tone control circuits 49 quency), at aperiodicity of approximately 7 C. P. 8., this means comprising an inductance Ll, which is normally connected in parallel with a section of the primary winding 34 by a two-way switch 42. When it is desired to provide the vibrato, the switch'42 is shifted to complete a circuit to a contact 44, which is connected to a contact 46, the latter being adapted to make contact with the reed 48 suitably actuated to vibrate continuously at vibrato periodicity or rate. The reed 48 is connected to ground so that it effectively intermittently connects and disconnects the inductanceLi in parallelwith the lowermost section of the primary winding 34 at the vibrato rate, and thus shifts the oscillator frequency through the previously mentioned range. The taps on the/primary winding 34 to which the vibrato apparatus is connected, are so chosen that the means frequency of oscillation when the vibrato switch 42 is turned to on is the same as the frequency of oscillation when this switch is turned to "03 position.

It is to be noted that the capacitative reactance of the timing mesh (inductance 34 and one of z Output and amplifier circuits The signal supplied to the grid of amplifier through a conductor 62.

shown in Fig. 2, and which may be controlled by manually operable switches 50, these'switches being normally closed and opened whenever any particular one or more of the meshes 49 is to be utilized. The details of the adjustable resonant and tone control circuits may be varied considerably to provide various voicings of the instrument, one such system being disclosed more fully in the aforesaid co-pending application, Serial No. 293,444.

The amplifier tube 48, which may be a triode of the 6J5G type, is self-biased by a resistor R1 by-passed by condenser CH]. The triode 48 comprises a cathode 40K, a control grid MG, and a plate 4UP. .The plate circuit of this tube includes the primary of a transformer 52 which is connected to a suitable source of B supply indicated as a terminal +258V. The secondary of the transformer 52 is center tapped in the usual manner for push-pull amplification, the ends of the secondary being connected to the control grids of push-pull amplifier tubes 54, 55, which preferably are triple-grid super-control amplifier tubes of the 6K7G type. The tubes 54 and 55 respectively comprise cathodes 54K, 55K, control grids 54GI, 55Gl, screen grids 54G2, 55G2, suppressor grids 54G3, 55G3 and plates 54P, 55P.

The center tap on the secondary transformer- 52 has connected thereto, through a resistor R8,

a mesh for expression or volume control. This mesh comprises a condenser CH having one terminal grounded and its other terminal connected to the resistor R8 at a terminal 56, and a resistor R9 which is connected between the terminal 56 and a conductor 58. The conductor 58 is connected to a terminal I58V of the power supply system through a resistor RIO and may, upon operation of an expression control contactor 60, be connected to a terminal +|88V through one or more of resistors RI 1 @to RI5 inclusive. When the expression control isshifted to its lowest volume position, it connected to +l00V It will be understood that upon operation of the volume control lever 68, the contacts on resistor R to RI'5 inclusive, and the conductor 62, are closed sequentially in the order named, and opened in the reverse order. In so doing, they change the potential upon conductor 58 from a maximum of 150 volts to a. minimum of 100 volts. The resistors RID to Rl5 inclusive thus form an adjustable voltage divider circuit.

.Resistor R9 and condenser CH function to limitv the rate of change of potential at terminal 56. Resistor R9 is in series with the volume contube 40 is likewise supplied to a plurality of ad- (5 to themaximum "resistance of said mesh, and

thus serves, in conjunction with condenser Cl I, to cause the rate of potential change at the terminal 56 to be approximately uniform as the volume control 60 is moved from step to step.

The potential at point 56 determines the grid bias applied to the control grids of the push-pull amplifier tubes 54, 55. These tubes are of the variable 1., or remote cutoff type, and advantage is taken of this feature to provide a simple expression or volume control system which will operate over a wideprange such as 48 db., by-

varying the grid bias upon tubes 54, 55.

By making gradual the rate at which the grid bias on' tubes 54, 55 changes, the expression or volume control may be simplified into a relatively small number of steps, and substantially secure a widerange of volume change without introduc- I hereinafter be made.

. current to a speaker I2.

ing undesirable sudden changes in volume, which would otherwise occur when the volume control 60 is moved from one position to another. Thus,

if resistors RII to Rl'inclusive are so chosen as to provide successive 8 db. changes in volume, a 48 db. range in volume is provided with only six contacts.

The value of condenser CII is such that the volume changes are gradual, but yet are not too slow to prevent-the volume from substantially following the control lever position to produce desired rapid dynamic changes in the musical tone being rendered. The control grid bias on tubes 54, 55 is determined not only by the volume control mesh previously described, but by the potential on the cathodes of the tubes 54, 55 which, as will appear hereinafter, is determined by the operation of the keys.

The screens 54G2 and 55G2 of tubes 54, 55 are connected to a suitable terminal +250V of the power supply system, while the suppressor grids 54G3 and 55G3 thereof are connected to a conductor 64, the potential of which is determined by the depression of keys as will be described hereinafter. However, it;may be stated here, that the suppressor grids are at a potential beyond cutofi whenever two or more keys are simultaneously depressed. The plates of tubes 54,

55 are supplied with current from a terminal +490V of the power supply through appropriate load resistors RIG and RII respectively, the signalsfrom the plates being impressed upon the control grids 66GI and 6IGI of power output tubes 66, 61 through blocking condensers CI2 and CI3 respectively. The tubes 66, 61 may be of any suitable type such as 7B5 power amplifier pentodes. Grid resistors RI8 and RI 8 are connected between the grids of tubes 66, 61 and ground, the cathodes being connected to ground through a self-bias resistor R20.

The values of the condensers CI2 and CI3 with respect to the values of resistors RI 8 and RI 9 are such that their time constants cannot cause appreciably long periods of cutoff for the tubes 66 and 61 because of transients occurring in the plate circuits of tubes 54 and 55, to which reference will The tubes 66, 61 are of the 6F6 type in which the suppressor grids 66G3 and 6IG3 are connected to the cathodes 66K and 61K respectively within the tube structures. The screens 66G2 and 6IG2 of the tubes 66 and 61 are connected to a terminal +300V of the power supply system, which is also connected to the center tap oi the primary winding 68 of an output transformer I0, the secondary winding oi the output transformer I6 supplying the voice coil The terminals of the primary winding 68 are respectively connected to the plates 66P and 61? of the tubes 66 and 61.

Key operated control circuits As previously stated, the instrumentwill usually comprise one or more octaves of keys which may to similar in arrangement to the keys of a piano. For the purposes of illustration herein, the instrument is assumed to have a range, of three octaves, C2 to B4, and reference characters corresponding to the notes controlled have been applied to. the keys illustrated in Figure 1.

Each of the keys, for example, the key 02, operates upon depression thereof successively to close three switches. The switches comprise four resillent contact arms I6, I1, lliand I8 bearing suitable contact pcints. An actuator 86 engages the uppermost contact arm I6 and is adapted topress this contact arm into contacting engagement with the arm 11. Thereafter, both the arms I6 and 11 flex further to bring the arm 11 into contacting engagement with the arm I8, and still further depression of the key causes the flexing oi the arms I6, I1 and I8 to bring the arm 18 into contacting engagement with the arm 19.

The arms 11 of all of the keys are connected to a common grounded conductor 82, while all of the contact arms 79 are connected to a common conductor' 84. Each of the contact arms I6 is connected to the common conductor 64 .through a resistor RZI. In order to reduce the number of resistors R2I required for the instrument, several keys which are separated from each other by more than an octave may have their contact arms I6 connected in parallel to the same resistance R2I since it is improbable, in the normal use of the instrument, that such keys will be simultaneously depressed, since 'in playing a melody, interval relations of greater than an octave rarely occur.

From Figure 1, it will be noted that the switch arm I6 for the keys C2, C3# and D4 are connected to a common resistor R2I, and the keys C2#, D3 and D4# are connected to a similar common resistor R2I. arms 16 for the following groups of keys may be connected to a resistor R2I, common to each of the keys of the group:' D2, D3#, E4 D2#, E3, F4: E2, F3, F4#; F2, F3#, G4; F2#, G3, G4#; G2, G3#, A4; G2#, A3, A4#; A2,'A3#, B4; A2#, B3;

and B2, C4.

The switch arms I6 for the keys C3 and 044? may each be provided with an individual resistor R2I, since they do not fit readily into any of the above groups of keys. In this Way, fourteen resistors R2! may serve the purposes for which thirty-six resistors would otherwise be necessary.

As will be explained hereinafter, the purpose of a contact arm 16 and its associated resistor R2I, is to prevent undesirable transients from occurring when the player, through a legato style of playing, inadvertently depresses two or more keys simultaneously.

The switch arm 18 of each of the keys is connected with its associated condenser CC2, CC3,

-etc., previously described, and serves, when the grounded contact arm 11 makes contact with the arm I8, to tune the oscillator 30 to the frequency corresponding with the key depressed.

It is to be noted that the control grid bias on tubes 54 and 55 is determined not only by the volume control mesh previously described, but by the potential on the conductor 88 which is determined by a voltage divider mesh consisting of 4 resistors R22 and R23.

The voltage of conductor 88 is however, not determined solelyby the resistors R22 and R28, but is also changed whenever any of the switch arms 11 and 19 are connected together, and thus connect a resistor R24 in shunt with R23.

When none of the keys is depressed, the voltage on the conductor 88 is such as to bias the tubes 54, 55 substantially to cutoff, irrespective of the set-' ting of the volume control. When, however, a key is depressed, the voltage on the conductor 88 becomes more positive because of the shunting ot.

In a similar manner, the switch substantially cutofi potential because condenser CIB is connected to-these grids directly through the transformer secondary and not through the resistor, which might limit the rate of change of potential. However, after the key has been depressed, the transient voltage which was supplied through condenser C will disappear, and the tubes 54, 55. This condenser will then be inparallel with condenser CI4, and thus the rate at which the operating bias is applied to the control grids 54G! and 556-! of tubes 54, 55 will be slower.

switch 90 may be manually operable by the player.

Upon release of the key, the rate at which tubes 54, 55 approach cut ofi is controlled by a condenser C15, which is in parallel with resistor R231 Resistors R22, R23 and R25 areof values 'chosen such that when contact arm ii is in contact with the contact arm 75, the potential of conductor 88 is at +150 volts with respect to ground potential. When no key is depressed, resistors R22 and R23 allow the voltage on the conductor B8 to be approximately 100 volts more positive than when a key is depressedthis potential then being +250 volts. Thus, the three resistors R22, R23 and R24 constitute a voltage dividing mesh operating to determine the potential of the cathodes 54K, 55K for tubes 55, 55. It will be noted that tubes 54, 55 are connected in push-pull arrangement, so that changesin the +150 volts, which is thesame as the potential on conductor 88, which connects to the cathodes with respect to the cathode, which is beyond cutofi for the type 6X76: tube. The rate at which this cutofi occurs is determined by the time constant of condenser Cl'l with respect to resistors R25 and R2l, the condenser Cl! being connected between the conductor 54 and ground. The value of condenser CI? is so chosen that the rate of D. C. component of the plate current in these tubes does not have any transient eiiect upon of these tubes are, however, maintained at a potential of substantially +400 volts, since the conductor 64 is connected through a resistor R25 with a terminal +400V of the power supply. Whenever one of the keys is depressed, the resistance R2! associated with that key, is eiiectively connected between th conductor 54 and ground, so that this resistance, together with the resistor R25, forms a voltage divider determining the potential upon the conductor 54, and hence upon the suppressor grids orthe tubes 54, 55. Assuming that the resistor R25 has a value of 500,000 ohms, and that the resistors R2I have a value of 300,000 ohms each, the potential upon the conductor 64 or upon the suppressor grids, will change from a value of +400 volts to a value of +150 volts when only one key is depressed. If, however, two keys are simultaneously depressed, there will then be'two of the resistors R2l of 300,000 ohms each forming the grounded leg of the above mentioned voltage divider. As a result, the potential on th conductor 64 will drop to 92.5 volts.

If more than two keys are simultaneously deprssed, the voltage on the conductor 64 will be correspondingly lowered. Thus, it will be seen that when one key is depressed, the potential on the suppressor. grids or the tubes 55, 55 will be change of the potential will be very rapid, but not sufliciently rapid to generate objectionable transients.

The condenser Cl! also controls the rat of attack of the second of two depressed keys after the first key has been released. These decay and attack rates are made clickless so as to provide a musically satisfactory transition from one note to the other as one of the two keys is released.

Operation The instrument is played in the usual manner of a melody instrument, by successively depressing the desired keys, C2 to B4, the musician preselecting the desired quality of tone by opening one or more of the switches 50, and controlling the intensity of the sound produced by moving the volume control 55. The musician may me select the desired rate of attack by operation of the switch 50, and ii a vibrato is desired in the tones produced, the vibrator switch 52 is operated .to make contact with the switch contact 44.

Having thus operated these various controls to condition the instrument to produce the tones desired, or suitable for the rendition of the particular musical selection, the instrument is played preferably in a legato organ-like manner. Upon the initial portion of the down-stroke of any key, for example the key C2, the potential on the conconductor 64 will be lowered as previously described to volts. the key will cause the grounded arm H to engage the arm 18, thus connecting the associated tuning condenser 0C2 in parallel with the primary 34-01 transformer 32, thereby to tune the oscillator to the frequency of the note C2,

It will be understood that the oscillator tube 30 and its associated circuits will substantially instantaneously shift to the frequency determined by the condenser C02, although such shift in frequency may be accompanied by undesirable transients. As will appear hereinafter, "the amplifier is paralyzed to prevent the transmission of such transients to the speaker.

, Further depression of the key C2 causes its switch arm 18 to contact with the switch arm 19. thus efiectively connecting the switch arm 19 to the grounded conductor 82 through switch arm 11. Connecting the switch arm 19 to ground causes the resistance R24 to be placed in shunt with the'resistance R23, thereby lowering the voltage on the conductor 88 which is connected to the cathodes 54K, 1 55K of the tubes 55, 55,

the rate at, which the change in the potential Further depression of As the potential of the cathodes of the tubes 54, 55 is thus shifted to a lower value by an amount of 100 volts, the potentials on th control grids 54GI, 55GI of tubes 54, 55 are'raised relative to the cathodes, to render these tubes conducting, at a rate determined principally by the condensers CI4 and CI5 and resistance R8, as previously described. The oscillations generated by th tube 30 are thus transmitted through the amplifier tube 40, push-pull amplifler tubes 54, 55 and power tubes 66 and 61 to the speaker.

The gain of the tubes 54, 55 may of course bevaried as desired by the player through operation of the volume control 60, which changes the potential of the terminal 56 at a gradual rate because of the condenser CII, as prevoiusly described, and thus determines the effective bias on the control grids 54GI, 55GI of tubes 54, 55, after the transient conducted through condensers CI4 and CI5 to the mid-point of the secondary of the transformer 52 has subsided.

If two keys, for example, keys C2 and C2#, are simultaneously depressed, the potential upon conductor 64 will be lowered to a. value of 92.5 volts, as previously described, since there will then be two resistances R2I connecting this conductor to ground. Under these-circumstances, as previously pointed out, the potential of the suppressor grids 54G3, 55G3 will be 57.5 volts negative with respect to the cathodes 54K, 55K, which will cut off the electron stream to the plates 54F, 55F, thus rendering tubes 54, 55 ineffective to transmit the signal. This is desirable because upon the simultaneous depression of two or more keys, two or more condensers CC2, CC2#, etc., will be connected in,parallel and tune the oscillator to afrequency which will not be at of undesired frequency, together with the transient due to shift in frequency of the oscillator, because of the insertion in the tuning circuit of a second tuning condenser, is prevented from being heard in the speaker of the instrument. Also,

upon depression of the second key while the first was depressed, the signal controlled by the first key decays at a non-objectionable transient producing rate because of the provision of the 55 condenser CI'I, which limits the rate of change of potential on the suppressor grids 54G3, 55G3 of the push-pull tubes 54, 55.

Upon release of one of two keys simultaneously depressed, the oscillator will first be shifted in frequency to that determined by the condenser associated with the key held depressed, and

thereafter, the potential on the suppressor grids of the tubes 54, 55 will return at a non-transient producing rate to the +150 volt potential, which is its normal operating potential, and the same as that upon the cathodes 54K, 55K of the tubes 54. 55.

Since the instrument becomes silent whenever two keys are held depressed at the same time, the player will find that to obtain a substantially legato'style of playing, the second key should m depressed as soon after the release of the first key as possible. On the other hand, to produce 70 "spurious, frequencies.

since this efiect may be obtained by playing the instrument with the usual staccato-detached touch, or may be obtained by playing it with an extreme legato touch, such that two keys are 6 frequently depressed simultaneously for short intervals' Modified form of oscillator tuning Figure 3, taken in'conjunction with Figure 2,

l0 discloses a modified form of the invention in which the oscillator is tuned by changing the inductance of its resonant circuit instead of changing the capacity in the circuit, as in the instrument previously described. All of the parts, with the exception of the oscillator tuning circuits and the vibrato apparatus, are identical with those previously described, and the same reference characters have therefore been applied to the elements of Figure 3, as have been used in describing the corresponding elements of Figure l.

taps (thirty-six in number for a three octave in'-' strument), which are at such points along the winding as will cause the oscillator to generate frequencies of the 'notes' of the musical scale.

when one of these points is connected to ground. The capacity of the resonant circuit for tuning the oscillator comprises a condenser C30 which is shunted by a small trimmer condenser C3I, for tuning purposes, and by a condenser C32, when a vibratocontrol switch I02 is in the position shown in Fig. 3. When the switch is moved to its opposite position, it connects with a condenser C34, the other terminal of which is periodicallyconnected to ground by a reed 48 of the vibrato apparatus. The value of the condenser C32 is such as' to causeloscillation at a pitch which is the mean of the two extreme pitches produced when the vibrato apparatus is in use.

The various taps on the primary winding I00 are connected by suitable conductors to the 5 switch contact arms I8 of the various keys. Thus,

upon depression of a key, such for example as the key C2, the inductance effective to determine the frequency of the oscillator consists of that portion of the winding I00 which is between the tap to which the switch arm 18 of the key 02 is connected and a terminal I04.

The oscillator tuning and keying circuits shown in Figure 3 are made with a minimum number of parts, since the individual capacities for each tion shown in Figure 1, are replaced by the single primary winding I00 provided with the necessary taps, which winding can be produced at relatively low cost. The instrument is played in the same manner'as previously described with reference to Figures 1 and 2, and the musical results obtainable are identical with those previously described. When two or more keys are simultaneously depressed, it will be apparent that some turns of I the primary winding I00 will be short-circuited through the key switch circuits, thus, introducing losses in the transformer 32, which may cause cessation of oscillation or the production of frequencies'and interruption in the oscillation of the tube 30' are not discernible, because while they are taking place, the amplifier is paralyzed by virtue of the utilization, of resistors R2I, and

a staccato eflfect, no special technique is required the suppressor biasing circuit elements associof the notes required in the form of the inven-.

However, such transition ated therewith, which operate in the manner previously described.

A further function of the resistors Bill and their associated circuits is to prevent transmission to the loud speaker of undesirable transients incident to making and breaking the key operated switch contacts in the tuning circuit of the oscillator. Thus, even though the tuning circuit of the oscillator were arranged so that simultaneous depression of two keys would not result in a spurious frequency, there would still be considerable advantage in using the resistors RH and their associated circuits, because of the fact that the latter circuits would be eiiective to prevent the transmission of key clicks or key noises to the loud speaker.

.While I have shown and described particular embodiments of my invention, it will be understood that the invention as illustrated and described is merely exemplary, and that I desire to include within the scope of my invention, defined by the following claims, all such similar and modified forms of the invention by which substantially the results of my invention may be obtained through the use of substantially the same or equivalent means.

I claim:

1. In an electrical musical instrument of the melody type, in which a signal frequency" generator is adjusted to supply any one of a plurality of frequencies, and in which the output of the generator is controlled by an electron discharge device having a control grid receiving a signal .i'rom the generator and a second grid efiective to cut ofi the electron stream through the discharge device; the combination of a voltage controlling network connected to the second grid of the discharge device to determine the potential there-' impulses of musical frequency, key controlled means to, adjust the frequency of said generator, an output circuit including an electron discharge device having a control grid and a second grid capable of cutting off the electron stream through said device upon having a predetermined potential impressed thereon, means for normally maintaining the potential on said second gridat a value which will permit substan-' tially unimpeded flow of electrons through said device, and means operated by the keys to change the potential on said second grid to a value suflicient to cut oil the electron stream to said device, said last named means being operativb whenever two or more keys are simultaneously depressed and being inoperative when but a single key is depressed.

3. In an electrical musical instrument having a tunable generator of electrical impulses of musical frequency, key controlled means for tuning said generator to any one of a plurality of musical frequencies, an output circuit for said generator, said output circuit including an electron discharge device having a cathode, a control grid, asuppressor grid, and a plate, means for normally maintaining a potential on; said suppressor grid suchthat the latter does not ap preciably afi'ect the electron stream through said device, and means operable upon the simultaneous depression of two or more keys to change the potential on said suppressor grid to a value at which said suppressor grid will out oh the electron stream through said device whereby, when a plurality of keys are simultaneously depressed, said electron discharge device will be efiective to stop transmission of electrical impulses from said generator.

4. In an electrical musical instrument, the combination of an oscillatory generator 0! musical frequency, an output circuit for said generator comprising a signal amplitude controlling electron discharge device having a plurality oi grids including a suppressor grid, a plurality of depressible keys, a. plurality of circuits successively completed upon depression of each of said keys, and voltage divider means effective upon the completion of the first circuits associated with two or more keys to change the potential upon the suppressor grid of said electron discharge device to a value at which it cuts ed the electron stream therethrough.

5. In an electrical musical instrument having a plurality of keys, an oscillator, means controlled by said keys for selectively tuning said oscillator to frequencies corresponding to notes of the musical scale, an electron discharge device having at least two grids, means for impressing the signal produced by said oscillator upon one or said grids, a voltage controlling circuit for determining the potential on the other of said grids, said circuit including a plurality of impedance elements capable of changing the voltage impressed upon said other grid, said impedance elements being of such value that when more than one of them is connected in said circuit, the voltage impressed upon said other grid will be sufiicient to cut off the electron stream through said electron discharge device, and means operated by each of said keys to connect one of said impedance elements in said voltage controlling circuit upon the initial portion of the down-strok of the key, whereby said elec-- -tron discharge device will be cut ofi whenever the simultaneous depression of two Or more keys may result in the generation of undesired irequencies by said generating system; the combination of an output circuit element the potential of which determines whether said output circuit shall be efi'ective totransmit signals from said generating system, means for normallymaintaining said element at a potential which renders said output circuit efi'ective, and means operated by said keys for changing the potential on said element in a sense toward rendering said output circuit inefi'ective, said potential changing means being of such value that the depression of more than one key will be required to change the potential on said element to a value rendering said output circuit ineflective.

7. In an electrical 'musical -instrument, the combination of a plurality of keys, a plurality of switches associated with each key and arranged to be sequentially operated upon the depression thereon, an output circuit including an electron discharge device having a suppressor grid, a'voltage controllin network connected to said sup said impedanc element in said network in a mannerto lower the potential impressed on said suppressor grid, an adjustable frequency oscillator, and a circuit completed by the closure of another of said switches to tune said oscillator to the note frequency represented by the depressed key.

8. In an electrical musical instrument having an adjustable frequency generator and an output circuit, an element in said output circuit, the potential of which determines whether a signal will be transmitted by said circuit, a network determining the potential of said element, a plurality of key operated switches, and means including, a potential source and circuits controlled by said switches and associated with said network to change the potential of said element in a direction to prevent the transmission of a signal by said output circuit, the potential change effected by completion of one of said circuits upon operation of one of said switches being insufiicient to prevent transmission of the signal, but the potential change efiected by the concurrent completion of two or more of said circuits by said switches being sufllcient to prevent transmission of the signal.

I 9. In an electrical musical instrument having an adjustable frequency generator and an out- .put circuit, said output circuit having an element the potential of which is a factor in determining the amplitude of the signal transmitted by said circuit, a source of potential of such value with respect to a fixed potential that when applied to said element it will cause transmission of the signal by said circuit, a resistor connecting said source to said element, a plurality of keys, switches operated thereby, a conductor maintained at said fixed potential, connections from said conductor to poles of said switches, and resistances connecting other poles of said switches to said element, said resistances being of such value relative to the parameters of said element and relative to the values of said potential source and resistor that connection of one of said resistances to said conductor will not cause said elementto prevent transmission of the signal, but the connection of two or more resistances to said conductor will cause said element to prevent transmission of the signal by said output circuit.

10. In an electrical musical instrument having an output circuit, the combination of an electron discharge device having a cathode, grid and plate; circuits connected to said device to cause oscillation thereof, said circuits including a condenser effectively connected between said grid and said cathode, a tapped tuning inductance having one terminal thereof connected to said grid, and a plurality of key operated switches eflective respectively to connect the taps of said inductance to said cathode, thereby to change the frequency of oscillation of said device to the frequencies of the notes or the musical scale upon closing the key operated switches.

.11. In an electrical musical instrument, the combination of an electron discharge tube oscillator having a tuning circuit including a condenser and a tapped inductance, an output apparatus therefor including an amplifier and electroacoustic translating means, a plurality of manually operable playing keys, at least two switches operated by each of said keys, connections respectively between the taps on said inductance and one of the switches of each of said keys for rendering efiective in said tuning (ill circuit a predetermined number of turns of said inductance depending upon the particular key which is operated, and a circuit completed by the second of each of said key operated switches to render said amplifier" effective to transmit signals from the oscillator to the electroacoustic translating means.

12. In an electrical musical instrument having an output circuit, an element in said output circuit the potent al of which determines whether the output circuit will be operative, means including a potential source connected to said element through a resistance for-normally maintaining, said element at a potential numerically greater than that required to render said output circuit operative, a plurality of key operated switches, and a plurality of current flow limiting circuits including said element and com pleted respectively by said switches to reduce numerically the potential on said element, said circuits having such current flow limiting characteristics that the completion of at least two circuits is required to reduce the potential on said element to a value sufiiciently low to render said output circuit inoperative.

13. In an electrical musical instrument havin an output circuit', an output circuit paralyzing element the paralyzing effect of which is dependent upon the potential impressed on said element, relatively high impedance means normally maintaining a non-paralyzing potential on said element, a plurality of'keys, a potential source, and relatively lower impedance circuits including said source and said element completed upon the depression of said keys respectively to change the potential on said means to a'paralyzing value whenever two or more of said keys are depressed at the same time.

14. In an electrical musical instrument, the combination of an electrical tone signal generator of variable frequency, a signal output circuit for said generator including an electron discharge device having a plurality of electrodes the relative potentials of which determine the signal amplitude, a plurality of playing keys, three switches associated with each of said keys and operable successively by depression of the key, circuits completed by said switches respectively, one of said circuits including means effective to change therelative potentials on two of said electrodes toward cut off value to an extent such that completion of a second such circuit upon contemporaneous depression of another key will' cause the relative potentials on said electrodes to exceed cutoff value, another of said circuits including means for determining the frequency of the signal supplied by said generator, and the circuit completed by the last operated of said switches including a pair of said electrodes and means controlling their relative potentials to determine the intensity envelope of the tone signal transmitted by said device.

JOHN M. HANERT.

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