US2357191A - Electrical musical instrument - Google Patents

Description

1944- J. M. HANERT 2,357,191

ELECTRICAL MUSICAL INSTRUMENT Filed May 29, 1941 4 Sheets-Sheet l //?V/? for John M Han e/"z 1944- J. M. HANERT ELECTRICAL MUSICAL INSTRUMENT Filed May 29, 1941 4 Sheets-Sheet 2 MBA NM Wm fifty.

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Aug. 29, 1944. J. M. HANERT 2,357,191

ELECTRICAL MUS ICAL INSTRUMENT Filed May 29, 1941 4 Sheets-Sheet 3 77 uy y W WW KEY KEV DEPRESSEO 85L E4550 TONE INTENSI T Y TONE INTENSITY T/M (PEfiCUSI/GN-FULL TOUCH RESPONSE? KSUSIfl/NED FULL 7' OUCH RESPONSE) 6Rcus5/0N-A/0 TOUCH RESPONSE) (isusrn/uso NO TOUCH RESPONSE) I fave/afar z/O/2/2 M Hara 9/" f Aug. 29, 1944. J. HANERT 2,357,191

ELECTRICAL MUS ICAL INSTRUMENT Filed May 29, 1941 4 Sheets-Sheet 4 Patented AIII,29,1944

EIEQTRICAL MUSICAL INSTRUMENT John M. Hancrt, Wilmette, 111., assignor to Hammond Instrument Company, Chicago, I11.I a corporation of Delaware Application May 29, 1941, Serial No. 395,754

30 Claims.

My invention relates generally to electrical musical instruments, and more particularly to a solo type instrument.

The invention hasfor its primary object the provision of an improved solo type instrument which may conveniently be played by means of a short range pedal clavier, and which may be used with great advantage as an instrument ancillary to the piano, or other musical instrument.

A further object is to provide an improved touch responsive keying system for instruments of the above mentioned type.

A further object is to provide an improved frequency generating system for electrical musical instruments with an improved frequency doubling apparatus, and improved amplification and tone intensity envelope controlling circuits and ,apparatus.

A further object is to provide an electrical musical instrument having a plurality of playing keys and a single tone intensity envelope controlling circuit and apparatus which is responsive to the velocity of depression of any one of the playing keys.

A further object is to provide an improved electron discharge device circuit for controlling the amplitude envelope of the tones.

A further object is to provide an improved electrical musical instrument capable of producing bass tones of adjustable tone quality which are especially adapted for the support and accompaniment of tones produced by the piano.

A further object is to provide an improved electrical musical instrument playable by keys in which the keys are touch responsive, and in which means are provided to predetermine whether the tones shall be sustained or percussive.

A further object is to provide an improved electrical musical instrument in which there is an octave coupler circuit, and in which means are provided to adjust the degree of such coupler effects.

A further object is to provide an improved adjustable symmetrical audio frequency push-pull oscillator employing remote cutoil electron discharge devices for producing a very desirable output wave form.

A further object is to provide an improved unitary quality control system for electrical musical instruments.

Other objects will appear from the following description, reference being had to the accompanying drawings in which:

Figure 1 is a perspective view of the instrument illustrating its method of use in conjunction with a piano; 1

Figure 2 is a wiring diagram of the complete instrumentexcept for the power supply system:

Figure 3 is a diagram showing th output wave form of the push-pull oscillator;

Figure 3a is a diagram showing the input wave to the control grids of the frequency doubler tubes.

Figure 4 is a diagram showing the wave form- Figure 7 is an intensity envelope diagram of the tonal output of the instrument with the controls set for percussion tones with the keys fully touch responsive, and illustrating the effects of different velocities of key depression;

Figure 8 is a diagram similar to Fig, 7 showing the tonal intensity envelopes produced upon various velocities of key depression when the controls are set for percussion with the keys partially touch responsive;

Figure 9 is a view similar to Fig. '7 showing the tonal output of the instrument when the controls are set for percussion without touch response;

Figures 10, 11 and 12 are intensity envelope diagrams similar to Figs. 7, 8 and 9 respectively. except that in each case the tones are sustained instead of percussive;

Figure 13 is asimplified wiring diagram of a single key controlled circuit, and is provided for more clearly illustrating the operation of this circuit;

Figure 14 is a left end elevation of the pedal clavier assembly with the enclosing casing removed; and

Figure 15 is a bottom plan view of a portion of the keyboard showing particularly the pedal key actuated switches.

Piano music has universal appeal, due particularly to the beautiful tone qualities of the strings and the variations in intensity of the tones which may be obtained by the touch of the player, that is, by the strength of the blows of the fingers upon the keys. However, it is impossible for the piano player to carry on a sustained melody with his left hand if low bass notes are also scored in the music. Thus, it is impossible for the left hand to play a deep base note and at the same time play a melody note in the middle register of the piano, due to the limited span of the player's hand.

By providing a simple foot-operated clavier, the pianist may play the bass note with his left foot while carrying on the melody and counter melody with his left hand, thereby greatly expanding the musical, and especially the contrapuntal, possibilities of the piano.

Another feature which has characterized piano music in the past is that on those musical beats which involve bass notes, the remaining accompaniment notes are noticeably thin tonally because the players left hand is already fully employed in playing the bass note, whereas in organ and orchestral music, those beats of the score which call for the bass notes frequently also call for pronounced full accompaniment notes. By means of this invention, the pianist not only plays the bass note with his left foot, but also is at liberty to play full and tonally rich chords simultaneously with his left hand.

The piano itself is a virtuoso solo instrument. Few music ans have been able to master fully the technique required. One of the greatest problems in piano technique is that of rapidly playing alternating bass notes and accompaniment chords.

The technique of rapidly shifting the left hand from the remote end of the keyboard for playing the deep bass note, to the middle register of the keyboard for playing the accompaniment chord is exceedingly difficult, and becomes increasingly difficult as the tempo increases. By the use of the invention, this dimculty is avoided, since the players left foot never is called upon to move over any substantial keyboard range, and the player's left hand may remain in the middle register accompaniment range of the keyboard. Under these conditions, the element of musical tempo is a relatively unimportant factor, and concert piano music may be performed at the composer's indicated tempo without difficulty. In general, the use of the instrument of this invention in conjunction with a piano makes it possible to render music of an orchestral character not previously playable by a single player upon a piano alone.

Furthermore, the tones of the lower octave or two of a piano contain noticeable enharmonic partials and are exceedingly bright. For many musical purposes, it would be more desirable to have these tones of a more harmonic and mellow quality. With the instrument of this invention the tone quality is controllable by the player to cover a range from a very deep mellow bass tone to an exceedingly brilliant tone comparable to the tone of the bass piano strings.

In addition, the instrument of the invention has provision for the production of octave coupler effects which, when used, effectively extend the frequency range of the bass tones up to the higher accompaniment range.

The instrument may be used to supplement any other type of solo or orchestral instrument, and is particularly useful in conjunction with accordions, since it enables the player to play the bass notes by means of the pedal keyboard, leaving the player's left hand free to play the chord accompaniment.

Referring to Figure 1, the instrument of the invention although physically separable from the piano, finds its greatest utility as a musical part of the piano, and is therefore shown in Figure 1 as associated with a piano ID. The instrument includes a keyboard designated generally by the reference character I! and a tone cabinet H which is connected to the keyboard I! by a multiconductor cable II. The keyboard comprises a base I! upon which pedals 20 are suitably mounted, the pedals illustrated extending throughout the range of an octave for the notes C0 to B0 inclusive (32.703 to 61.735 C. P. S. or 65.406 to 123.47 if the switches I and I are open).

The keyboard assembly also includes a casing 22 which may enclose some of the electrical elements of the instrument, and serves as a support for the adjustable controls. A disc 24 having suitable radial graduations is adapted to control the quality of the tones, as will hereinafter appear, while a similar disc 28 is rotatably mounted on the casing 22 for the control of the output volume. The control discs 24 and 26 are made sufficiently large and protuberant so that they may be readily operated by the foot.

There are provided a plurality of snap switch actuators 28, 29 and 30 which are of rugged construction so that they may be operated by the toe, being swung latterly from one side to the other. The switch actuator 28, may, for example, determine whether the tones produced shall be sustained or percussive. The switch actuator 29 may be of a three-position type which, when in central position, causes the instrument to be non-touch responsive, and when moved to the right may cause the tones produced to be fully responsive to the velocity of the key depression, while when it is swung to the left the instrument may besemi-touch responsive. The lower switch actuator 10 may control merely an "on" and off" switch to connect a supply of current to the instrument as a whole. A three-position switch actuator 3| may be mounted in a convenient position on the tone cabinet H to control octave coupler effects.

In order that the player of the instrument may conveniently swing his foot with the heel of the shoe clearing the floor, it is usually desirable that the piano bench be raised somewhat above its normal height. In Fig. 1, the piano bench 32 is shown as having its legs resting upon a frame 34 which raises the bench a few inches so that the player does not have to lift his thigh from the bench in order to enable him to swing his foot freely over the pedal clavier. Such frame 34 is not necessary for a person having limbs which are of average length. On the other hand for childen, it may be desirable to raise the whole keyboard I! by inserting a suitable sup port beneath it.

Referring more particularly to Figs. 14 and 15, the keys 2!] are each riveted or otherwise suitably secured to a relatively heavy leaf spring 36, each having an opening to receive a stud 38, the studs 38 projecting upwardly from a main frame plate III adjacent the rear edge thereof. The springs 36 for all of the keys rest upon a half round bar 42 which is suitably secured to the main frame plate 40 through a spacing and supporting strip ll of wood. Each of the studs 38 has a nut 46 threaded thereon, the nut having a generally spherical surface on the bottom thereof to provide substantially a point contact for the spring 36. By adjustment of the position of the nut 46, the initial stress of the spring '38 may be adjusted, the nut ll being adapted to be locked in adjusted position by a Jam nut 48.

The casing 22 is supported from the main frame plate 40 by a plurality of studs 50. The strokes of the keys are limited by strips of felt 52 and N. A guide comb 56, provided with slots 51 to receive the keys 20, is secured to the main frame plate 40, and may have its slots lined with felt 55 to eliminate noise.

Each of the keys has an insulating actuator 50 or otherwise suitably secured thereto, as by screws or rivets 59. Each of these actuators (except that for the C key) is adapted to operate switch fingers 60 to 83. The switch fingers ill to 63 are riveted or otherwise suitably secured to insulating sheets 64 which in turn are secured to the main frame plate 40 extending across a wide slot 66 in the plate 40. The switch fingers 60 to 63 are so arranged that the ends thereof overlap, and that the downward movement of the associated actuator 58 will flex first the switch fingers 80 and BI and thereafter the switch fingers 62 and 53 away from contact with the corresponding switch fingers of the next adjacent key. Switches 60 and 61 may be arranged to open shortly after the commencement of the down stroke of their associated keys, while the time of opening of the switches 62 and 63 will be determined by the velocity with which the key is struck.

It will be noted from Fig. 15 that there is no switch 60 for the lowermost key, the key C. However, each of the other keys is provided with four switches 60 to 63, as shown in Fig. 2.

Frequency generating system As shown in Fig. 2, the frequency generating system comprises a low frequency oscillator, including a pair of pento'des Ill and H which may be of the '6K7G or 7A7 type. These tubes are connected in push-pull arrangement, their cathodes 12, 13 being connected to ground through a self biasing resistor RI and their control grids 14, 15 being connected to the ends of an oscillation coil Ll. The center tap of the coil Ll is connected to ground. The inductance of Li may be varied, preferably by changing the air gap in the iron laminations of the core thereof, to tune the oscillator initially.

The screen grids 16, ll of the tubes Hi and H are connected to a terminal +300 v. of the power supply system having a voltage of 300 volts positive with respect to ground, this connection being made through a voltage dropping resistor R2. The suppressor grids 18, 19 of the tubes 10 and H are connected to their cathodes. The plate 80 of the tube Hi is connected through a blocking condenser Cl and a feedback resistor R3 to the grid 15 of the tube H. Similarly, the plate 8| of the tube H is connected through a blocking condenser C2 and a feedback resistor R4 to the control grid 14 of the tube 10. Current is supplied to the plates of the tubes 10 and H from the terminal +300 v. of the power supply through resistors R5 and R6 respectively.

Conductors 9i and 92 are connected respectively to the grids l4 and 15. As will more fully appear hereinafter, the capacitance between the conductors ill and 92 is controlled by the key operated switches 60, and thus such capacitance is in effect connected across the ends of the winding Ll to form therewith a resonant circuit for substantially determining the frequency of oscillation of the oscillator composed of the tubes II and II. a

It will be noted that the plate ll of the tube 10 feeds energy into the grid ll of .the tube H,

. of the tube II, thus decreasing the plate curtil rent through tube II and increasing the potential of the plate 8! of said tube. This increased plate potential of the tube II results in increased potential of the grid I4 of tube Ill. thus further increasing the plate current through the tube ll until a further increase in grid voltage does not result in an increase in plate current through the tube 10. At such time, the potential of the grid 15 of the tube H begins to drop, and a reverse sequence of events takes place. Of course the increase and decrease of plate current through the tube II takes place in the same manner but 180 out of phase with respect to the corresponding changes in current through the tube III, the tubes 10 and H thus producing output voltages across the resistors R5 and R6 which are substantially 180 out of phase.

The tuning capacitance between the conductors 9| and 92 is controlled by the key operated switches which, as previously stated, are connected in series so that the highest key of a plurality of keys depressed determines the frequency of oscillation of the oscillator. The capacitance is provided by a plurality of condensers C8 of suitably chosen values, each of these condensers having one terminal thereof connected to the conductor 9|,- while its other terminal is connected to one of the switch fingers 60.

The condenser C3 associated with the key B is of relatively large value and the remaining condensers C3 therefore may be relatively small, since they are adapted to be connected in parallel with each other and therefore need merely provide the incremental capacitance necessary to change the frequency of oscillation of the oscillators 10, H through successive semitone intervals. This is a feature of considerable economic advantage since the total capacitance required is relatively large due to the low frequency of oscillation.

Since the tuning of the oscillator for the semitone intervals is accomplished by changing the capacitance of the resonant circuit of the oscillator, changing the inductance (by varying the reluctance of its core) in tuning the instrument as a whole, results in changing the pitch range of the oscillator without affecting the semitone interval relationship.

When none of the keys is depressed, the oscillator will operate at the frequency of the note CO (32.703 C. P. 8.). Such oscillation will, however, not be transmitted to the output circuit because of the provision of control circuits hereinafter to be described.

The wave shape of the output of oscillator 10, H is shown in Fig. 3 as measured from a fixed potential such as ground, to the plate of tube 10. The positive amplitude portion of the wave of Fig. 3 is comparatively flat-topped because of the saturation of the tube, for under these circumstances, the tube I0 is delivering its maximum plate current, whereas the negative portion of the wave is limited by the cut off of the tube.

portion of this wave is not sharply peaked. This is because of the remote cut of! characteristics of the tubes and II. This is desirable for musical purposes inasmuch as the high frequency harmonic content is thereby very much reduced over and above what it would be if sharp cutofl oscillator tubes were employed.

Frequency doubler and preamplifier A pair of sharp cutoii pentodes 93, 94, which may be of the 707 or 6J'7G type, have their control grids 95, 96 connected to the condensers CI and C2 respectively through decoupling resistors R1 and R8. The control grids and 96 are connected to ground through condensers C1, C0. These condensers affect the wave impressed upon the grids of the tubes 93, 94, to cause it to assume the shape shown in Fig. 3a. The values 01' the condensers Cl and C8 are so chosen with respect to the values of resistances R1 and R8 that the peak positive amplitude is substantially the same for each of the twelve semitone intervals of the instrument. These condensers further serve to render the shape of the positive peak of desirable rounded contour, which is desirable since it represents, in the ultimate output of the instrument, a mustical tone in which the successive harmonies are of substantially uniformly decreasing relative amplitude.

The cathodes 91, 98 of the tubes 93 and 94 are connected to a negative terminal -50 v. through high value self biasing resistors R9 and RIO, the cathodes also being connected to ground through by-pass condensers C9 and CIO. The screen grids of the tubes 93 and 94 are maintained at a predetermined potential determined by a voltage divider network comprising resistors RI I, BIZ and RI3, the resistor RI3 being in the form of a potentiometer, the slider of which is connected to ground and operated by the control disc 24. (Fig. 1.)

By adjustment of the value of the potentiometer RI3, the screen bias may be readily varied between limits of +35 volts and +100 volts. The plates 99 and I00 of the tubes 93 and 94 are connected through plate load resistors RI 3a, RM to a terminal I02, which terminal is connected to a suitable constant potential source, indicated as +300 v., through a filter resistor RI5. The terminal I02 is also connected to a point of constant potential, indicated as ground, through a filter condenser C l I.

The efiective value of the plate load resistance for the tube 94 may be decreased by closing a switch I04 which connects a resistance RIB parallel with the resistance RI4, or the resistors RI4 and RIG may be shunted by closure of a switch I06. The switches I04 and I06 are operated by the switch actuator 3i. (Fig. 1.) The outputs of the tubes 93 and 94 are connected to a common terminal I08 through decoupling resistors BIG and R20. The terminal I08 is connected to the input of a preamplifier tube H0 indic ted as a triode, which may be of the 7A4 or 6J5G type, through a blocking condenser CIZ. The grid of the tube H0 is connected to ground through a grid resistor R20a, while the cathode of this tube is connected to ground through a self biasing resistor R2I. The plate of the tube H0 is connected through the primary winding L2 of an interstage transformer TI to the source of plate current indicated as +300 v. The primary winding L2 may be shunted by a resistor It is to be noted that the shape of the negative R22 which serves to improve the frequency response of the transformer TI.

The signal supplied to the transformer TI differs appreciably irom that generated by the oscillator tubes 10, II, and the character 0! the signal wave depends not only upon the characteristics of the tubes 91 and 94, but upon the potentials of the screen grids of the tubes 04, 94, upon the positions of the switches I04, I", and, of course, upon the general circuit arrangement.

As has been previously stated, the self biasing resistors R9 and R.I0 are extremely high in value, being in the order of 4 megohms. Under these conditions, the grids of tubes 93 and 94 will be highly biased in a negative direction, and consequently only the highest positive peak signals will be effective to cause pulsations of plate current to flow, in a similar manner to that more fully disclosed in the patent to Gilbert Smiley Re. No. 20,831.

Operation 0/ octave coupler Suppose now that switch I00 is closed and therefore the output of tube 94 does not contribute any signal at the terminal I00. Under these conditions, the signal upon terminal I00 will have the wave shape shown in Fig. 4. An inspection of Fig. 4 will show that the wave shape consists of a series of separate peaks having desirably rounded apices. Taking into consideration the non-linear operation of the tube 93, it is apparent that the curvature of these apices is related to the curvature of the positive peaks of the input wave shown in Fig. 3a. For musical purposes, it is very desirable that the wave peak have a rounded apex, and this is one reason for providing the condensers C1 and C9 and resistors R1 and R8, since they cause the input wave shape to have smoothly rounded apices. It is also apparent that negative pulses have no effect on the output signal as it appears at the common terminal I00 because of the large negative bias on the control grids of the tubes 93, 94.

The width of the bases of the waves shown in Fig. 4 is controlled by the potential on the screen grids of the tubes 93 and 94. As the screen voltage is increased in a positive direction, the width of the bases of the waves is increased and therefore the tone is made more mellow (less bright). This effect differs materially from an ordinary one stage resistance capacity tone quality controlling mesh, because the adjustment of the screen grid potential by means of the potentiometer resistance RI3 results in a change in the db./H factor (as is more fully explained in the patent to Laurens Hammond No. 2,126,464) instead of producing attenuation which is proportional to frequency only.

Upon opening the switch I06 and closing the switch I04, the wave shape of the signal at the terminal I08 will be that represented in Fig. 5. The waves shown in Figs. 4, 5 and 6 are drawn to the same time scale, and it will therefore be apparent that the wave of Fig. 5 comprises a high amplitude signal of the same frequency as that of Fig. 4, plus an additional lower amplitude signal occurring 180 out of phase with respect to the high amplitude signal. The amplitude of this I signal is less than the amplitude of the original signal because the combined resistance of resistors RI4 and RIB in parallel is lower than the resistance RIia, and because tubes 93 and 94 are pentodes with constant current output characteristics, The wave shown in Fig. 4 is derived exclusively through the tube 03, while in Fig; 5 the high amplitude peaks are derived from the tube 93 and the low amplitude peaks are derived from the tube 84.. The tube 33 carries on th push-pull arrangement of the oscillator so that the peaks of the signals provided by the tubes 33 and 34 are necessarily in push-pull or 180 phase relation.

When both switches I04 and I06 are open, the signal between the terminal I08.-and ground will have the wave shape indicated in Fig. 6. In this figure alternate peaks are derivedfrom the tube 33, while the intermediate peaks are derived from the tube 94. Since the resistors RI3a and RI4 are of the same value, the amplitudes of both series of peaks are the same. Under these conditions, all of the odd harmonics present in the output wave of the tube 93 (whose wave shape is shown in Fig. 4) are substantially cancelled by the identical (but out of phase) output of the tube 34. As a result, the apparent frequency of the tone produced will be raised one octave as may be deduced from an examination of Fig. 6.

It is to be noted that the tonal brightness of the wave (of Fig. 6 as compared with Fig. 4) is considerably reduced because of the fact that the distance between waves is made one half as great, but that the width of the bases of the waves is substantially the same as before. tubes 93 and 94 were operating in a linear manher (for instance as in a push-pull amplifier), and the signal to them was derived across a center tapped transformer, the resulting signal would be zero because of the complete cancellation produced by connecting the plates 99 and I together at the junction point I08. It is only through the fact that these tubes operate nonlinearly that complete cancellation does not occur, and that only the odd harmonics of the distortion terms produced are cancelled, thus leaving the even harmonics uncancelled.

Thus, Fig. 4 represents a complex musical tone of a certain low pitch, whereas Fig. represents a composite musical tone consisting of this low pitch tone together with a complex tone one octave above, and has the effect, for example, of sounding a note and its octave, as is obtained by the use of an octave coupler in an organ. Fig. 6, on the other hand, represents a musical tone of the octave only, and contains none of the frequency of the lower octave indicated in Fig. 4.

Output control circuits The secondary winding L3 of the interstage transformer TI has its end terminals connected to the grids III and H2 of pentodes H3 and H4 respectively, these pentodes being preferably of the variable mu or remote cutoff type such as the 7A7 or6K7G type. The functions of these tubes are to control the transmission of the signal produced by the oscillator to the output with a selected intensity envelope, and to prevent signal transmission when no keys are depressed. The suppressor grids of these tubes are externally connected to their cathodes II5, II6 respectively. The plates Ill and H8 of these tubes are connected through load resistors R23 and R24 respectively to a suitable plate current source indicated as +300 v.

The screen grids H9 and I20 of the tubes H3 and H4 are connected together and to a terminal I22, the potential of which is determined by a voltage divider network comprising a resistor R25, a fixed resistor R26 and a variable 'of resistors R25, R26 and R21 that the screen grids H9 and I20 are normally maintained at a potential sufllciently negativeas to substantially cut oil plate current through these tubes.

Upon opening any one of the switches 33 by the depression of one of the playing keys, the potential of the screen grids will tend to rise to its normal operating value, and th rate of this increase in potential will be in part determined by the value of the condenser CI4, which effectively connects the screen grids H9, I20 to a suitable point of fixed potential, preferably to the cathodes H5, I I3.

The cathodes I I5, I I6 are maintained at a constant positive potential such as volts by a low impedance voltage divider consisting of a resistor R30 connected between the cathodes and the source of positive potential, indicated as +300 v., and resistors R3I and R32 connected in series between the cathodes and ground. The cathodes H5, H6 are also connected to a center tap I23 on the secondary winding L3 through a relatively large condenser CI6. The center tap I23 is connected by a conductor I24 with the stationary poles of the switches GI 'and 62 for the lowest key, the key C, and when all of the switches SI and 62 are closed, the center tap I23 is thus connected to a terminal I26. This terminal I26 constitutes the junction of resistor R3I and R32 and is normally maintained at a suitable positive voltage such as +68 v., thereby producing an effective negative grid bias of 7 volts on the control grids III and H2 of tubes I I3 and H4. The center tap I23 is likewise connected through conductor I24 and switches 62 to the movable arm I28 of a three-position switch, and thus may be connected to terminal I30 or terminal I32, or may be disconnected from both of these terminals. The terminals I30 and I32 effectively form the junctions of a series of resistors R34, R35 and R36 which individually and collectively are of high impedance with respect to the resistor R32.

The resistor R38 is adapted to be connected between the conductor I24 and ground upon closure of a switch I34. When the switch I34 is closed, the resistor R38, of Very high value, effectively determines the rate at which the control grid bias on the tubes H3, H4 approaches a cutoff value.

Power amplifier circuits The output of the tubes H3, H4 is transmitted to a pair of triodes I40, I42 which may be in a single envelope such as provided in the 7N7 or 6F8G type tube, the connection being through blocking condensers CI9 and C20. The grids of the triodes I40, I42 are connected to ground through grid resistors R4I R42 respectively, while the cathodes of these tubes are adapted to be connected to ground through a self biasing resistor R43 which is connected to ground through a variable resistance R44, by which the bias on the control grids of the tubes I40, I42 may be varied to make adjustment for the degree of amplification of these tubes which is desired.

The plates of the tubes I40, I42 are coupled by a condenser C2I which serves to remove undesirable high frequencies of the signal, and also high frequency distortion products which may be introduced in these and other preceding tubes. The plates of the tubes I40 and I42 are also connected to the end terminals of the primary winding L4 of a push-pull transformer T2, the center tap of which is connected to a plate current supply indicated as a terminal +300 v. The secondary L5 of the transformer T2 is connected to the input grids of class 38 output tubes I43, I44, preferably of the 6N7G type, which is a twin triode type. The center tap of the secondary winding L5 is connected to ground, and the oathodes of tubes I43, I44 are also connected to tacts 6i and 02, which in turn is a measure of the ground. The plates of the tubes I43 and I44 are I Operation of key circuits Th operation of the key switch controlled circuits may best be described by reference to the simplified diagram of Fig. 13 in which representative parts of the complete circuit of Fig. 2 are illustrated and bear corresponding reference characters.

Assuming that the player wishes to produce percussion tones with full touch response to obtain tone intensity envelopes of the type indicated in Fig. 'I, he will close switch I34 and set switch I28 so as to make contact with the ter-- minal I32. When no keys are depressed, all of the switch fingers 60 to 53 will be closed and the resistance R28 will then be connected to ground. Connecting resistor R28 to ground lowers the screen potential at the terminal I22 to a value such that plate current cannot flow becaus of the negative bias on the screen relative to the cathode. Thus, no signal can be transmitted from the oscillator to the speaker. Also, under these conditions, the control grids III, II2 are at a potential of +68 volts, this potential being determined by the voltage divider network consisting of the resistors R30, R3I and R32, being the potential of the terminal I 26. Since the oathode is maintained at a fixed potential of +75 volts, the effective bias on the grid is '7 volts. With this 7 volt bias, the-amplification factor of the remote cutofi tubes H3, H4 would be at a maximum if the screens were not negatively biased.

As previously described, depression of a key, such as the key F in Fig. 13, results first in the opening of switches 60 and El, the opening of the switch 00 thereby tuning the oscillator to the pitch of the note F. The opening of the switch 6i permits the potential of the center tap I23 to drop'toward the potential of the terminal I32 through the closed switch 62. The rate at which this change in potential on the center tap I23 occurs is controlled by the time constant of the mesh consisting of condenser CIS and resistor R36.

After a time interval determined by the velocity of key depression, the switches 52 and 63 are opened. Upon opening of the switch 62 the change of potential of the center tap I23 toward the potential of the terminal I32 ceases. Opening the switch 53 results in the screen potential of the terminal I22 rising to its normal operating value of approximately 175 volts. thus velocity with which the pedal key F is depressed. After the key has been fully depressed, further reduction in the control grid bias occurs through the discharge of condenser CIB through resistance R38 which is connected to ground through closed switch I34. As this condenser slowly discharges to ground, the effect of a slowly decaying percussion tone is produced.

The level to which the tone finally decays is determined by the precise adjustment of the screen voltage on tubes II3, H4. This adjustient varies depending upon the particular tube characteristics, and is preferably made adjustable in the form of a variable resistor R21. For a given pair of tubes, this adjustment is made by holding down a pedal key until the tone is no longer decaying in amplitude, and then adjusting R21 to the desired minimum volume level.

Release of the key, closing switch 63, causes the screen voltage at the terminal I22 to drop from its full operating potential towards ground potential at a rate determined principally by the time constant of CI4 and R28. Closure of the switch 02 results in reconnecting the center tap I23 to terminal I32 which does not appreciably affect the functioning of the tubes H3, H4. As the key moves further upwardly, the switches 80 and SI will be closed. Closure of the switch 6| results in connecting the center tap I23 to the terminal I26 which is at +68 volts, so that the condenser CIS is again charged to its full potential, which represents the minimum '7 volt control grid bias. At this time, however, the tubes are already cut of! and consequently no sound is heard in the speaker coincidental with the closure of switch GI. Closure of switch 60 causes the oscillator to be tuned to its lowest C pitch (32.703 C. P. S.). This change in oscillator frequency, however, is not heard in the speaker as the screens oi the control tubes have already cut off plate current.

Under the above assumed conditions of operation when the switch I34 is closed and the switch I30 is connected to the terminal I32, tone intensity envelopes represented by the curves of Fig. 7 may be produced. The curve 1H represents the envelope when the key is struck with a relatively heavy blow and consequently moves at high velocity, the curve IL represents the envelope when the key is struck with a light blow and moves at low velocity, while the curve IM represents the envelope when the key is struck with a medium blow and moves at medium velocity. The curves of Fig. 7 show that the tones produced may be made similar in intensity envelope to those characteristic oi piano tones.

Upon release of the key it will be noted that the tone decays very rapidly, in a manner similar to the decay oi. a piano tone upon release of the key when the damper again engages the string. The mass of the keys 20 is so related to the stiffness of their supporting springs 23 that they will return to normal position at a predetermined speed when they are released. This speed is such that the time interval between the closure of switches 62 and 03 and the closure of switches 34 and BI is in the order of .020 second. This interval of time permits substantially transientless decay rateso! the tones.

In Fig. 7, the distance between the peaks of the curves II-I and 'IL is a measure of the degree of touch responsiveness of the instrument, since is maintained at a potential slightly above ground (ground being the bias which causes the lowest amount of plate current to flow), and corresponds to the minimum volume of tone which is obtained upon depression of a pedal at an indefinitely low velocity. The difference between the maximum intensity peaks of the curve 1H and IL may be in the order of 26 db., which is approximately the dependable dynamic range or a piano and is suflicient for all musical expression purposes.

When it is desired to decrease the touch responsiveness of the instrument so as to make it unnecessary for the player to strike the pedals with accurately determined velocity blows, the switch I28 is shifted to make a connection with the terminal I30, the switch I34 of course remaining closed. Under these circumstances key strokes of the same velocities which previously produced the intensity envelopes represented by the curves 1H, 'IM and IL will produce tone intensity envelopes represented by the curves 8H,

M, 8L respectively. It will be noted here that the db., range between the maximum intensities of the 8H and BL curves is considerably reduced and may be in the order of 8 db.

This reduction in the deg'ree of touch responsiveness is due to the fact that the voltage towards which the condenser CH5 changes'upon opening of the switch BI is more nearly the same as that of terminal I26, and therefore the rate at which the potential drops on the condenser CIB is not so rapid. Also, the minimum intensity of the tone produced upon a low velocity blow is considerably greater when the switch I28 connects to terminal I30, because the potential at the terminal I30 produces a grid bias corresponding to relatively high plate current and correspondingly high level acoustic output. It is to be noted, however, that the potential towards which the condenser CI8 decays after both switches BI and 62 are open, is the same as before, and therefore the tones decay to the same level as that of the fully touch responsive conditions illustrated in Fig. 7.

In the rendition, of certain musical selections, especially when the instrument is used to provide orchestral accompaniment, the tones may be of uniform relatively high intensity, and it is not essential for the player to be able to control the intensity by the velocity at which he depresses the key. When the instrument is used under such conditions, the player will move the switch I28 to an intermediate position in which it does not make contact with either of the terminals I30 or I32. Under these circumstances, the tone intensity envelope produced will be of the character illustrated in Fig. 9. This figure shows that the envelope produced is identical for heavy,

' medium and light key blows, the curves for such different intensity blows coinciding, and being marked 01!, 0M and 0L, to illustrate this fact;

The reason why the tone intensity produced is independent of key velocity lies in the fact that the connection to switch 02 is open circuited.

Therefore, after opening switch 6|, no reduction in potential across the condenser CIG occurs other than the slow drift toward ground due to the slow discharge through resistor R38. Thus, the charge on condenser CI6 is substantially the same regardless of the velocity with which the key is depressed. Under these conditions, the player need not depress the key with a heavy blow to obtain a powerful tone, but need merely depress the pedal so as to open the contact 03 in much the same manner as an organist depresses the pedals on an organ. In both cases the action is non-touch responsive. As was previously mentioned, this operating condition is most useful when playing with a relatively large orchestra in which the piano is called upon to play at a high level for relatively long periods of time. If the volume produced is too loud under these conditions, the player may reduce this volume by operation of the variable resistor R which controls the over-all gain of the system. This is accomplished by rotating the dial 26 by the player's foot.

Figs. 10, 11 and 12 show the intensity envelopes of tones produced when the switch I34 is open, so that the tones are sustained, that is, retain their initial intensity level as long as the key is held depressed. In Fig. 10, the curves illustrate the envelopes produced when the switch I34 is open and the switch I28 is connected to the terminal I32.

The operation of the circuit is identical with i that described in connection with Fig. '7 with the exception that after both switches 6i and 63 are opened, the charge on condenser CI6 is maintained because there is no path by which this condenser may be discharged except through minor leakage paths between the wiring and between the tube elements. In practice, these leakage paths are negligible in effect. The gridsli II and H2 of tubes H3 and N4 of course are alw ys biased negatively with respect to the cathode arid thus draw no current.

The conditions of sustained full touch response, as represented in Fig. 10, are particularly useful when the instrument and piano are used with an orchestra of sustained tone instruments, in which case it is highly desirable for the bass notes to be sustained to provide adequate support for the other orchestral sustained tone instruments.

Making the instrument touch responsive adds greatly to the artistic merit of the instrument because delicate shades of nuance and expres-- sion produced by the other orchestral instruments may be perfectly accompanied by corresponding shades of nuance, and through artistic playing on the instrument, make it possible to produce subtle variations in intensity levels, all for the ultimate purpose of enabling the player to express his musical feelings with a high degree of artistic fidelity.

It will be noted that in Fig. 10, the difference in maximum intensity between the curves ML and MK is again in the order of 26 db., which provides a sufiicient range of musical expression for all purposes.

It is sometimes desirable that the variation in intensity be less directly responsive to the velocity of key depression, and such' results are obtained by shifting the switch 28 to a position in which it makes contact with the terminal I30. The tone intensity envelopes will then be represented by the curves HH, HM, and HL. Under these conditions, the operation corresponds to the conditions previously described with reference to Fig. 8, except that the tones may be indefinitely sustained by holding the keys depressed. This adjustment is particularly useful to players who have not fully developed the technique of depressing the keys at predetermined controllable velocities.

When the instrument is to be used to accompany a full orchestra playing in a large auditorium and rendering music of a sustained organlike character, the switch 28 is adjusted to its intermediate position where it does not make contact with either of its terminals I30 or I32. This leaves the switch 62 open circuited so that operation of this switch does not have any effect whatsoever upon the charge of the condenser CIS and hence does not aifect the tone intensity envelope. As a result, the tone intensity envelopes for high, medium, and low velocity key strokes will'be identical, shown by the coincidence of the curves I 2H, HM and ML of Fig. 12.

From the foregoing it will appear that the control tubes I I3, I I4 have the multiple functions of controlling the absolute amplitude, and also controlling the amplitude envelope of the transmitted signal. This control is effected by circuits associated with different electrodes of these tubes, each of which is efiective to change the gain of these tubes. In the circuits illustrated, the change in amplitude of the signal transmitted is controlled by circuits determining the control grid bias and screen grid bias. It will be understood, however, that these control circuits could be effective to operate upon the suppressor grid bias, upon the plate current supply, or upon the cathode heater or filament current, as well as upon the control and screen grids as described. Furthermore, it will be evident that it is not essential that there be two control tubes, but that a single control tube could be used. However, the use of two control tubes in push-pull relationship is desirable as providing an additional means for the suppression of unmodulated plate current transients due to completion and breaking of the circuits controlling the potentials on the tube elements.

It is to be noted that when the control grid bias is made relatively high by relatively slow depression of the pedal, to play a soft note, the unmodulated plate current transient due to changing the potential on the screen grids of tubes H3, I will be correspondingly small. In order more clearly to describe this phenomenon, assume that the oscillator is not operating, but that the pedal key controlled circuits are operating in a normal manner. In this case, upon depression of a pedal key, unavoidable late current transients would be produced by the speaker, due to the impossibility of securing perfect matching of tubes H3, H4. If, under these assumed conditions, the pedal is depressed slowly, the audible transient produced by the speaker will be weak as compared with the transient as heard when the pedal is depressed rapidly. If now the oscillator is started, the signal supplied by the oscillator will very effectively mask the small transient produced by the unavoidable unbalance of tubes H3 and H4. The fortuitous condition prevails that when the control grid bias is very large, the amount of plate current turned on through changin the potential 011 the screen grid is very small, and the transient occurring at low volume levels is proportionately small.

It will be noted that the pedal clavier is raised a minimum distance above the floor so that the keys thereof may be conveniently operated by the player. The spacing between the keys is preferably the standard spacing used in organ pedal claviers. However, both the natural and sharp keys are much shorter than customary so that when the pedal clavier is used in conjunction with an upright iano, it may readily be positioned between the front board of the piano and the piano bench, with the latter in its proper position with respect to the piano keyboard. The pedal clavier structure is not claimed herein but forms the subject matter of the copending application of Laurens Hammond and John M. Hanert, Serial No. 497,022, filed August 2, 1943.

While I have shownand described a particular embodiment of my invention, it will be apparent to those skilled in the art that the invention may be embodied in a variety of forms. I therefore desire, by the following claims, to include within the scope of my invention all such variations and modifications by which substantially the results of the invention may be obtained through the use of substantially the sameor equivalent means.

I claim:

1. In an electrical musical instrument, means for controlling the transmission of the tone signal comprising, a pair of electron discharge devices of the remote cutoff type, each having a control grid, a cathode, a second grid, and a plate; a circuit impressing the tone signal on the control grids of said devices in push-pull manner; a condenser connecting the cathodes of said devices to the control grids thereof; a first circuit normally maintaining a direct current potential on said control grids of such value that said devices are substantially cut off; a normally ineffective second circuit for increasing the direct current potential on said control grids; a high impedance third circuit capable of increasing the direct current potential on said second grids to their normal operating voltage; a low impedance fourth circuit normally rendering said third circuit ineffective and maintaining said second grids at direct current potentials of value such that said devices are cut off; three normally closed switches respectively in said first, second and fourth circuits, a lplaying key operable to open the switch in said first circuit and after an interval determined by its velocity of operation to open the switches in said second and fourth circuits.

2. In an electrical musical instrument, means for controlling the transmission of the tone signal comprising, an electron discharge device of the remote cutoif type, having a control grid, a cathode, a second grid, and plate; a circuit impressing the tone signal on the control grid of said device; a condenser connecting the cathode of said device to the control grid thereof; a first circuit normally maintaining a direct current potential on said control grid of such value that said device is substantially cut oif; a normally inefi'ective second circuit for increasing the direct current potential on said control grid; a high impedance third circuit capable of increasing the direct current potential on said second grid to its normal operating voltage; a low impedance fourth circuit normally rendering said third circuit ineffective and maintaining said second grid at a direct current potentialof value such that said device is cut off; normally closed switches respectively in said first, second and fourth circuits; a playing key operable to open the switch in said first circuit and after an interval determined by its velocity of operation'to open the switches in said second and fourth circuits.

3. In an electrical signal amplitude envelope controlling apparatus for musical instruments, the combination of an electron discharge device having an input circuit for connection to a signal source and an output circuit for connection to a means for utilizing the signal, said device having electrodes the direct current potentials of which determine the gain of said device, a plurality of signal controlling keys, means common to all of said keys including a resistance-capacitance mesh responsive to the velocity of operation of any one of said keys for determining the potential on one of said electrodes thereby to control the maximum amplitude of the signal transmitted to the output circuit of said device, and a circuit including a resistance-capacitance mesh and controlled by said key for determining the direct current potential on one of the electrodes of said device to control the attack and final decay portions of the intensity envelope of the signal transmitted to the output circuit of said device.

4. The combination set forth in claim -3, in which said electron discharge device is of the variable mu type having a control grid and a second grid, and in which the key velocity responsive means operates to control the potential on said control grid while the attack and decay controlling circuit operates on said second grid.

5. The combination set forth in claim 3, in which an additional optionally completed circuit is provided to cause a gradual change in the potential on the amplitude controlling electrode, thereby to cause gradual decay of the signal between the attack and final decay portions of the intensity envelope of the transmitted signal.

6. In an electrical musical instrument, the combination of a musical tone frequency oscillator, a control tube coupled to said oscillator to receive the signal output thereof, a .playing key, means operated thereby to control the frequency of oscillation of said oscillator, a control tube having a control grid and an additional grid, said tube being connected to receive the oscillator output, a pair of switches sequentially operated by said key. a circuit controlled by the first operated switch to cause changes in the potential on said control grid in a direction to decrease the gain of said tube at a predetermined rate, and a circuit controlled by the second operated switch to change the potential on the other of said gridsin a direction to increase the gain of said tube.

7.,In an electrical musical instrument having an output system including lectroacoustic translating means, the combination of a single generator of electrical impulses capable'of being adjusted to produce any one of a plurality of musical note frequencies, a plurality of playing keys, means controlled by said keys to adjust said generator for the production of a frequency corresponding to an operated key, an electron discharge device forming an electrical impulse transmitting means between said generator and said output system, and gain controlling circuits operatively associated with said device, said circuits being common to all and controllable in part by any one of said keys to determine the maximum amplitude of the output of said device and the attack and final decay characteristics of the tone produced.

8. In an electrical musical instrument having means for amplifying an electrical signal and translating it into sound, the combination of a source of musical frequencies, an electron discharge devic for controlling the transmission of the signal from said source, said electron discharge device having a cathode, control grid, screen grid and plate and having its output coupled to said amplifying means; means to maintain said cathode at a constant direct current potential; a playing key; three switches operable by said playing key at intervals determined by the velocity of key operation and with the first of said switches operated first; a circuit controlled by th first of said switches for causing thepotential upon the control grid of said device to change gradually from substantially a cutoff value to a value at which said device transmits a signal; a circuit controlled by the second of said switches for arresting the gradual change in the potential of said control grid; a circuit normally maintaining said screen grid at a cutoff potential; and means controlled by the operation of the third of said switches for causing the potential on said screen grid to increase to a value at which said device will transmit the signal.

9. In an electrical musical instrument having an amplified and, electroacoustic translating means, the combination of a variable frequency oscillator; circuits for tuning said oscillator foroperation at any one of a plurality of musical tone frequencies; a control tube connected to re ceive the signal produced by said oscillator and having its output coupled to said amplifier; a plurality of keys; four switches operated by each of said keys, said switches being arranged to be operated in a predetermined sequence upon depression of said keys with the first and second switches operating prior to the third and fourth switches; means for connecting the first of said switches in said oscillator tuning circuits; means controlled by the operation of the second switches for initiating a gradual increase in the gain of said control tube; means operated by the third of said switches for arresting the change in gain of said control tube; and means operated by the fourth of said switches for rendering said control tube effective to transmit the signal from said oscillator to said output circuit.

10. In an electrical musical instrument having a single variable frequency generator and an output circuit therefor including a signal transmission control device in combination, a plurality of depressible keys, circuits controlled by said keys for tuning said oscillator to any one of a plurality of musical note frequencies, and means common to all of said keys and responsive to the velocity of depression thereof for determining the maximum amplitude of the signal transmitted by said control device, said last named means including circuits common to all of said keys and comprising switches operable by each of said keys.

11. In an electrical musical instrument having an electrical musical signal generating system and an output system including amplifying and electroacoustic translating means; the combination of a multielectrode electron discharge device for transmitting signals from said generating system to said output system, said electron discharge device having a cathode, control grid, screen grid and plate; a circuit normally maintaining said control grid at a value close to cutoff potential; a circuit normally maintaining said screen grid at cutoff potential; 8. playing key; three switches operable upon depression of said key in a manner such that said first switch is operated prior to the operation of said second and third switches; a circuit controlled by the operation of said first switch for gradually increasing the potential on said grid; a circuit controlled by said second switch for arresting the increase in the potential on said control grid; a circuit controlled by the third switch for increasing the potential of said screen grid to a value rendering said device operative; and high impedance means independent of said key operated switches for decreasing the potential on said control grid.

12. In an electrical musical instrument, a multielectrode electron discharge device for controlling the signal output of said instrument, said device including a cathode, control grid, screen grid and plate, means normally maintaining said control grid and screen grid substantially at cutoff potentials, a key controlled circuit for increasing the potential on said control grid at a predetermined rate, a second key controlled circuit for arresting the change of potential caused by said first key controlled circuit, a third key control circuit for increasing the potential of said screen grid to a value above its cutoff potential,

a playing key, and means responsive to the velocity of operation of said key to render effective said first and said second key controlled circuits at successive time intervals.

13. In an electrical musical instrument having means for generating electrical musical tone frequencies and having an output circuit including electroacoustic translating means, the combination of means for controlling the transmission of the tone frequencies from the generating means to the output circuit comprising a multi-electrode electron discharge device of the variable mu type having at least two grids capable of controlling the amplitude of the signal transmitted therethrough, one of said grids being determinative of the variable mu characteristics of the tube, a circuit associated with said latter grid to determine its potential and thereby control the amplitude of the signal transmitted, and a circuit associated with the other of said grids to control the potential thereof in a manner to determine attack and final decay portions of the amplitude envelope of the transmitted signal.

14. In an electrical musical instrument having an electrical musical tone frequency sourc and an output system including electroacoustic translating means, the combination of an electron discharge device having a plurality of electrodes and an input circuit receiving a signal from said source and having its output connected to said output system; means effective to apply potentials on the electrodes of said device of value to cause said signal to traverse a substantially linear portion of the grid-voltage plate-current characteristic curve of said device; a playing key; a plurality of sequentially effectiv control circuits associated with said playing key, the first of said control circuits including a capacityresistance network and a pair of switches sequentially actuated by said playing key and oper.

ating on said electron discharge device in a. manner to control the electron stream to the output thereof, thereby determining the maximum value of the intensity envelope of the tone produced by the eiectroacoustic translating means; a second control circuit -rendered eflective in timed sequence with respect to the first of said control circuits, said second control circuit including a capacity-resistance network also operating to affect the same electron stream as was affected by the first of said control means; both of said control circuits operating to regulate the gain of said electron discharge device and thereby control the power output of said electroacoustic translating means.

15. In an electrical musical instrument having a single electrical musical tone frequency source capable of providing frequencies of the notes of the musical scale, and an output system including electroacoustlc translating means; the combination of an electron discharge device having a plurality of electrodes and an input circult receiving a signal from said source and having its output connected to said output system; means. effective to apply potentials on th electrodes of said device of value to cause said signal to traverse a. substantially linear portion of the grid-voltage plate-current characteristic curve of said device; a playing key; a circuit operated initially upon depression of said key to determine the note frequency provided by said source; a plurality of sequentially effective control circuits associated with said playing key, the first of said control circuits including a capacityresistance network and a pair of switches sequentially actuated by said playing key at intervals determined by the velocity of key depression and operating on said electron discharge device in a manner to control the gain thereof, thereby determining the maximum value of the intensity envelope of the tone produced by the electroacoustic translating means; the second control circuit being rendered effective in timed sequence with respect to the first of said control circuits, said second control circuit including a capacity-resistance network also operating to affect the gain of said electron discharge device, and thereby to control the power output of said electroacoustic translating means.

16. In an electrical musical instrument having an electrical musical tone frequency source and having an output system including electroacoustic translating means, the combination of an electron discharge device having a plurality of electrodes and having an input circuit receiving a signal from said source and having its output connected to said output system, each of said electrodes being capable of determining the rate of electron flow to the output of said device; a playing key; a circuit including a condenserresistance network controlled by said playing key and operating on at least one of said electrodes in a manner to control the electron stream to the output of said device and thereby determine the attack portion of the intensity envelope in the tone produced by the electroacoustic translating means; and additional means for controlling the amplitude of the signal transmitted by said device, said additional means operating on another of said electrodes to affect the electron stream to the output of said device for controlling the maximum intensity of the acoustic output of said electroacoustic translating means.

17. In an electrical musical instrument having a ource of electrical musical tone frequencies and having an output circuit including electroacoustic translating means, the combination of a muitielectrode electron discharge device of the variable mu type having at least two grids capable of controlling the gain thereof, said device being connected to form a signal transmission path between said source and said translating means, a playing key, and a plurality of circuits controlled by operation of said key, said circuits being effective respectively to determine the potentials on said grids between values preventing substantial signal current fiow through said device and values which cause said device to contranslating means, the combination of a pair of electron discharge devices connected to receive the push-pull output of the generator, output circuits for said devices including a common terminal, and means to change the output of one of said devices without affecting the output of the other, whereby the signal at said terminal may selectively include either the fundamental frequency and harmonics of said generator frequency, or a frequency the fundamental of which is twice that of said generator.

20. In an electrical musical instrument, the

next higher octave frequency to the oscillator fre-' quency.

23. The combination set forth in claim 21 in which manually operable means are provided to change the degree of rectification of said rectifying means, thereby to change the quality of the tones produced.

24., The combination set forth in claim 21 in which said rectifying means comprises an electron discharge device having a screen grid, and manually operated means are provided to vary the potential on said grid, thereby to control the degree of rectification of said device and thus affect the quality of the tone signal output thereof.

25. In an electrical musical instrument havin an output system including a terminal the direct current potential of which determines the amplitude of the sound signal transmitted thereby, means to control the potential on said terminal comprising a first potential source of value such that when applied to said terminal it will cause transmission of the signal at substantially maximum amplitude, a second potential source of value such that when applied to said terminal the signal will be transmitted at substantially minimum amplitude, a relatively low impedance connection including a first manually operable switch connecting said first potential source to said terminal, a relatively high impedanc connection including a second manually operable switch and connecting said second potential source to said terminal, a condenser connected between said terminal and a point of relatively fixed direct current potential, and means for opening said first and second switches in timed sequence responsive to the velocity of operation of the means.

combination of a push-pull master oscillator,

v melody type having an output system including electroacoustic translating means, the combination of a variable frequency oscillator having push-pull output, a keyboard, key operated means for tuning said variable frequency oscillator to musical frequencies corresponding to the note frequencies of said keys, and push-push non-linear rectifying means transmitting the output of said variable frequency oscillator to said output system the non-linear'characteristics of said rectifying means resulting in the introduction of musically desirable harmonics enriching the signals transmitted thereby to said output system, said rectifying means causing'substantial cancellation of the odd harmonic partials in the tone produced by said electroacoustic translating means.

22. The combination set forth in claim 21 in which a manually operable control is provided to adjust said rectifying means to produce doubling of the oscillator frequency or the addition of the.

26. The combination set forth in claim 25 in which the output system includes a multi-electrode electron discharge device having a negatively biased control grid and a connection between said control grid and said terminal, whereby said grid does not appreciably discharge said condenser and affect the potential of said terminal with the passage of time.

2'1. In an electrical musical instrument having an output system including aterminal the direct current potential of which determines the amplitude of the sound signal transmitted thereby, means to control the potential on said terminal comprising a first potential source of value such that when applied to said terminal it will cause transmission of the signal at substantially maximum amplitude, a second potential source of value such that when applied to said terminal the signal will be transmitted at substantially minimum amplitude, a relatively low impedance connection including a first manually operable switch connecting said first potential source to said terminal, a relatively high impedance connection including a second manually operable switch and connecting said second potential source to said terminal, a condenser connected between said terminal and a point of relatively fixed direct current potential, a circuit including a third switch for paralyzing a part of said output system at a point at which there appears the signal controlled by the potential of said terminal, and a key for operating said first and second switches in timed sequence responsive to the velocity of operation of the key and operating said third switch after said first switch has been operated.

28. In an electrical musical instrument having a musical tone frequency generating system, an electroacoustic translating means, and a signal transmission system for coupling said generating system to said translating means, the combination of a plurality of playing keys, a variable gain electron discharge device forming a part of said transmission system and being common to all of said playing keys, said device having an electrode the direct current potential of which determines the gain of the device, a potential storing condenser coupled to said electrode, a first circuit including a potential source for maintaining a predetermined potential on said condenser, the potential being of such value that when applied to said electrode it causes said device to operate at high gain, a second circuit for changing the potential on said condenser at a controlled rate toward a value which applied to said electrode causes said device to operate at low gain, and means operable by any one of said keys to render said circuits sequentially ineffective at a rate which is a function of the velocity of key operation.

29. The combination set forth in claim 28 in which each of said keys is provided with a pair 0! sequentially operated switches, in which the first operated switches of said pairs are connected in series in said first circuit, and in which the subsequently operated switches of said pairs are connected in series in said second circuit.

30. In an electrical musical instrument having a musical tone frequency generating system, a signal transmission system, and an electroacoustic translating means, the combination of a plurality of playing keys, a variable gain electron discharge device forming part of said transmission system and having an electrode the direct current potential of which determines the gain oi the device, a potential storing condenser coupled to said electrode, a first circuit for maintaining a predetermined potential on said condenser corresponding to high gain of said device, a second circuit for changing the potential on said condenser at a controlled rate toward a value corresponding to low gain of said device, and means operable by any one of said keys to render said circuits ineffective one after the other during an interval which is a function of the velocity of the key operation.

JOHN M. HANER'I'.

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