US2233258A - Electrical musical instrument - Google Patents

Description

Feb. 25, 1941. L HAMMOND ETAL ELECTRICAL MUSICAL INSTRUMENT 3 Shuts-Sheet 1 Filed Se t. 5, 1939- [/2 were fo /*5 La uf'ens' Hammond a'Jok/%1Zanerz" M 1% w. uqli 2 2 c w o I 7 M v LM 2 M 0W 3 Feb. 25, 1941. L HAMMOND ETAL 2,233,258.

ELECTRICAL MUSICALYINSTBUMBNT Filed Sept. 5, 19:59 a sheets-sheet a QM E 17: 65'

j/OO 7 [rm ere zors' a arena/Yam? on d e. Jo/m Mf/anerz" Patented Feb. 25, 1941 UNITED STATES ELECTRICAL MUSICAL INSTRUMENT Laurens Hammond, Chicago, and John M. Hanert,

Wilmette, 111.; said Hanert assignor to Hammond Instrument Company, Chicago, 111., a corporationot Delaware Application September 5, m. Serial No. 203,444

23 Claims.

Our invention relates generally to electrical musical instruments and more particularly to musical instruments of the melody type, and constitutes an improvement on the instrument disclosed in the application of John M. Hanert,

Serial No. 274,325, flied May 18, 1939.

It is an object of our invention to provide an improved melody type instrument having a keyboard of a range oi several octaves by the use 01 which any melody may be played.

A further object is to provide an improved electrical musical instrument of the melody type employing a plurality of generators, and utilizing an improved keying system whereby undesirable j transients are suppressed both when playing legato and non-legato.

A further object is to provide a keying system whereby the touch of the player exercises a control over the envelope of the tone.

A further object is to provide an improved electrical musical instrument of the melody type having a plurality of frequency generators, a keyboard 0! a range greater than that of any one of the generators, and key circuits controlled by the keys for selectively keying the signal from one or more of the oscillation generators.

A further object is to'- provide an improved keying system for an electrical musical instrument in which relays are employed and in which the current flow through the winding of the relay has' an effect upon the amplification of the output of the instrument independent of the completion of circuits by the relay.

A further object is to provide an electrical 35 musical instrument having a variable gain electron discharge tube amplifier in its output, in

which the key completed circuits energize relays for causing the transmission to the output oi the desired signal, and in which the energization 40 of the relay, independent. of the switch contacts operated thereby, controls the gain of said electron discharge device.

A further object is to provide an electrical musical instrument employing an oscillator, controlling the irequencies produced by the instrument, with an improved vibrato producing means. v

A further object is'to provide an oscillator having improved means for periodically shifting its frequency between selected predetermined limits, in which the mean frequency remains constant, irrespective of which limits are selected.

A further object is to provide an improved relay keying system in connection with a keyboard,

all the keys of which are free to be depressed at all times, without, requiring the use of mehaving no associated mechanically operated switches common to a plurality of said keys, to retard the maximum speed of action on said keys.

A further object is to provide an improved relay keying system for use with a keyboard in which no serious restrictions are imposed on the player concerning the number of keys which may be simultaneously depressed in playing a melody such as, for instance, two keys being momentarily simultaneously depressed in playing from one to the other or said keys.

A further object is to provide an improved keying system for an electrical musical instrument in which the time delay between the initial current flow through the relay energizing coil and the subsequent operation of contacts associated with the armature of said relay is utilized in a manner to eliminate undesirable transients from the output of the instrument.

A further object is to provide an improved keying system for electrical musical instruments in which no substantial time delay occurs between breaking a contact in series with a relay winding, and a change in voltage occurring in a circuit associated with the relay winding.

A further object is to provide an improved keying system for electrical musical instruments in which simplification and reduction of cost is efiected by providing a relay in which one function associated with the energization of the relay is accomplished by taking advantage of a change in'voltage occurring in a circuit associated with the winding of the relay.

A further object is to provide an improved relay keying system for an electrical musical instrument in which there is no period of silence occurring when two keys are simultaneously depressed in adjacent selected groups of keys.

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

Figure 1 is a perspective view of the instrument shown attached to a piano;

Figure 2 is a wiring diagram of the power supply system for the instrument; and

Figures 3 and 3a together constitute a wiring diagram of the instrument.

General Description As shown in Figure 1, the keyboard for the instrument comprises three octaves of the customary piano keyboard, the keys being designated Ci to B8 inclusive. if The actual pitches of the tones produced upon depression of these keys may be varied as desired by register changing switches such that the notes produced upon depression of the keys may be selectively in the bass, tenor, or treble registers. As a result, the keyboard may, by the use of four such register controlling switches, be made to sound any note within a six octave range,

In general, the instrument comprises a single master oscillator l0 which is connected to control a series of six relaxation oscillators II to I! inclusive, which are connected in cascade and have their frequencies of oscillation stabilized by a signal derived from the master oscillator ll. Instead of the relaxation oscillators disclosed herein, any other suitable type of oscillator whose frequency may be readily stabilized such as gas tube oscillators, multivibrators and the like. might be utilized. The frequency of oscillation of the master oscillator It may be periodically shifted at a vibrato rate by means of a vibrato mechanism 20, and this frequency shift of the master oscillator is of course proportionally present in the frequencies of oscillation of the controlled oscillators i! to H inclusive. I

Each of the keys Ci to B3 controls the performance of several functions:

(a) It selects a tuning circuit for the master oscillator l0, and at the same time selects the appropriate grid bias potential for the controlled oscillators ii to IT inclusive, so that these controlled oscillators are conditioned readily to be stabilized by the frequency of the master oscillator;

.(b) Thereafter, it closes a,contact which energizes a relay, the energization of which starts to turn the amplifier on with a predetermined rate of attack; a

(b) The energization of the relay causes completion of circuits which render the preselected tuning circuits effective, keys the signal from the appropriate oscillator to the amplifier, and causes a circuit to be broken which renders ineflective the depression of another key in an adjoining octave group, which may -be depressed at the same time.

The instrument also includes a selectively operable switching mechanism 24 for determining the register in which the instrument is played, and for providing musical effects in which one or more octaves of the note may be simultaneously sounded. The output of the instrument is supplied to an amplifier 28, volume control 21, and

electroacoustio translating means. Power for operation of the various circuits is supplied by a power pack shown in Fig. 2.

Frequency Generating System a condenser C4.

pending upon the number of these condenser r ps which are connected in series, tune the oscillator II to one of the semi-tone notes of an octave. The frequencies thus generated constitute the semi-tones of the highest octave of frequencies available in the instrument which are 2093 to 3951 c.p.s.

Bias to tube ll is supplied by means of grid leak RI which is in parallel with a grid condenser Cgl. B creen bias to tube 3| is supplied through voltage dropping resistor R2 and filter condenser Csl. Plate load on tube II comprises resistor R3, in series with winding II of oscillation transformer 32, and resistance R4 which is connected to filter condenser Cpi and filter resistance R5 to a terminal 300V of the power supply, at approximately 300 volts potential. The suppressor grid of tube 30 is connected to the cathode.

The output of the oscillator Iii is coupled to the grid circuit of the controlled oscillator i2 through The oscillator i2 comprises a triode 38 of the 6.156 type. the grid circuit of which includes a protective resistor R6, a secondary winding I of an oscillation transformer 42, and grid resistor R1, which is connected to ground. The cathode of the tube 3| is connected to a terminal HIV in the power supply system which is at a potential of 10 volts positive with respect to ground, there thus being 10 volts negative bias on the grid of tube ll.

Current is supplied to the plate of tube 38 through a conductor 44, which is connected to terminal 280V of the power supply system, through a timing resistor R8. The rate of relaxation of the oscillator I2 is controlled by the relative values of the timing resistor R8 and the capacitance of a timing condenser C! which is in series with condenser C6 through the primary winding 4' of the transformer 42.

An output signal is derived from the oscillator l2 through a conductor I which is connected to a point intermediate the condenser CO and the secondary winding N. The output of the oscillator l2 appearing across the condenser Cl will be generally of saw-tooth shape, thus comprising a fundamental frequency with a long series of harmonic over-tones of progressively decreasing amplitude. The value of condenser CC is large relative to that of Cl, and thus does not have an appreciable eflect upon the rate of relaxation of the oscillator i2- The constants of the oscillator circuits i2- and the potentials supplied thereto, are such that the relaxations occur at the same frequency as the master oscillator II. In other words, the signal derived from the master oscillator I0 and impressed upon the grid of the tube 38 through the coupling condenser Cl serves to trip the relaxation oscillator for each positive impulse received.

The transformer 42 has a tertiary winding 5|! which is shunted by a resistance R9, of a value in the order of approximately ohms. The secondary ll may have approximately one half, while the tertiary winding SI may have approximately one eighth as many turns as the primary 46. The resistor R9 shunted across the tertiary winding is reflected into the plate circuit of the tube 3! as a load impedance.

The oscillator I 3 may be substantially identical with the oscillator i2 except that it will be noted that it is coupled to the oscillator I! through an adjustable tap 52 on the resistance R9. The adjustment of the tap I! will be such adjustment i made when the instrument is assembled and willnot ordinarily 'need to be readjusted. The biasing potential for the oscillator I3 is supplied from a conductor I4 through a filter resistor RIO. A filter condenser C8 has one terminal connected to a fixed potential terminal IV of the power supply, and its other terminal connected to the tertiary winding I8 of transformer 42. The potential on the conductor 84 is varied depending upon the key depressed, as will appear more fully hereinafter, so that the oscillator i8 will relax at one half the rate of oscillators i8 and I2, regardless of the frequency to which the master oscillator I is tuned.

The relaxation oscillator i4 is similar to oscillator l8, except for differences in the values of the timing resistors R8 and condensers C5, and in the number of turns in the windings of its transformer 42. The relaxation oscillator i4 also diiler from the oscillator I8 by the inclusion of a small inductance Li in series with the primary winding 46 of the transformer 42 associated with the oscillator i4. This small inductance serves to remove musically undesirable high frequencies in the generally saw-tooth wave shape occurring across its condenser C8.

The oscillators i5, i8 and I! are similar to the oscillator l4, being similarly connected in cascade, and having circuit elements of such values that they relax at frequenciesin octave relationship. It will'be noted, however, that transformer 42 associated with the relaxation oscillator I! does not have a tertiary winding but instead, has a high resistance RH shunted across its primary winding to provide the plate load corresponding to that provided by the resistance R8 for the relaxation oscillator II.

The bias potential'upon the conductor 54, in order to make certain that the relaxation oscillators i8 to II inclusive safely follow changes in frequency of the relaxation oscillator i2, must be varied as the frequency of the master oscillator in and of the oscillator I2 is varied. The conductor 54 is connected to a point intermediate resistors RiZ and Rl3 which together with a resistance R form .a voltage divider connected between the terminal 380V of the power supply and ground. Associated with these voltage divider resistors is a further voltage divider resistance system consisting of resistors RCii to RB inclusive, connected in series between Ri2 and ground. The voltage appearing intermediate resistances RH and RI! will be determined by the particular key which is depressed, as will be described hereinafter.

It is to be noted that tuning is accomplished by connecting selected condensers CC to CB to ground, and that such connection likewise determines which of the resistors R08 to RB are effective in the voltage dividing mesh provided by these resistors as well as the resistors Rl2, RI: and R. All of the resistors in this voltage dividing mesh are so chosen that resistors R03 to R3 inclusive do not constitute appreciable load (to alternating current of the master oscillator frequency) upon tuning condensers CCt to CB, and Cct to Cb. In this way, the frequency. of oscillator ii) is determined substantially by the reactive elements of the tuning system and not appreciably by the resistances RC5 to RB, RH, R13, or R, and thus good frequency stability for oscillator i8 is assured, be-' causeof the high Q (sharpness of resonance) of this tuning circuit.

It will be noted that inductances L2 and L8 are inserted between the tuning condensers CD -and CD3, and CF and CH respectively, and that these inductances are wound together so as to form a mutual inductance M between them. These inductances are very small compared to the inductance of oscillation transformer 32, and thus have inappreciable effects upon the tuning of oscillator it. Their function is to suppress the spark incident to tuning the oscillator i0.

It will be noted that when the number of effective condensers CC to GB in the tuning circuit is changed, the circuits by which such changes are made are subjected to the residual D. Q. voltage on such condensers as are shunted out of the circuit by virtue of the bias voltage divider resistors m to BB connected in shunt with them. The inductance L2 and L8 either singly, or both mutually. depending upon which group of condensers is shunted, thus serve as a means to limit the discharge of the condensers to a rate such as will not cause excessive sparking at the tuning contacts.

Vibrato mechanism The frequency of the oscillator III,' and hence of all of :the controlled oscillators I! to I! inclusive may, whenever desired, be periodically shifted throughout a small range, in .the order of 3%, at'a vibrato periodicity of approximately 7 c. p. s. to provide the vibrato in the tones ultimately produced.

The vibrato mechanism comprises a vibratory magnetic metal reed 58 secured to a suitable rigid grounded support 50 and having a'weight 82 adjustably secured thereto. so as to permit variations in the vibrato periodicity. The reed 58 is maintained in vibration by an electro-magnet 64, one end of the winding of which i connected to the terminal llillV of the power supply system. The other end of the winding of the electromagnet 84 'is connected to a contact 66 which makes contact with the reed during the upper portion of its vibratory oscillation, and since the reed is grounded at the support 60, current flows through the coil of the electro-magnet 64 and attracts the reed 58, whereupon its contact with the contact 66 is broken.

A second contact 68 is provided for engagement with the reed 58 and is thus periodically grounded through the reed at the rate of vibration of the reed. This contact 88 is connected to single-pole double- throw switches 10 and 12 by which the contact 88 may be made to interrupt the circuit through a coil 14 of relatively high inductance, or through a coil 18 of lesser inductance. The coils "I4 and 16 are connected in parallel with approximately 13% of the turns of the primary 34 of transformer 32 at a tap l8.

When either of the switches I0 or I2 is in its oif position, the inductance I4 or I8 associated therewith is connected to a tap 88 on the primary 84 so as to cause the oscillator ill-to oscillate at a frequency which is the mean of the frequencies at which it oscillates when the inductance 14 (or 16) is periodically connected between the tap l8 and ground by the contact 68. The number of turns between ground and tap I8 is 29.1% of the number of turns between ground to tap 80. The switches 18 and 12 are manually operable, and may be used singly or together, to provide three different degrees of vibrato. The taps I8 Key circuits As previously stated. and as shown in Fig. i, the instrument is composed of a plurality of octaves of keys CI to B3 preferably of the general arrangement utilized in piano and organ keyboards, and is herein illustrated. as having a gamut of three octaves, although the keyboard could be reduced or extended to any desired length. Since the instrument is of the melody type in which more than two keys are seldom, if ever, simultaneously depressed intentionally, the keys CI to B3 may be very much shorter than the standard piano keys. By virtue of the size and positioning of the keys they may readily be depressed simultaneously with the depression of the piano keys.

The keys CI to B3 are mounted in a suitable keyboard frame III which may be conveniently attached to the front rail III of the keyboard of a standard piano, organ, or similar instrument. The frame ill carries a plurality of pivoted tablets it: which are utilized for the manual operation of the various control switches,such as the vibrato switches 10, I2, and the various other switches, described hereinafter, whereby the tone quality is varied, the rate of attack of the tones controlled, and the instrument otherwise conditioned to produce tones of the desired characteristics.

Above the tablets Ill is a channel-shaped guide I to receive blocks ll! bearing indicia identitying the tablets I83 as to the nature of the controls effected thereby.

It will be noted that the keys CI to B8 are spaced to register with the corresponding piano keys, but are very much shorter than the piano keys. The two sets of keys are closely adjacent one another so that the musician may conveniently play simultaneoysly on both keyboards with one hand.

While the type 01 keys and keyboard disclosed herein is particularly well adapted tor the control of a melody instrument, any other suitable keyboard may be used. It should be noted, however, that when a piano type of keyboard is used, the black keys should be made narrower than usual so that a white key located between two adjacent black keys may easily be depressed without depressing either of the black keys.

Each of the keys CI to At! (except keys BI and B2), operates two switches in sequence. The switches which are closed first upon depression of these keys are illustrated in Fig. 3 as contacts C In to Alla respectively, while the contacts which are subsequently made by keys CI to B3 are designated Clb .to Bib respectively. The contacts Old to Mia are adapted to make contact with a bus bar 8|, while the contacts Cla to Aila are adapted to make contact with bus bar I2, and similarly, the contacts Clo to Mia are adapted to make contact with a bus bar 83. All of the second made contacts of each of the-keys, i. e., .the contacts Clb. to Blb are adapted, upon depression of the key, to contact with a grounded bus bar It.

The contacts Cla to Aiila are connected respectively in parallel with the contacts Cla to Mia and C3a to A330, and respectively to the corresponding terminals of the condensers CC to CM. When no key is depressed, the oscillator II is tuned to oscillate at the frequency of the note B (3951 c. p. s.) as determined by the capacitance of all of the tuning condensers taken in series. Therefore, when depressing a B key, it is not necessary to provide tuning contacts for making connection with the buses ll, I2 and 83. However,

it is necessary, for reasons which will appear for the second octave are connected together by a conductor 80, and those for the third octave of keys by a conductor 81.

Associated with the first octave of the keyboard is a relay 8|, having a break contact Ola and two make contacts llb and Ole. A similar relay 82 having a break contact "a and two make contacts 02b and lie is associated with the second octaves of the keys, while a relay 93 having a similar break contact "a and two make contacts 83b and Me, is provided for the third octave of keys. The windings of the relays ll, 92 and 93 each have one end thereof connected to the conductors 85, 88 and .1 respectively. The other end of the winding of relay II is connected through the normally closed contact 92a and conductor III to a conductor Bl which is connected to a positive potential terminal 300V of the power supply through one section R98 of a voltage divider, the other section R88 of which is connected to ground, the section R99 being shunted by a conductor Cll.

Thus, upon depression 0! any of the keys in the first (low) octave, the relay 0| will be energized, providing the relay 92 has not been energized, and its contacts 92a are thus closed. Energlzation of the relay 9| results in the clo sure of contact Slb which connects the bus ii to ground, thus rendering effective the tuning of the oscillator III by the particular condensers taken in series, dependent upon the particular key of the first octave which is depressed. At the same time, the closure of the contacts lb affects the grid bias of the controlled oscillators II to II inclusive, since the grid biasing resistors R08 to RB are connected in parallel with the tuning condensers. Due to the fact that these biasing resistors are in parallel with the tuning condensers, there will be a certain time lag whenever one of the playing keys is depressed, before the biasing potential determined by the depressed key is impressed upon the conductor 54, since a very small fraction of a second is required for the discharge of the tuning condensers through the biasing resistors. This delay is, however, so short as not to be of any significance because the resistors RCt to RB are of very low value, in the order of 5,000 to 15,000 ohms, and thus form a relatively low resistance path for the discharge of these tuning condensers. On the other hand, the biasing resistors RC3 to RB cannot be made of too low value compared to the impedance ofiered by the tuning condensers, as these resistors would then lower the Q of the oscillator tuning circuit.

Energization of the relay 9! also results in the closure of the contacts Ilc, one of which is connected to the output signal conductor I" which is connected to the amplifier as will be more fully described hereinafter. The other con tact of the contacts tic is connected to a con- Gil ductor IOI which is adapted to receive signals from the controlled oscillators I4 to I1 inclusive,

depending upon which oil a plurality oi register the signal output conductors oi the controlled oscillators I2 to I1 inclusive through amplitude controlling and non-robbing resistors RI04a, etc., associated with the switches I04a, etc., respectively.

'There is a conductor I02 tor the second or middle octave of keys, which corresponds to conductor IOI for the first or low octave of keys, and a similar conductor I06 for the third or high octave of keys. When, for example, the register control I04 is moved to the position in which it is shown in Fig. 3, the signal from the controlled oscillator I4 is impressed upon conductor ml, the signal from the oscillator I3 is impressed on the conductor I02, and the signal from the oscillator I2 is impressed upon conductor I03.

Upon operating the register control I05, it will be noted that the conductors IOI, I02 and I03 will receive signals from controlled oscillators each one octave lower than when the register control I04 was operated. In a similar manner, operation of the register controls I06 and I01 will cause signals to be impressed upon the conductors IOI, I02and I99 which are each two and three octaves lowerrespectively, than are connected to these conductors, when the register control I04 is operated. It will be noted that conductor I02 is connected to relay contact 920 while conductor I03 is connected to relay contact 93c.

As previously pointed out, depression of a key in the lower octave, or first octave of keys, will not result in the energization of the relay 9i if the relay 92 is energized. Similarly, the depression of a key in the highest, or third octave of the keyboard, will not result in the energize,-

tion of the relay 93 it the relay 92 is energized,

since it will be noted that one terminal of the winding of relay 93 is connected to the conductor 61, while the other terminal thereof is connected to the break contact 92a which, when relay 92 is energized, is open.

Similarly, relay 92 cannot be energized unless both relays 9| and 93 are de-energized, because its energizing circuit leads from conductor 66 through the winding 01 relay 92 to the normally closed contact 9I a through conductor IIO, through the normally closed contact 93a and 9 hence through conductor II2 to the conductor Amplifier and volume controls The conductor 96 leads to the; amplifier 26,

being connected to the cathodes of an intermediate push-pull stage thereof. The amplifier receives its signal through conductor I which is connected to the grid 01' the first stage amplifying tube illustrated as a triode I20, and which may be of the 6J5G type, through a blocking condenser CI2, the conductor I00 being connected to terminal IOV of the power supply through a load resistor RI4. The grid of the tube I20 is connected to terminal 2V oi the power supply through a grid resistor RIS.

The cathode of tube I20 is connected to terminal NW of the power supply, while its output or plate circuit is connected to the primary or a transformer I22, the other end of the primary being connected to the terminal 250V of the power pack. The secondary winding of the coupling transformer I22 has its terminals connected respectively to the grids of push-pull amplifier tubes I24 and I26, which may be pentodes of the 6K'7G type, having a remote cut off control grid characteristic. The cathodes oi the tubes I24, I26 are connected'to a center tap of the secondary oi transformer I22 through a condenser CI4, the cathodes also being directly connected to the conductor 96.

The center tap on the secondaryof the transformer I22 has connected thereto, through a resistor RI 6, a mesh for expression or volume control. This mesh comprises a condenser CI6 having one terminal grounded, and its other terminal I21 connected to the resistor RI6, and a resistor RI1 which is connected between the condenser CI and a conductor I28. The conductor I28 is connected to terminal'50V of the power pack through a resistor RIB. The conductor I28 is connected to ground through a. resistor Rlll and may, upon operation of the expression'control lever I29, be connected to ground through one or more of resistors R20 to R24, and when the swell pedal is closed (i. e. at lowest volume p0- sition), is connected to ground through a conductor I30.

It will be understood that upon operation of the swell pedal, .the contacts on the resistors R20 to R24, and on the conductor I30, are closed sequentially in the order named, and opened in the reverse order. In so doing, they change the potential uponthe conductor I26 from a maximum of approximately 50 volts to a minimum of ground, or zero potential. The resistors RI8 to R24 inclusive thus form a variable. voltage divider circuit.

Resistor RI1 and condenser CI5 function to limit the rate of change of potential at terminal I21. Resistor RI 1 is in series with the swell pedal resistor mesh, and serves in conjunction with condenser CH, to cause the rate of potential change at the terminal I21 to be approximately uniform as the swell pedal is moved from step to step. The potential at point I21 determines the grid bias applied to the control grids of push-pull tubes I24 and I26. These tubes are of the variable mu or remote cutoil type, and advantage is taken of this feature in providing a. simple swell or volume control system which will operate over a wide range such as 48 db., by varying the grid bias to tubes I24 and I26.

By making gradual the rate at which this grid bias changes, the swell or volume control-may be simplified into a relatively small number of steps, and still secure a wide range of volume without undesirable sudden changes in volume occurring when the swell or volume control lever I I! is moved from one position to another. Thus, if resistors RIO to R24 inclusive are so chosen as to provide successive 8 db. changes in volume, a wide range of volume is provided with only six contacts. CI! is so chosen that volume changes are sufliciently gradual, but yet are not too slow to prevent the volume from substantially following the swell pedal position to produce desired rapid dynamic changes in the musical tone being rendered. The control grid bias on tubes I24 and I28 is determined not only by the swell pedal mesh previously described, but by the potential on the conductor 98 which, as previously described, is determined by the voltage divider mesh consisting of resistors R98 and R99. The

voltage of the conductor 98 is, howe er, not determined solely by the resistors R9! and R99, but is also changed whenever any of the relays 9 I 82 and 93 is connected in shunt with R99 upon depression of any one of the keys.

When none of the keys is depressed, the voltage on the conductor 96 is such as to bias the tubes I24 and I2. beyond cutoff irrespective of the setting of the volume control. When, however, a key is depressed, the voltage on the conductor 88 becomes more positive because of the shunting of the relay-winding impedance across RM, and thus the eilective bias on the grids of tubes I24 and I2! decreases, with resultant increase in the signal output of the tubes I24 and I2.

The rate at which the signal increases in the outputs of tubes I 24 and I26 is determined by the time constant 01' CM and RIG. When a playing key is depressed, the control grids of tubes I24 and I24 remain at cutoil' potential because condenser CI4 is connected to these grids through the transformer secondary and not through a resistor, which might limit the rate of change or potential. However, after the key has been depressed, the transient voltage which was supplied through condenser CI4 will disappear and the bias upon the grids in tubes I24 and I2. will.

CI4, and thus the rate at which the operating.

bias is applied to the control grids oi tubes I24 and I2. will be slower. Switch I32 may be manually operable by the player. when releasing the key, the rate at which tubes I24 and I26 cut oil is determined by condenser CIII which is in parallel with resistor R99. It will be noted that tubes I24 and I2. are connected in pushpull so that changes in the D. C. component 01' the plate current in these tubes does not haveany transient eilect upon their signal output.

The screen grids of the tubes I24 and I2! are connected to a terminal IUOV of the power supply system, while the suppressor grids thereof are connected directly to the cathodes. Plate current is supplied to the tubes from a terminal IIIV through a conductor I34, which is connected to the plates of the tubes I24 and I2 by load resistors R04 and R86 respectively. The signal output of the push-pull amplifiers I24 and I2! is supplied to the grids of the power output tubes I40 and I42 connected in push-pull arrangement through blocking condensers CI! and CIB.

The cathodes of the tubes I40 and I42, which may be pentodes oi the 6M6 type, are sellbiased above ground potential through a common resistor R28, while the grids of the tubes I4! and I42 are connected to ground through grid resistors R30 and R12 respectively. The values of the grid resistors R" and R32 with respect to the values or condensers CH and Cl! respectively should be such that the sudden changes occurring in the potentials on the plates of the tubes I24 and I26 will not result in altering the potential on the grids of tubes I40 and I42 for an appreciable length of time.

The suppressor grids of the tubes I44 and I42 are internally connected to their cathodes, while their screen grids are connected to the terminal 300V of the power supply system. The plates of the tubes I40 and I42 are connected to the terminal 300V through a connection to the mid-point on the primary 0! push-pull output transformer I44. The secondary oi the transiormer I44 is connected to the voice coils of a speaker I48. The field coil 01' the speaker I46 may constitute an inductance in the power supply as will appear hereinafter.

Having described in general the construction 01' the amplifier, it will now be more clearly understandable why the relays SI, .2 and 23 are so connected that energizing one relay makes it impossible to energize adjacent relays by depressing keys in adjacent octave groups of keys.

The reason for providing these mutual interlocking circuits is to eilectivelysuppress undesirable transients which might occur when the player, through a legato style of playing, causes two keys in adjacent octave groups to be simultaneously depressed. 11' these interlocking circuits were not provided, undesired signals or undesired changes in tuning would occur. It will be noted that each time a player plays across from one octave group of keys into an adjacent octave group of keys, the amplifier will momentarily be cut oil when the first mentioned key is released, and then again rendered eiiective when the relay associated with the second mentioned key is energized. Thus, the old signal is turned off with a controlled decay rate and the new signal is turned on with a controlled attack rate, as determined by the condenser Cll, resistors R99 and RI. and condenser CI4. It will be noted that each of the relays II, 4! and N is interlocked with its adjacent relay; but that the relays 9| and I! are not interlocked and it is therefore possible, it the player should depress keys in both the lowest and the highest octave, to energize both relays II and I3. Such energi.

zation of these relays will cause the potentials on the cathod of tubes I24 and I2. to become very positive, causing tubes I24 and I2! to distort the signal. It is not contemplated, however, that the player in the normal use or the instrument shall play legato from a note in the lowest octave to a note in the highest octave.

As previously described, upon depression of a key-for example, CI-its switch CIa will be closed before its switch Clb is closed, and upon release oi the key, the switch CI b will open before the switch Cla opens. By having the biasing potential on tubes I24, I24 controlled by the current flow through the windings of the relays SI, 82 and 93,the bias potential begins to change immediately upon opening of the switch Clb, and will substantially reach cutoff potential at a non-transient producing rate by the time that switch contact Chi is opened, and by the time that the contacts 9| b and 3Ic open.

If the tubes I24, I25 were not biased substantially to cutoff by the time switch CIa is opened, the oscillator I0 would begin changing its frequency to that of the note B. and thus an undesirable pitch change would be heard during the interval between the opening of the switch CIa and the time that the biasing potential on tubes I24 and I 26 reached cutoff.

Tone control system As shown in Figure 3, this tone control system comprises a plurality of resistors R34 to R39 inclusive, a plurality of'inductances LIO to LI3 inclusive, a-plurality of condensers C20 to C23 inclusive, and controlling switches I50 to I55 inclusive. The switches I50 to I55 inclusive operate individually and selectively to permit the signal impressed on conductor I00 to develop across the capacity-inductance resistance mesh associated therewith.

For example, when switch I50 is open, the signal on conductor I00 will have no frequency characteristic impressed upon it because element R34 is a pure resistance. When switch I5I is opened, the signal on conductor I00 will be altered in a manner to emphasize the low frequencies relative to the high frequencies. When switch I52 is opened, the signal on conductor I00 will be altered in a manner to emphasize the high frequencies relative to the low frequencies.

When switch I53 is opened, the signal on conductor I00 will be modified such as to resonate a band of frequencies lying adjacent to the resonant frequency of condenser C2I and inductance LII. The sharpness of resonance is controlled by resistance R31 which is in parallel with C2I and LI I. This resonant frequency may preferably be in the order of 3000 c. p. s. Similarly, when switch I54 is opened, the signal upon conductor I00 will be resonated by condenser C22 and inductance LI2 and R38. The resonant frequency here preferably is in the order of 1000 c. p. s. In a like manner, when switch I55 is opened, a resonant effect in the order of 200 c. p. s. will be impressed upon the signal on conductor I00. It is to be noted that these switches may be opened by the player individually or in any desired combination.

Operation of keying system I Upon depression of a playing key, for example the key G2, the contact G2a is first closed, thus connecting all of the condensers CC and Co to CG and Cg, together with associated biasing resistors RCii to RG to the movable contact arm of relay switch 92b.

However, such connection does not change the tuning of the oscillator I0, since at this instant the relay contact 92b is open. Further depres sion of the key G results in closure of the switch (32b, whereupon a circuit from the grounded bus bar 84 through the switch 62b, conductor 08, winding of relay l2, closed relay switch 9Ia, conductor IIO, closed relay switch 03a, conductor II2, common conductor 38 and voltage divider resistance R30 to power supply terminal 300V is completed. The flow of current through this circuit results in a lowering of the positive voltage across the voltage divider resistance R99, and consequent reduction in the positive control grid bias on tubes I24 and I24, which causes plate current to start to flow through said amplifier tubes. 1

The energization of the relay 92 causes closure of the relay switch 32b, thus connecting the bus 32 to ground and thereby tuning the oscillator I0 to the frequency determined by the previously mentioned condensers, which are thus effectively in the grid tuning circuit of the oscillator tube 30. This grounding of the conductor 32 also results in changing the ground point of the series of biasing resistors RC1! to RB, and thus lowers the biasing potential on the conductor 54 to a' value sufficiently more negative to condition the relaxation oscillators I2 to H to follow. safely the change in frequency of oscillator I0.

It will be understood that the grid bias on the relaxation oscillator tubes 38 is an important factor in determining the rate of relaxation, and that it is necessary to change the rate of relaxation of each of the oscillators I2 to I! through a range of substantially an octave. Changing the grid bias on the tubes 38 of the relaxation oscillator-s affords a simple method of simultaneously changing the frequency range through which the relaxation oscillators I3 to I! may be effective. The exact frequency at which the os clllators I2 to H will operate is determined by the amplitude and frequencies of the signals impressed upon the grids of their tubes. The condenser C4 may be of relatively low value (e. g.

.003 mfd.) thereby to limit the amplitude of the signal impressed upon the grid of the tube 30 forming part of oscillator I2, and to impart a desirable sharply peaked wave shape to this sig- 1311, to improve the stabilization of the oscillator Since the oscillator I2 operates at the same frequency as the oscillator I0, the coupling afforded by the condenser C4 is not very critical and in practice need not be made adjustable. The amplitude of the signals impressed upon the grids of the remaining relaxation oscillators I3 to I! is adjustable during the construction of the instrument, by-adiustment of the potentiometers R3 having the movable contacts 52. In practice, this adjustment is made such that equal increments of bias may be added and subtracted from that impressed upon the conductor 54 without causing loss of the frequency stabilization control, thus conditioning the relaxation oscillator for the greatest safety as regards following the frequency changes of the preceding oscillator, and maintaining exact frequency division by the factor 2. For example, a small variable bias battery of 1 volts may be connected in the conductor 54 adjacent the voltage divider RI 2 and RI3, and said battery alternately inserted with its potential such as to add or subtract from the potential provided by the voltage divider system previously described. If all of the relaxation oscillators I2 to I! continue to operate when the bias on the conductor 54 is thus changed through a range of 3 volts, the oscillators may be considered to be operating safely. 1! any of the oscillators I2 to I! fail to divide properly under the conditions where the voltage on the conductor 54 is changed through this range of 3 volts, adjustment of the associated potentiometer R8 and R82 is made until such condition does obtain.

Thus, upon closure of the relay switch 821:, oscillator I8 will be tuned to the note G of the highest octave (3136 c.'p. s.), and the oscillator I2 will likewise follow this frequency while the oscillators I3 to I I inclusive will omillate at frequencies of 1568, 784, 392, 198 and 98 c. p. a. respectively Assuming that register control I88 has been operated. signals from the oscillators I8, I4 and I3 will be impressed upon the conductors I8I, I82 and I88 respectively. when the relay 82 is operated and-the relay switch 820 closed, the signal from the oscillator I4 will be supplied through theconductor I82 to the conductor I00, and hence be impressed upon the grid of the amplifier tube I20 through the condenser CI2.

Energization of the relay 82 also opens relay switch 82a, thus opening the current supply circuit tor the windings oi relays 8i and 88. As a result, assuming that key G2 is held depressed, further depression of a single key in either of the adjoining octave groups of keys CI toBI, or

C8 to B3 will have no effect. Depression of an additional key of the group C2 to B2 may, however, havea tuning eirect ii the second depressed key is such as to lower the irequency of oscillator I8.

. It is of course contemplated that the player shall never press more than two keys at one time. If, for example, while holding the key G2 depressed the player should depress the keys C2 and 032, the condensers CC! and Get as well as the resistor RCt would be shunted through the bus 82 and thus would alter the tuning of the oscillator I0.

Closure of relay switches 82b and 820 causes the signal of the proper frequency to 'be impressed on the grid of the amplifier tube I28.

Also, the amplification of tubes I24 and I28 is rising because of the change in potential of conductor 88 at a desired rate of attack. Inasmuch as contacts 82b and 820 are made relatively soon after the voltage is applied to relay winding 82,

.the amplification of tubes I24 and I28'will not have reached a substantially high value at the time that closure of said relay contacts occurs.

If, however, the relays operate relatively slowly, the potential on the conductor 88 and hence on the cathodes of the tubes I24, I28, may be of such low value, when none of the relays is energized .(by proper selectionof the values of the resistors R88 and R88), so that the tubes I24. I28 are normally biased considerably beyond cutofl. Thus, upon energization of any of the relays 8|, 82, 88, a certain length of time will be required for the potential on the cathodes of the tubes I24, I28 to build up to the cutoff potential due to the time delay afforded by the condenser CI4 and resistor RI8. This time delay may be correlated with the speed of operation of the relay armatures so that a condition cannot arise that the amplifier tubes I24 and I28 have appreciable gain before the relay switches Me, 820 or 83c, as well as the tuning relay switches 8Ib, 82b or 8217, have operated to complete their respective circuits. Thus, any transients incidental to the closure of said contacts ThlB,

gizing relay-82 and causing th potential on conductor 88 to rise to cutoiI value for tubes I24 and'l28. This rate of decay of operating bias on tubes I24 and I28 is controlled by condenser Gill and resistance R88 and is of such value as to provide a click-less decay rate. Thus, the ampliiier'begins to decay immediately upon the opening of contact G2b. Following this operation and at a time when the amplifier is substantially cut oil, the contacts 82a, 82b and 82c associated with relay 82 are operated. In this way, all transients due to changing the tuning of the generator system and keying of the signals supplied by said generating system are eliminated, thus making the attack and decay musically acceptable.

When the player plays a note such as the note G2 with the results Just described, and while holding this key depresses another key, such as, for example, the key C8 in the higher octav of keys, the depression of the key C3 does not have any efiect until the key G2 is released.

The relay contacts are preferably so arranged that upon energization of the associated relay, the contacts 8Ia 82a and 83a are opened after the make contacts have been made, and may preferably take place as near as possible to the end of the relay armature movement. Similarly, upon de-energization of the associated relay, its contact 8Ia, 82a or 83a will complete the circuit after the other relay switches have opened. By reason of this sequence of operation of the relay switches, the shift of the tone from the note G2 to note 03 takes place in a minimum interval of time-because relay is energized very shortly after relay 82 is de-energized upon release of the key G2. As a result, relay switches 82b and 820 open substantially simultaneously with the closure of relay switches 83b and 830. There mustbe' a slight interval between these operations in order to provide time for the change in bias on the push-pull tubes I24, I28 through the conductor 88, and the amplifier is substan tially cut of! during this interim oi time. The relays 8|, 82 and 88 may be ordinary telephone type relays, but are, preferably provided with platinum or some other precious metal contacts.

For example, if the'player, while still holding the key G2 depressed, depresses another key in the same octave-for example key A2-the relay 82 is not de-energized and upon release of the key G2, the oscillator III will have its frequency shifted to the new frequency substantially instantaneously upon release of the key G2. On the other hand, if, while holding the key G2 depressed, he presses a lower note in the same o tave-for example the note D2-the frequency of the oscillator I8 will immediately be shifted to the new frequency of the note D even before the key G2 is released.

It is intended that the instrument be used to signals will appear on conductor I00 which will be associated with the signal contacts of each of the "two relays BI and 03. Thus, the D note will sound as a low D note and another D note two octaves higher.

As a result of depressing these two keys, the voltage on conductor will be more negative than ordinarily (because of the lowered impedance of the two windings of the into the signal, which distortion'products, however, willbe harmonics of the lowest of the two D signals. ,Under' these. conditions, the tones heard will be very loud, and produce a very bizarre efiect which will ordinarily not be desired, but may be utilized upon rare occasions.

During the playing of the instrument, the swell or volum control may be'operated at will successively to connect or disconnect the resistors R to R24 in parallel, thus changing the, grid bias on the tubes I24 and I20, which results in a change in the amplification" factor of these remote cutofi' tubes I24 and I20. Since the change in grid bias effected by the volume control is made gradual by virtue of the condenser CI! and resistor RI1, the changes in the volume of the output are correspondingly gradual. Furthermore, any odd harmonic distortion introduced by the curvature oi the grid characteristics of the remote cutofl tubes I24 and I20 tends to be substantially eliminated due to the push-pull arrangement utilized.

Whenever the player desires to change the register in which the tone sounds, he may operate the desired register controls- I04 to I01 and thus shift the tonal register of the three octaves of keys provided. If desired, two or more of these register controls I 04 to I01 may be operated simultaneously, thus providing an effect similar to that obtainedby the use of coupler switches in pipe organs, or that of more than one instrument.playing simultaneously in octave or multi-octave relationship. These register controls when two. or

more are used simultaneously, may be thought of as eilecting changes in the tone quality. It will be understood that the register controls may beof the type, such as pipe organ stop tablets, which remain-in either of their two positions to which they are moved.

The resistors RI04a to RI01a,'RI04b to R.I01b, and RI04c to RI 010 may be of different values so 'as to provide means whereby the compounding of' tones produced through simultaneously using a plurality of register controls may be-varied such that the octave coupling may be more or less than that which one would expect were it possible to play an octave on the keyboard.

It will be understood of course that resistors RI04a, Rl04b and RI04c must be of such value as to match in intensity the lowest note of one octave group to the highest note in the adioin-. ing octave group. However, resistors Rl04a and .RIIlSa, for example, may be chosen on the basis Power supply system In the previousdescription of the instrument, various terminals have been referred to from which voltages and values represented by the reference characters of the terminals are supplied. Any suitable power supply capable of providing these voltages with reasonably good regulation may be used. In Figure 2, we have, however, shown a pmticular form of power supply system which is well adapted for this purpose. It comprises generally aninput power transformer I60 having a primary IOI which is supplied with 115 volt alternating current, being controlled by an on and 01 switch I02. The secondary winding I62 of the transformer I80 has its terminals connected to the plates of a full wave rectifier tube 104 of a suitable type such as the 5U4G tube, the filament of which is supplied with current by a tertiary winding I. The heaters of the various tubes oi the instrument are supplied by an additional secondary winding I68, the center tap of which is connected to terminal I017.

The center tap of the winding I62 is impressed upon an input filter system comprising condensers C24 and C26, and an inductance LI4 which may be the field winding of the speaker I46. A protectiveresistance R40 is connected between the filament of tube I64 and the condensers C24 and C28, which are connected to Terminals V and I50V' are connected to suitable taps on' the voltage divider resistance R,

while the terminal 300V is directly connected to the conductor, I10. The terminals 50V, I 50V and 300V are connected as previously described, to

supply power for the amplifier 28. It is to be expected upon closure of one of the playing keys,

that the current drawn by the amplifier will change because" of the changing load. This change in load will refiect itself as a change in voltage between the conductors I10 and I12.

Relaxation oscillators I2 to 11 will relax at substantially the same frequency provided that the ratio at their grid bias and plate bias remains constant. In order that the relaxation oscillators I2 to I1 will have the ratio between their plate voltage and grid bias constant, a separate voltage divider R42 is provided for the relaxation oscillators, being tapped at point I16, connected to terminal IOV, which serves as a source of cathode potential for the tubes 28. A condenser C28 is connected between the tap I10 and the conductor I12 serving as a filter for the from a terminal 2V, being provided by divider resistance R44, tapped at its mid-point, and filtered by resistor R48 and condenser C30.

The terminal 250V is connected to the conductor I10 through a filter resistor R48, and a filter condenser C32: connects the terminal 250V to ground. The terminal 250V is utilized to supply the plate current and voltage for tube I20.

.The terminal 280V is connected to the conductor I10 through a filtering resistor R50, and is connected to ground through a filtering condenser C34. The terminal 280V is the source for plate voltage for the tubes 38 of the relaxation oscillator circuits. J

The terminal IO0V is connected to the conductor I10 through a filtering resistance R52 and connected to ground through afiltering condenser C36. the temiinal, IO0V being utilized to supply current-to the vibrato mechanism for the vibration of the reed thereof.

General operation oscillator I0 will, provided no keys are depressed,

commence oscillation at the frequency of the note B in the highest octave range, namely, at 3951 c. p. s., and the controlled oscillators I2 to I1 inclusive will of course oscillate at corre-,- sponding octave frequencies below that of the oscillator I0.

As previously noted, the inductance 34 is variable. This inductance may be adjusted preferably by changing the size of the air gap in the core of the transformer. Such tuning will ordinarily be unnecessary since the oscillator ,I0 is of very stable construction and has excellent frequency stability, but it may be necessary when it is desired to tune the instrument to the pitch of some other instrument which cannot be readily tu'ned.

The player may then select the register in which he wishes to play, by operatingone or more of the register selectors I04 to I01. Thus, signals from oscillators selected from the group I2 to H will ,be impressed selectively upon the conductors IOI, I02 and I03, which lead to the relay operated switches 8Ic, 92c, and 930 respectively. I

After making a suitable selection of the resonant filter circuits to be employed, by opening one or more of the switches I50 to I55, the instrument will be ready for use. When closing the on and ofi switch I62, the player will either independently or incidental to the operation of said switch, flick the vibrato reed 58 to cause it to commence vibrating. Thereafter the .reed

. will automatically continue its vibration. Before playing, the operator, if he desires the vibrato tone, will operate either one or both of the switches I0 and I2 to cause the desired vibrato frequency shift in the frequency of oscillation of the oscillator I0. Likewise, if a slow attack of the tone is preferred for the particular selection to be rendered, the musician will close the switch I32.

Having thus set the various controls, the instrument is in condition for playing, and is played as a melody instrument usually with an organlike, legato touch. 1

As previously mentioned, any suitable frequency generating system using any one of various types of oscillators may be employed in the instrument in place of the frequency generating system disclosed herein. The frequency generating system per se does not form a part of our invention claimed herein, except in combination with the remaining parts of the instrument.

Our invention has been described as relating to a melody instrument, by which we mean an instrument in which ordinarily but one note is sounded at one time so that it is not .generally capable of playing chords. The instrument disclosed herein is thus primarily a melody instrument as thus defined, although, as previously stated, chords consisting of notes in octave relationship may be played upon the instrument. Furthermore, by the use of several manuals and duplication of other parts of the instrument, it will be readily apparent that an instrument capable of playing chords could readily be constructed. It is with these facts in mind that the instrument is generally described herein as a "melody instrument.

While in a number of instances various elements of the circuit have been specified as being of particular values, these values may, as is well known in the art, be varied considerably, especially if compensatory changes are made in the arrangement or values of other parts of the instrument.

It will be apparent to those skilled in the art that numerous variations, modifications claims. to. include within the scope of our invention all such modified constructions whereby substantially the results of our invention may be obtained by substantially the same or equivalent means.

We claim:

1. In an electrical musical instrument, the combination of a plurality of groups of keys, 9. relay associated with each group of keys, circuits for energizing each of said relays upon depression of any of the keys of'its associated group, and means operated upon energization of any one of said relays to open the energizing circuits of relays associated with the groups of keys located immediately adjacent the group of keys associated with the energized relay.

2. In an electrical musical instrument, the combination of three groups of keys, a relay associated with each group of keys, circuits for energizing said relays respectively upon depression of any key in-the group associated with the relay to be energized, and means operated by the energized relay'for preventing the energization of the relay associated with each group of keys located adjacent the group of keys associated with the energized relay.

3. In an electrical musical instrument, the

combination of three groups of keys, three relays associated respectively with said groups of keys, a relay energizing circuit for each of said relays, switches operated by the keys of each group for completing the circuit for energization of the relay'associatedsuch group, normally closed switch contacts operated by said relays respectively, said normally closed switch contacts of one relay being in at least one of said circuits for energizing another of said relays.

4. In an electrical musical instrument, the combination of three octaves of keys, a relay associated with each octavecf keys, switch contacts associated with each relay and opened upon energization thereof, and an energizing circuit for each of said relays arranged to be completed upon depression of, any one of the keys of the octave associated therewith, each of said circuits including at least one of said switch contacts of one of the other relays.

5. In an electrical musical instrument, the combination of a plurality of keys divided into groups, a relay associated with each group of keys, circuits for energizing said relays upon depression of any key of the group associated with the particular relay to be energized, switch contacts opened upon energization of said relays respectively, and circuits connecting the switch contacts of the relay associated'with one group of keys inthe relay energizing circuits for relays associated with adjacentgroups of keys, whereby upon energization of a relay associated with one group of keys the depression of a key in an adjacent group will be melody type, the combination of a plurality of octave groups of keys for controlling the transmission to the output oi the instrument of signals irom sources supplying signals of musical. irequency, a relay for each octave group of keys, an energizing circuit for each oi said relays arranged to be rendered efi'ective upon operation of any 0! the keys inv its associated octave group of keys, and means operated by each of said relays upon the energization thereof to prevent the energizetion of a relay associated with an adjacent octave group of keys.

'I. In an electrical musical instrument having a plurality of inter-stabilized electrical impulse generators providing impulses at frequencies bearing octave relationship with respect to one another, and having an electron discharge tube amplifier and electroacoustic translating means supplied with the output of said amplifier, the combination of a plurality of manually depressible keys, a pair of switches operable sequentially by each of said keys, a relay, an energizing circuit for said relay including one of said key-operated switches, means responsive solely to current fiow through said relay energizing circuit to cause a controlled rate of change in the amplification by said electron discharge device, and a switch operated by said relay to connect one of said sources to the input circuit of said electron discharge device.

8. In an electrical musical instrument of the melody type having a plurality of manually operable keys, tunable sources of electrical impulses o! diilerent musical frequencies, and having an amplifier and loud speaker; the combinationof a switch operated by each of said keys and effective to condition said-sources to be tuned to frequencies correlated with the nominal frequency corresponding to the note of the key depressed, a relay having an energizing circuit and a pair of contact switches which are closed upon energization of the relay, an energizing circuit for said relay including a second switch operated by said key, means controlled by one of said relay contact switches to conduct a signal from one oi! said sources to said amplifier, a circuit controlled by the other of said contact switches for rendering effective the tuning of said sources, and means rendered efiective by current flow through the energizing circuit of said relay to change the gain of said 9. In an electricalv musical instrugnent havin to connect one of said generators to the input circuit of said electron discharge tube amplifier;

10. In an electrical musical instrument, the combination of a plurality of groups of keys, a relay associated with each group of keys. circuits controlled by said keys of each group for energizing the relay associated therewith, and means operated by each relay upon the energization thereof to open such or said circuits as energize the relays associated with adjacent groups of keys.

harmonicaily related frequencies, means to tune said generators to frequencies or harmonically related notes 01' the musical scale, a plurality of groups of keys, a switch operated by each of said keys respectively to condition said tuning means to tune said generators to predetermined frequencies, a relay having a winding and a pair of contact switches closed upon energization of said winding, a circuit including one of said contact switches for rendering effective said tuning means, a circuit completed by said second contact switch to supply electrical impulses from one of said generators to said amplifier, means normally effective to bias an electron discharge tube of said amplifier beyond cutoi'i', means to complete an energizing circuit through the winding of said relay upon depression of any one of the keys of one of said groups, and means for changing the bias on said tube at a controlled rate as aresult of current fiow through the winding of said relay.- I

12. In an electrical musical instrument having an output amplifier including amulti-electrode electron discharge device, a voltage divider circuit normally supplying a potential to said device sufiicient to bias said device substantially to cutoil', a'relay controlling the transmission of a signal to said amplifier, and a key controlled energizing circuit for said relay, said circuit being in'parallel with a portion of said voltage divider circuit, the impedance 01 said energizing circuit 1 being oi. such value with respect to the imped ance of said portion of said voltage divider circuit that the bias on said electron discharge device will be reduced to a value above cutoff whenever said energizing circuit is completed.

.13. In an electrical musical instrument of the melody type having a muti-octave range key-- board, a plurality oi electron discharge device generators of musical signals bearing octave frequency relationship to one another, means controlled by keys of corresponding note designation of difi'erent octaves simultaneously to tune all oi said generators to frequencies corresponding to such note designation, an amplifying electron discharge device, means normally biasing said device substantially to cutofl, a relay having an energizing circuit closed by depression of any of the keys '0! an octave of said keyboard, means affected by current flow through said circuit for decreasing the bias on said device at a predetermined rate, and a circuit completed upon energization of said relay to impress a signal from one of said generators upon said amplifying device.

14. In an electrical musical instrument having a plurality of octave groups of keys, a plurality of generators, an amplifier, a plurality of circuits controlled by said keys for connecting said generators to said amplifier, and means operated upon completion of one of said circuits under the control of a key of one of said groups to prevent the completion of another of said circults under the control of a key in an adjacent group.

15.,In an electrical musical instrument, an amplifier including an electron discharge device having a cathode, plate and control grid, a voltage divider determining the operating potential of said cathode. a relay, a circuit controlled by said relay for impressing a musical signal upon the grid and cathode of said device, and a keycontrolled circuit for energizing said relay, said key-controlled circuit being in parallel with a portion of said voltage divider, whereby the potential of said cathode is changed upon completion of said key-controlled circuit.

16. In an electrical musical instrument having means for generating and controlling a musical signal and having an amplifier includinga multi-electrode electron discharge device for receiving a signal from said generating and controlling means and transmitting the signal to electroacoustic translating means; means to control the amplitude of the signal output of said device comprising, a terminal, a source of direct current potential adjustable by relatively few and large increments, said adjustable source being connected to said terminal, means readily operable by the player of the instrument for adjusting the potential of said source, a reactive element connected between said terminal and a point of fixed potential, said element being of sufiicient reactance to cause the rate of change of potential on said terminal to change smoothly despite abrupt changes in the potential of said source, and means connecting said terminal to one of the electrodes of said device to determine the bias thereof.

1'7. In an electrical musical instrument having means for generating and con-trolling a musical signal, an amplifier including a variable-mu vacuum tube for receiving said signal and transmitting it to electroacoustic translating means; means to control the amplitude of the signal output of said tube comprising, a terminal connected to the control grid of said tube, a voltage divider having a connection to said terminal for determining the potential thereof, said voltage divider being adjustable only by relatively large steps, means [readily operable by the player of the instrument for controlling the adjustment of said voltage divider, and a reactive element connected between said terminal and a point of fixed potential, said reactive element being of sufilcient value to cause the potential on said terminal to change gradually from one potential value to another upon abrupt adjustment of said voltage divider from one position of adjustment to another.

18. In an electrical musical instrument, the combination of an oscillator having an adjustable tuning circuit including a plurality of condensers connected in series, voltage dropping resisters connected in parallel with said condensersrespectively, key operated switch controlled cir cuits for selectively shunting groups of said condensers, and a plurality of inductances of relatively low impedance connected in series with said condensers between groups thereof, whereby the discharge of condensers shunted by said key controlled circuits will be impeded to reduce arcing upon opening and closing said key operated switch controlled circuits. v

19. The combination set forth in claim 18 in which two of said inductances are provided and lay energized upon closure of said second key operated switch, said relay being eifective to close the other switch of said pair of switches.

21. In an electrical musical instrument having means for generating and controlling a musical signal, and an amplifier including an electron discharge device for receiving said signal and transmitting it to an electroacoustic translating means; means to control the amplitude of the signal output of said device comprising, an element readily operable by the player of the instrument, a plurality of fixed impedances forming part of a voltage controlling circuit and arranged to be successively connected in the circuit by said element, a terminal the potential of which is determined by the number of said impedances which are connected in said circuit, a reactive element connected between said terminal and a point of fixed potential to limit the rate of change of potential on said terminal, and means connecting said terminal to one of the electrodes of said electron discharge device to determine the bias thereof.

22. In an electrical musical instrument having an oscillator the frequency of oscillation of which is determined in part 'by the amount of inductance in a tuning circuit thereof, the combination of a coil having end terminals and intermediate taps, said coil forming the tuning inductance of the oscillator, a pair of inductance elements normally connected in parallel across said intermediate taps, a periodically operating interrupting switch having one contact thereof connected to one of said terminals, and manually operated switches separately operable to disconnect a terminal of each of said inductance elements respectively from an intermediate tap of said coil and connect it to the other contact of said interrupting switch.

23. In an electrical musical instrument having an oscillator the frequency of oscillation of which is determined in part by the amount of inductance in a tuning circuit thereof, the combination of a coil having end terminals and intermediate taps, said coll forming the tuning inductance of the oscillator, a pair of inductance elements normally connected in parallel across said intermediate taps, and means selectively rendered effective to connect either one or both of said inductance elements periodically in parallel with the portion of said coil between one terminal thereof and the tap thereof nearest said terminal.

LAURENS HAMMOND. JOHN M. HANERT.

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