Patent GB-529304-A (GB)
Title: Improvements in electrical musical instruments
IPC: G10H1/00
Filed: 1939-03-29
Abstract: 529,304. Electric musical instruments. HAMMOND, L. March 29, 1939, No. 9841. Convention date, April 2, 1938. [Class 88 (ii)] [Also in Group XL] An electrical musical instrument designed to simulate a piano but capable of adjustment to give effects similar to an organ or a xylophone, banjo or other stringed instrument and to produce novel musical effects has a keyboard controlling the output from 88 alternating- current generators continuously operating to produce frequencies of a tempered musical scale corresponding to the 88 notes of the standard piano. Thermionic valve oscillators may be used and the output is so controlled and adjusted that low notes have a greater harmonic development than high notes and the harmonic development of each note increases with the intensity of the note so that the character of the note changes as the note swells and fades away. A touch control provides that the loudness of a note, and its " brightness" depends upon the speed at which the corresponding key is depressed. The note then fades logarithmically until the key is released upon which it rapidly fades to zero unless a sustaining pedal has been depressed. The attack, the rate of fading and final intensitv of the notes can be adjusted by the player for the instrument as a whole in order to produce various effects. The twelve highest notes are provided by vacuum oscillators but the remaining 76 notes are generated by gasfilled relaxation oscillators of which one is shown at 280, Fig. 1. The control grid is fed from a transformer 288 the primary of which is connected to the output of the oscillator of the octave above it. The plate circuit includes a resistance 282, an inductance 284 forming part of a transformer for feeding the grid of the oscillator of the octave below, and a condenser C15. There is also a source of current 286. The cathode circuit includes two resistances R16, R17the latter being shunted by a condenser C14. A condenser C16 may be cut in by a ganged switch 290 under control of the player to vary the brilliance of the notes. The voltage developed across the resistance R17 is fed to the control grid 292 of a control valve 294 which is preferably a pentode, the screen and suppressor grids of which prevent any capacity effect between the control grid and plate. The plate P49 is supplied with current from a source 312, Fig. 2, through the lead 306. The cathode is connected to a condenser C4 and a point N is normally maintained by a source of current E at a voltage which completely cuts off the output from the valve 294. The values of the condensers C4, C14 are determined for each note when the instrument is first " voiced". A source D is connected through a high resistance R6, e.g. 20 megohms, to a point M which is connected to the point N through a resistance R7, e.g. of 4 megohms. Each key carries a pivoted switch arm 238, Figs. 1 and 13, permanently connected through a resistance R12 to a condenser C8. This condenser is also connected through a resistance R11, of about the same value as the resistance R7, to an adjustable current source C. The tip of the switch arm 238 normally rests on a bus-bar 244 connected to an adjustable current source A, normally of high negative potential. As the key is depressed the arm 238 successively makes contact at 246, breaks contact at 244, breaks contact at 246 through encountering an insulating strip 248 and finally makes contact at 250 so as to connect the condenser C8 to the point M which is connected to ground through a condenser C7 of similar capacity. The contact 246 is connected through a resistance R13 to a current source B which is normally at a less negative potential than the source A. Before the switch arm makes contact at 250 a projection 268 on the key moves a flexible plate 262 out of contact with a contact 266 connected to the point M through a relatively low resistance R10, and just before the bottom of the stroke of the key a switch arm 252 carried by the key and working through a hole in a fixed insulating strip 258 momentarily makes contact at 260 to bring the point M to the potential of an adjustable source F. A sustaining pedal 270, by rocking a shaft 272, may prevent the reclosing of the contact 266 when the key is released. The condenser C8, initially negatively charged from the source A, is partially discharged as the arm passes over the contact 246 by an amount depending upon the speed of depression of the key, and the residual charge is then shared with the condenser C7 and slowly leaks away through the resistances R6, R11 and through the valve 294 which transmits only a series of impulses corresponding to the most positive excursions of the grid 292. When the key is released the charge on the condenser C7 is rapidly though not too abruptly dissipated through the resistance R10 unless the sustaining pedal is in use, in which case only the high resistance R6 and valve current is available for the purpose. The contact 260 ensures that the point M is brought to a normal potential at each stroke when repeat notes are played with the sustaining pedal in use. Contact is not made on the upstroke as the arm 252 is frictionally engaged with its pivot. The potential at C determines the level of audibility to which the note sinks before the key is released. By setting the voltages at A, Band C at the same level, e.g. -100 volts, and the voltages at D, E and F at +6 volts sustained organ notes are obtained but with the added feature that if the sustaining pedal be used the notes will fade after release of the keys in the same way as in a piano. If the potential of D is made equal to C the sustaining pedal maintains the note at its original level. If the potential of C is made less than A and B the effect is an organ note with pianistic attack. If the potential of C is made relatively high the notes may grow in intensity after being struck. The resistance R9 between contact 250 and the point M which may be omitted for the lower notes reduces the attack on high notes and therefore reduces the keythump effect present in standard pianos. Since the voltage at B determines the minimum intensitv with a soft blow a note is sounded however light the blow. If the voltages at A and B are reversed the harder the key is hit the softer is the note. For xylophone and banjo effects a ganged switch 296 connects the point M through a resistance R14, one for each note, to an adjustable current source G so that the notes fade rapidly after being struck. The outputs from the plates of the valves 294 are connected in parallel in groups of four and led through the adjustable tone-control network shown in Fig. 2 to the amplifier and loud speaker, ganged switches 326 determining the quality of the output by putting condensers and inductances of different values in circuit. Other capacities and inductances are controlled by a swell pedal or by hand also is provided. The voltages at A,B,C,D and F which control respectively the maximum intensity with a hard blow, the minimum intensity with a soft blow, the final intensity with key held down, the final intensity with key released but damper off, and the intensity just prior to the bottom of the stroke, are controlled by five multiple switches mounted on a panel with appropriate inscriptions, one positive and a series of negative voltages being available. The voltages at E are individually adjusted for each note to ensure complete cut-off of the corresponding valve 294 when the instrument is not being played. Fig. 28 shows a simplified form of instrument without touch control. In this case the key, when depressed, moves a flexible strip 426 out of contact with a bus-bar 434 and into contact with a conductor 428 connected to the point M, a strip 424 being simultaneously moved out of contact with a bus-bar 436 at cut-off potential. a sustaining pedal 438 can act on the tip of each strip 424. References are made to Specifications 454,502, 494,917 and 499,370. Instead of coupling the valves of successive octaves in cascade as described the frequency dividing circuit disclosed in Specification 527,058 may be used. Upward movement of each key is limited by a flexible strip 218, Fig. 13, which is connected to a flexible strip 222 passing over rods 226, 228 to a spring 224 so as to damp the upward movement of the key. Downward movement is limited by a felt strip 230.