Patent GB-527058-A (GB)
Title: Improvements in electrical musical instruments
IPC: G10H5/00
Filed: 1939-03-30
Abstract: 527,058. Electric musical instruments. HAMMOND, L. March 30, 1939, No. 9952. Convention date, April 2, 1938. [Class 88 (ii)] [Also in Group XL] An electrical musical instrument playable from a keyboard has its upper octave of notes generated by an electric frequency generator system, the remaining notes being produced by stable non-linear amplifiers each receiving signals from the amplifier or generator one octave above and so adjusted as to produce a frequency of one half the input frequency. These frequencies are fed to a normally inoperative control circuit selectively rendered operative by the keyboard to transmit the desired signals to an output circuit and loud speaker. The notes may be caused to simulate a piano, organ, or other stringed instrument at will, and means are provided for producing a vibrato effect and for varying the harmonic development of the notes, and for producing novel musical effects. The upper twelve notes are generated by high vacuum oscillating valves which in practice would be double triodes but the generator for note number 85, top A, of the musical scale, is shown as two valves 90, 92, the plate of the latter being back coupled to the grid of the former through a condenser 94 and variable resistance 96. The grid is tuned by a variable inductance 98 and a fixed condenser 100. Vibrato control. Four other condensers 120 ... 123 can be selectively put in parallel with the condenser 100 by switches 124, 125 and two of these, 122, 123 are connected to earth through a vibrating reed 126 actuated by an electro-magnet 130 and circuit breaker 134 so that when either or both of the condensers 122, 123 are in use the pitch of the note varies above and below the normal pitch to an extent depending on the condenser or condensers in use. The vibrating reeds for other notes preferably have a different frequency. Frequency dividing system. The output from the valve 92 is resistance-capacity coupled to the grid 102 of a pentode DT73 corresponding to the octave below, note number 73, through a resistance R3. The screen and suppressor grids, at fixed voltages, prevent back coupling, and the cathode 104 is strongly biassed negatively through a high resistance R6. The grid is connected to the cathode through a condenser Cb and a condenser Cc is in parallel with the resistance R6. The impedance of the grid circuit is so adjusted that only alternate peaks of the positive grid excursions cause current in the plate circuit and as the valve is working close to the cut-off, this frequency, which is one-half the grid frequency appears as a fundamental frequency accompanied by a series of diminishing harmonics. This valve is similarly coupled to the valve of note 61 except that an additional condenser Cd is placed across the plate circuit. The valves for the lower notes such as note 13 and note 1 shown in Fig. 1b have each an additional condenser Ce and resistance R1 in the grid cathode circuit. A modified circuit is described with transformer coupling between the stages. Control circuits. The signal from each valve is fed to the grid of a control valve, preferably a pentode, normally retained at the cut-off point. The signal from the valves 90, 92 is fed through a resistance 144, condenser 142 and resistance 150 to ground and the voltage across the resistance 150 is fed to the grid 140 of the control valve CT85. An additional resistance 148 controlled by a switch controls the brightness of the note. Each of the remaining valves is connected from the cathode 104 through a condenser Cf and resistance 212 shunted by a condenser 214 to ground and the voltages across the resistance 212 appear on the grid 140 of the control valve. A condenser 216 controlled by a switch varies the brightness of the note. The cathode of each control valve is connected through a resistance R7 to a point 164 connected through a condenser 166 to ground. The point 164 is connected through a resistance R8 to a. rigid switch arm 168 and through a lower resistance R9 to a flexible arm 170 normally contacting with a busbar 182 maintained at cut-off potential. A current source Bc, normally at zero potential, is connected through a resistance R11 to a condenser 186 which is also normally connected through a resistance R10, flexible arm 172 and contact 174 to a busbar 176 maintained at a negative potential from a source Ba. On depression of a key K85, K73 &c. the strip 170 is first moved out of contact with the busbar 182 and the strip 172 is brought into contact with the arm 168 so that the charge on the condenser 186 is shared with the condenser 166 and signals appear in the plate circuit of the corresponding control valve. The signals fade logarithmically as in a piano to a level determined by the voltage at Bc and when the key is released the point 164 is rapidly but not too abruptly brought to the cut-off voltage by leakage through resistance R9 and switch arm 170 to the current source Bb. A sustaining pedal 194 actuates an insulating strip 200 which depresses the ends of all the strips 170 so that the note continues to fade logarithmically or remains at its final level until the pedal is released. There may be two pedals, one for the treble and the other for the bass sections. By making the voltage Bc equal to Ba sustained organ notes are produced and by making the voltage Bc higher than Ba notes which increase in intensity from the instant the keys are struck, are produced. The outputs from the plates of all the control valves is led through a wire 158 to adjustable resonant output circuits 160, volume control 204, which may be actuated by a swell pedal, and amplifiers 206, 208 to a loud speaker 210.