US3026758A

US3026758A - Tremolo producing means for an electrical musical instrument

Info

Publication number: US3026758A
Application number: US745564A
Authority: US
Inventors: Robert J Ziehlke
Original assignee: Gibbs Manufacturing and Research Corp
Current assignee: Gibbs Manufacturing and Research Corp
Priority date: 1958-06-30
Filing date: 1958-06-30
Publication date: 1962-03-27
Anticipated expiration: 1979-03-27

Description

March 27, 1962 R. J. ZIEHLKE 3,026,758

TREMOLO PRODUCING MEANS FOR AN ELECTRICAL MUSICAL INSTRUMENT Filed June 50, 1958 INV EN TORI ROBERT J, ZIEH LKE United States Patent ice 3,026,758 TREMOLO PRODUCING MEANS FOR AN ELECTRICAL MUSICAL INSTRUMENT Robert J. Ziehlke, Janesville, Wis., assignor to Gibbs Manufacturing 8: Research Corporation, a corporation of Wisconsin Filed June 30, 1958, Scr. No. 745,564 3 Claims. (Cl. 841.25)

This invention is concerned with an electronic musical instrument and more particularly with tremolo producing circuits for an electronic musicalinstrument.

Some musical instruments, as for example organs, are provided with means for producing'a tremolo efiect, i.e. an efiect in which the intensity oramplitude of the tone is varied at a relatively rapid rate as compared with variations eifected by the swell or volume control of the instrument. In pipe organs, this efiect is sometimes achieved by rapidly varying the pressure of the air supplied to the pipes. The present invention is concerned with a circuit by which a similar elfect is achieved in an electronic musical instrument such as an electronic organ.

A principal object of the present invention is the provision in' an electrical musical instrument having means for producing bass and treble musical tone signals of an improved tremolo producing circuit in which the internal cathode-to-anode resistance of an electron discharge tube is varied at a desired tremolo rate to effect a corresponding variation in the amplitude of the bass and treble musical tone signals.

Another object is the provision in the musical instrument of the preceding object of means reducing the tremolo efiect on the lower bass signals.

Another object is the provision in the musical instrument of the preceding objects of a feedback circuit having a time constant longer than the period of the lowest tone signal and shorter than the period of the tremolo rate for minimizing noise in the tremolo producing circuit and assuring a substantially sinusoidal variation in the inter nal cathode-to-anode resistance.

The various features and advantages of the invention will readily be apparent from the following specification and from the drawing, in which:

The FIGURE represents a schematic circuit diagram of an embodiment of the invention.

The tremolo circuit which forms the basis of this invention is designed for use in an electronic organ, i.e. an organ in which electrical signals representing the various tones are generated, amplified and broadcast through speakers. It will be apparent from a consideration of the following description that the present invention may be utilized with any musical instrument which produces tone representing signals, regardless of the manner in which they are initially produced, or the nature of their end use.

Turning now to the drawing, an embodiment of the invention will be described in detail. During the course of the description, various components will be assigned specific values or identified, as by particular electron tube type designations. It is to be understood that these values and type designations are intended to disclose an operative circuit and are not to be construed as critical, except where specifically stated to the contrary. Many alterations and modifications of the specific circuit disclosed will readily be apparent to those skilled in the art.

Briefly, electrical signals representing musical tones are produced by tone generator 1, which may be of any suitable type. The musical tone signals produced by the tone generator are coupled to a tremolo producing circuit indicated generally as 2, a tone modifying circuit 3, a voltage amplifier 4, driver stage 5, power amplifier 6 and speaker 7. The tremolo producing circuit 2 includes an amplifier 8 which forms a" variable impedance which 3,026,758 Patented Mar. 27, 1962 is shunted across the output of the tone generator, and an audio oscillator 9 which provides a control signal for the amplifier, varying the impedance thereof at an audio rate, and thus varying the impedance of the tone generator output at the same rate. This causes a corresponding variation in the amplitude of the tone signals at the audio rate of oscillator 9. p

More particularly, the tone signals from the tone generator 1 are coupled through a D.C. blocking capacitor 15, 0.1 ,uf. (microfarad) to a load impedance com-prising a potentiometer 16, 500,000 ohms, having a terminal 16b connected to a'referen'ce or ground potential 19 and a series resistor 17, 560,000 ohms, connected to the potentiometer by the tap 16a and having a terminal 17a. This arrangement permits adjustment of the output level of the tone generator. The tremolo producing circuit 2 is connected across the output terminals 16a and 17a.

The terminal 17a is connected by contacts 20a of a jack 20 to a series resistor 21, 560,000 ohms which in turn is connected to a shunt bass boost circuit 3. The bass boost circuit comprises a pair of resistors 22, 56,000 ohms and 23, 390,000 ohms, resistor 23 being shunted by capacitor 24, 0.02 ,uf.

The modified tone signal is coupled to the grid 25a of amplifier tube 25, a 12AX7. Self-bias for the amplifier is provided by unbypassed cathode resistor 26, 2700 ohms which is returned to ground 19. The anode 25b of the amplifier is connected through load resistor 27, 220,000 ohms and a decoupling circuit including dropping resistor 28, 3,300 ohms and

capacitor

29, 15 ,uf, with a 13+ supply of 320 volts. The amplified output of voltage amplifier 4 is coupled through a blocking capacitor 30, 0.01 f, with a control grid 31a of output tube 31, a 12AT7. The anode 31b of driver stage 5 is connected directly to 13+ through the decoupling network made up of resistor 28 and capacitor 29. The output signal from the stage is developed across cathode load resistor 32, 33,000 ohms, while an unbypassed cathode resistor 33 provides self-bias. Control grid 31a is returned to the junction between

resistors

31 and 33, by resistor 34, 220,000 ohms.

Returning now to the tremolo producing circuit 2, the amplifier 8 includes a triode 40, a 12AT7, having its anode 40b through a blocking capacitor 41, 0.025 ,uf, to the terminal 17a. The cathode 40a of the triode 40 is maintained at positive potential by a voltage divider including resistor 42, 150,000 ohms and adjustable resistor 43, 10,000 ohms. Adjustable resistor 43 shunted by capacitor 44, 4 i, connects the cathode 40a to the terminal 16a. Anode 40b is connected through load resistor 45, 330,000 ohms and decoupling circuits including resistor 46, 10,000 ohms, capacitor 46a, 15 [.tf., resistor 28, capacitor 29 to the B+ supply. The control grid 40c of the triode 40 is returned to ground 19 through a pair of series connected resistors 47 and 48, 1 megohm each. Inverse feedback for the amplifier 8 is secured by means of capacitor 49, 0.01 ,uf. connected between the anode 40b and the control grid 40c thereof.

The internal cathode-to-plate resistance of the triode 40 'is connected in parallel with the impedance of the grid circuit, made up of capacitor 49 and resistors 47 and 48 and varies in accordance with the amplitude of the current passing therethrough. With the tremolo circuit inoperative, this current is adjusted to a median value by proper setting of adjustable resistor 43 in the cathode circuit of the amplifying device.

In order to produce a tremolo effect, it is desired to vary the impedance, that is, the internal cathode-to-plate resistance, of the triode 40 above and below its impedance with the tremolo circuit inoperative, at a subaudio rate, thus varying the amplitude of the tone representing signal at the terminals 16a and 17a above and below its normal 3 value and at the same rate. Subaudio oscillator 9 includes a triode 50, a 12AX7, the anode of which is connected with 13+ through resistor 51, 150,000 ohms,

resistors

46 and 28. The cathode 50b is returned to ground through adjustable resistor 52, 10,000 ohms, shunted by

capacitor

53, 4 ,uf. A resistance-capacitance feedback network connected to the oscillator 50 includes resistor 54, 3.3 megohms, capacitor 55, 0.01 ,uf., resistor 56, 680,- 000 ohms, capacitor 57, 0.01 ,uf., resistor 58, 470,000 ohms, capacitor 59, 0.01 ,uf. and capacitor 60, 0.01 pf. Feedback from the anode 50ais made to the juncture between

capacitors

59 and 60. The subaudio frequency 'output of oscillator 50 is obtained across resistor 61,

10,000 ohms, connected in parallel with cathode resistor 52.

Blocking capacitor

62, 4 ,uf., prevents direct current from flowing through load resistor'61. The audio output of oscillator 50 is coupled through capacitor 63, 0.1 ,uf., to the juncture between resistors 47 and 48 of the grid circuit of tube 40.

The frequency of oscillation of oscillator 9 depends upon the values of resistors and capacitors in the feedback network. With the figures given, the oscillator operates at four or five cycles per second. The value of resistor 56 rnaybe altered to change this frequency, although for proper tremolo efiect the frequency should be kept in the range of four to eight cycles per second. The amplitude of the control signal coupled to gate tube 40 may be varied by adjustment of variable resistor 52 in the cathode circuit of the oscillator. For a heavy tremolo effect, this resistor should be set to produce an amplitude variation of the tone representing signals in the signal channel of plus and minus 3 db. The operation of the tremolo circuit is controlled by on-olf switch 64 in the cathode circuit of the oscillator, which shorts resistor 61 when in the off position.

Transient elfects in the tremolo circuit are reduced by the feedback capacitor 49 connected between the anode .40b and control grid'40c of variable impedance device 40.

The time constant of this circuit, which'includes the grid resistors 47 and 48, is selected to be greater than the period of the lowest audio frequency signals handled by the channel (16 cycles per second) and smaller than the period of the tremulant control signal from oscillator 50. The value of capacitor 41 (0.025 ,uf.) is such that it has an appreciably higher reactance at the lower audio frequencies than at the high frequencies, reducing the tremolo effect on the lower bass frequencies of the tone representing signal.

The heavy feedback provided by capacitor 49 also compensates for the non-linear characteristic of the gating device 40,.keeping the variation in impedance, which. produces .the'tremolo, substantially sinusoidal at the anode 40b of the-gate circuit.

The signal appearing across load resistor 32 of output stage 5 is connected through acoupling transformer 65 with a power amplifier 6. Capacitor 66, connected in series with the primary winding 65a of the coupling transformer, is selected to tune the primary circuit of the transformer to series resonance at a frequency of about thirty-two cycles per second. Thus, the load on output stage'31 acts as a high pass filter, eliminating the tremulant frequency from, the signal coupled to the power amplifier 6 Without affecting the desired tone representing signals.

While I have shown and described certain embodiments of my invention, it is to be understood that it is capable of many modifications. Changes therefore, in the construction and arrangement may be made without departing from the spirit and scopeof the invention as disclosed in the appended claims.

I claim:

1. In an electrical musical instrument having a tone 7 comprising an amplifier including an electron discharge tube having an anode, a cathode and a grid, a coupling capacitor connecting the anode directly to one of the terminals, a parallel connected capacitor and first resistor connecting the cathode to the other terminal, a second resistor connecting the grid to the other terminal, and an oscillator producing signals at a desired tremolo rate and including a third resistor across which the tremolo signals are obtained, the third resistor being connected across the second resistor thereby applying the tremolo signals to the gridto vary the internal cathode-to-anode resistance of the tube at the tremolo rate, whereby the amplitude of the bass and treble signals at the output terminals of the load impedance are varied at the tremolo rate.

2. In an electrical musical instrumenthaving a tone generator producing bass and treble signals and having a load impedance connected to the tone generator and including output terminals, a tremolo producing means comprising an amplifier including an electron discharge tube having an anode, acathode and a grid, a coupling capacitor'connecting the anode directly to one of the terminals, a parallel connected capacitor and first resistor connecting the cathode to the other terminal, a second resistor connecting the grid to the other terminal, and an oscillator producingsignals at a desiredtremolo rate and including a third resistor across which the tremolo signals are obtained, the third resistor bein'g'connected across the second resistor'thereby applying the tremolo generator producing bass and treble signals and having.

a load impedance connected to the tone generator and including output terminals, a tremolo producing means comprising an amplifier including an electron discharge tube having an anode, a cathode and a grid, a coupling capacitor connecting the anode directly to one of the terminals, a parallel connected capacitor and first resistor connecting the cathode to the other terminal, second and third resistors connecting the grid to the other terminal;

a feedback capacitor connecting the anode directly to the grid, the feedback capacitor and the second and third resistors having a time constant which is longer than the period of the lowest pitch bass signals and shorter than the period of the tremolo signals, and an oscillator producing signals at a desired tremolo rate and including a fourth resistor across which the tremolo signals are obtained, the fourth resistor being connected across the third resistor thereby applying the tremolo signals to the grid to vary the internal cathode-to-anode resistance of the tube at the tremolo rate, wherebythe amplitude of the bass and treble signals at the output terminals of the load impedance are varied at the tremolo rate.

References Cited in the file of this patent UNITED STATES PATENTS 2,138,500 Miessner Nov. 29, 1938 2,147,948 Kent et al. Feb. 21, 1939 2,221,188 Hammond et al. Nov. 12, 1940 2,322,884 Roetken June 29, 1943 2,485,538 Rowe Oct. 18, 1949 2,534,342 Daniel Dec. 19, 1950 2,817,708 Fender Dec. 24, 1957 2,835,814 7 Dorf May 20, 1958 2,835,870 Fretz May 20, 1958 2,892,373 Bauer June 30, 1959