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Hammond Solovox

Technical Description
Richard H. Dorf

Hammond Solovox
Fig. 1 Typical Solovox Installation.

THE Hammond Solovox (Hammond Instrument Company, Chicago, Ill.) is and has been for a number of years the best-known monophonic electronic musical instrument made in this country. While it permits the playing of only a single note at a time and has neither the types nor variety of tone colors to appeal to most serious musicians, it has found extensive use in popular-music ensembles and particularly in the home among amateurs. It is ordinarily used with the piano - the player's right hand playing the melody on the Solovox and the left hand playing an accompaniment on the piano - where its sustained tones contrast with the percussive effects of the piano to produce pleasing sounds with a minimum of study, effort, and expense.

Unlike the Hammond organ, the Solovox is completely electronic, with no moving parts other than the keys and controls. (To be accurate, the older models J and K had a vibrating-reed vibrato system, but later models are electronic in even this respect.) This chapter describes the model L.

A typical Solovox installation is shown in Fig. 1. The two principal units are the keyboard and a tone cabinet. The tone cabinet (not shown in the, illustration) houses the speaker and all vacuum tubes. The keyboard, attached to the front of the piano, holds the switches which are actuated by the keys, and all control circuits.
Hammond Solovox block diagram
Fig.2 Solovox block diagram

Fig. 2 is a block diagram of the Solovox which shows the principal components and their relationships. A master oscillator is tuned through a three-octave range by the keys, and its frequency may be varied cyclically to produce vibrato. The oscillator synchronizes a frequency divider Which produces similar tones one octave lower down. This, in turn, synchronizes a second and third frequency divider.

One output is taken from the oscillator, which operates in the highest three-octave range of the instrument, and one from each of the frequency dividers. In accordance with their pitch ranges the

outputs are labelled SOPRANO, CONTRALTO, TENOR, and BASS. Any one or a combination may be switched into a common output bus which goes to a series of five tone filters. Whatever tone qualities are switched on are passed to a preamplifier, thence to a push-pull control stage. In the control stage the output volume is controlled by a knee lever fastened under the keyboard, and the attack of each note is slowed down by using an extra set of keyboard contacts to remove cutoff bias on the stage at a controlled rate. From here the signal passes to an output stage and loudspeaker.

Hammond Solovox master oscillator
Fig. 3 The Solovox master oscillator

The master-oscillator circuit is diagrammed in Fig. 3. It is basically very simple and capable of wide-range operation. The output of V1a is R-C coupled to the grid of V2a. The plate of V2a is R-C coupled back to the grid of V1a. Since the circuit is re-entrant and the feedback phase is positive, the combination oscillates. The frequency of oscillation is controlled by capacitance and resistance placed between the grid of V1a and ground.

Two step-type controls are provided. C1 is the main tuning capacitor, and C2, C3, and C4 are added to give the exact value required for the fixed capacitance. The next bank of six .002-µf capacitors is so arranged that as the arm labeled ROUGH TUNING is moved upward, it contacts the capacitors one by one and at maximum position all six are across the main tuning capacitors. Six steps of 400µµf each are available with a similar switching arrangement for the fine-tuning adjustment.

The inductance part of the tuned circuit is used to vary the frequency to give the pitch required for each key over the three-octave range of the oscillator (528.8 to 8951 cycles). Thirty-six small inductors, each with a movable core for individual adjustment, are wired in series, with a switch contact connected between each adjacent pair. When a key is pressed, the corresponding contact is grounded, decreasing the net inductance between the grid of V1a and ground. When the lowest C key is pressed, no contact operates - the full value of all the series inductors is in place and the lowest frequency sounds. (Actually this frequency is being generated all the time, even when no key is being pressed. The control tubes. however, block the amplifier until a key is pressed, as described later.) When the topmost key (B of the top octave) is pressed, all inductors but the one nearest the grid are shorted to ground and the remaining inductance is just large enough to tune the circuit to the topmost note. With this arrangement, no spurious notes are heard even when two or more keys are pressed at once by accident; only the uppermost of the notes played is heard.

Fig. 4. Frequency dividers of the Solovox

The vibrato circuit includes the two remaining triodes of the 6SN7-GT and 6SL7-GT used in the oscillator. V2b is the 6-cycle oscillator, a standard phase-shift unit, the output of which is fed to Vlb, a switching tube. The plate-supply potential for V1b is only 40 volts, obtained from the drop across the frequency-divider cathodes (see Fig. 4). With the low plate potential of 10 volts and zero bias, the comparatively small signal from the vibrato oscillator is sufficient to cut off the switching tube when the signal is in the negative direction.

With the vibrato switch on, paralleled 400-µµf and 800µµf capacitors (total value of .0012 µf) are connected from one side of the audio tuned circuit, through the switching tube, to ground. On the half-cycles of vibrato frequency during which the switching tube conducts, the capacitance is across the audio tuned circuit, changing its frequency to some degree. On the half-cycles during which Vib is cut off, the audio frequency is unchanged. The frequency is thus switched at the vibrato-oscillator rate of 6 cycles per second, giving a vibrato effect.

With the vibrato switch off, a permanent capacitor of 400µµf is connected across the tuned circuit. The 400µµf capacitor between V1b plate and the audio tuned circuit is still in place and the switching tube is still doing its job. The vibrato effect is therefore still present to a very slight degree. This is desirable, for it is enough to destroy the "perfection" of the electronically generated tones, which would otherwise be so perfectly steady as to lack interest.

This is an interesting point in all electronic instruments - perfection is undesirable! A pipe organ, a wind instrument, a violin - all of them have inherent random irregularities of pitch and volume caused by small variations in the wind supply or slight unsteadiness in the player's control. One of the essential factors in art appreciation by the emotions is variation; monotony is inartistic and unpleasant. Thus the natural slight unsteadiness of acoustic instruments is welcome, and to attain a really ideal musical instrument, the electronic engineer should deliberately avoid the perfection which we normally look for in engineering. In one very practical sense this is a major difference between a set of code-practice oscillators tuned to musical pitches and a good electronic musical instrument. The code oscillators have constant pitch and no variation in tonal quality. The musical instrument must have at least a vibrato and a selection of tone colors.


Frequency dividers are very common in electronic musical instruments. In polyphonic instruments there is usually one set of them for each of the twelve notes of the scale and each divider need work at only one frequency. In the Solovox there is only a single set of three dividers, each of which must work over a three-octave range. To fulfill this requirement they are designed to be not particularly frequency-selective and they are of the non-oscillating multivibrator type. This means that in the absence of a synchronizing signal they do not oscillate but remain in one or the other of their two stable conditions.

The frequency-divider section of the Solovox appears in Fig. 4. The first tube V3a is a rectifier which rectifies the output of the master oscillator. The grid is coupled to the oscillator plate through C1. The waveshape of the master oscillator output is roughly symmetrical and contains principally odd harmonics, sounding like a muted instrument, or a woodwind, or stopped pipe. The rectified output from V3a is no longer symmetrical and contains even harmonics as well as odd ones.

V3b is a pulse rectifier. Because of the capacitive coupling between V3a and V3b, the rectified output of V3a appears as a.c. on the grid of V3b Due to the average plate currents of the tubes in the dividers the cathode of V3b is 40 volts positive, giving the tube a high negative grid bias. Negative input to the grid therefore has no effect, but positive input increases the plate current and produces negative pulses at the plate.

V4a and V4b are the first frequency divider. In the resting condition one of the tubes is cut off and the other is conducting. When the driver V3b puts out a negative pulse, it passes through C3 and C4 to the divider grids. The negative pulse has no effect on the tube which is cut off, for instance, V4a. However, it causes the tube which is drawing current, V4b in this case, to put out a positive pulse at its plate. The positive pulse is transferred to the grid of V4a through the R-C plate network, causing V4a to conduct and put out a negative pulse of its own. This negative pulse is transferred to the grid of V4b, adding to the negative input signal. In a very short time V4b is cut off and V4a is conducting, the reverse of the original state. At this point the circuit is again stable. However, the next negative pulse to come from the plate of V3b starts things all over again and in the same manner V4a now cuts off and V4b conducts.

In this way, it takes two input pulses from V3b to make the V4 circuit execute a complete cycle of change, returning to its original condition. Since the frequency of pulses from V3b is the oscillator frequency, the contralto output taken from the plate circuit of the divider at the junction of R1 and R2 is one-half the oscillator frequency and has a rectangular waveshape.

Each of the two following dividers works in the same way and includes a driver-rectifier triode (V5a and V5b) and two multivibrator triodes (V6a and V6b, V7a and V7b). In each case the input frequency is divided in half, so that there are four outputs an octave apart from the generating section of the instrument - one from the oscillator and three from the dividers.


Two circuits are shown in Fig. 5. The first is the register-control section of the Solovox and the second the tone controls.

The register controls do two jobs. The first and most obvious is the selection of which ranges shall be sounded. The entire instrument covers six octaves. When the soprano switch is closed the three highest octaves are fed to the output bus. When the contralto switch is closed, each note keyed sounds one octave lower, which adds one lower octave to the instrument, and so on down to the bass switch.

The outputs of the frequency dividers are symmetrical in waveform. This is a "muted" tone carrying only odd harmonics. To add even harmonics to the tones of the bass register, a certain amount of tenor output can be added, since the fundamental of the tenor is the second harmonic (one octave above) of the fundamental of the bass. The amplitude of the added tenor must be kept down so that it does not sound to the ear like an additional octave repetition. If, for example, the original bass tone has ½ as much third harmonic as fundamental, 1/9 as much fifth harmonic as fundamental, and so on, then adding a tenor fundamental about ½ as loud as the bass fundamental will give a second harmonic to the bass which will simply fill out the harmonic content of the bass in the correct proportion. The effect is to get rid of the muted tone quality.

With the mute switch of Fig. 5 in the off position the bass receives some additional tone for this purpose from tenor; tenor receives some from contralto; and contralto receives some from soprano. Since there is nothing higher than soprano the special oscillator rectifier of Fig. 10-4 supplies even-harmonic additions for the soprano tone. With the mute switch on, these additions are removed and the muted quality is sounded.

There are five tone-control switches in Fig. 5. All the filters are in series across the register-control output bus. The tone switches are normally closed; when a tone quality is selected the switch is opened. With all but the DEEP TONE switches closed, the tone develops across a highly capacitive load and the high-frequency harmonics are very much reduced. With the FULL TONE switch open, frequency response is almost flat, but with some attenuation of treble to round off the sharp edges of the waveform. FIRST VOICE and SECOND VOICE are roughly 400-cycle and 800-cycle resonant circuits which give brassy and reedy tones, while the BRILLIANT filter is a high-brass filter with a very bright tone. The filters are in the grid circuit of the 6J5 preamplifier, the plate of which couples to the control stage through a transformer.
Hammond Solovox Register and Tone Controllers
Fig. 5. Register and Tone Controllers

The control stage, diagrammed in Fig. 6, consists of a pair of push-pull, remote-cutoff 6SK7's. The two cathodes are connected to the center of a voltage divider consisting of R1 as one leg and R2, R3, R4, and R5 in parallel as the second leg. Normal voltage at the cathode tap is 145 because of the connection of plus 290 volts to the input of the voltage divider. The center of the voltage divider is also connected through R6 and R7 to a bus running the length of the keyboard. One of the contacts under each key is permanently grounded. When any key is pressed the bus is grounded, placing R6 and R7 across the lower (cathode) leg of the divider and reducing cathode voltage to about 45. The rate of drop of the voltage is controlled by the 6-µf capacitor, which causes a time delay.
A second voltage divider is placed across the 290-volt supply, with R8 as its upper leg and R9 as its fixed lower leg. The center of this divider goes to the center-tap of the input transformer and thence to the grids. Shunted across the lower leg of the divider is the network of resistors and the selector switch shown, which varies volume by varying the d.c. grid voltage. At maximum volume the grids are about 30 volts above ground. Since the cathode, when a key is down, is about 45 volts positive, the net grid bias is about 15 volts and the tubes operate normally. As the volume control is closed the lower leg of the grid divider is shunted more and more and the grids become less positive. They therefore become more and more negative with respect to cathode. When no key is pressed the cathodes are about 145 volts positive, enough to cut off the tubes no matter what the position of the volume control.
Hammond Solovox Control stage delays attack and decay
Fig. 6 Control stage delays attack and decay

The fall of grid-cathode voltage when a key is pressed is normally slowed up additionally by the .07-4 capacitor between transformer center-tap the keying circuit. For faster attack the switch is opened and a 22-megohm click-suppressing resistor is in series with the Capacitor.
The output stage is conventional, using a pair of 6K6 tubes.

Extract:- Electronic Musical Instruments by
Richard H. Dorf
Richard Henry Goldfogle Dorf
14th Mar 1921 - 21st June 1989

Richard H. Dorf was an electronic engineer, prolific author on the subject of vacuum tube electronics and electronic organs, and the head of the Schober Organ Corporation – a supplier of self-build electronic organ kits (using patents licensed from Baldwin organ Co.).