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Hammond Chord organ
Technical Description
Richard H. Dorf

 Hammond Chord Organ
Fig 1. The Hammond Chord Organ

The Hammond Instrument Company has an interesting history which not only includes being among the first to popularize the nonacoustic musical instrument but also emphasizes the unconventional. The Hammond organ with its drawbars and the Solovox with its original idea of a melody instrument for use with the piano are the most outstanding items; and the Nova-chord, with its complex combinations of effects, though no longer manufactured, is remembered with pleasure by many players and listeners.

The newest Hammond contribution is the Chord Organ shown in Fig. 1. Though it adds little to the art of music as such, it is designed as an instrument which can fulfill the dreams of many innately musical people who have not had the opportunity to learn to play normal instruments. The Chord Organ is primarily for one-finger artists, and it gives them, with small practice, the ability to play full musical selections, complete with harmonies. To do this it resorts to more complexity than most electronic instruments (which are usually compound-having many similar circuits) but it is easy to understand and is so ingeniously designed that the Rube Goldberg aspect disappears after thorough examination.

Examination of Fig. 1. shows that the organ has a 37-note key manual, a board at the left with 96 buttons, a row of control tablets above the manual and buttons, and a pair of pedals.

There are four divisions. The solo division operates in the same manner as a
Solovox; Hammond Solovox
Hammond Solovox
it is suitable for one-note-at-a-time melody playing, using the keyboard. BASS, TENOR, and SOPRANO tablets control the registers, somewhat as the 4', 8', and 16' registers are selected in a normal organ. FAST ATTACK and ACCENT tablets give the solo tones a fast attack or a percussive quality. A WOODWINDS tablet gives a symmetrical waveform emphasizing odd harmonics. And DEEP TONE, FULL TONE, FIRST VOICE, SECOND VOICE, and BRILLIANT tablets give various tone colors.

The organ division, played on the same keyboard at the same time, gives polyphonic music - several notes simultaneously. String and flute tone tablets are provided to call forth either quality or both together. Thus, when the manual alone is played with several notes simultaneously, the organ division is heard on all notes and the solo division on the top note only.
The Hammond Chord Organ's button board
Fig 2. The Chord Organ's button board

The chord division is the main distinguishing feature. Fig. 2 is a drawing of the button board. When a single button is pressed, a chord sounds. There are 96 buttons; for each of the twelve musical keys there are eight chords -from minor seventh to major plus sixth. Button caps are provided so that before playing a selection the chords which will be used may be marked with the caps to make recognition quicker. In Fig. 2, for instance, a typical selection in the key of C calls for F major (subdominant), G seventh (dominant seventh), D seventh (dominant seventh of dominant), and so on. The caps are movable, of course.

A SUSTAIN-CANCEL tab is provided for the chord division. With the switch on, pushing a chord button brings in the chord at moderate volume; when the chord bar (see Fig. 1) is pressed at the same time with the palm or thumb, the chord gets louder. With the tab off the chord will not sound at all unless the chord bar is pressed. A MUTE tablet makes chords more "mellow."

The pedal division consists of the two pedals and a FAST DECAY tab. When the left pedal is pressed, a low-octave tone similar to the bass note of the chord being played is heard; when the right pedal is played, the pedal tone is a fifth higher; the two give variety. The FAST ATTACK tab makes pedal tone disappear almost at once; without it, "the melody lingers on."
Block diagram of the Hammond Chord Organ
Fig 3. Block diagram of the Chord Organ

The block diagram of Fig. 3 gives an idea of what is in the organ behind the panelling, though the diagram is very much simplified. There are three separate generating systems, all using vacuum-tube oscillators. The solo and organ generators are controlled by the keyboard, after which the selected tones go to the tab controls, thence to a pair of volume controls called balancers, the amplifier, expression control, and built-in speaker.

The chord generators are controlled by the chord button, the chord bar, and the pedals, as well as by the tabs. Chord-button tones have no balancer as they are fixed in relative level; pedal tones can be balanced. In the following we shall describe the divisions separately, which will clear the cobwebs of complexity.
Partial schematic of the Hammond chord organ solo division
Fig 4. Partial schematic of the solo division
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The solo division of the Chord Organ is really a complete Solo-vox. It is shown schematically in Fig. 4. V1-C2 is the oscillator, which is tuned over the range from F-349.2 cycles to F-2794 cycles. The tuning is done by the 37 tuning inductors, which are connected in series between the grid of V1 and ground. When a key is pressed, a corresponding key contact connected between two of the series coils shorts the junction to the solo tuning bar, which is grounded. This reduces the net inductance between V1 grid and ground, raising the oscillator frequency to that of the key. The lowest F has no tuning contact. since with all the inductors working the oscillator tunes to the low F. C84 is the main tuning capacitor, while the two groups C1 to C5 and C7 to C12 are for coarse and fine tuning respectively. They tune the entire range, of course, not the individual notes. The latter may be tuned by sliding the cores in and out of the inductors.

The oscillator rectifier V32 creates the sharp positive pulse necessary for the driver, the second triode of V3. The latter drives an aperiodic flip-flop circuit which reverses its condition once per pulse - once per oscillator cycle. Output taken from one side of the flip-flop is an almost square wave of frequency half that of the oscillator, or one octave below. The output of the first flip-flop frequency divider is taken from pin 10f V4 through R20.

A second driver and flip-flop divider is driven by output from the first, so that for each oscillator tone, there are three octavely related frequencies made available.

For each frequency there are two tones, woodwind tone and a complex one. The highest-tone woodwind output is the cathode of oscillator V1, a sine wave. The next two are square waves from the plates of one triode of each divider. The complex tones are taken from the plates of the rectifiers and driver stages. Waveforms are shown in Fig. 4.

The six outputs pass through modifying networks of resistors and capacitors to take some of the "edge" off the tones and make them more pleasing. The BASS, TENOR, and SOPRANO switches - double-pole, double-throw - switch the three registers as desired to the WOODWIND switch. With the latter on, the grid of solo preamplifier V8a is connected to the woodwind modifying networks; when the tab switch is off, the more complex tone goes to the preamplifier grid.

The preamplifier plate circuit feeds a tone-color network containing five sections in series between the plate line and ground. Each section is normally shorted by a tab switch when the switch is in the nominal off position - contacts closed on one side. When, for ex ample, the 1ST VOICE tab is placed in the on position, the parallel combination of L40, C54, and R68 is placed across the signal line. This gives the tone a peak near 750 cycles, imparting to it a horn-like quality. The 2ND VOICE section peaks at around 1,000 cycles. DEEP TONE places a capacitor across the line to cut highs and make the tone more "mellow," while FULL TONE has only a resistor and gives flat response. BRILLIANT shunts the line with an inductor, reducing bass to give a rather piercing quality.

The solo control stage V9-V1 exists to allow control of the tonal attack. Normally the cathodes are at about plus 65 volts, obtained by voltage division from the 270-volt point shown in the diagram. This cuts off the stage. When any key is pressed a solo control line connected to point X is shorted to ground by the solo control busbar under the keys and the key contact. This shorts the bias voltage to ground. With the switches in the positions shown, C58 makes the attack fairly slow because a sudden decrease in the cathode voltage causes a negative surge through the capacitor, charges C(() negatively, and moves the grid in the negative direction, which remains until the charge on C00 leaks off through R77. When the fast attack tab is operated its switch opens, disconnecting C58. With the SOLO ACCENT switch on not only is C58 disconnected but Cr52 is connected across R80. For the sudden decrease in cathode voltage caused by pressing a key, C62 effectively shorts the resistor and reduces the bias for an instant, causing the note to be loud at first and giving a rather percussive effect.

Output from the solo division is controlled by a balancer potentiometer, the arm of which goes to the main organ preamplifier in common with outputs of the other divisions.
Partial circuit diagram of the Hammond chord organ divisions
Fig 5. Partial circuit of the organ division

A simplified circuit of the organ division is shown in Fig. 5. The generator system for the 37 tones consists of sixteen L-C oscillators, each of which can be tuned to either of two frequencies, except for the lower four and upper one, which can be tuned to three frequencies.

The tuning is done automatically when a key is pressed. Normally the lowest oscillator, for instance, the first triode of V12, is tuned to the frequency of low G by C87 across the tuning inductor. When the F# key is pressed a contact connected to the lower end of C90 strikes the upper busbar, connecting C89-R1 17-C90 across half the coil and lowering the frequency. When the F key is pressed a contact connects only C89 across the lower half of the coil, lowering the frequency to F.

The organ oscillators are normally not operating because they have no plate voltage. Whenever a key is pressed, a contact in the lower row strikes the lower busbar. This carries the B-supply of 280 volts to the plate of the corresponding oscillator through a simple R-C network which softens the attack somewhat. Each organ oscillator has two outputs, one from the upper end of the tuned circuit giving a sine-wave or flute tone, and the other from the lower end giving a waveform like that shown, known as the string tone. All similar outputs of all oscillators are paralleled and brought through tab switches and tone modifying networks to the organ division balancer, the arm of which goes to the common preamplifier grid. It should be noted that four busbars run under all the keys. Two are shown in Fig. 5 and two in Fig. 4. The idea of using one oscillator for two or three notes is that not too much music calls for simultaneous playing of two adjacent notes, especially the fairly simple music which a typical Chord Organ player would probably use. The frequencies covered by the organ oscillators are 174.6 to 1397, the F just below middle C to that three octaves higher.
Hammond chord organ Pedal and chord divisions
Fig 6. Pedal and chord division and amplifier
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Figure 6 shows the pedal and chord divisions, as well as the amplifier section, which is simple.

There are six chord oscillators, each tuneable to two frequencies, making a full octave of twelve tones available. No chord has more than four notes and all use tones between F-174.6 and E-329.6 cycles. The chord oscillators also supply the tones which pass through frequency dividers and give the pedal notes.

Each oscillator has four contacts, one associated with each of four busbars. When a contact is touched to the upper busbar the oscillator is moved down one-half tone by connecting a capacitor from the tap on the tuning inductor to the grounded bar. When a contact is touched to the second bar the oscillator plate output is connected to that bar. This second busbar carries the chord output signals.

The lower two busbars carry pedal output signals taken from the oscillators in the same way.

The chord actuating system is a mechanical assembly which cannot well be shown here. Each of the 96 buttons selects the right three or four notes for the chord and the correct two notes for the pedals. The notes are predetermined by the positions of small projections on 96 pivoted levers underneath the buttons. The projections press actuators to operate the required contact springs.

Let us take an example, say the button which creates the C-major chord consisting of C-E-G. When the button is pressed one lever projection actuates the contacts connecting the B-C oscillator to the chord signal busbar. (This oscillator need not be tuned since it is normally running at the frequency of C.) A second projection connects the D#-E oscillator output to the chord signal busbar. A third tunes the G-G# oscillator to G by closing the tuning contact for that oscillator. A fourth closes the contacts carrying G-G# oscillator output to the chord signal bar. This completes the formation of the chord.

In addition a fifth lever projection connects a B-C oscillator output contact to the left pedal signal busbar and a sixth connects the G-G# oscillator output to the right pedal signal bar.

The chord output signals from the chord signal busbar go to the chord control tube, shunted on the way by the mute switch which places a capacitor across the line to produce more "mellow" quality when desired. The tube, V1, normally has 85 volts of negative bias on the grid from a fixed source, cutting off plate current. When the chord bar is pressed the bias disappears, allowing the chords to come through.

Chord signals also go through the SUSTAIN CANCEL switch to the input of the preamplifier, pin 70f V20, to the same point reached by the outputs of the solo and organ divisions. The preamplifier output goes to the second half, V20a bypassing the chord control tube. Thus when the SUSTAIN CANCEL switch is closed a reduced-level chord signal comes through even though the chord bar may not be pressed.

The outputs of the two pedal signal busbars go to the two pedals, which are mechanically interlocked. Considering only the signal contacts of the pedals, for the moment, output from whichever pedal is pressed goes to the input of a two-stage frequency divider exactly similar to those used in the solo division. In this way pedal tones two octaves below the chord tones are produced. The output of the frequency dividers goes through an R-C tone modifying network to the grid of the pedal control tube, pin 2 of V8b. The grid is normally at 85 volts negative. When the pedal is pushed, a control contact on it removes the tube bias, allowing the tone to come through. The mechanical interlocking of the pedal signal contacts is such that when the pedal is released the last signal contact made is maintained. This keeps the tone going while the bias on the pedal control tube slowly returns through the time-constant network and the tone dies away. The pedal fast attack switch modifies the time constant to make for faster attack and decay.

The output of the pedal control tube joins all the other signals at the grid of amplifier V20b, pin 2, after passing through a pedal balancer potentiometer. The VOLUME SOFT switch in this grid circuit simply shunts the line to ground through the 18,000-ohm resistor, reducing the volume of the entire instrument.

The output of the amplifier triode goes to the expression control. This control is a special variable air capacitor with two stator plates. One stator is connected directly to' the amplifier plate through a blocking capacitor .004-pi. The other stator plate is connected to the amplifier plate through a tone-compensated attenuation network. The rotor is connected to the No. 2 grid of a phase splitter of the common-cathode type. The position of the rotor, controlled through a knee lever extending from the underside of the organ keyboard, controls the level of the signal reaching the phase splitter.

The phase splitter is the driver for a conventional 6V6 output stage. Radio-phonograph inputs are provided at the phase splitter grid. The amplifier has a negative-feedback connection from output transformer to phase-splitter grid, the frequency characteristic of which can be adjusted somewhat by a variable capacitor. The vibrato of the Chord Organ is similar to that in the Solovox - a phase-shift oscillator and switching tube connected to the grids of the oscillators through a switch which grounds the grids when vibrato is not desired.

Despite the apparent complexity of the Chord Organ it is extraordinarily compact. Figure 1 shows the main controls. Figure 7 is a rear view showing how the electronisms are mounted in the case. Figure 8 is taken from above the rear of the organ with the top removed and the upper chassis swung down for test or repair. The organ works with the chassis in this position, except that the expression control lever will not actuate the control. The linkage for it can be seen in the chassis. Figure 9 is the front of the organ with top removed, and the board holding the tab switches taken off and lying upside down.
Hammond chord organ interior
Fig 7. Rear view shows how parts are mounted

Rear of the Hammond Chord organ
Fig 8. Photo from rear, with top removed and upper chassis swung down

Front of the Hammond organ
Fig 9. Front of the organ. Top is off and the switch board is upside down

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.).