51 Detailed Theory Of Operation


5-3. The main tone generator assembly consists principally of 48 rotating sub-assemblies (each subassembly consists of a shaft, 2 disks called tone wheels, and a bakelite gear), and a drive shaft which extends the entire length of the generator. This drive shaft is resiliently coupled at one end to a starting motor and at the other end to a synchronous run motor (reference paragraph 5-12), and is divided into several sections connected by semi-flexible couplings. (See figure 1-5.) A series of 24 driving gears, 2 each of 12 sizes, is mounted on this shaft.

5-4. Twenty-four of the 48 rotating subassemblies are mounted on each side of the drive shaft so that each of the driving gears engages 2 bakelite gears associated with opposite rotating subassemblies. These bakelite gears rotate freely with the tone wheels on separate shafts and are connected to their respective assemblies by a pair of compression-type springs. The bakelite gears are provided in 12 different sizes corresponding to the 12 driving gears of different sizes. Consequently, 4 of the tone wheel subassemblies, each containing 2 tone wheels, operate at each of 12 different speeds. Each driving gear, with its associated bakelite gears and 4 tone wheels, is contained in a separate compartment, magnetically shielded from the rest by steel plates which divide the generator into a series of bins. (See figure 5-2.) All four tone wheels in any one compartment run at the same speed.

5-5. Each tone wheel is a steel disk about 2 inches in diameter and contains a predetermined number of high and low points on its outer edge. (See figure 5-1.) Each high point is called a tooth. There are 12 wheels with 2 teeth, 1 wheel to operate at each of the 12 speeds (reference paragraph 5-4); similarly 12 wheels each have 4 teeth, 8 teeth, 16 teeth, 32 teeth, 64 teeth, and 128 teeth; also 7 tone wheels have 192 teeth. A 2-tooth wheel and a 32-tooth wheel form an assembly, giving 2 frequencies, 4 octaves apart. The 4- and 64-tooth wheels



Figure 5— I. Construction of Main Generator


Figure 5— I. Construction of Main Generator

Hammond Organ Tone Generator



Figure 5 2. Tone Wheel Tooth Cuum hi Generator

4 AND 64 16 AND BLANK 4 AND 64 16 AND 192 4 AND 64 16 AND BLANK 4 AND 64 16 AND 192 4 AND 64 16 AND BLANK 4 AND 64 16 AND 192 4 AND 64 16 AND 192

4 AND 64 16 AND BLANK 4 AND 64 16 AND 192 4 AND 64 16 AND BLANK 4 AND 64 16 AND 192 4 AND 64 16 AND 192

2 AND 32 8 AND 128 2 AND 32 8 AND 128 2 AND 32 8 AND 128 2 AND 32 8 AND 128 2 AND 32 8 AND 128 2 AND 32 8 AND 128 2 AND 32 8 AND 128 2 AND 32 8 AND 128 2 AND 32 8 AND 128 2 AND 32 8 AND 128 2 AND 32 8 AND 128 2 AND 32 8 AND 128



Figure 5 2. Tone Wheel Tooth Cuum hi Generator

are assembled together, as are the 8- and 128-tooth wheels and the 16- and 192-tooth wheels. Five 16-tooth wheels are mounted with blanks to maintain the balance of the rotating unit. (See figure 5-2.) Only 91 frequencies are required for the organ; for identification purposes these frequencies are numbered 1 to 91 inclusive.

5-6. A magnetized rod, about 4 inches long and 1/4 inch in diameter, is mounted near each tone wheel. (See figures 5-1 and 5-2.) A small coil of wire is wound near one end of the magnet. The tip of the magnet at the coil end is ground to a sharp edge and mounted near the edge of the associated tone wheel. Each time that a tooth of the wheel passes the rod, the magnetic circuit changes and a cycle of voltage is induced in the coil. The voltage is very small and is of known frequency. The frequency is predetermined by the number of teeth and the speed of the rotating tone wheel. Larger coils are used with tone wheels of lower frequencies to provide good low frequency output, but smaller coils are used with tone wheels of higher frequency to prevent excessive losses.

5-7. Copper rings are mounted on certain low frequency coils for the purpose of reducing harmonics. The eddy current loss in such a ring is small for the fundamental frequency of the coil, but is high for its harmonics. As a result, the the relative intensities of any harmonics which may be produced by irregularities in the tone wheels are reduced.

5-8. The edge of each tone wheel and the tip of each magnet are coated with lacquer to prevent corrosion, for, should oxidation set in, the change in tooth shape would introduce undesirable frequencies.

5-9. Filters for eliminating spurious harmonics from the generated simple tones are loeated on the top of the main tone generator, and consist of filter capacitors and reactors. (See figure 3-4.) (These capacitors and reactors are tuned units and are called tone generator filters.)

5-10. The tone generator filters have a single tapped winding. This tap is grounded and one side, which is connected to the associated coil assembly through a capacitor, forms a resonant circuit for the fundamental frequency of that coil. Harmonics are supressed. The capacitors for frequencies 49 to 54 inclusive are 0.255 mf, and the capacitors for frequencies 55 to 91 inclusive are 0.105 mf. Both capacitors and reactors are used with frequencies numbered 49 to 91 inclusive. On frequencies 44 to 48 inclusive, the capacitors are omitted, but the reactors used have a greater number of turns. Below frequency 44, neither capacitors nor reactors are used; a length of resistance wire shunts each generator output. This resistance wire is wound on the appropriate magnet coil.

5-11. The tone generator filters are mounted on top of the generator at an angle to minimize reaction between them. Wires connect the filters to the coil assemblies and to the terminal strip on the generator. Ninety-six terminals are provided on this strip; 3 terminals are grounded to the generator frame and serve to ground the manuals and pedals, and 91 terminals carry the various frequencies.

5-12. The start motor is a shaded-pole induction motor. The synchronous run motor (used on 60 cycles) has a 2-pole field and 6-pole armature, and a synchronous speed of 1,200 rpm (revolutions per minute). For 50 cycles, a

4-pole armature is used which has a speed of

1,500 rpm. When the organ is placed into operation, the start switch is first operated to apply power to the start motor. The rotor of the start motor slides endwise and engages a pinion on its shaft which a gear on the generator drive shaft. (See figure 5-3.) When the "RUN" switch is operated, while the start switch is held in "ON" position, power is applied to the synchronous run motor and a 250-ohm resistor (1,000 ohm for 234 volts) is connected in series with the start motor, thus reducing the driving power of the start motor. Because of the braking action and the loss of power of the start motor, the system slows down to, and locks into, synchronous speed; the run motor then begins to carry the load. When the "START" switch is released and springs back into position, the start motor disengages from the drive shaft by action of a spring assembly, and stops.

5-13. The spring couplings of the motor shaft, the flexible couplings between the sections of the drive shaft, and the tone wheel spring couplings are provided to absorb the variations in motor speed. The synchronous motor operates with a series of pulsations, one each half-cycle. If the tone wheels were coupled rigidly to the motor, this irregularity would carry extra frequencies into each tone wheel. The spring suspension system for supporting the main tone generator minimizes the transmission of mechanical <vibration between the console cabinet and the main generator.


5-15. The vibrato effect is created by a periodic raising and lowering of pitch, and thus is funda

Hammond Syncronous Runmotor Inside
l-ïgure 5 3. Starting Mu tor

mentally different from a tremolo or loudness variation. Il is comparable to the effect produced when a violinist moves his finger back and forth Oil a string while playing, varying the frequency while maintiuninp. constant volurni,

5-16. IliC Hammond Organ vibralo equipment, as shown in simplified block Uiugram. figure 5-4, varies the frequency of all tones by continuously shifting their phase. It includes a phase shift network or electrical time delay line, composed of a number of low pass filler sections, and 3 capacity type pickup or scanncr, which is motor-driven so thai itoscans back and forth along the fine.

S 17. Electrical waves fed into the hue are Shifted in phase by each line scctioil (the amount per section being proportional to frequency), so that at any tap Oil the line, the phase is retarded relative to the previous lap.

5-18. The scanning pick-up traveling ulong ihe line will ihus encounter waves increasingly re-

Drawbar Hammond Organ Schematic Diy

Fifiwc 5 4. Fundamental Diagram of Vibrato System larded in phase at each successive lap, and the signal it picks up will continuously change hi phase. The rate at which this phase shift occurs will depend Oil how many line sections arc scanned cach second.

5-19. Since a cycle is equivalent to 360 electrical degrees, a frequency shift of I cycle occurs for each 360 electrical decrees scanned per second. For example, if the scanner passes over the line at such a rale that 3,600 electrical degrees are scanned each second, there will be a frequency change of 10 cycles.

5-20. For the widest vibrato, the whole line is scanned from beginning to end hi about 1 ,'14 second, and ihis rate of change of phase causes about 1-1/2 percent decrease hi frequency. Note that the frequency remains constantly 1-1 ¡2 percent low as long as the moving pick-up retards the phase at a constant rate.

5-21. Since the pick-up sweeps from start to end of the line and then back, it increases the frequency by an equal percentage on its return trip, the average output frequency remaining equal to the input frequency. 'ITie exact amount of frequency shift depends not only on the amount of phase shift in the line but also on the scanning rate. This rate, however, is constant because the scanner is driven by lite synchronous running motor of the organ.

5-22. The degree of vibrato (or amount of frequency shift) may be varied by a switch (not shown ill figure 5-4) which causes the whole line to be scanned for No. 3 (wide) vibrato, about half of it for No. 2: and about one-third for No. 1.

5-23. A vibrato chorus cffcct, similar to the effect of 2 or 3 slightly oul-of-Uine frequencies mixed together, is obtained when the vibrato output signal is mixed with a portion of signal without vibrulo. For vibrato chorus, part of the incoming .signal appears across the vibrato line and the rest across a resistor in series with the line. As the vibrato effect is applied to the part Of the Signal appearing across the line, but not to the part, appearing across the resistor, the combination produces a cho/US effect. For normal vibrato, this resistor is short-circuited. In the Model G3 console the vibrato effect can be applied to either manual separately or to both at once.

5-24. Figure 5-5 shows the vibrato line box. Each of the inductance coils is connected with one or more capacitors to form one filter section.

5-25. Figure 5-7 shows the construction of the vibrato switch.

5-26. The scanner, shown in figure 5-6, is mounted on the main generator synchronous motor and driven at 41 2 revolutions per minute. It is a multi-pole variahle capacitor with 16 sets of stationary plates and a rotor whose plates mesh with the stationary ones. In figure S-7, Index R, two sets of plates have been removed to show the rotor.

5-27. Signals coming from the line through the vibrato switch appear on the stationary plates and a/C picked up, one at a time, by the rotor. Connection to the rotor is made by carbon brushes, as shown in figure 5-6. Index A. Two brushes touch the sides Of the contact pin and a third presses on the end, in order to eliminate the possibility of contact failure.

5-28. Figure 5-8 shows the vibrato circuit.

5-29. The vibrato switch lias no "OFF" position, and 3 vibrato chorus positions (CI, C2, and C3j are included in it as well as the 3 vibrato positions (VI, V2, and V3). 'ITie vibrato effect is turned "ON" and "OFF" for cach manual separately by means of "VIBRA IO SWELL"and "VIBRATO CRKAT" tablets on the manual assembly.

5-30. The preamplifier used with this circuit has two separate channels into which signals from the "VIBRATO OREAT" and "VIBRATO SWELL" tablets are fed. (Reference paragraph 5-37.) The "VIBRATO"signal goes through a preliminary amplifier, through Ihe vibrato

Figure 5 5. Vibrato Line Box

END BStjSrt vCWtfTEO <M ITAf at>ftiMG




END BStjSrt vCWtfTEO <M ITAf at>ftiMG



£ sets plates remcveoto sho* rorcn."



Figure 5—6. Vibrato Scanncr



Hammond Tonewheel 192 Teeth



(bottom view with coven removes)



Figure 5—8. Schematic system, and then into additional stages of amplification. The "NO VIBRATO" signal also has a preliminary amplifier, but bypasses the vibrato system and goes directly into the additional amplifier stages.


5-32. The 9 contact springs on each key are connected by resistance wires to the proper terminals on the terminal strip and carry the harmonics of the particular note with which they are associated. (Reference paragraph 1-20.) The resistance wires avoid overloading of the generators and thus allow each generator to be used independently to feed a number of key circuits. All key contacts are alive at all times. When a playing key is depressed, its 9 frequencies are impressed on the 9 bus bars of the manual. No wires are connected to these bus bars; a preset or adjust key must be depressed to complete the circuit. (See figure 3-3.) Each preset or adjust key is provided with 9 contacts identical to those on the playing keys and is further provided with a locking and tripping mechanism, the purpose of which is to permit only 1 preset or adjust key to be in operation at a time. (See figure 1-6.) The cancel key releases a depressed preset or adjust key; this cancel key has no contacts.

5-33. Flexible wires connect the 9 contacts of each adjust key (A# and B) to the 9 drawbars contollled by the key. The wires are color-coded

Diagram, Vibrato System for identification. Each drawbar makes contact (according to the stop position to which it is drawn) with any one of 9 bus bars connected to taps on the mixing transformer. (See figure 3-3.) The bus bars correspond to different intensities of sound.

5-34. The 9 preset keys (Cff to A) are connected by flexible leads to the preset panel in the back of the console. (See figure 1-4.) The preset panel consists of 2 sets of 9 bus bars which correspond to those in the drawbar assembly and which are connected to the same taps on the mixing transformers.

5-35. The mixing transformers are mounted on the manual chassis assembly as shown in figure 1-4. Shielded leads carry the signals from the secondaries of these transformers to the preamplifier.

5-36. PEDAL SWITCH ASSEMBLY. The pedal switch assembly is similar in operation to the manual chassis assembly (reference paragraphs 5-31 to 5-35 inclusive); the pedal switch assembly, however, contains only 4 bus bars instead oft9- A flat spring at the end of each pedal of the detachable pedal clavier depresses a small plunger, as shown in figure"3-2, on the pedal switch assembly and actuates a double set of contact springs, thus making eight contacts available for each note The pedal contact springs are connected by decoupling resistance wires to terminals. A cable connects these terminals through a wiring lube to lite proper rerminals on the main tone generulor strip. Tnc pedal switch bus bars are connected, by means of four colored wires, through u filter reaclor and resistor network to the pedal drawbars. < See figure .v9.'l The reactor and resistors filter out undesirable higher harmonics and serve to balance the pedal tones.

Figure 5 9. Pedal Switch Contacts


5-38. Typical Circuit Before Pre-amplil'ier.

5-39. Each voltage of predetermined frequency produced by the lone generator is connected to one or more key contacts. When the associated playing key is depressed, this voltage is impressed upon the hits bar and is carried through the pre set key switch to the preset panel. 'Ilic- voltage is then fed to one of Ihe several taps of the mixing transformer which is^associatcd with the manual being played. From the high impedance secondary of the mixing transformer, this voltage (combined with others which may ha fed through simultaneously) passes to one of the preamplifier input circuits. (Vibrato "ON" or "OFF" circuit).

5-40. Power to operate the preamplifier and power amplifier is supplied through the ran switch circuit as shown in figure 3-3.

5-41. Preamplifier Circuit. Input.

5-42. The signal from each mixing transformer is sent to ihe Vibrato "ON-OFF" tablet associated with its particulai manual, and is then carried to the "VIBRATO" or "NO VIBRATO" preamplifier input, depending on the position of the tablet.

5-43. The input circuits arc similar, with one extra stage of amplification in the "VIBRATO" channel to compensate for the loss that occurs througll the phase Shift network and associated scanner. 'I"he input tube V4 receives the signals from "VIBRATO" a:id "NO VIBRATO" circuits and further amplifies them. The signal then is impressed on the "I.Ol.'D" stator of the volume control, and on the "SOFT" stator through a compensating network,

5-44. Volume or Swell Control.

5-15. The volume control is activated by the swell pedal connected by an appropriate linkage. (See figure- 1-4.) The volume control assembly consists of two sets of stator plates, similar to those used in the scanner assembly. (See figure 5-6.) A rolor assembly of similar size is moved by the swell pedal and is capable of meshing with either stator or a portion of each. Tile degree of mesh determines the strength of the entire signal.

5-4f>. Ihe signal is further amplified by the second scction of V4 and sent to driver tube V3 which in turn drives the I2BII7 output tube.

5-48. The "Touch Response" percussion feature is controlled by four tilting tablets (figure 1-1). It is available only on the Upper manual and only when the "U" adjust key is depressed. Percussion tones are produced by borrowing the second or third harmonic (depending 0:: position of the "Percussion Harmonic Selector" tablet) from the corresponding drawbar of the Upper manual "B" adjust key group, amplifying it, returning part of it to the same drawbar, and conducting the balance through push-pull control tubes which, when keyed, cause the signal to fade away at a pre-dctcrmincd rate.

5-49. With the percussion tablet on. "B" adjust key pressed, and an upper manual playing key pressed, the second or third harmonic signal goes to percussion input terminal II on the preamplifier chassis and is amplified by T4 and TS. Ihe percussion input transformer T5 not only provides push-pull signal for the control tube V7 but also has a third winding which feeds signal back to the 2nd or 3rd harmonic drawbar tluough equivalent key circuit resistor R50 and terminal "J".

5-50. When a key is depressed, the note first sounds loudly, after passing througll the control tube V7, transformer T6, a high pass filter, and terminal I) to the grid of V4. Immediately, capacitor (31 in the control tube firid circuit begins to discharge, causing the signal to lade away.

Hammond Busbar Wiring






5-51. This circuit works as follows: Terminal K (Approximately -25 volts) is connected to the 8th harmonic "B" adjust key drawbar wire, which is connected through the adjust key contact to the manual bus bar. Pressing any upper manual key connects this bus bar to a tone generator terminal and virtually grounds terminal K through the tone generator filters. This virtually grounds the plate of V6, stops conduction, isolates the cathode of V6, and thus isolates the grid circuit of control tube V7. The grid then drifts from about -25 volts to about -15 volts, at a rate determined by the time required for C31 to discharge through R57 and R58. At the completion of this sequence, the percussion signal is blocked so that it is no longer audible.

5-52. No further percussion signal can be heard until all keys of the upper manual are released so that the control tube V7 grids can again drop to -25 volts (the rate of this drop is fixed by the time required to charge C31 to -15 volts through R55 and R56). Thus the percussion effect is heard only when keys are played in a detached manner; that is, when all keys are released before pressing the next one.

5-53. REVERBERATION UNIT. (See figure 1-10.) This device simulates musically desirable echoes in a large room. An electrical signal from the amplifier is applied to the driver coil in the reverberation unit, which converts the electrical signal into a twisting movement of 3 coil springs. This motion is transmitted along each spring to a pickup unit, where part of it is converted back to electrical energy. The remaining portion is reflected back to the driver and again back to the pickup after a time interval determined by the spring length. This reflection process continues until the signal level is reduced to about one millionth of its signal value so that it is no longer audible. The springs are different in length and thus there are 3 separate sets of echoes, each repeated a number of times. Electronic amplification circuitry associated with the reverberation unit is contained in the power amplifier, described below.

5-54. POWER AMPLIFIER. (See figure 5-10.)

5-55. This is a 3-channel amplifier with 2 treble channels (one for non-reverberated and one for reverberated signal) and a bass channel, with a cross-over point of 200 cycles. Each channel has two 6BQ5 output tubes with self-bias. Each treble channel drives a 12" speaker, and the bass channel drives two 15" speakers in parallel.

5-56. The power supply unit is a separate chassis housing the power transformer, rectifier tubes, filter, and input connections for power and signal. A 6-pin plug engages the console cable, and a 5-pin receptacle is provided for plugging in additional tone cabinets. The console cable consists of 5 conductors; 2 for AC power, 2 for push-pull signal, and ground.

5-57. The push-pull signal from the console (G1 and G2) drives treble input tube VI. Resistance-capacitance filters ahead of V1 filter out signal frequencies below 200 cycles. VI drives output tubes V2 and V3 of the treble direct channel. It also drives double triode tube V9 which, in turn, drives the reverberation unit.

5-58. The output of the reverberation unit passes through transistor TR-1, and part of the signal goes to the treble reverberation switch. This adjusts the amount of reverberated signal going into VI0, which drives output tubes VI1 and VI2 of the treble reverberation channel. The switch, in its "off' position, picks up signal from input terminal Gl, in order to make use of the channel for non-reverberated signal when the treble reverberation is off.

5-59. Both treble channel output transformers have tertiary windings which supply inverse feedback signal to the cathodes of the output tubes.

5-60. A portion of the output of transistor TR-1 goes to double triode tube V4, which is connected as a phase splitter to drive the push-pull bass channel. The output of V4 goes to the bass reverberation switch, which is also connected to the input terminals Gl and G2. The bass channel receives a large amount of reverberated signal along with some direct signal in the "III" position, only non-reverberated signal in the "OFF" position, and varying mixtures in the intermediate positions.

5-61. A filter network following the bass reverberation switch filters out signal frequencies above 200 cycles. Following it is a "room size" switch which can be used to provide better balance by reducing the bass volume when used in a small room. The signal then feeds push-pull tubes V5 and V6, which drive the bass output tubes V7 and V8.

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