Hammond Organ Technical Information
Vibrato Equipment

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5-l5. The vibrato effect is created by a periodic raising and lowering of pitch, and thus is fundamentally different from a tremolo or loudness variation. It is comparable to the effect produced when a violinist moves his finger back and forth on a string while playing, varying the frequency while maintaining constant volume.

5-l6. The Hammond Organ vibrato equipment, as shown in simplified block diagram, 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 filter sections, and a capacity type pickup or scanner, which is motor-driven so that it scans back and forth along the line.

5-17. Electrical waves fed into the line are shifted in phase by each line section (the amount per section being proportional to frequency), so that at any tap on the line, the phase is retarded relative to the previous tap.

5-l 8. The scanning pick-up traveling along the line will thus encounter waves increasingly retarded in phase at each successive tap, and the signal it picks up will continuously change in phase. The rate at which this phase shift occurs will depend on how many line sections are scanned each second.

5-19. Since a cycle is equivalent to 360 electrical degrees, a frequency shift of 1 cycle occurs for each 360 electrical degrees scanned per second. For example, if the scanner passes over the line at such a rate 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 in about l/14 second and this rate of change of phase causes about 1-1 /2 percent decrease in frequency. Note that the frequency remains constantly l-l /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. The 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 the 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 in 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 effect, similar to the effect of 2 or 3 slightly out-of-tune frequencies mixed together, is obtained when the vibrato output signal is mixed with a portion of signal without vibrato. 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 chorus effect. For normal vibrato this resistor is short-circuited. In the Model C-3 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.

S-26. The scanner, shown in figure 5-6, is mounted on the main generator synchronous motor and driven at 412 revolutions per minute. It is a multi-pole variable capacitor with 16 sets of stationary plates and a rotor whose plates mesh with the stationary ones. In figure 5-7, Index B, 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 are 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 has no "OFF" position and 3 vibrato chorus positions (Cl, C2, and 'C3) are included in it as well as the 3 vibrato positions (Vl , V2, and V3). The vibrato effect is turned "ON" and "OFF" for each manual separately by means of "VIBRATO SWELL" and "VIBRATO GREAT" tablets on the manual assembly.

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

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