X 66

These images are all of an X66 vibrato scanner in for a rebuild. The celeste scanner is virtually identical with the exception of a reduction gear to reduce its speed to 2/3 revolution per second. The celeste scanner also works in conjunction with a phase-shifting line box similar to the vibrato line box, but with fewer sections. What it does is to output two signals with a very slow and subtle two phase vibrato. Because the speed is much slower, the frequency change is likewise proportionally less, and it does not sound like vibrato.

The idea with the celeste is to introduce two signals which likewise are amplified separately and combined acoustically after they leave the speakers. The two celeste signals when mixed in the air imitate the effect of a Leslie speaker on the chorale mode, which turns the Leslie rotors around 40 revolutions per minute as opposed to the 360 RPM of an X66 vibrato scanner, or a similar speed for the Leslie rotors when on the “tremolo” mode.

The reason for doing this is to reproduce the celeste effect of a real pipe organ, where very slight tuning variations between different ranks of pipes produce a slow and random slight unsteadiness and also the effect of motion to the tone. The X66 celeste does a credible imitation of the Leslie chorale effect, which is yet another reason for my premise that although Laurens Hammond wasn't going to allow Leslie speakers on his instruments, he seems to have devoted a lot of time and effort to reproducing electrically several important aspects of Leslie speaker operation. Might we say “imitation is the sincerest form of flattery [or even endorsement in this case?]”

The slow scan celeste of an X66 is not of course random, because it is the same for each revolution of the celeste scanner. However, it does affect different frequencies differently and the interaction between the two signals it outputs is different for every pitch and is therefore very useful musically. The slow rotation as compared with the usual duration of tones held during normal playing of the instrument therefore makes the repetitive aspect of the Hammond celeste unnoticeable and adds to its usefulness.

The musician can turn the celeste effect on or off as he desires, however a very small amount of the celeste effect is permanently added to the percussion section of the X66 and also to a very slight degree, to all of those voices that are derived from the sawtooth wave outputs of the composite tone generating system. It is generally true that the overtones of many real bell-like instruments are in reality partials and not true harmonics which would be by definition not perfectly in tune. It is also true that bells like those in church steeples are often sounded by swinging them on special mountings so that they are in motion when sounding which likewise creates a similar effect. Also, in a piano, most of the notes use three strings sounding together.

However, as any piano tuner can tell you, it is impossible to attain true zero-beat absolute tuning perfection in piano strings and once again, a slow celesting type of effect is present in real pianos as well. Since the X66 does include an imitation piano, celesting the percussion voices adds realism. Therefore, Hammond's deliberate routing of a portion of the percussion through the celeste system is indeed a beneficial effect.

Turning once again to the vibrato in an X66, I mentioned that the vibrato scanner which I showed you in these pictures affects all pitches from Middle G# to the very highest C. The pitch of that G# is 415.30 Hz, and likewise the G just before it is 392 Hz. Although the crossover point in a real Leslie is 800 Hz, Hammond's crossover point is about an octave lower, however he once again parallels a real Leslie speaker by using a different vibrato system [with a correspondingly different type of vibrato effect] for all frequencies below Middle G#. On the next page we'll look at the circuit which does this and see how it works. It's also worthwhile to note that in a real Leslie speaker, although the crossover takes place at a stated frequency of 800 Hz, it is not a sudden change. As you get up around 550 Hz, you will begin to get a little bit of the signal entering the treble rotors, and likewise even up around almost 1000 Hz, a trace of signal still goes through the bass speaker and its rotary element. I have not, by the way, done actual measurements to verify these figures. rather I have listened closely to a real Leslie to see what it does at crossover, and then knowing what notes I played, I looked them up on a frequency chart. However in listening close up to a real Leslie it is probably not possible to determine exactly which speaker is contributing the greatest signal at a particular frequency and thus we must take this preceding observation as exactly that, a somewhat subjective observation by a person relying on his hearing and not upon accurate instrumentation.