X 66

Here's another closer look at the vibrato or celeste scanner from an X66 Hammond. With the top plate removed, this gives a better look at the sets of stationary plates around the central rotor which has two sets of plates that mesh with the stationary plates. Each of the two sets of rotor plates is roughly the same size as any set of stationary plates. However, in a standard Hammond, the scanner has one set of plates on the rotor. In the X66 there are two sets and they are 180° apart. On each end of the rotor assembly there is a brass disc and each of the two sets of plates on the rotor is connected to one of these discs. Right next to each of these two brass discs on the rotor, there is a corresponding stationary brass disc which is insulated from any metal of the scanner. These two pairs of brass discs each constitute a capacitor, inasmuch as that is what an elementary capacitor is. For a more detailed explanation of a basic capacitor, see the description in our Wurlitzer 4600 series article here.

Without going into too technical an explanation, let's just say that in a circuit in which there is a capacitor, alternating current will flow, even though the two plates of a capacitor are separated from each other. Each set of stationary plates all around the scanner, and the two sets on the rotor are also capacitors during the time per revolution that these plates are meshing, and likewise will allow the flow of alternating current in the circuit.

The reason for such an arrangement is that it makes it possible to, in a sense, connect to each section of the vibrato phase shift line box in sequence as the scanner rotor turns, and extract the signal that is present on the stationary plates. And, this is done with no sliding or mechanical contacts which might otherwise eventually wear out. Whenever an X66 Hammond is powered up, the vibrato scanner is rotating at approximately 360 RPM. (which is the vibrato rate for this section of the instrument's tones.) By using the principle of a capacitor, it's possible to contact or access each section of the vibrato line box and yet not have to close any physical contacts, or to use sliding mechanical contacts, all of which in such an application would eventually wear out and create problems.

The signals on the stationary plates are virtually identical, but the phase of each is different, having been phase shifted by the components in the vibrato line box. The scanner "picks off" signals from each line box section in sequence, because the AC audio signal effectively flows from each line box tap to the rotor plates. It is here in the scanner where the actual vibrato is generated. The resulting vibrato signal then appears on the corresponding disc on one end of the scanner rotor. Again by capacitive action, it then effectively flows to the stationary disc and from here it goes through a shielded cable to the vibrato recovery amplifier which makes up for the losses incurred in the scanner and sends the signal off to subsequent sections of the instrument.

The reason for the two sets of plates exactly 180° opposite on the rotor is to generate TWO vibratos simultaneously which are likewise 180° out of phase. That is, when the pitch of one vibrato signal is decreasing, the pitch of the other vibrato signal is increasing. Now if you read my description of what happens in a typical Leslie speaker, you will see that as the Leslie top rotating horn moves towards a listener, the listener will hear (by means of the Doppler effect) a slight increase in the apparent frequency. At the same time, the horn is moving away from the closest wall to the Leslie, and the sound reflected off the wall is DECREASING in pitch (again because of the Doppler effect.)

So we may assume that Hammond wanted to duplicate this characteristic because as we know, he said, "I don't want any of Don Leslie's speakers on MY Hammond organs!" But he WANTED the EFFECT of Don Leslie's speakers, and by generating two opposite phase vibratos simultaneously, he went a long way forwards as far as mimicking that effect.

However, it was not quite that simple. The Leslie tremolo effect is entirely acoustical; sound waves mix in the air. Hammond very quickly found that he could NOT take two 180° out of phase vibrato signals and mix them electrically in a circuit because the result sounds terrible; Do this and you get a vibrato at double the normal rate that sounds like a cross between a really bad, nervous singer and a sheep. So Hammond had to keep these two vibrato signals electrically separate. This is why there are two different coupling disc capacitors in the scanner, each of which sends its signal to independent sections of the vibrato recovery amplifier, and the signals remain segregated right to the speaker cabinet where once again, separate audio channels and speakers handle each one independently. This is why on an X66 Hammond you will find two audio channels designated as either "Tibias" A & B, or "Drawbars" A & B depending on who is describing it.

However, no matter what you call it, the two vibratos from the scanner must be kept entirely separate and only allowed to combine AFTER they leave their respective speakers as sound waves in the air.