X 66

~ BASS VIBRATO ~

The picture above shows the circuit card which produces what Hammond refers to as "bass vibrato." This vibrato affects all frequencies on the drawbar section from bottom C to the G above Middle C. Again I will insert my opinion here, which is that Hammond probably did this for two reasons. First, the traditional Hammond vibrato system using a vibrato line box and a scanner tends to produce more vibrato as you ascend the scale, until you approach the cutoff frequency of the filter sections in the line box. From that statement we may infer that as you go lower in pitch, the vibrato effect is less. Of course if you are emulating what happens in a real pipe organ, then having less intense vibrato on lower pitches is what happens anyhow, and in most pipe organs, all pipes below the pitch of Tenor C (130.8 Hz) are not tremmed at all.

Reason two of course is [my opinion] that Hammond was trying to make the X66 sound like it was playing through a Leslie speaker without having an actual Leslie speaker in the deal. Leslie speakers run all lower frequencies through a 15" speaker with a rotary scoop-like baffle after the speaker which produces a somewhat different sounding vibrato-tremolo mix from that produced by the rotary treble horn.

In a real Leslie speaker, the Leslie Tremolo effect tends to disappear on really low frequencies such as bass pedal tones. This, however is a good thing, because vibrato on bass tones sounds terrible for some reason. Once again I'll insert my opinion. The reason why vibrato on low bass sounds bad is because the frequencies of the bass tones begin to approach the vibrato frequency, or at least that's what I think anyhow. Interestingly, because of the technology used, the Hammond bass vibrato system likewise after a certain point produces a less intense vibrato as you get into the bass range, but in the range from around GG (98 Hz) to Middle G (392 Hz) , it produces a better sounding vibrato than a scanner system would. So once again it seems that Laurens Hammond was doing electrically what Leslie speakers do acoustically, and I'd have to say from playing several X66s including our group's instrument, that he succeeded in emulating the effect of a Leslie quite well.

There is also a vibrato tab to place vibrato on the bass pedal tones of an X66. This vibrato comes from a different type of scanner/linebox system which also makes the vibrato that the X66 uses on all voices that are produced by the formant system, and we'll get to that shortly. Some people erroneously think that the bass vibrato card also affects the pedals but in fact it does not.

The operation of the bass vibrato card is to shift the phase of the affected frequencies by a certain amount which is essentially the same for all affected frequencies. It then selectively gates these tones on and off, and the result of combining these tones results in a signal which has vibrato. One of the differences in the Hammond system as compared with Wurlitzer is that it produces a 180° phase shift in the phase of the affected signals throughout a significant portion of its frequency range instead of 90° like Wurlitzer's system. Another significant difference is the way that it gates the signals, and the third difference is that it has three phase shifting/gating circuits in series.

The Hammond system uses three saturable reactors to do the gating, and the reactors are controlled by a DC voltage which is applied to their control coils, and which pulsates sinusoidally at the vibrato rate. For a little rundown on what a saturable reactor is and how it works, go here. Also, by the nature of inductive reactance vs frequency, the effect of saturable reactors on lower frequencies is less than that on higher frequencies, so the Hammond system for producing vibrato on lower frequencies does indeed produce a less intense vibrato as you get into really low frequencies which is exactly what happens with real pipe organs and also in Leslie speakers.

Initially, beginning with G above Middle C, the highest frequency that goes through this bass vibrato system, the effect would be to increase the vibrato intensity as the frequency gets lower. However, the effect of the saturable reactors decreases as you lower the frequency so that the bass vibrato does not in fact increase in intensity but actually, when you get to the really low frequencies, decreases in effect, which is the desirable outcome, because vibrato on very low bass tones does not sound good.

By its design, the Leslie speaker's effect on really low bass tones becomes significantly reduced. Therefore, we may conclude that Laurens Hammond was electrically imitating the Lesie effect as closely and in as many ways as he possibly could.

The operation of the bass vibrato system in an X66 is best understood by referring to the schematic below, along with the description in the X66 service manual.

The bass vibrato is produced by a reactor circuit which varies the frequencies of the tones by means of a continuous phase shift. The circuit is comprised of three series-connected shifter stages, each with its own transistor and saturable reactor. A single oscillator and a driver provide the in­put to the phase shifters. A voltage amplifier is used in the output.

A low frequency oscillator establishes the vibrato rate. Pulses from the oscillator are supplied to the driver, the emitter of which is in series with the three saturable reactors. The output of the final phase shifter is amplified by another transistor before being routed to the expression preamplifier board. The second stage (01-2108) of the driver draws current when the first stage is off, thereby providing a constant current drain. The constant drain prevents oscillator pulses from adversely affecting the power supply.

The continuous phase shift is accomplished by using 180° out-of-phase signals from the collector and emitter of each shifter stage and controlling them with the saturable reactors. The reactors serve as a means of providing a varying composite of signals from each stage, ranging virtually from full emitter to full collector signal.

What all this stuff [which I copied out of the X66 service manual] simply means is that signals on the bases and collectors of transistors in audio circuits like these are 180° out of phase, and if you gradually go from one to the other, you'll get a continuous and gradual phase shift which results in a frequency change while the phase is changing. Since the signal goes from one to the other and back again, that means that the frequency will alternately decrease and increase slightly, and the result of that of course is vibrato.

~ FORMANT VOICE VIBRATO ~

There is also a third rotary vibrato scanner in the X66. This scanner is exactly like those that you would find in any standard or traditional Hammond organ such as a B3 or C3. This third scanner runs at a slightly higher speed, around 400 RPM, and it works with an additional vibrato line box with fewer sections and develops the vibrato effect for the tab voices on the X66 which are derived via the formant system from the sawtooth wave outputs from the tone generator.

There are additional resistors associated with this line box so that in addition to phase shifting, there are also slight volume changes as well for the signals that appear on different sections of this vibrato line box. The resistors account for the production of a slight amplitude varying tremolo in the resulting tone, as well as the frequency varying vibrato developed by the phase shifting. Hammond and his people evidently studied real pipe organs very closely, and they found that in real pipe organs, when you apply the tremulant effect to reed and particularly to string pipe voices, the tremulant not only varies the pitch but also the volume. With regard to this vibrato effect for these formanted voices, Hammond was not trying to imitate a Leslie speaker, but rather to reproduce an effect found on some voices in a pipe organ.

The principal of the formanted voice vibrato scanner is the same as that for the main vibrato scanner, however there is only one set of plates on the rotor, so it develops just a single phase vibrato with some tremolo superimposed on the vibrato. Because the phase shift produced in the formant voice line box is a lot less, the vibrato component of this effect on the resulting tones is likewise less intense than that of the main vibrato when on the maximum setting. On the X66, Hammond referred to this effect as "tremulant" borrowing the term from the pipe organ field.