WURLITZER 4600 Series Instruments

North Suburban HAMMOND ORGAN Society

signal from a reed pickup with vibrato applied

This video shows you the end result, with the vibrato applied to the electrical signal.

This is the resulting signal after the two signals, phase A and phase B (which are constantly varying in amplitude) are electrically combined. The gradually changing length of the wave indicates a frequency change. As is typical of many of these Flash animations, I have greatly slowed it down to make it easy to see. In actuality, the wave would be jiggling along at 6 to 7 times a second and would be much harder to follow. This is the waveform from a pickup which is over the approximate center of a reed. This animation will not work on some cellphones. For best results, view these pages on a computer.

The Wurlitzer vibrato system is actually fairly simple and straightforward. It does, however, have one characteristic which may or may not be a problem depending on an individual listener's or player's viewpoint. When a singer adds vibrato to her voice, or a violinist adds vibrato by wiggling his fingers on a string, these actions change the frequency of the tones right where they are being produced, and the percentage of frequency deviation is equal for all frequencies. For instance, if a soprano sings A 440, and her average vibrato goes ± 1.5%, then on the low pitch excursion, she will be singing A 433.4, and on the high excursion, she'll do A 446.6. Now, the second harmonic of her voice which is nominally 880 Hz for the note A 440 would drop to 866.8 when going low and on the high side, the second harmonic would increase to 893.2 Hz. At A 440, when her voice is on the low part of a vibrato excursion, she drops pitch by 6.6 Hz, and when going high, she adds 6.6 Hz. The second harmonic of her voice, however, drops by 13.2 Hz, and likewise increases by 13.2 Hz on the high side.

This same rule holds true for all musical sound sources that incorporate vibrato in the normal way, which is to vary the frequencies of the tone sources directly. Human beings indeed are used to perceiving vibrato as an equal percentage phenomenon or as a ratio entity where, as the frequency of a tone increases, the number of cycles by which the vibrato causes the tone to deviate from steady-state increases proportionally, but the percentage of pitch variation remains constant.

The Wurlitzer vibrato system however, works somewhat differently in that it increases or lowers the frequencies of all tones fed into it by a constant number of cycles, and not by a constant percentage. If you give a little thought to this, you will soon realize that as you ascend the scale on the Wurlitzer instrument, the amount of vibrato actually decreases. The reason for that is that if, for example the Wurlitzer system changes frequency by ± 2 Hz, then 2 Hz is a much larger percentage of a tone of say A 220 than it is for a tone of A 880. This is contrary to the way that normal musical instruments and voices work when producing vibrato, and indeed the Wurlitzer ES vibrato is not satisfactory in some cases, and it lacks much of the richness and "vibrancy" that we would expect from the vibrato effect.

Several musicians who have in the past performed on these ES instruments, such as Ken Griffin and also Sy Mann, resorted to Leslie speakers to overcome this difficulty. Ken Griffin even took this one step further. Ken used only the treble rotors in the Leslie speakers associated with instruments he played, because indeed, in the low and mid range of the Wurlitzer 4600 series instruments, the instrument's vibrato is perfectly satisfactory. But on the high end of the frequency spectrum the Leslie effect obtained by using only the treble rotors would overcome the much weaker treble vibrato very nicely.

Today, of course, we can use modern digital signal processors to produce virtually any type of vibrato that we might wish to use, and when suitable enhanced in this manner, then these Wurlitzer instruments take on a new dimension and really can sound quite impressive.

The remainder of the Wurlitzer 4600 is fairly conventional [with one notable exception] in that it uses a typical push-pull tube amplifier with standard power pentodes such as 6L6s for driving the speakers. The exception is the inclusion of a gating circuit in the amplifier. Every key and pedal has, in addition to the normal key contacts, a gate triggering contact. The gating circuit is normally cut off. No signal of any kind gets out of the amplifier. However, when you press a key or a pedal, the gate conducts and allows the signal to get through. When you let go of a key or pedal, the gating circuit gradually shuts off, the time for it to do so being determined by a capacitor discharging through a resistor. (In the instruments I have worked on, the gating time seems to be about 2 to 3 seconds.)

The purpose of this gating circuit is to eliminate any slight extraneous noises through the speakers when the instrument is on but no notes are being played. The theory is that when you play, the music is sufficiently loud that it masks any residual background noise. The gate could cut off almost immediately, but it has to remain on for a while so that if the musician is using percussion sustain, the gate remains open while the sustaining tones are fading out.

There is a slight adjustable positive bias voltage that is applied to all of the reeds through their commoning wires. If the bias equals the residual leak-through voltage that can exist on the pickups, then both the reeds and the unused pickups are at exactly the same potential and there should be no signals generated other than when called for by the musician. If, however, a potential difference does exist even when no pickups are energized, then all of the reeds will produce a slight audio signal and the result is a steady noise in the background. There is an adjustable "pot" on the keying assembly on top of the reed unit where you can adjust the bias voltage to a level that eliminates just about all of this background noise, and likewise, the amplifier's gating circuit should take it out completely.

Because of the high voltages and resistances and low signal levels that we find in an electrostatic tone generating system, it becomes very sensitive to external conditions. In the damper air in the summertime, you may need to adjust this bias pot for a different setting from what you would have it set at in winter when the air is very dry. Fortunately, the bias pot is easily accessible just be removing the back cover of the console which is easily accomplished by giving a quarter turn to the five locking fasteners on the back.

We hope that you have found this tour through two representative examples of what is today an extremely rare and unusual musical instrument of interest. Understanding these early electronic instruments is very helpful for those of us who play, because the knowledge of the workings of the instrument allows us to take better advantage of all of its capabilities while avoiding, or at least not being limited by any shortcomings.

From listening to much of Ken Griffin's recorded work and also from my own efforts, I would definitely conclude that the 1950s Wurlitzer electrostatic instruments are best suited for ballads and more ethereal type music, whereas the typical Hammonds are more suited to fast, jazzy and novelty type music, although both instruments can satisfactorily overlap these categories. The Wurlitzer ES instruments produce some tonalities which are very different from those of other electronic organs. Although they had both clickless keying and terrific pedal bass and could also include sustain and percussion, they never really caught on with public, who during the decade of the 1950s, were enamored with the Hammonds. The last Wurlitzer ES organs were made in 1961 and then the company changed to a more modern technology which marked the end of the line for what was not only a very unusual instrument, but an instrument with some very advanced features for its time as well as a truly distinctive tone that was unlike any other.

The next instrument which we shall look at in detail will be the Hammond X66 The X66 article is a work-in-progress, but you'll find at least some basic information already in that article. In the meantime, if you haven't already done so, go to the Hammond article on this web site and read it carefully as a great deal of specific information about the X66 will be much more understandable with the background that you'll obtain from reading about the standard traditional tone-wheel Hammonds.

 Previous Page   Page 14.   Home Page