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Wurlitzer Electronic Organ

Technical Description
Richard H. Dorf

Fig. 1. Wurlitzer Series 50 organ

"AND then came a crashing chord from the mighty Wurlitzer!" In the days of our fathers and grandfathers the "mighty Wurlitzer" was as much of a byword in the realm of popular organs as the Hammond organ is today in the soap opera radio studio. Wurlitzer has been in the music business longer than most of us can remember, making and selling musical instruments.

Today the mighty Wurlitzer is a streamlined reed organ actuated and heard with the help of electronics, and sounding like a small pipe organ, with nary a reed wheeze to be heard.

The author has often been asked, "What ever happened to the Everett Orgatron?" The answer is that Wurlitzer took it over, refined it, and built it into a line of several large electronic organ models. The one pictured in Fig. 1, for instance, is a Wurlitzer Series 50, a complete two-manual organ with 32-note radiating pedal clavier, AGO dimensions, and 22 stops. More and less elaborate models are made, as well.
Wurlitzer Electronic Organ
Fig. 2. The model 44 Wurlitzer spinet organ.

The newest Wurlitzer is possibly the most interesting of the lot, for several reasons. It is the Model 44 organ, shown in Fig. 2. As the photo shows, it is a spinet, but has two short manuals in what is coming to be something of a style among modern spinet electronics, each with 44 notes, from F to C. It also has a toe-pedal clavier of 13 notes, C to C. The power amplifier and speaker are built into the console, which is a complete organ with nothing coming out but the a.c. cable and the music.

What particularly distinguishes the Model 44, however, is the fact that all the reeds are blown all the time, at low air pressure. The keying is done entirely electronically, with consequent elimination of the characteristic slow speaking of reeds which has bothered some people. The system tends to reduce mechanical ciphers, too, because dirt has little chance to accumulate on a reed that is waving in the breeze all the time. Electronic keying also makes the keying much simpler, doing away with the direct action and its Rube Goldberg lever system or electrical keying with its solenoids by the ton. Another advantage is that the attacks may be made as sharp or as delayed as the designer wishes.
Wurlitzer Electronic Organ tone generator
Fig. 3. Wurlitzer tone generator is a brass air-blown reed

The Wurlitzer uses the same basic principles as most other electrostatic transducer devices, as illustrated in Fig. 3. The basic component is the reed of brass. A stream of air strikes the reed through a slot in the reed cell. If the stream comes upward, the first breath pushes the outer end of the reed upward. Since the reed is a high-Q mechanical resonator, the burst of pressure tends to make it overshoot and go past the point to which the wind pressure would push a nonresonant body. Then the energy stored in the reed because of its elastic properties pushes it down, and again it overshoots the initial resting position, after which it springs back up again. Because of its resonance, the angular velocity of the movement is a function of the frequency of resonance; and the reed always finds some air pressure at the proper point in its cycle to give it back the energy it expended in the last cycle, thus keeps moving at its resonant frequency.

The electrostatic transducer system is necessary to convert the reed motion into audio voltage since the reeds themselves are enclosed in a soundproof compartment. The reed and the pickup screw are the two "plates" of a capacitor whose capacitance varies with the position of the reed. With each variation in capacitance there is a rush of electrons from one plate to the other. The electrons pass through the load resistor and create an audio-frequency voltage drop through it, which is transferred to the grid of a tube. The Model 44 has a bank of 73 reeds which provide all the tones and tone colorings for the organ.
Wurlitzer Electronic Organ reeds with tone pick up screws
Fig. 4. A group of reeds with tone pick up screws.
Wurlitzer Electronic Organ trumpet pickup
Fig. 5. A trumpet pickup

Two types of tone are available directly from the reed pickups. A flute-type tone (not a sine wave, but only moderately rich in harmonics) is picked up by the tone screws directly above the reeds. Most of the reeds have at least two pickup screws, for reasons which will appear. A horn-type tone is picked up by special so-called trumpet pickups, which are bent pieces of sheet metal. Figure 4 shows a group of four reeds with two tone screws each (except for the C, which has three) and the trumpet pickups. As Fig. 5 makes clearer, the latter are located a very small distance away from the end of the reed and, as can be imagined, give a rather sharply peaked waveform.
Wurlitzer Electronic Organ complete reed unit
Fig. 6. The complete reed unit

The entire tone unit, containing reeds, blower motor, and wind-chest, is shown in Fig. 6, as it looks when removed from the organ case with the rear of the tone unit taken off for access. Each of the six pans can be removed separately to give access to the group of reeds and pickups under it. Figure 7 shows the tone unit with its soundproofing cover in place. This makes it impossible to hear the reeds acoustically. Figure 8 shows one of the pans removed to reveal the reeds and pickup screws. Note in Fig. 7 that the compressor motor is outside the tone unit but attached to it.
Wurlitzer Electronic Organ sound proofing
Fig. 7. Tone unit with sound proofing in place
Wurlitzer Electronic Organ Cover removed from a pan to show reeds
Fig. 8. Cover removed from a pan to show reeds and screws

The basic keying system for the Model 44 is shown in the drawing of Fig. 9, which illustrates keying for one pickup of one reed. Reference to Fig. 10, a photo-of the bass end of the upper manual keying and stop mechanism, will help clarification.

Wurlitzer Electronic Organ Diagram of the keying system
Fig. 9. Diagram of the keying system

The keys are mounted on a strip of rigid material at the rear of which is attached a short strip of spring metal, the rear of the latter being screwed down to a transverse rail which is a part of the formed metal key support. When the key is pressed therefore, all parts back to the end of the spring holding the assembly to the rail are depressed. The usual bearing and levelling provisions are toward the front of the key to keep it in line and equalize its action with the others.

When the key is pressed it pushes down the activator, a vertical strip of insulating material with a hole for each of the 10 key contacts (9 for stops and the tenth for gating the amplifier, as we shall see later). Each key contact is a spring wire fastened at the rear to the key contact mounting block- free at the other end. Since each contact passes through a small hole in the activator, when the key is pressed and activator lowered, the contacts are pushed downward.
Wurlitzer Electronic Organ upper-manual key and stop mechanism
Fig. 10 . Bass end of the upper-manual key and stop mechanism.

Beneath each contact is a rod of phenolic material with a heavy nichrome wire running its length. See Fig. 10 for the manner in which the nichrome wire is mounted on the rod. Each rod is pivoted axially (pivots not shown). Each is controlled by a stop tablet, through the mechanical linkage system shown. With the tab in the OFF position (face most nearly horizontal) the corresponding phenolic rod is so rotated that the key contact above it strikes the phenolic instead of the nichrome. In the ON position, the rod is rotated so that the nichrome wire is uppermost and is contacted by the key contact.

A supply of plus 810 volts is fed from the amplifier power supply to the nichrome wire on each of the rods through a 47,000-ohm resistor and a bronze spring which serves as a flexible element in view of the rod rotation. When the key is played each contact picks up this positive voltage from its stop rod (assuming the particular stop is pulled) and feeds it through a time-delay filter to the appropriate reed pickup. If the d.c. were suddenly applied to the pickup there would be a distinct click. The filters allow the d.c. voltage to be built up gradually on the pickup, resulting in a smooth keying characteristic. The filters are printed circuits. There are 165 filters, each going to a pickup screw or strip. Seventy-three are for flute-tone pickup screws; 36 are for accompaniment pickup screws, which are similar to the flute pickups but are not so close to the reeds and are therefore softer; 43 are for trumpet pickup strips; and 13 are for pedal pickups, which are similar to the flute-tone pickups for the manuals.

All the reeds for the lowest octave of notes are connected together and through 9.1-megohm and 3,900-ohm resistors to ground. The series combination is the load across which audio is developed. From the top of the load resistor the signal goes through a 0,1-μf capacitor to the bass input of the amplifier. The remainder of the reeds are connected in common to another 9.1-megohm resistor which goes through the same 3,900-ohm resistor to ground to make up the treble load. The signal passes through another 0.1-μf capacitor to the amplifier treble input. A 50,000-ohm potentiometer with its arm connected to the top of the 3,900-ohm resistors provides 0-5.7 volts which polarizes the reeds slightly to prevent any signal when no keys Are pressed.

The available variety of tone colors for both manuals is achieved through mixing stops, since actually there are only two varieties of Color - flute and trumpet. This scheme can be shown readably in diagram form only on a page several times the size of this one, so it has been tabulated and condensed into the chart of Fig. 11.
Wurlitzer Electronic Organ; Chart shows which tone screws are used for which keys and registers
Fig.11. Chart shows which tone screws are used for which keys and registers
Larger image click here

Let us first take the upper manual, which is equivalent to the swell on a usual organ. The keys are shown in pictorial form, numbered from F18 to C61. C25 corresponds to middle C, 261.7 cycles. Notice the reed chart above the upper-manual chart. This shows the 73 reeds and their notes, beginning with C1 at 65.41 cycles to C73 at 4186 cycles.

Now let us see what happens when we play the upper manual with the SOFT FLUTE 8' stop pulled. Start by pressing Middle C, key 25, and find it on the manual picture in Fig. 11. Now note that a horizontal column of figures extends to the right from the SOFT FLUTE 8' marking on the chart. Follow a line of boxes upward from Key 25 until it intersects the SOFT FLUTE 8' line; you will find a box with the number 25. This indicates that when C25 is pressed with the SOFT FLUTE 8' stop pulled, a pickup on reed No. 25 is energized.

Next we must know which pickup on that reed is energized. To find out we note that to the left of the SOFT FLUTE 8' marking is a solid square box symbol. At the bottom of the chart is the legend which makes it clear that all pickups for this stop are the soft flute pickups - tone screw immediately over the reed, with greater distance (and therefore less effect) than the other similar screw on the reed which is for normal flute tone.

Thus, from the chart of Fig. 11 we know that under these circumstances we will get only a soft flute tone from the organ, at the pitch of middle C.

Now, leaving the SOFT FLUTE 8' tab in the ON position, we also pull the FRENCH HORN 8' tab. Again following the boxes up from key 25 to the FRENCH HORN 8' line, we find the number 25, indicating the same reed. But at left of the FRENCH HORN 8' title we see the black triangle, which, after reference to the legend at bottom, shows that a normal flute pickup screw has been energized. Now both screws on reed 25 are energized.

Now suppose we want to add some interest to the tone quality. Let us add to the former two stops the one called TONE COLORING 22/₃. Again following the boxes upward to the TONE COLORING 22/₃ intersection, we find the number 44 and referring to the reed chart we see that we have energized a normal flute pickup (triangle to left of TONE COLORING 22/₃ designation) on the reed supplying note G one and one-half octaves above C25. This is approximately the third harmonic of middle C.

Let us add a fourth stop, TONE COLORING 13/₅. By the same process we find that we have added the normal flute pickup on reed 58, the E about two and one-quarter octaves above the middle C, approximately the fifth harmonic of middle C.

Thus we can see that pulling TONE COLORING 22/₃ adds the third harmonic to any notes played at 8' pitch and pulling TONE COLORING 1%' adds the fifth harmonic. This is carried on as far as possible until the TONE COLORING stops run out of reeds, after which octaves are repeated, giving sub-third and sub-fifths. This scheme is simply a harmonic synthesis system somewhat similar in principle to the more complex Hammond system.

The TONE COLORING stops are not the only adjuncts of the harmonic synthesis system, taking care of only the fifth and third harmonics. The second harmonic, an octave above the 8' note, is handled by the ORCHESTRAL FLUTE 4' stop. When we play middle C with that stop pulled, we energize the flute pickup on reed 37, which is C an octave above middle C. The fourth harmonic is supplied by the PICCOLO 2' stop, which energizes reed 49, two octaves above middle C. Thus the system gives us control over the fundamental, second, third, fourth, and fifth harmonics. Note that there is also a BASS 16' stop which gives us in addition the sub-fundamental, one octave below the key struck. Naturally, these stops can be manipulated in many different ways to give different total tone colors.

The upper manual also has two trumpet-tone stops which have no relation to the harmonic synthesis system, though they work in the same way. The first is the TRUMPET 8' stop. Assuming we have struck middle C, key 25, with the TRUMPET 8' stop pulled, we find that we have energized a pickup on reed 25. From the black semicircle opposite the stop name and the legend below we see that it is the trumpet pickup - the strip of metal close to the end of the reed - which is energized. The second trumpet stop is the BASSOON 16', which energizes the trumpet pickup on the reed one octave below the key struck. Though the trumpet stops are not related to the harmonic synthesis scheme they can, of course, be added to any combination of flute stops to produce still more combination.

The lower manual operates in the same way, except that there is no fifth harmonic, but a sixth instead, plus an eighth. The lineup is as follows:
  • Subfundamental - BASS 16'
  • Fundamental- HORN 8'
  • 2nd Harmonic- FLUTE 4'
  • 3rd Harmonic- TONE COLORING 22/₃
  • 4th Harmonic- PICCOLO 2'
  • 5th Harmonic- none
  • 6th Harmonic- TONE COLORING 11/₃
  • 8th Harmonic-FIFE 1'

In addition, the lower manual has a TENOR TRUMPET 8' which uses the trumpet pickups and an ACCOMPANIMENT 8' which uses the soft-flute pickups.

The keys of the lower manual energize many of the same pickups as those of the upper manual, and do not add anything. That is, if key 25 on the upper manual is pushed with the TRUMPET 8' stop pulled, pushing key 25 on the lower manual with the TENOR TRUMPET 8' stop pulled will simply energize the same pickup, but will produce no additional sound.

The pedals have no selection of stops, with a single tone quality obtained by making each one energize a separate pedal pickup screw on the lowest thirteen reeds. An additional time-constant network is used at each pedal contact to make the pedal tones speak and decay more slowly than those of the manuals.
Wurlitzer Electronic Organ Connection Board
Fig. 12. Connection Board with printed circuit filters mounted on the rear

Figure 12 shows the connection board on the reed chamber, to which all the keying leads go. This board on its inner side contains all the 165 printed-circuit time-constant filters. It should be noted that each keyed pickup has 160 volts d.c. on it (dropped from the original value of 310 volts by incidental action of filters. The first eight notes of the trumpet pickups have only 50 volts, brought about by special dropping resistors at the bass-end entrances of the corresponding stop rods and splitting of the nichrome wires. Figure 11-18 shows the rear of the console with the upper-manual key action picked up and brought into sight. Note the 12-inch speaker secured to the board beneath the manuals in front.

Wurlitzer Electronic Organ Console rear
Fig. 13. Console rear. The upper-manual has been raised to make it visible

The amplifier system of the organ is not particularly notable, but the vibrato circuit may well be called exciting by many people who have sought a practical way to introduce genuine vibrato - frequency shift - into systems where the original tone source must remain at a constant frequency. We may dismiss the amplifier after noting that one of the stages is used for gating; it normally has cutoff bias which is removed through a time-constant circuit by a series of paralleled contacts under the keys whenever any key is pressed. This eliminate noise of any kind in the absence of a signal.
Wurlitzer Electronic Organ phase-shifter
Fig. 14. The phase-shifter which produces vibrato

The heart of the vibrato circuit (aside from the low-frequency oscillator) is shown in Fig. 14. The treble input from the reed pickups containing frequencies between 138.6 and 4186 cycles is fed to the grid of a 6SQ7 preamplifier in a standard cathode-biased circuit. (The bass signal is fed through a fairly similar stage directly through the main amplifier system without vibrato.) From the plate circuit of the 6SQ7 it goes through C1 to the grid of V2, half of a 6SN7-GT.

V2 is a phase splitter of the "long-tailed" type, with one signal taken from the cathode circuit, across R7, and the other from the plate, the two signals 180 degrees apart in phase. At the plate the signal is divided into two parts, one part through C3-R8 and the other through C4-R). The cathode signal is also divided in two paths, one through R10-C, and the other through R12-C7. The plate and cathode signals in each leg are then combined, one set through C6-R11, and the other through C8-R13.

The entire purpose of the six legs enumerated above is to act as a phase-shift network, producing two signal outputs which have a constant phase difference of about 90 degrees. These two outputs appear at the points marked X. Their phase relationship to the original V2 input signal changes, of course, with change in frequency. But they maintain a difference between themselves of about 90 degrees between about 500 and 15,000 cycles. It is appropriate to use the language of, for instance, phase modulation transmitters and call them quadrature voltages, for the vibrato of the Wurlitzer 44 is actually a phase-modulation system!

The two quadrature voltages are fed to the grids of V3 through blocking capacitors C9 and C10. The two signals are mixed at the plate of V3 and the mixed output is again a single signal taken from C11.

Vibrato-frequency voltage at either 5.7 or 6.7 cycles is obtained from a low-frequency oscillator and phase inverter; it appears in push pull on the grids of V3, as indicated in Fig. 14. It causes the two triodes to conduct singly. When the phase of the low-frequency signal makes the left grid positive and the right negative, the left triode conducts. When the phase is reversed, the left triode conducts. This causes a continuous change in the phase of the audio signal coming out of V3 over approximately a 90-degree range.

When the left triode conducts, one signal of given phase goes through. When the right triode conducts the other quadrature signal comes through and there has been approximately a 90-degree phase shift. And the shift is smooth, for the signal emerging from the mixer is the vectorial sum of the two voltages at the plates of the two triodes.

If both are conducting equally, as is the case at the 0-, 180-, and 360-degree times of the low-frequency push-pull signal, the mixer output is 45 degrees away from either extreme. Thus the phase at any instant is dependent on the relative contributions of the two triodes, which is a function of their relative low-frequency grid signals at that instant. The latter vary in a sine manner, giving a phase swing that is smooth and natural.

Needless to say, phase modulation, is equivalent to frequency modulation; it is, so to speak, a reciprocating electronic Doppler effect, as if the classic train blowing its whistle were to come toward and retreat from the listener again and again, with consequent apparent up-and-down change in the pitch of the whistle.

The output of the mixer is connected through C11 to a filtering stage which, by using frequency-selective negative feedback, cuts off sharply below 130 cycles so that none of the vibrato-frequency signal can affect the amplifier and speaker.
Wurlitzer Electronic Organ oscillator and phase-splitter
Fig. 15. The oscillator and phase-splitter provides switching voltage for vibrato circuit.

The oscillator is shown in Fig. 15; it is one-half of a 6SL7-GT operating as a standard phase-shift oscillator. The primary control is the two-circuit, three-point VIBRATO SPEED switch. In the FAST OS1tion R2 is selected as the second resistor of the phase-shift network, determining the frequency at about 6.7 cycles. The second section of the switch connects the grid circuit of the other half of the 6SN7-GT, which is a phase splitter, to the arm of the VIBRATO DEPTH switch. The latter determines how much oscillator signal is sent to the phase splitter by tapping at one of three points on a voltage divider carrying oscillator output signal from the plate.

When the VIBRATO SPEED control is at SLOW, R1 is selected for the phase-shift network; this makes the oscillator frequency about 5.7 cycles. When it is at OFF, the output section of the switch disconnects the phase-splitter grid from the oscillator output.

Since the rate of phase swing and apparent music signal frequency swing depends on the oscillator frequency, the speed switch determines the vibrato rate in the music. And because the oscillator output determines how much total phase shift will occur the DEPTH switch determines how deep or wide the vibrato will be.

This vibrato circuit, it may be remarked, effectively does the same job as the Hammond vibrato scanner, but it does the job electrically, without moving parts, in a manner which can only be called elegant. Such a vibrato, with its ease of construction and compactness, does an almost impossible job which has puzzled many people who wished to add automatic vibrato to guitar amplifiers, amplified string instruments, and the like, only to be forced to settle for amplitude tremolo.

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