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Lowery Organo

Technical Description

Richard H. Dorf

Fig 1. Organo installed in an ordinary piano

The Lowrey Organo made by the Lowrey Organ Division of Central Commercial Industries Inc., Chicago, gives the player the facilities of a complete, small, one-manual organ but has no console of its own Its unique feature is that it utilizes the keyboard of any piano without impairing the regular operation of the piano. Fig. 1 shows a complete Organo installation on a standard piano. The cabinet at the right houses the speaker, amplifier, and generators the attachment on the piano's front is a small control panel; the long, narrow frame across the back of the keyboard contains the mechanism by which pressure on the piano keys brings forth organ tones from the instrument.
Lowery Organo functional block diagram
Fig 2. Organo functional block diagram

The Organo is an electronic instrument which generates its tones in vacuum-tube oscillators. Fig. 2 is a block diagram showing in a general way what the instrument contains.

The range is five octaves - the standard organ-manual compass - from two octaves below middle C to three octaves above. The tones are generated by frequency-dividing multivibrator chains, twelve in all, each synchronized by a master oscillator and vibrato-controlled by a reactance tube fed by a single vibrato-frequency oscillator for all the chains.

The generators are connected to 60 key switches actuated by the piano keys. The output points of the lower 25 switches are connected in parallel. This line is the lower-register bus which carries all the tones up to middle C. The remaining 35 key-switch outputs are paralleled to form the upper-register bus, which carries all the tones above middle C. Two register switches determine whether one or both of the two registers is to be heard, since the player may wish to hear the piano only in the lower register and the organ tone in the upper register, or vice versa. These switches appear on the control panel attached to the front of the piano.

The tones from upper or lower registers, or both, are passed through six R-C and L-C filters with paralleled inputs and separate outputs. Each imparts a different quality to the tone. These qualities are selected by the stop switches which also appear on the control panel. Three different qualities or stops are available principal, horn, and string each loud (forte or f) or soft (piano, or p). The solo switches on the panel enable the player to make either register somewhat louder than the other so that a melody may be made to predominate over an accompaniment. The vibrato switches select either a light (small-amplitude) or heavy (large-amplitude) frequency variation at about 6 cycles per second.

The tones selected with the stop switches pass to the amplifier located in the tone cabinet there is a preset volume control between two of the amplifier stages; and between two other stages is a second potentiometer-type volume control, the leads of which are brought to the control panel. This is the expression control, operated by a knee lever.


Lowery Organo Schematic of tone generator chassis
Fig.3 Schematic of tone generator chassis

The heart of the Organo is the tone-generator system with its 60 tubes and 12 chassis. Each note, A through G sharp, is generated in the third octave above middle C by an inductively tuned master oscillator. A typical generator string is shown in Fig. 3, with the master oscillator the second triode of the 12AX7. The oscillator may be tuned by adjusting the slug within the coil form.

Each of the following twin-triode 12AU7's is a multivibrator with the customary cross-connected plates and grids. The first multivibrator is synchronized to the master-oscillator frequency by the capacitive connection between the oscillator plate and the resistive network in the multivibrator plate. The remaining multivibrators are synchronized, each by the preceding one, in a similar way. The unmarked values on the schematic, Fig. 3, differ according to the notes generated by the various strings. Table 1 shows the missing values.

The master oscillator and each multivibrator furnish an output lead, the five outputs being separated by an octave each. Each output is taken from a plate through a capacitor and resistor. These outputs are not single-frequency tones, however. Each of the lower (1 through 4) stages borrows some tone from the stage above it through resistors R2, R14, and the other similarly placed. In addition, each of the higher four stages (2 through 5) borrows some tone from the one below it through R3, R12, and so on. Thus each output contains principally the tone which it is supposed nominally to supply, with the addition of some tone an octave higher and an octave lower.

Table 1
Values for Fig. 3. All Resistances in Megaohms.

NOTE R1 R2 R3 R4-R11 R12 R13 R14 C1
C 0.22 0.33 08.2 0.82 2.7 0.56 1.0 .039
C# 0.22 0.33 08.2 0.82 2.7 0.56 1.0 .039
D 0.22 0.33 08.2 0.82 2.7 0.56 1.0 .033
D# 0.22 0.33 08.2 0.82 2.7 0.56 1.0 .033
E 0.15 0.22 5.6 0.68 1.8 0.56 0.82 .027
F 0.15 0.22 5.6 0.47 1.8 0.56 0.82 .027
F# 0.15 0.22 5.6 0.47 1.8 0.56 0.82 .027
G 0.15 0.22 5.6 0.47 1.8 0.56 0.82 .022
G# 0.1 0.15 3.9 0.47 1.0 0.47 0.68 .022
A 0.1 0.15 3.9 0.47 1.0 0.47 0.68 .022
A# 0.1 0.15 3.9 0.47 1.0 0.47 0.68 .022
B 0.1 0.15 3.9 0.47 1.0 0.47 0.68 .022

The first triode of the 12AX7 is a reactance tube placed across the master-oscillator tuned circuit. The grid is controlled by a signal from the instrument's 6-cycle vibrato oscillator. Varying the pitch of the master oscillator varies that of all the tones in the string because of the synchronism.
Lowery Organo  Key switch frame with cover off
Fig.4 Key switch frame with cover off

Figure 4 is a photograph of a portion of the key switch frame with the cover removed. The frame is placed over the rear of the keyboard and is fastened down with adjustable end brackets. Sixty small plungers project downward and rest on sixty of the piano keys. The shorter plungers in Fig. 4. are those which touch the black keys.

Each switch is a single-pole, double-throw unit, the arm of which is a small coil spring which moves up and down with the plunger when the key is pressed. The spring, connected to a generator output, is grounded to the topmost bus bar (see photo) when the key is up, short-circuiting the tone to ground. When the key is pressed, the spring contacts the lower bus bar. The lower bar is divided into two parts, the lower part encompassing the first 25 notes - the lower register - and the upper part of the remaining notes - the upper register. These are collector bars on which all generator outputs switched by the player appear.
Lowery Organo The vibrato oscillator
Fig. 5 The vibrato oscillator

The vibrato oscillator is shown in Fig. 5. It is a simple phase-shift unit located on the power-supply chassis. The left triode of the 6SL7-GT is the oscillator itself, with its grid connected to the second triode which acts as an amplifier. Output is taken from the plate of the amplifier to modulate the twelve reactance tubes in the generator strings.

The degree, of vibrato (its amplitude, not frequency) is controlled by a pair of switches and three resistors located in the control panel (drawn as part of Fig. 5. for easier understanding). With both HEAVY VIBRATO and LIGHT VIBRATO switches in the OFF position the amplifier output goes through a voltage divider consisting of the 560,000-ohm resistor as one element and a total of 10,000 ohms in the switching network as the second element, across which the reactance-tube grids are connected. The reduction in output voltage reduces the vibrato effect to negligibility.

If the LIGHT VIBRATO switch is placed in the ON position, the net resistance of the switching. network rises to 10,000 plus about 9,600 (27,000 and 15,000 in parallel) ohms or about 20,000 ohms., which raises the network output sufficiently to obtain a moderate vibrato effect. When the HEAVY VIBRATO switch is on the total in the switching network is 27,000 plus 10,000 or 37,000 ohms, which raises the vibrato effect to a large value.


Lowery Organo Tone-color filter circuit
Fig.6 Tone-color filter circuit

The key switch upper- and lower-register collector bars are connected to the circuit shown in Fig. 6. the components of which are located in the control panel. Each register goes to a switch which either grounds the bus or passes it on to a voltage divider network and solo switch. If both registers are to be played at the same volume the solo switch is in the normal position, as the lower-register solo switch is in Fig. 6. The upper element of the voltage divider is the resistor from the plate of a multivibrator tube to the octave output terminal in Fig. 3; the lower element, across which the output is taken, is whatever appears between the register switch and ground in Fig. 6. With the solo switch at normal, the lower element is 4,700 ohms. With the switch in solo position, it is 22,000 ohms. Thus, the register selected is louder than the other and a solo melody may be played on it, with a softer accompaniment on the other. This is one form of "split keyboard" very often employed on single manual organs. The split keyboard is common in non electric instruments such as the old-fashioned harmonium, where it usually has a separate group of stops - tone colours - as well as different volume level.

The tones emerging from the solo-switch network are fed to two buses, one for each register. The buses are brought together through 500-μμf capacitors to the first stop switch, labelled STRING p. The small value of capacitance tends to filter out low frequencies from the complex waveform of each note, producing. pulse-type waveforms with fairly sharp "spikes." This simulates a string tone, which has this character because the hairs of the bow have comparatively rough surfaces and pull the string in little jerks.

The buses are connected through a second pair of capacitors with a value of .0018μf to the STRING f switch. This gives the tone a bit more body, changing the quality somewhat and in the process allowing more of the fundamental to come through with an effect of greater volume.

All horns are resonant in at least one frequency range. The horn effect is simulated in the Organo by an L-C resonant filter composed of two .0027-μS capacitors and a 24-henry inductor in series. This is the piano or soft horn. For the forte or loud horn one of the capacitors is increased to .01 μf, with a lowering of resonant frequency.

The principal tone is simply the normal output waveform of the generators, with some of the higher harmonics reduced. The principal networks consist of a pair of resistors and a capacitor in a T configuration. The second resistor value is lowered from the value used for piano to obtain the forte volume.

Each stop switch is single-pole, double-throw and the arm is connected to ground for the OFF position and to an output bus for the ON position. The bus is connected to the amplifier.

Lowery Organo amplifier
Fig. 7 The amplifier

The amplifier is pictured in Fig. 7. and diagrammed in Fig. 8.
Lowery Organo Diagram of organ amplifier
Fig. 8 Diagram of organ amplifier

Tubes V5 and V6 in the photograph are the power supply rectifier and the vibrato oscillator.

Referring to Fig. 8, the first triode of a 6SN7-GT is grid-excited by the output of the stop-switch bus in Fig. 6. Between this and the second triode is a volume control which is preset at installation for the maximum volume desired in the room. The second triode is a cathode-follower, included to obtain a low-impedance line feeding to the expression control. The expression control, a 2,000-ohm potentiometer, is located in the control panel, the inside of which appears in Fig. 9.
Lowery Organo Control panel
Fig. 9 Control panel on piano front includes a knee action volume control

The lever and spring mechanism operated by the knee-lever turns the potentiometer. The low-impedance cathode line and the low resistance of the potentiometer prevent hum pickup or appreciable loss of treble in the line between the amplifier (which is in the tone cabinet) and the control panel on the piano.

The next stage is a phase inverter of the classical tapped-grid resistor type except that the cathodes of the two triodes are commoned to an unbypassed cathode resistor for better balance. The two plate outputs are fed to the grids of the 6V6 output tubes through 0.1-μf blocking capacitors. The .0047-μf capacitor across the output of the first phase-inverter tube reduces the high-frequency response slightly to eliminate some noise. The loudspeaker is an electrodynamic whose field coil is used as a filter choke. The power supply is conventional.

The layout of the Organo components is simple and straightforward. The control panel shown in
Fig. 9
contains all the tone-coloring components as well, as the expression control and the switches.
Lowery Organo Rear view of the tone cabinet
Fig.10 Rear view of the tone cabinet

Figure 10 is a rear view of the tone cabinet with the covers removed. The numbered components are as follows:

  • 1. Auxiliary power switch (there is a switch and pilot light on the front of the cabinet).
  • 2. and 18. Power and filament transformers.
  • 3. and 14. Cabinet portions.
  • 4. and 12. Rear covers.
  • 5. Component frame.
  • 6. Cable for speaker, power-switch, and pilot-light.
  • 7. Terminal board.
  • 8. Pilot light.
  • 9. Output transformer.
  • 10. Speaker.
  • 11. Generator tubes.
  • 13. Speaker plug.
  • 15. Power cord.
  • 16. Rectifier tube (a pair of 6W4's has been included in the newer models using 12AU7 generator tubes instead of 12AX7 as formerly).
  • 17. Filter capacitor.

  • All connections to and between the various units are brought to the terminal boards at the center, so that all units are completely accessible and may be removed readily for repairs.

    An interesting variation of the usual components scheme has been made available in the Janssen piano, of which the Organo may be made an integral part. Figure 11 shows a piano so equipped. There is no separate tone cabinet; all units of the Organo are built into the piano case. The oscillators, amplifiers, and other heat-producing units are installed along with the speaker on the inside of the piano kneeboard. A ventilating fan is provided to keep the interior cool enough to prevent damage to the piano mechanism. The key switch frame is placed inside the piano out of sight toward the rear ends of the keys.
    Lowery Organo mounted in a Jenssein Piano
    Fig. 11 A Jenssein Piano with built in Organo

    In playing an ordinary piano equipped with an Organo attachment, striking the piano keys with normal pressure will actuate both mechanisms and produce both piano and organ tones simultaneously. However, in experimenting with the instrument, the writer has found that it is possible to play the Organo without appreciable "interference" apparent from the piano strings. It is a matter of regulating touch, since the key switches are adjusted so that tone will be heard when the key is depressed about halfway. Full depression sounds both piano and organ tone, the effect of which can be quite pleasing. However, a damper bar may be obtained and installed in the piano. It has a lever at the end, with which the bar can be rotated on its axis, placing damping material on the strings so that they do not sound. The action then produces only organ tones. Damper bars are not common, however, and users do not seem to have felt a great need for them.

    Extract:- Electronic Musical Instruments by
    Richard H. Dorf
    Richard Henry Goldfogle Dorf
    14th Mar 1921 - 21st June 1989

    Richard H. Dorf was an electronic engineer, prolific author on the subject of vacuum tube electronics and electronic organs, and the head of the Schober Organ Corporation – a supplier of self-build electronic organ kits (using patents licensed from Baldwin organ Co.).