Minshall Model L organ
Technical Description
Richard H. Dorf


Fig 2. Minshall Model L decribed in this artical

Fig 1 Model LC

Fig 3. Model S


Fig 4. Chord Organ
One of the principal problems which manufacturers of organs have is economy of design consistent with good performance. When the market was confined almost exclusively to churches this problem did not loom so large because churches, while most of them are by no means wealthy, can at least, in effect, pool the funds of many individuals to pay for an instrument. The home organ buyer, however, is using only his own money and now that he is an important customer prices must be scaled down. This is a problem because organs are inherently somewhat expensive. They must, for example, provide at least one separate tone generator for each of at least 60 notes, and this alone leads to volume consumption of tubes and other components.

One of the most economical and interesting designs is that of the Minshall electronic organ (made by Minshall Organ, Inc., Brattleboro, Vt. -formerly Minshall-Estey but now having no connection whatever with Estey). The Minshall tone generators employ only half a tube, 4 resistors, and 3 capacitors per note. Similar economies occur throughout the instrument. Yet it produces a great variety of tones which are imitative of pipe-organ sounds.

The company is one of the youngest in the field and this was its first commercial realization. Since then, however, a good deal of change has been made to overcome faults which extensive field reports brought to notice and to increase very greatly the quality and variety of tonal resources. In this article we shall see the organ and its components and point out improvements.

There are several standard Minshall models. Top of the line is the LC shown in Fig. 1. This is a full 2-manual unit with 25-note pedal clavier. It is distinguished from the Model L, shown in Fig. 2, by the fact that the LC has a second 4-octave set of tone generators which may be tuned slightly sharp or flat of the main generator set and can be switched in to give a "celeste" effect. The two models are otherwise identical. Figure 3 shows the spinet Model S, which has two offset 44-note manuals and 13 pedals; the S has about the same registration as the larger models despite its smaller size and lower price, which makes it one of the most versatile spinets offered. The Chord Organ is shown in Fig. 4. This is a single-manual unit. It may be played in the standard way, but for chord use ten chord buttons are provided. These are used with the lower-octave keys of the manual to produce ten chords in each of the 12 keys, 120 in all. In this article we shall describe the Model L in detail. All the other models are essentially similar except for the chord organ which would require a separate article.

OVER-ALL SCHEME

Fig. 5 Block diagram shows the essential sections of the Minshall organ

A simple block diagram in Fig. 5 gives a general idea of the layout of the organ sections. Each of 12 tone-generator chasses produces five notes separated by octaves, so that 60 notes are generated in all. These are wired to a key-switch assembly for each manual. Each key-switch assembly has three output busses, one each for 4-, 8-, and 16-foot tones. Each bus goes to an amplifier, after which the tones are fed through tone filters. The original tones are roughly sawtooth in shape and contain considerable harmonics; the tone filters, selected by tablet switches, shape the tones to imitate various organ voices. The outputs of the filters are combined and fed through a preamplifier which incorporates the swell-shoe attenuator, after which the tones are amplified by a power amplifier and fed to a speaker. The speaker is contained in the console of the Chord Organ and the Model S; the larger models employ separate tone cabinets. The Model H, a one manual organ (discontinued c.1963), also uses a self-contained speaker.

TONE GENERATORS

Fig 6. The new tone generators are similar to the old ones but have several improvements
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The basic distinction of Minshall electronic organs has always been the tone-generator system. The principal virtue of the circuit is that it is inexpensive and yet reliable.

The complete circuit of a tone-generator chassis appears in Fig. 6. Each of the frequency dividers requires only one-half of a 12AX7, with no transformers or other expensive components. The resistors and capacitors used are all standard 10 per cent tolerance components, none having to be specially selected except for R1 through R4. The frequency ranges over which a generator with one set of values will operate and divide properly is over half an octave. There are four dividers on a chassis, however, and the useful ranges overlap somewhat so that the total range for the chassis may possibly be less than five semitones. For this reason, the required total 12-semitone range for all the generators has been divided into four sub-ranges of three semitones each and four sets of values are used to cover the total. The values for these are all shown in the table at the bottom of Fig. 6.

The master oscillator which may be tuned to set the organ in tune is a well designed Hartley. To allow for vibrato creation by variation of d.c. element voltages, the time constant of the grid-leak network R7-C3 has been made slightly smaller than would be required for optimum stability and an a.c. path has been added between plate supply and grid consisting of C2 and R6. When the plate supply voltage is varied at a rate between 5 and 8 cps, the frequency varies at the same rate. The use of the L-C oscillator is a part of the new models and is an improvement over the old design in which R-C phase-shift oscillators were used. The R-C oscillators tended to be somewhat unstable over a period of time because of change in tube plate resistance.

The frequency-divider 'stages are effectively voltage amplifiers in which the plate output is used to charge a capacitor between plate and cathode. The value of the capacitor is so chosen that it can charge only at a rate in the neighbourhood of half the input frequency. The fundamental component of the plate voltage is then fed back to the grid in such phase as to cause the tube to cut off during every other input cycle; this causes the alternate positive peaks of the input wave to make the tube conduct and produce plate-current pulses at half the input frequency. The action, like that of most feedback systems, is hard to describe in a few words; complete details can be obtained elsewhere.

Fig 7

Waveshape at (A) is that given by the generators proper. The shape at (B) has higher harmonic content and is obtained by differentiation.

The plate output is the result of a capacitor which is charged and discharged, and therefore takes the waveform of a sawtooth, as illustrated at (A) in Fig. 7. As can be seen, the flyback time of the saw-tooth is quite large - at least 20 per cent - and the harmonic content is not very great. This was a fault in earlier models and made it impossible to secure any really bright tone qualities or, indeed, to have any really satisfactory variety of tone colors. In addition, the high-impedance key-switch system had to be of the shunt type because of leakage through the capacitance of open switches.

In the new circuit the plate outputs are not used directly. They are first passed through differentiators, which may be looked on as high-pass filters. They consist of C8-R13, C11-R16, etc., in Fig. 6. They have two functions. First and most important, they change the harmonic structure of the waves, making the harmonics much more prominent with respect to the fundamental, as may be seen in the resulting waveform of (B) in Fig. 7. With this improvement, the new models have very satisfactorily bright and interesting reeds and strings and a very good variety of colors. The second function, incidental but useful, is in voicing. By selection of the capacitor elements of the differentiators, the over-all level of the higher notes is made greater than that of the lower ones. When all tones are passed through the later formant filters which are mostly of the low-pass type, the total scale tends to have more even loudness from top to bottom than if all incoming tones to the filters were of the same level. This is the same job done in the Baldwin organ by networks between octaves in the keying bus outputs and in the Schober Organ Kits by varying-value resistors in series with each key switch.

There are two additional improvements in the new Minshall generators. The first is that a cathode-bias resistor has been added in each divider stage. There is now less possibility than before that a change in some component or voltage will cause a misfunction, since an unbypassed cathode resistor tends to be a compensating factor, holding the tube at about the same operating point despite changes in other factors. The second improvement is that all the divider grids are direct-coupled. In the former design the grid impedances were extremely high and weather variations would sometimes cause trouble. The new arrangement brings grid impedances down to normal equipment values and this problem is eliminated.

Fig 8. Side view of the tone generator chassis shows the printed circuit which holds most of the components

Figure 8 shows how a tone-generator chassis looks. Notice that almost all the resistors and capacitors are mounted on a printed-circuit panel. The printed circuit makes for neat and inexpensive production and easy servicing since each component may be lifted or removed without disturbing others and may even be put back again without harm if found to be good. Figure 9 shows the rear of the organ. To remove a generator, the divider strap is loosened and the generator is simply pulled out; it is held in place only by the power and output plugs on its ends.

Fig 9. Rear view of the Model L. showing the tone generators, power supply and power amplifier, and tablet-board chassis

The key-switch circuitry of the new models is much changed from the old system, both electrically and mechanically. The high impedances of the old generator outputs made shunt keying mandatory; that is, switches were normally closed, grounding undesired tones. Not only did this require more components and more labour, increasing the cost, but a switch with non-functioning contacts would cause a "cipher," a continuous sounding of the tone. Inevitably any switch will once in a while fail to work, but it is far less disturbing to have a failure of a tone than to have it sound continuously.

Fig 10. Key-switching diagram for the swell manual. Great wiring is the same, with addition of plugs and cables to carry tome up to swell
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The new system, drawn in part in Fig. 10, employs the series keying method, with normally open switches. Each tone is brought to the appropriate switch through a 0.1 megohm isolating resistor. When a key is played the switch closes, passing the tone to output busses which run the length of the assembly. The switches are actually three circuit ones, so that when a key is played tones of 4-, 8-, and 16-foot pitches are brought to the respective busses. The resistor-capacitor networks to which the busses are connected are key-click filters, tailored in values to the specific ranges which they cover, so that clicks are almost eliminated but minimum harmonic structure of the tone is affected.


Fig 11. One end of a key-switch assembly, showing the switch blocks printed-circuit output busses and seperators.

The key switches themselves are no longer the fiat blade type. When used for series switching these units have too large a capacitance between the opened blades, and there is leakage of unkeyed tone into the rest of the system; the result is an annoying whine in the background. Part of a key-switch assembly, opened and fanned for view, appears in Fig. 11. The switches themselves are blocks of phenolic in which three silver-alloy fingers are set. A small phenolic actuator is placed over the fingers and when the key comes down it hits the actuator, which forces the fingers down. Each finger strikes a gold bus wire running at right angles to it. Because the entire switch consists only of two thin crossed wires capacitance across an open switch is negligible and there is no audible leakage whatever. The combination of materials results in trouble-free contacting over a long period.

PEDAL TONE GENERATOR
The main tone generators provide only five octaves of tones equivalent to the 8-foot range of the standard 61-note manuals. The 4-foot tones for the upper manual octave are repeats of the upper-octave 8-foot tones and the lower-octave 16-foot tones are repeats of the 8-foot lower-octave tones. In the earlier models 16-foot pedal tone was derived from a "resultant bass" arrangement which mixed the lowest 8-foot tone with its musical fifth and produced a beat note an octave below the fundamental. The actual 16-foot component of this system was small and the idea unsatisfactory from every angle but cost.

In the new models actual 16-foot tone is derived from a special pedal generator which requires only two tubes for the entire pedal section. It is actually not a generator at all but a wide-range frequency divider.


Fig 12. Diagram of the pedal-switch assembly. Tones from the main generator are fed through the large connector to the switches.

Figure 12 is a schematic diagram of the pedal keying switches. Each switch is a single-pole double-throw unit and the diagram shows the switches in normal (unkeyed) position. Eight-foot tone from the main generators is fed through the connector and cable to each of the switch contacts as shown. When any pedal is pushed, its switch changes positions and the corresponding 8-foot note is brought to the output. Because of the switching arrangement, only one tone at a time can appear at the output, the lowest of any number played simultaneously.

Fig 13. The pedal generator, consisting of amplifier, wave shaper and flip-flop

The pedal generator is shown schematically in Fig. 13. The 8-foot tone from the main generators through the pedal switches goes to the first grid of a 12AX7 amplifier and from its plate to the second half of the tube which is a wave shaper giving the tone the proper shape to trigger the following flip-flop circuit. The second 12AX7 is an aperiodic flip-flop which is nonfrequency -sensitive over a wide range. The 16-foot output is taken from one plate circuit so that for every two input cycles there is one output cycle. The 8-foot output is taken from the second plate circuit of the first tube. Both are fed to the tone-color section. This is an extremely neat, inexpensive, and effective method of deriving real 16-foot tone.


Fig 14. The tablet-board holds the tab switches and the chassis containing bus amplifiers, tone filters and preamplifiers.


TABLET BOARD CIRCUITRY
The stop filters, bus amplifiers, and preamplifier are all located on the tablet board assembly, which consists of the wood board above the swell manual on which the stop tablets and other controls are mounted, to which is attached a metal channel containing the circuitry. This is shown in the photograph of Fig. 14. Figure 15 is the schematic diagram of the bus amplifiers, whose function is to amplify preliminarily the voltage appearing on each of the manual keying output busses before it is applied to tone filters.

Fig 15. Bus amplifier and coupler system

Each of the triode voltage amplifier grids is fed signal from one keying bus, the bus being terminated by a 12,000-ohm resistor. The 16-foot tones from both manuals go through to the grids without isolating resistors, but the others have resistors between bus and grid. Each triode has voltage feedback, a capacitor and resistor from plate to grid; the purpose of this is to give an effectively low output impedance.

There is one coupler on the organ, a Swell to Great. This means that when the coupler switch is closed, as it is in the diagram, 8-foot and 4-foot tones keyed on the great manual will pass through the 8-foot and 4-foot filters associated with the swell manual. This means, of course, that they must be mixed into the swell busses. Note how the 4-foot great tones are handled for this purpose. Tone from the keying bus is fed to its tube through a resistor and to the coupler switch which, when closed, injects 4' great tone into the grid circuit of the 4' swell amplifier tube. An important point in this process is that the 4-foot swell output from its amplifier and the 4-foot great output from its amplifier must not change in level with operation of the coupler switch. Effects on the 4-foot great amplifier are prevented by taking the coupling line directly from the bus ahead of the isolation resistor and making sure that when the switch is closed this point is shunted by nothing which would be comparable to 12,000 ohms.

Preventing some effect on the 4-foot swell tone is not so easy, but it is done here in a very neat way. The output of the 4-foot swell amplifier depends on the magnitude of the feedback. This depends on the total value of resistance between grid and ground, since this resistance is the shunt leg of a voltage divider for feedback. The value of the 4-foot swell voltage reaching its grid is also dependent on the total value of the series resistors since these are the shunt leg of a signal voltage divider. When the coupler switch is closed, the 4-foot swell voltage at its grid is decreased because of the lessening of the total resistance between the grid and ground. However, this decreases the feedback voltage fed around the 4-foot swell tube, so that its gain rises just enough to restore the former output level, now with the addition of the 4-foot great tone. The same process applies to the 8-foot coupling.

Fig 16 Tone filters and tab switches for the great

Figure 16 shows the tone filters for the great stops, of which there are eight. The idea here is the same as in the Baldwin and the Schober organs - formant filters which simulate the acoustical effects of the various spectrum characteristics of different instruments or types of organ pipes. These filters are less complex than those in the Baldwin and Schober because the initial tone is not so complex and not as much filtering can be or need be done.

Fig 17. The swell registration circuit
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The swell registration is shown in Fig. 17, and again the principle is the same. Here, however, two little tricks have been used. Among the 12 stops there is a 2-foot salicet, which seems surprising in view of the fact that there is no 2-foot keying bus. The salicet is actually operated from the 4-foot line, but it is a very stringy tone (note the high-pass filter) almost devoid of fundamental content. Because of the lack of 4-foot fundamental it actually sounds an octave higher than, say the 4-foot flute.

The second trick is in the clarinet filter. The normal clarinet tone is almost devoid of even harmonics. In the Baldwin organ the symmetrical tone. without even harmonics is obtained for this purpose by an outphasing circuit, which is patented. In the Minshall the same effect is achieved (over a limited range) by so designing the tuned circuit that its phase shift with respect to the tone fed directly through tends to cancel even harmonics. The resulting tone is not quite authentic but is improved. Figure 18 shows the pedal registration schematic with its four stops.

Fig 18. Circuits of the four pedal tone filters

The preamplifier, also located on the tablet board, is diagrammed in Fig. 19. Outputs from the swell and great registration sections go to the grid of the first tube. The pedal output passes through a "balancing" section so that the pedal level can be adjusted for any set of auditorium conditions with respect to the manual levels. This first triode has feedback around it. Volume of the organ is controlled between the first two stages by a swell-shoe control which varies the impedance of the shunt leg by a voltage divider, being compensated for loudness by the capacitor network which raises the comparative level of the bass as volume decreases. A brilliance control in the plate circuit of the second stage is simply an old-fashioned tone control. The third stage is a cathode-coupled phase splitter and the preamplifier output stage is push-pull with feedback around each half. The "chimes" input is to be used with any of the commercial electronic chime devices, most of which consist of struck bars whose vibrations are picked up electrically and amplified.

Fig 19. Fig. The preamplifier circuit, with swell-shoe and brilliance controls and push-pull output.
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Figure 20 is a schematic of the amplifier and supply sections. The amplifier, rated at 15 watts output, consists of a pair of 6L6's with cathode feedback from the transformer secondary. The power supply is standard, furnishing B-plus and filament voltage to generators and tablet board through connectors as shown. The tablet-board connector also carries preamplifier output to the power amplifier grids and to a pair of sockets into which lines to booster amplifiers may be plugged.

Fig 20. Schematic diagram of the power amplifier and power supply with vibrato oscillator.
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The vibrato oscillator, a 6SN7, is also in the power supply. This is a simple feedback oscillator the frequency of which can be adjusted over a range of about 5 to 8 cps by switching different resistors across R9. This is done at the tablet board and a line from the cathode for this purpose is carried up through the large connector. Regulated B-plus is applied to the vibrato-oscillator plates (note the VR tubes) and the "vibrated" voltage is carried to the master oscillator plates through the generator plate supply.

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