The following few chapters don't have much to do with the actual restoration of the Orchestrelle, so feel free to skip to Chapter 21.1.  But during the course of assembling the upper parts of the organ, it became apparent that it would be very awkward to reach up and work on this area, and pump at the same time.  Also, quite frankly, I am a lazy sod.  I decided to add a good blower, but it had to have four essential attributes.

    First, it had to be quiet.  I selected OSI Item 8618.10, a "Ventola" blower made by Laukuff in Germany.  This blower specs out at:
        0.18 HP at 120 VAC (The motor is actually a three-phase, with a huge capacitor to fake the third phase; not very efficient, but three-phase motors are the quietest design available)
        3360 RPM
        3-1/8 inWC static pressure at cutoff.
        3 inWC at 106 CFM
        2 inWC at 171 CFM
This blower is indeed spookily quiet.  It has a reverse curve, asymmetrically spaced blade, perfectly balanced, plate enclosed impeller.  A bypass, sidestream flow is passed around the double walled volute and out the little opening beside the inlet.  This flow cancels some of the wind rush noise, and prevents surge and overheating.

    Second, it must have a flat curve.  That is, the pressure must not change much between low and high flows.  The reverse blade curve and large ratio between the impeller diameter and thickness, makes this blower have a much flatter curve than any ordinary industrial centrifugal fan.  Cheaper blowers often have a 5-to-1 or greater pressure range between minimum and maximum flow.

    Third, a high accuracy regulator must be fitted, to hold the pressure as close to 2-3/4 inWC as possible.  Even the shallow curve of the Ventola is not adequate.  Reed organs have to have a fairly steady pressure to sound the best.  Free reeds do not change pitch very much with changes in wind pressure.  But higher than design pressure causes the reeds to oscillate at too high amplitudes.  These large reed motions cause the reed to go too far in and out of the slot, creating distorted waveforms.  Then the instrument sound too loud and screechy, with poor definition of the different voices.  There have been reports in MMD, that organs that have been fitted with unregulated blowers, just to make up for leaks, sound harsh.

    Fourth, the regulator should have a system of high capacity and leak tight check valves, so that when the blower is not used, the organ can be pumped correctly, without loss.

    The design of this regulator is based on the Aeolian Company regulators furnished on their early pipe organs.  The theory of operation, and another, simpler design, is described very thoroughly, in a paper by Johan Liljencrants, in the MMD Technical archives.  This cross section shows the details of my design.

    This drawing shows details of the two big check valve assemblies that swing closed when the blower is off, to permit manual pumping.

    The framework of the box is made, from 1 x 6 poplar.  To give an idea of the scale, the box outside dimensions are 24" square.  All the joints are sealed with strips of cheap kidskin skiver.

    The inlet section of the box and the box bottom, is lined with felt.

    The inlet opening from the blower is lined with felt, and an additional noise reducing felt sock is tacked to the inlet.

    To cut the tapered edges of the ribs, made from 3/8" Finnish plywood, a guide for the table saw is made.  The eight ribs are cut out, all the same size.

    The blower is connected to the organ temporarily with 4" PVC drain pipe and OSI foil reinforced paper flexible trunking.  On the rear cover of the big bass end "elbow box" connecting the reservoir to the left stop tower, there is a large hole, with a wooden cover.  That is where I attached the blower hose.  According to another owner, other Orchestrelles, even much older ones, have this apparently useless opening.  I suspect that this is where the factory attached a hose from the plant wind supply, to power the organ during voicing and tuning.  It would be very awkward and tiring to work on the ranks, while standing back and pumping, and trying to reach forward and up, to reach the reeds.

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