Improving Bellows and Reservoir

by Steve Bowman

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Improving Bellows and Reservoir
In my experience working with a John Smith Busker organ, it seems that the bellows and reservoir are barely big enough to do the job. So, unless everything is air tight the air pressure is unstable. This is revealed by a noticeable (and to me, objectionable) pulsating in the sound, particularly noticeable in sustained notes. This is not an uncommon problem with small organs which are engineered to be as small and light as possible. I have been troubleshooting my John Smith Busker organ until finally I am now almost satisfied with its performance. Here are some of the things I have done:

First, I tried to eliminate possible leaks as has been suggested in other articles. Then, I checked to be sure there was no lost motion when the bellows stroke begins. The greatest amount of air movement is in the middle of the stroke. By the end of the stroke no air is being pumped, therefore it is wise to keep that time to a minimum. Then, I recovered the pumping bellows and reservoir until I was satisfied that they were as air tight as possible. In my organ the air outlet is in the movable board of the reservoir. Since the space is not needed on the side of the reservoir for the air outlet, I added a 3/4 inch wide strip screwed and glued to the side of the movable reservoir board and the box before recovering it.

When I recovered the bellows and reservoir, I also increased the span on the open end about 1/2 inch to allow for additional reserve and pumping capacity. It appears that the overall reserve air capacity was increased by 25%. Now it is over 15 seconds after the reservoir is fully open before it completely closes. My reservoir will now hold air for over 45 seconds when sealed off from the organ.

The increased tension on the reservoir spring caused by the reservoir opening wider could be a potential problem with this modification, but has not been a negative side effect for me. To get much benefit from this modification of the pumping bellows, it would be necessary to also change the crank shaft to provide for a greater stroke of the bellows rods which I have not yet done. I would try for at least 1/4 inch of additional movement.

Another thing I did and I am convinced also helped stabilize the air pressure was that I increased the size of the overflow valve hole from about 3/4 inch to 7/8 inch to minimize the chance that there would be additional inflation of the reservoir due to air being pumped in slightly faster than it could be released. I have thought of adding 3/4 inch more width to the outside edges of the pumping bellows also, but limited space in the organ cabinet has discouraged me from trying that modification. At some point I guess it just makes more sense to build a whole new larger bellows unit. But if you are short on air and want to give what you have a boost, try some of these ideas.

Steve Bowman,  Oxford, NC

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