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
Steve Bowman, Oxford, NC