Building an organ requires a variety of skills and, as I have found myself,
these require a little practice to make perfect. My Organ Building packages
describe, as well as I can, how to achieve a working organ, as you would
have seen there are test procedures throughout the building process which
should ensure immediate success.
Unfortunately not all the organs built have been as good as others. This
normally shows up when a builder who is happy with their organ hears another
identical one playing much better. By the very nature of the project there
are bound to be variations, but there is no reason why there should be wide
differences. Having now seen a lot of the organs that have been built,
and helping those with problems, I have put together this list of common
mistakes and causes of weakness:
First of all it is important to remember that the design as offered is a
fully tested and proven one. Many of the problems I have seen are caused
by modifications, in particular to the bellows and spill valves. I
can understand this as many of the builders are used to steam models and
by their nature have practical enquiring minds. Lets face it most of
us are well practiced at using a different material or method if we can not
put our hands on the specified part. Unfortunately an organ is like
a living thing and any one part not functioning correctly can upset everything
else, hence the testing procedures. Also once the organ is assembled it is
not at all easy to know where to start looking, especially if there is more
than one problem. Remember that a small leak on its own is insignificant
but many leaks are a disaster.
Remember that all wood is porous, some more so, and MDF can be dreadful.
You cannot detect air leaking through a porous board so it is essential to
seal it all just in case. What I recommend is to dismantle your organ
to its main components. You will now have a kit of all the ready-made parts,
which can easily be reassembled after testing.
Start with the Bellows. Follow the testing procedure given, Check each individual
feeder for air leaks, you may have to remove the reservoir to do this. Did
you use clamping board when gluing on the cloth? Make sure the valves are
built as shown and sealing fully. With the outlet holes blocked, each feeder
should be totally airtight. If the reservoir does not stay up under pressure
for a minimum of 5 seconds then find out why. Using blackout cloth I expect
mine to stay up for at least 10 seconds, and they do. Some common causes
of poor bellows performance I have seen are:
1. Cloth sticking out at corners, or not wrapped tightly around the hinge
2. Leather valves not lying flat, or fitted with springs or even with a ply
backing stuck on them.
3. Rough surface on board under bellows. If the ply is not smooth then
glue a piece of smooth card on it.
4. Valves in reservoir catching on side of box.
5. Spill valve leaking.
When you are sure that the bellows are up to spec they can be reconnected
to the pressure box. But for the time being with the outlet only feeding
into the water gauge. If you are fairly sure that your bellows were
OK to start with then only carry out the following checks before dismantling:
With the spring in place you should now check for correct operation
of the spill valve and spring tensions. The reservoir should only open the
specified distance before the spill valve opens. It is essential to use a
water gauge (a length of pvc tube stuck to the side of the organ will do).
With everything airtight, the Busker organ can be run on 5 to 6 inches WG.
It is not enough to simply set the maximum output to this amount, but now
spend time to make sure that pressure remains essentially stable over 90%
of the reservoir travel. The length and tension in the spring affect this.
Unfortunately my early plans showed a different size spring, or a large fold
back paper clip, both of these gave good results in my prototypes, but I
have now gained a little more experience. The correct spring should be made
from 2.5 mm diameter piano wire, as sold in model shops. This is folded round
a ½" bar one and a half times and with 3 ½" long arms. At rest
there should be 5 ½" inches span. The block on which this rests on the
bellows should be about 3 inches long and countersunk at ½" intervals
for adjustment. Fit one block on the reservoir lid at the extreme opening
end, and another under the pressure box. The blocks should be made
to give a gap between them of 3 1/4" with the reservoir closed. Make
sure that the spring is free to move in space over its total travel as you
fix the blocks in place. This arrangement should give a virtually constant
pressure over the range of the reservoir. Movement of the spring in the adjusting
holes should give from about 4 to 6 inches WG. Check that the spill valve
is not leaking and that the spring for it does not cause too much effect
on the WG when the bellows are just spilling out, which is the normal running
position of the reservoir. The bellows should provide a virtually constant
pressure over the full span of the reservoir with the spill valve having
no more than about ½" increase in this. Finally crank the bellows slowly,
at about 18 turns per minute the reservoir should stay inflated and you should
feel each bellow doing the same amount of work. It is only when the bellows
are performing as described that the pipes can be properly voiced and tuned.
When you have connected the tubing make sure that the bellows still
have full and free movement.