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Repair of the bellows of a swell shutter engine English text Nederlandse tekst

July 2017


A conventional swell device

All romantic organs have at least one division (played from the second or third manual) of which the organ pipes are placed inside a more or less sound insulated space: the swell box. The sound intensity is governed by ten to thirty shutters, made of thick wood, that can be opened by the organist by pushing his right foot on the swell pedal. In most organs, swell shutters are connected to each other via wooden rods and a rod to the swell pedal. Here you see the swell shutters of the Saint-Anna organ I dismantled. (The door in front is of course normally closed.)

Zwelinstallatie Sint-Anna

If the action is not direct (mechanical) but electrical, a 'swell engine' is used that pushes the rod by inflating several bellows, as in the organ above (not visible).

Swell shutters individually governed by bellows

The pipework of the positif manual of the Hilboesen/Pels organ of Saint-Boniface (Sint-Bonifatiuskerk) in Alphen-on-Rhine (Alphen aan den Rijn) is also placed in a swell box. The sound intensity is governed by twelve shutters, made of 40 mm thick oregon pine. However, in the Boniface organ, built in 1939 by Bernard Pels of Alkmaar, the twelve shutters are moved individually by a bellow. Every shutter is fully open or fully closed. The intensity of sound is not determined by the opening angle of all shutters together, but by the number of shutters fully open. This has advantages and disadvantages, but the crescendo effect is certainly not worse compared to the conventional 'linear' system (as I can testify as this is the organ I played on for more than 25 years).

Zwelwerk met balgjes die individueel de jaloezieŽn bedienen

The bellow of each swell shutter is normally closed and a spring pulls it so, in order to open the shutter if the organ is switched off. This ensures that the temperature of the organ is homogeneous and the swell division is not getting out of tune compared to the rest of the organ. If the organist closes the swell pedal with the heel of his right foot, an electrical contact is made, a magnet is energised and a pair of disc valves is opened so that organ wind is inflating the bellow. Via a steel wire, the shutter is pulled to close.

(By the way, the green felt on the upper (and lower) side of the shutters was applied by myself some thirty years ago, when I was organist of this church, in order to improve the swell effect by improving the acoustic insulation of the closed swell. This and other measures worked out very well and the crescendo and in particular decrescento effect of the Saint Boniface organ were amazing.)

Balgje en stangetje naar jaloezie zwelwerk

The swell pedal is connected to a contact rail in the console via a wooden rod, seen directly to the right of the centre of this photo. By moving the rod, ten contacts are closed or opened (numbered). The swell division has twelve shutters, so?! The four shutters opening last are switched in pairs of two. That's smart, because the dynamic difference between 8 open and 4 closed shutters is almost negligible compared to 9 open and 3 closed. On this photo, we see the general combinations in the centre, the soldered contacts to the organ at the left and the new panel for the detached choir console on the right.

Contacten van de zwelinstallatie in de speeltafel

More details of the construction below, if I show the work done on the swell assembly.

Operation and removal of the bellows

The bellows are made using rubberised canvas linen, glued onto the edges of the wooden bellow boards. The folds consist of thick cardboard glued onto the linen. Because there were no laths to secure the linen to the boards, the glue had come loose and the bellows were severly leaking. This not only caused noise, but more importantly the swell dynamics were severly limited (actually the swell effect was close to zero).

Het canvas van de zwelbalgjes laat los van de balgbladen en ook de kartonnen vouwen laten los Het canvas van de zwelbalgjes laat los van de balgbladen en ook de kartonnen vouwen laten los

It was not easy to determine how to remove the bellows or the swell engine as a whole. In the end, the bellows turn out to be fixed to the action windway with three screws, from the inside! In order to reach the screws, the closing panels had to be removed, from the back of the windway, near the swell shutters. Some shutters were removed and doing some acrobatic climbing was inevitable.

One shutter removed, spring detached. Rear panel of the windway removed. The pneumatic action is visible. Two shiny screws can be seen.

Een zweljaloezie en achterpaneel van windkanaal zijn verwijderd

The pneumatic pistons inside the windway can be seen after the panels have been removed. Also visible are the horseshoe magnets and two of the screws unscrewed. And the hand of my assistant.

De pistons zijn zichtbaar na ver verwijderen van de achterpanelen van het windkanaal

The operation of the mechanism can be explained using the photo below. Its level of complexity is typical for the electropneumatic or pneumatic organ action. As said above, normally the bellow is folded in, also by the action of a spring, keeping the swell shutter open. If the electrical contact is closed by movement of the organist's feet on the swell pedal, the horseshoe magnet is energised. It attracts a small steel disc, closing a small windway to a little square bellow glued to the roof of the channel (the dark brown thingy). Now the wind pressure in the large windway presses the small brown bellow empty (in the surrounding air) and the steel rod connected to the brown bellow is lifted. It opens the large circular valve on the bottom of the windway. A similar disc valve under it closes the way to the surrounding air. Via the open disc valve, pressurised organ wind is flowing in a small channel and this in turn inflates the large black bellow to which the shutter is connected, as seen above. So the action in fact consists of five levels …

De Een zweljaloezie en achterpaneel van windkanaal zijn verwijderd

The horseshow magnet attracts a small circular steel disc, closing the two tiny cirular windways.

De hoefijzermagneet

Repair of the worn bellows

The twelve bellows were taken to the workshop. In general, the linen, dating from the 1984 restoration, was in good condition, it was only separated from the bellow boards. Of some bellows, the folding edges were worn out and some of the cardboard 'folds' had also separated from the linen. It was decided to renew only the severly worn out linen, to re-glue all linen, and to apply reinforcement patches.

Een deel van de verwijderde balgjes

The cardboard 'folds' that keep the bellow in shape had separated from the linen in some instances.

De kartonnen vouwen zijn in sommige balgjes losgelaten van het linnen

Sometimes only the linen needed to be re-glued to the boards. In all cases reinforcement linen patches were applied, seen in the upper middle.

Soms hoefde het linnen alleen maar opnieuw De kartonnen vouwen zijn in sommige balgjes losgelaten van het linnen Versterkingsstukken op de hoeken van de verder nog in goede staat verkerende linnen bekleding

The bellow is ready.

Het balgje is klaar

Some bellows had to be re-covered completely. Here you see the removed linen (below) and the new patch of rectangular linen to which the cardboard folds are glued (middle).

Versleten balgbekleding en het nieuwe stuk linnen

After glueing, the linen is cut to fit.

De balgbekleding wordt na het plakken op maat gesneden langs de balgbladen De balgbekleding wordt na het plakken op maat gesneden langs de balgbladen

It is wise to secure the glued edges of the linen cover to the bellow boards by fixing laths to the edges. In 1984 for one reason or another, this was not done.

De opgelijmde linnen bekleding wordt verzekerd door er latjes overheen te schroeven

Then the twelve swell bellows were screwed to the windway and swell engine. The swell engine was closed, springs and wires were assembled etcetera. My mate Koos is assisting. It looks like he is preparing to read the Lecture from the Bible lachebekje

De gerestaureerde zwelbalgjes worden weer op de zwelmachine geschroefd

This is the result.

Zwelwerk met balgjes die individueel de jaloezieŽn bedienen

There are two bellows still not functioning. One has an electric fault which is difficult to trace down. It can even have to do with the electronics of the link to the choir console. The other is malfunctioning because the little brown bellow is leaking. As we wait for approval to rectify this, these two bellows were fixed in their closed position.

Indeed, the swell dynamic range is very large again! This was a rewarding job.

Kind regards,
Rens Swart