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32-keyless Trumpet Organ

4 Action Relay

The drawing above (not to scale) demonstrates the main organ's action. The top section is the relay, and the bottom section is the distribution board. The relay section shows two identical actions; elements (from top to bottom) are the inlet tube from the keyframe, membrane chamber, and wooden tube with the valve sitting on its top, in a pressurized chamber. On the left is a small regulation bleed hole in a disc of plastic card. The space to the left of that is an extension of the pressurised space below.

How it works

The main chamber is constantly under pressure, which extends through the bleed hole and tiny channel to the membrane chamber. Pressure is maintained so long as music card is sealing the other end of the tube at the top. When that tube is opened by a hole in the music the pressure is released, causing the membrane to be pushed upward by the pressure below. This opens the valve on top of the wooden tube so letting wind pressure run into the tube and thus through the distribution board to the relevant pipe(s). When the advancing music card seals the top tube the membrane drops back onto the valve seat at the top of the wooden tube, using pressure bled through the hole in the plastic card and into the membrane chamber, so the pipe no longer plays.


The relay consists mainly of its top board and the valves it holds. This board has recesses to take the zephyr membranes, connected to the pressurized interior by long 3mm holes running horizontally through the board. These holes had to be drilled at a precise angle to avoid running into recesses that didn't belong to them. Firstly, I don't possess a 3mm drill over 4½ inches long, and even if I had, there was no way to ensure it would run straight. Also, I could not guarantee to be able to clean out the junctions between the horizontal holes and their short vertical counterparts after drilling.

My solution was to make the board from four pieces of mahogany, mark and drill them individually, and then laminate them together to form one board. The joins between the boards coincided with the centres of the vertical holes, so they could be cleaned out before the laminating process. The picture here shows these separate board elements before laminating.

Above left: the finished board from the top. This shows the inlet holes which will be connected to the keyframe, and the bleed holes on the front edge. These bleed holes have a small 10mm diameter recess to take plastic-card discs. The discs will be individually drilled to precise sizes to regulate the amount of bleed required by each individual action. I had decided against adjustable bleeds for two reasons. Firstly, atmospheric conditions change according to the weather, and adjustable bleeds would be necessary to counter the natural bleed variations through membrane leather; however I am using zephyr for the membranes, and zephyr is virtually airtight, so there would be no variation occurring here. Secondly, a bleed with a circular hole has a larger aperture than a thin ring round a tapered screw in a hole. This will allow larger dust particles to flow through, preventing a gradual clogging of the bleed aperture over time.

Above right: the board looking from below. The large holes are recesses to take the zephyr membranes for the 32 note actions in the organ.

Zephyr membranes were cut into hexagons for maximum glueing area; then pallet leather discs were glued to one side while discs of lead were glued to the other. The lead serves two purposes; to keep the valve flat, and to provide a little weight to let the valve to close promptly and positively.

Small pieces of felt were glued into the recesses to limit the valve movement to one-quarter of the diameter of the valve aperture. Any more movement would be a waste, and reduce the rapidity of operation. A channel needed to be provided between the keyframe hole in the centre and the bleed hole at the side.

Above: Two methods of producing the wooden tubes from dowel. I made a good few spares, using both methods and chose the neatest batch for the final product. The most important aspect of these is for the ends to be perfectly flat, level and smooth.

Below left: The tubes were glued over the holes on the top of the distribution board. Two of the actions were for register control, and needed exhaust valves. Wires attached to the membrane valves would be extended through the distribution board and through a valve guide (below right). Exhaust valves will be mounted on the wires above the guide. Those valves will be pulled closed when the membranes lift, allowing pressure to operate the register action. When the membranes collapse the exhaust valves will open, allowing the register pneumatics to return to normal.

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