Randy A. Bergum
Keeping an older organ maintained properly can be very rewarding, although sometimes challenging, especially when many of the original vendors have gone out of business. Because some of the repairs required of any installation require parts that are no longer made, the technician must seek creative solutions.
A few years ago Bob Trousdale faced the problem of repairing several large scale chimes at Plummer Auditorium in Fullerton, California. Each of these chimes had a vertical crack about six inches long starting at the top end near the plug and catgut hanger, adversely affecting the sound quality. Bob had asked members of the Plummer crew if anyone knew of a solution to this problem. There had been a few articles published in Theater Organ magazine, as well as the Glue Pot newsletter, but these repair schemes yielded various results, from Pretty Good to Not so Good.
There have been many attempts at restoring the sound of cracked chimes. Hose clamps have been installed on the crack to pull it closed, but this does not work completely. The tone is not fully restored, and a low frequency undulation can result. Filling the crack with epoxy and wedges has been tried, although this still permits a discontinuity along the crack surface, which interferes with the vibrations.
The original chimes used by Wurlitzer and other makes were supplied by J. C. Deagan of Chicago. They were made of nickel-plated brass, and they tended to crack during the 60 to 70 years of hard use. The typical crack occurs at the end of the tube in a longitudinal direction, and tends to dull and shorten the strike tone by interfering with vibration propagation.
A possible cause of cracking could be found in the way the tubes are formed during manufacture. The better grades of chimes - Class A, B, and C were drawn, but the Class R tubes were rolled from a brass sheet, shaped into a tube and joined at a seam. Solid plugs are then pressed into one or both ends, which affects the tone quality of the chime. This added mass acts to lower the pitch of the fundamental and the first few harmonics, effectively stretching the perceived pitch to one that is more pleasant to the ear. Pressing the plugs into the tube with an interference fit is necessary to match the impedence between the tube and the plug, making the assembly behave like one solid system. However, this interference fit causes a tensile hoop stress in the tube, much like that found in a rain barrel full of water, where the weight of the water places a bursting force in the outward direction. Steel hoops are usually installed around the circumference to contain these outward forces.
Local stress at the tip of a crack is extremely high, and easily exceeds the tensile strength of the base metal, causing the crack to propagate quickly once formed. In addition, microscopic imperfections in the base metal and seams made during manufacture cause local stress risers, which encourage the formation of cracks. Furthermore, the typical chime is struck repeatedly by a hammer, encouraging crack growth.
One method of chime tube repair has been to cut off the offending part and turn the tube into a higher note. This is not usually the best way to solve the problem for two reasons: First, the scale of the trimmed tube usually won't match the rest of the original set, and second, there are no guarantees of finding a larger set that is close in scale.
We have found that the chimes can be repaired by welding with a TIG welder (Tungsten Inert Gas). Silicon Bronze rod is used as the filler metal because it closely matches the parent metal mechanical properties, and it has very good welding characteristics.
First, a bit about brass. Made of varying parts of copper and another element, such as zinc or sometimes silicon, brass is easy to machine and finish. The zinc contained in the brass will vaporize at an amazingly low 1,663 degrees F, and is very toxic, so you'll want to have adequate ventilation or the proper respirator for metal vapors.
The chime shown in this article is cracked at the bottom end, and has a recessed bottom plug inserted. Double plugged tubes usually have a very small hole drilled to allow for pressure escape during manufacture.
Fig. 1. Crack in tube before preparation.
Fig. 2: Tapping the Bottom Plug
Fig. 3: The Bottom Plug
Fig. 4: Prepped tube showing stop drilled hole and chamfered edges
Fig. 5: The Welder
Fig. 6 - Completed weld bead
Fig. 7 - Bead filed off and tube polished.
The completed chime has the tone restored, and matches the timbre of the other tubes in the set. The appearance of the chime reveals that the repair site is visible, but not overly noticeable. The nickel plate has been removed in the area adjacent to the welding, and the underlying brass has been polished. This may not be a problem since many chimes are hung vertically with the tops hidden by the hammer action, so your handy-work might be covered up in the end. The actual weld bead can be distinguished by its color, which has slightly more copper in it than the native brass in the tube, since some of the zinc has boiled off during welding.
Fig. 8 - Installed Chime Tube back home with the others
To date we have about 10 chimes in service, the longest one being in use since 1997 with no observable failure. The projected service life is not known, but it seems that the repaired area should be stronger than the original seam.
Many thanks to Bob Trousdale, John Ledwon and Al Sefl for their help in preparing this article.
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Copyright © September 1997 and Beyond. Randy A. Bergum - All Rights Reserved