From: email@example.com (D. L. Bullock) To: "MMD"
Date: Fri, 22 Apr 2005 09:38:42 -0500 Subject: Duo-Art Cross Valves This is something I got from a former rebuilder friend from another forum along with my response. I thought he said it very well. Here is the item followed by a source footnote and my response. It has been forwarded with his permission. - - - Currently at auction on eBay are Duo-Art "replacement" valve plates. These are beautifully made and are sold as a replacement for "those notorious cross valve seats that always leak". That is the good news... However, I have restored hundreds or thousands of cross valves and had no problems. There is an issue with replacing the valve seats that many do not understand and that it is the relationship between the air flow and "pluck" of the valve. Pluck is the resistance to activation from the "off" (or closed) position. At the reduced valve gaps utilized by player stack valves, flow becomes primarily dependent on orifice perimeter. Under these conditions, an orifice with a multi-faceted perimeter (such as a star or a cross) will have greater flow than a circular orifice. At such small valve travel (gap), orifice area is less important to flow than orifice perimeter. However, in player stack valves, the total area of an orifice is a major factor in pluck because it determines the area of the valve's surfaced exposed to vacuum. In the same application, the total area of an orifice becomes a limiting factor in flow only when its area is exceeded by that created by the dimensions of the valve gap and perimeter multiplied. When utilized in player stack valves, a circular orifice yields _minimum_ perimeter (i.e., flow) and _maximum_ area (i.e., vacuum resistance). It is the worst possible of shapes. In the same application, a cross-shaped orifice provides _more_ perimeter and _less_ area. It has a much better perimeter-to-area ratio. Player stack valves employing seats with circular orifices are perfectly acceptable, as is, in most non-critical applications. Yet, the demanding repetition requirements of reproducing pianos dictate that such valves be used in conjunction with primary valves or variable bleeds (e.g., Ampico's model B ball-check-type bleed) if optimum performance is to be attained. And, indeed, several manufacturers of reproducing actions chose those routes. Aeolian's method of optimizing the performance of a premium single-valve system for its Duo-Art mechanism was to equip it with the cross-type valve plate which maximizes flow and minimizes pluck. The only possible downsides of this design might have been the critical tooling requirements of the plate and the need for a highly skilled and patient workforce in assembly. Many complaints of the replacement seats are that the pianos tend to play too loud. This is just one of several problems caused by eliminating original cross valve plates. Proper replacement valve facing leather, careful cleaning, not bending them during removal, plus sealing the bottom of your valve wells with shellac like Aeolian did, will result in superior Duo-Art valve work and greater stability by your Duo-Art at lower vacuum levels. This equals better (original factory) performance. It is true that Aeolian went to the round valve plate, but stayed with the cross valve in the Steinway and high end pianos. The less expensive pianos used the round plates and there were different sizes of them depending on the model and year. Also, keep in mind that when the valve plates got smaller, so did the valve itself. It is a delicate balancing act. There is definitely a use for the new valve seats, but it will not improve performance by simply using them as a Band-Aid or panacea to circumvent proper restoration. Restoration? Do it the way the factory did and it will function like new. Robin PS. The cross valves don't always leak! In fact they are extremely reliable and function beautifully. Info included from AMICA: Thanks to the AMICA Bulletin too! Sept-Oct 1999 by Jeffrey Morgan, "Wondrous Cross Revisited". - - - My response to him was: I agree completely, Robin. I love the cross valves. Years ago, I changed my cross valves to round when I was having trouble with Durrell's leather leaking. This was before I understood that most leather today is too leaky to use on Duo Art. I got worse problems and changed them all back when I found good leather to replace his with. The only time I had a set of cross valves leak is when I had bad leather that leaked with the round valves as well. D.L. Bullock St. Louis www.pianoworld.us --------------------