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MMD > Archives > August 1996 > 1996.08.17 > 06Prev  Next

Re: Duo-Art Playing Too Loud
By Craig Brougher

I think Spencer Chase covered about all of the things in his list that can cause a Duo-Art to play too loudly. It never is quite possible to explain everything fully in a letter when talking generally about a problem of this nature.

I will say, however, that cross valves will cause this same problem, although round valves will not. Here is how it works: All Duo-Arts rely on a little bit of leakage in the stack valves to prevent building up pressure when no notes are being played. (That is when they leak just a little-- at the low intensity.) This could not happen with a self- compensating mechanism like the Ampico, but in the Duo-Art, the stack pressure is created through a knife valve.

Consider for a minute how this looks to the pump. Visualize an in-line valve that cannot get perfectly airtight, in series with another valve that can get just as tight as it can. Close them both off. Place a gauge between the two in-line valves and what do you read? Zero? Nope. You read exactly 1/2 the pressure at the pump! Now suppose your first in- line valve (the knife valve) was just a little warped, curved away from its slide. After all, it's maple, and maple can do that. Now you read, say, 15" when both are closed, with very, very little extra leakage from the knife valve (that is, when the pump is at 20").

With cross valves, you will have a number of valves which have taken a "set", shaped like the cross valve itself. When new leather is used, it will take a number of years, but will do the same thing even worse, because it is GARMENT LEATHER, tanned to be soft. Original Duo-Art valve leather was pre-stretched, industrially staked leather designed for the trade! That is a tremendous compromise, and very few people realize it.

Only with a round valve are you going to permanently prevent this problem. Later Duo-Arts ALL used round valves, with the same expression system and worked just fine, but the cross valve was retained in the larger players whose stacks had been built earlier and were still waiting for a piano order to place it into.

Just as a pouch would hold a few of the valves up just high enough to leak, so also will a few valve leathers which have taken an impression of the cross seat and have gotten rotated, sitting cross-wise so that it can leak when the vacuum level called for is low. Now when the vacuum called for is high, those valves get "sucked" down tight and don't leak at all. But the only adjustment is to the zero intensity, so you have to set the lowest intensity higher just to get the notes to play. That raises everything, and really blasts you out of the room once the intensities get over #10.

The normal valve leakage in a Duo-Art is required, as I said, but if only one valve is being suspended even 1/64-inch on one corner, either by a pouch or an embossed valve leather sitting crooked, this is what you will have. Sometimes you can fix this cross valve problem temporarily by replacing the expression box with a suction box of vacuum cleaner on each side, separately. Keep the piano hammers from playing or dampen the strings with towels and run your fingers back and forth over the tracker bar. Do this enough times and sometimes cantankerous valves will re-seat. It takes about 60 inches of vacuum and usually about 5 minutes of back and forth on each side to do it well. If the problem is a pouch though, you won't be able to fix it that way.

To find a tight pouch, put a gauge on that half of the stack and suck on each pouch nipple momentarily when you have the zero intensity set to a normal 5" or so -- the pressure it should be working at but doesn't. Each time you find a tight one, you'll see the gauge move up. If it is the only valve on that side leaking, the gauge will stay up. If you have more of them, the gauge will drop back down. Granted, your own vacuum adds a little, so just factor that out. You'll see the difference after a few valves.

I assume here that everything else has been tested and is okay, because this is the only symptom given. An overly large motor pulley was actually used on concert grands, by the way. It doubled the power.

The linkage referred to by Robbie is a regulation problem, but also a real trapeze act that would be difficult to explain in a letter, and would most probably be misunderstood if I were to get into it. Let me make it more simple: Given that everything else is right about the box, look at the photograph of the Duo-Art Expression box in the reprint sold by Player Piano Co. You will see the normal position of the lever which is operated by the accordions. It sits at about a 10-15 degree back angle. Start with that. From there, if you still have the problem, you are close enough to conclude that that isn't the problem. Doesn't mean it's perfect, but as close as Aeolian ever got one.

Spencer also mentioned the Theme valves leaking. When the theme valve(s) leak, the accompaniment zero intensity will not go low enough to really set the instrument softly enough. To make sure this is not the case, set the solo side of the box to about as low as you can get it. That takes it out of the circuit. Then set the accompaniment side to whatever produces a soft but even response. Now raise the solo side to, say 7-8 inches of water on the gauge when the theme valve is opened. Now return again to the accompaniment side and see if your setting has raised. If it raises over about 1/8" your one-way theme isolator valves may need some attention. By the way, do not use suede both sides to replace these, because suede leaks unless it has a sealing side.

Craig B.

(Message sent Sun 18 Aug 1996, 02:09:41 GMT, from time zone GMT.)

Key Words in Subject:  Duo-Art, Loud, Playing, Too

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