NOTE: I now use Phenoseal to seal the wood inside the unit.

HI Bill, 

While I would never call myself an expert at anything, I have 
spent many days perfecting the rebuilding of the Simplex block 
valve/pneumatic to the point where they work every single time.

The vast majority of the units that failed early on did so 
because of leakage through the wood or through the mounting 
screw holes. Depending on the technique that you used to take 
the blocks apart, there could be microscopic cracks between 
the upper and lower valve chamber.

To eliminate the possibility of such leakage, I reseal every 
square millimeter on the inside of the unit. In the pouch well 
and the lower half of the valve chamber, I use shellac, two 
thick coats. I also seal the hole leading to the pouch well 
and the vacuum channel with shellac. And, after the blocks are 
back together, I seal the area on the block that's inside 
the pneumatic and the vacuum channel leading from the exhaust 
side of the valve to the inside of the pneumatic, including 
the slit section. To seal the area in the upper half of valve 
chamber, around the metal valve seat, I use Phenoseal. But I 
don't just paint it on. After I apply a thin coat, I close 
off seat with my finger and suck gently on the supply channel. 
This sucks the Phenoseal into the wood. After that's dry, I 
apply another thin coat and then blow gently into the vacuum 
channel. If there is any wood leakage, the Phenoseal will 
bubble up, identifying the leaking area. In each case where 
leakage was detected, the air was leaking through the chamber 
walls, and not around the valve seat (since it is sealed on 
the underside with shellac). Quite honestly, I've never been 
able to pin point exactly where the air is coming in, but my 
belief is that it is coming in through the open mounting 
screw holes. 

(I made a 3-perspective drawing of the block to see how 
close the screw holes are to the valve chambers and pouch 
wells and it's measured in millimeters. So such leakage is 
extremely plausible. You might also find that if that is indeed 
the problem, it will magically disappear when the blocks are 
installed into the piano, since those holes are totally 
closed off from outside air by the screw.) 

Now I'll back up a little. Unlike most rebuilders, I do not 
seal the pouches. And it is for that reason that I am  
responding to your posting privately. We could get into a very 
long draw out discussion about the benefits and drawbacks of 
sealing the pouches, but common sense tells me that if you 
add anything to the pouch leather, the flexibility of the leather 
decreases. The question I asked myself from square one was:
if sealing the pouches is so important, why didn't the 
manufacturer seal them? Since they felt no need to do it, I 
don't either.

Moving on. When I put the block back together, the two halves 
are dry fit to see if there is any warpage. And if there is, 
the warped half or halves are lightly sanded. I also use a 
fairly substantial amount of hot hide glue when glueing the 
pieces back together, especially around the air channel 
leading into the pneumatic. I found that since the edge of 
the pouch is dangerously close to that opening, air can 
seep underneath (or through the edge) of the pouch, allowing 
atmospheric air into the pouch well. This causes the pouch 
to rise when being tested, like the note is turned 'on'. When 
the pouch is also sealed (on top), making it more air tight 
than ever intended, the whole situation becomes even more 
critical, and even the slightest leakage through the edge 
of the pouch will trigger the note.

One thing I know for absolute positive is that a pouch will 
not move if there is an equal amount of vacuum on both sides 
of the leather. And only two things can change that state. 
One, the entrance of atmospheric air (or reduced vacuum) 
on the underside of the pouch. Two, the entrance of a higher 
vacuum level above the pouch. Since #2 is basically 
impossible, that leaves #1. The question then becomes: where 
is the air leaking 'in'? Eliminating every possible place 
(even through the wood) where air can leak 'in' eliminates 
the possibility of a problem.

Musically, 

John A. Tuttle
===========================================================


One of the real headaches about the Simplex unit is 
the proximity of the pneumatic air channel and the 
pouch chamber (both above and below the pouch). They 
are quite close to each other and even the slightest 
leakage between the two will render a unit useless. 

I noticed when opening the blocks that Simplex was not 
adverse to using copious amounts of hide glue. The 
evidence is everywhere. They knew how critical it was 
to have complete separation between the two spaces 
and that each space had to be air tight. Glue filled 
any and all of the voids and regularly was seen all 
around the edges on the pouch, infringing on the 
operating area by as much as 1/8" nearly all the way 
around.

The reality that Simplex was in no way concerned about 
the 'over-flowing' glue onto the operating portions of 
the pouch, got me to wondering about the often overstated 
importance of pouch placement. It's often been said that 
a pneumatic-operated pouch-setting tool must be used to 
reduce the possibility of dragging the glue into the 
operating portion of the pouch well. While this is reasonable 
in instances where the pouch is 'free standing' on a pouch 
board, it is a totally unnecessary concern when applying 
the pouches in the Simplex block. 

Simplex obviously designed the pouch to be big enough in 
total size that losing 1/8" all the way around was of no 
significance at all. In fact, when you examine the physics 
of the matter, a smaller pouch will react faster than a 
larger pouch (all other factors being the same). So one 
has to consider that Simplex actually planned on using 
lots of glue to put the blocks together, knowing that, in 
a sense, 'more was better'.

I noticed during the rebuilding of an entire set; the 
units I had problems with were the ones where I had been 
skimpy with the hide glue, when putting the blocks back 
together. Also, the consistency of the glue was definitely 
a factor. I constantly added small amounts of water to keep 
the glue at just the right thickness.


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