I hesitate to start a war, but a few further comments about the Duo-Art
expression box seem in order.

The long regulator springs were indeed doubtless intended to maintain a
linear rate, but that does not mean that the expression curve was
intended to be linear, and on British Duo-Arts it is demonstrably not.
Witness the intentional design of some British upright boxes which
makes it possible to have only one position for the accordion/spill
linkage, and this automatically gives an accelerative movement to the
knife valve so that the resulting expression curve is somewhat concave.

The gaps between successive levels increase throughout, markedly so
above power 10 providing that the pump has sufficient tension and flow
to minimise the inherent 'droop'.  Power 10 is approximately mf, so
there are more gradations below mf than above.  Instruments (even
American) set up like this certainly sound musically convincing, with
fine pianissimo delicacy and yet ffff fortissimos.  Where settings give
a flat, linear curve instruments sound unsubtle at the lower dynamics,
very middly and by contrast seem to lack intended fortissimos.

It should be noted that the chosen Duo-Art method of adjusting the
regulator spring by means of a collar gives a different result from
what would be achieved by moving its fixed end.  Shortening the theme
regulator spring by its collar both increases its initial tension and
its effective rate.  This gives perhaps +0.25" H2O (theme to
accompaniment) at power 0 and +1.5" at power 15, and ensures that
the theme can more than bisect accompaniment levels if required and
be clearly differentiated all the way up.

Reiterative adjustment of the theme spring collar and the knife-valve
minimum setting is required to obtain the necessary curve, and this was
the adjustment that surely eluded technicians.  Much easier for good
Customer Relations to factory-fit different rate springs with similar
initial tension and order in the Service Manual that they just have
sufficient resting tension not to rattle, adjust the minimum setting so
notes don't miss and all should be fairly OK...

On British Duo-Arts the crash valve was not done away with: it was
never envisaged, but its absence in no way means that ffff passages are
not reproduced.  British boxes in practice give negligible constriction
and the selected regulator springs match the full expression
curve/regulator span closely to the natural dynamic range of the
instrument, so a sudden step-change crash is superfluous and musically
an affront.

In Britain Duo-Arts were always expensive, the grands particularly, and
they were invariably found in rich people's drawing-rooms which, even
in our poor little country, are not known for being small.  Miniature
grands of the sort that became quite common in the US were hardly
produced here and Steinway Os and similar sized Webers are the norm.
Readers may rest assured that properly-voiced such pianos will produce
astonishing sound pressure levels in large rooms even before 'clanging'
-- the point on any instrument which sensitive pianists and clever
Duo-Art expression boxes regard as over-playing.  I may add that I have
yet to hear a Duo-Art in the US that plays as loud, even using a crash
valve, as a number of similar instruments here in the UK while still
being able to match their ravishing pianissimos!

Interestingly, I have encountered several unrestored continental
Duo-Art grands (e.g., Gaveau, Ibach) which are fitted with US-made
expression systems but which have identical regulator springs.  And
one had had its crash valve disabled from new.

Being a professional designer and manufacturer of loudspeakers I know
not to venture into the minefield of decibels (power or intensity?) as
relating to pianos...

A happy and Holy Easter to all.

Patrick Handscombe
Wivenhoe, Essex, UK