The patented Centripetal Tension Resonator in Mason & Hamlin grands
was intended to preserve the crown of the soundboard in a new
instrument and raise it, if necessary, after the piano had settled
following several changes of season.  It's probably best to leave it
alone except in the course of doing a complete rebuilding, unless
you were a factory-trained service person.

During the summer of 1971 I worked at the New England Conservatory
of Music and Boston University, cleaning and regulating grand pianos
of many sizes and makes, mostly Steinways and Masons.  Pianos at the
Conservatory ranged from full plate antique to ones of fairly recent
vintage, all but one Steinway "M" ex-Duo-Art having never contained
players.  I imagine most of these were donated over the years, with
only the finer examples offered being accepted.

The idea of the tension resonator fascinated me, and I studied the
undersides of a good many Mason & Hamlins.  There was just one Mason
that had only 3 tension rods, and that was made in the 1930's.  From
the placement of the wooden braces under the soundboard, it was obvious
that it was built to accommodate the Ampico, which, no doubt due to the
slackening demand during the depression, was never installed, the piano
instead being finished as a "straight" instrument with single legs and
the keybed not being cut away at the back where the stack would go.
All the other "A"s had radial braces and six tension rods except for
the old "A"s which had eight.  The "AA"s all had eight, and the "BB"s
and "CC"s had ten.

I would say that three tension rods is an inadequate number, little
better than none at all.  The reason for the small number was to permit
the pump and motor to be fitted closer to the soundboard, which made
the underbelly of the piano less apt to resemble that of a pregnant
hippopotamus.

Jeffrey Wood