My old Dallas friend, Bill Flynt, wrote, "We found that if we did the
tuning as usual with the trem 'off', all was okay until the trem was
turned 'on' (the usual, with tibia pipes).  The rank sounded badly out
of tune, although I don't remember whether sharp or flat.  We decided
to do the tuning with trem 'on' and hope for the best.  The result
seemed quite satisfactory to the listener's ear."

This is sometimes a problem found in theater organ tuning.  Quite often
Tibias have this malady.  The situation of the orchestral tremolo
equates to a sine wave.  In electronics, a sine wave travels up into
the positive voltage side from 0 volts, then peaks, and travels down to
0 volts.  It then passes it to peak (or valley) on the negative voltage
side, then travels back up to 0.  The two peaks both up and downward
travel from 0 will be equal.  In other words, up 12 volts and down 12
volts with a variation of 24 volts from top to bottom of the sine wave.
Is this clear as mud?

In an orchestral (theater organ) tremolo, the same sine wave occurs
with the wind pressure and pitch of pipework.  Instead of 0 volts the
sine wave centers around the calm pressure at which the pipe is winded
for tuning.  When everything works right the tremolo being activated
will cause the pressure to immediately drop below tuning pressure,
peak, then travel up to peak at a pressure higher than the pressure at
tuning.  It then falls back to the lower pressure, thus tracing the
sine wave pattern in wind pressure.

When the reservoir is not correctly regulated, the sine wave will be
there but it will not center on the tuning pressure.  Some of them
will center above tuning pressure and some will center below tuning
pressure.  When this happens the pipes will seem to be higher or lower
in tuning than the rest of the organ when trem is on.

To regulate a theater reservoir, the ball valve must open first,
the small flap opens next and the large flap opens next.  When at rest
the Wurlitzer reservoir should measure, I believe, 7.5" from top to
bottom board (outside measurement.)  For more information check the
technicalities books from the American Theater Organ Society
publications.  This is covered extensively.  This will correct any
deficiencies from my memory.

D.L. Bullock
St. Louis
www.thepianoworld.com