Driving piezo scanner tubes using the driven internal electrode configuration
Out5 with -10 volts at its input becomes -250 volts DC. This sets the bias point and prevents any of the AC drives from creating any net positive signal. The manual says

Care must be taken not to apply positive voltages to the internal
electrode since this can lead to depolarisation.

So you have -250 volt on the inner electrode and that's all it says. If in doubt set In5 to -10 volts because the TD250 amplifier can't produce AC signals bigger than +/- 250 volts: -

By applying the full-scale negative voltage to the internal electrode,
a contraction equal to half the vertical scan range is obtained. This
method exploits the higher positive electric field strength of the
piezoelectric material, which is usually five times the negative
electric field strength.