  A prototype of a dual-readout calorimeter using brass capillary tubes
surrounding scintillating and clear plastic optical fibres was tested using
beams of particles with energies between 10 and 100 GeV produced by the CERN
SPS. The scope of the test was to characterise the performance of the
tube-based detector response to positrons in terms of linearity, energy
resolution, and lateral granularity. After calibrating the detector and
processing the output signal to correct for the energy dependency on the
particle impact point, the linearity of the measurement was found to be better
than 1\%. The positron response was compared to that predicted by a
Geant4-based simulation, finding good agreement both in terms of energy
resolution and shower profile. The detector resolution was estimated to be well
described by a stochastic term of 14.5\% with a negligible constant term.
