The local three-dimensional (3-D) ideal magnetohydrodynamic (MHD) stability properties of a 12-field period torsatron device of moderate aspect ratio are investigated as a function of an externally applied quadrupole magnetic field. It is demonstrated that with a parabolic pressure profile the volume-averaged beta limit due to the ballooning instabilities decreases from beta congruent-to 2% to beta congruent-to 1.5% as the current I(mid) in the quadrupole field coils normalized to that in the main helical coils increases from 0 to 0.19.