  We consider the linear stability to three-dimensional perturbations of
two-dimensional nonlinear magnetohydrodynamic basic states obtained from a
specified forcing function in the presence of an imposed initially uniform
magnetic field of strength $B_0$. The forcing is chosen such that it drives the
CP flow of Galloway & Proctor (1992) when $B_0=0$. We first examine the
properties of these basic states and their dependence on $B_0$ and on the
magnetic Reynolds number $Rm$. The linear stability of these states is then
investigated. It is found that at a given $Rm$ the presence of a background
field is stabilising. The results also allow us to speculate that at a fixed
value of $B_0$ the growth of the unstable perturbations is `fast', in the sense
that the growth rate becomes independent of $Rm$ as $Rm \to \infty$.
