  This article deals with the approximation of a stochastic partial
differential equation (SPDE) via amplitude equations. We consider an SPDE with
a cubic nonlinearity perturbed by a general multiplicative noise that preserves
the constant trivial solution and we study the dynamics around it for the
deterministic equation being close to a bifurcation.
  Based on the separation of time-scales close to a change of stability, we
rigorously derive an amplitude equation describing the dynamics of the
bifurcating pattern.
  This allows us to approximate the original infinite dimensional dynamics by a
simpler effective dynamics associated with the solution of the amplitude
equation. To illustrate the abstract result we apply it to a simple
one-dimensional stochastic Ginzburg-Landau equation.
