  Let $K$ be a number field and $A/K$ be an abelian variety of dimension $g$
with $\operatorname{End}_{\overline{K}}(A)=\mathbb{Z}$. We provide a
semi-effective bound $\ell_0(A/K)$ such that the natural Galois representation
attached to $T_\ell A$ is onto $\operatorname{GSp}_{2g}(\mathbb{Z}_\ell)$ for
all primes $\ell > \ell_0(A/K)$. For general $g$ the bound is given as a
function of the Faltings height of $A$ and of the residual characteristic of a
place of $K$ with certain properties; when $g=3$, the bound is completely
explicit in terms of simple arithmetical invariants of $A$ and $K$.
