  We have studied the upper critical field, Bc2, in poly-crystalline MgB2
samples in which disorder was varied in a controlled way to carry selectively p
and s bands from clean to dirty limit. We have found that the clean regime
survives when p bands are dirty and s bands are midway between clean and dirty.
In this framework we can explain the anomalous behaviour of Al doped samples,
in which Bc2 decreases as doping increases.
