DEFAULT PO TYPE IN MD04

Hi all, I am in ECC6 and I have the following problem:

purchase requisition type NB is used for all the companies. This type links a lot of different Purchase order types.

When I try to convert PR into PO from MD04, the system takes automatically PO type NB even if in the user parameter and in default setting of PO the user has different type.

How can I do to select a different PO type?

regards