  We prove a modified version of Previdi's conjecture stating that the
Waldhausen space (K-theory space) of an exact category is delooped by the
Waldhausen space (K-theory space) of Beilinson's category of generalized Tate
vector spaces. Our modified version states the delooping with non-connective
K-theory spectra, almost including Previdi's original statement. As a
consequence we obtain that the negative K-groups of an exact category are given
by the 0-th K-groups of the idempotent-completed iterated Beilinson categories,
extending a theorem of Drinfeld on the first negative K-group.
