  A new amorphous alloy system $(TiZrNbCu)_{1-x}Co_x$ covering a broad
composition range from the high-entropy (HEA) to Co rich alloys (x$\leqslant$
0.43) has been fabricated, characterized and investigated. A comprehensive
study of the chemical compositions, homogeneity, thermal stability, electronic
structure and magnetic and mechanical properties has been performed. All
properties change their variations with x within the HEA range. In particular,
the average atomic volume deviates from the Vegard's law for x$\ge 0.2$, where
also the average atomic packing fraction suddenly changes. The valence band
structure, studied with ultraviolet photoemission spectroscopy, shows a
split-band shape with 3d-states of Co approaching the Fermi level on increasing
x. Due to onset of magnetic correlations magnetic susceptibility rapidly
increases for x$\ge 0.25$. Very high microhardness increases rapidly with x.
The results are compared with those for similar binary and quinary metallic
glasses and with those for Cantor type of crystalline alloys.
