The effective QCD Hamiltonian is constructed with the help of the background
perturbation theory, and relativistic Feynman--Schwinger path integrals for
Green's functions. The resulting spectrum displays mass gaps of the order of
one GeV, when additional valence gluon is added to the bound state. Mixing
between meson, hybrid, and glueball states is defined in two ways: through
generalized Green's functions and via modified Feynman diagram technic giving
similar answers. Results for mixing matrix elements are numerically not large
(around 0.1 GeV) and agree with earlier analytic estimates and lattice
simulations.Comment: LaTeX2e, 23 pages, 4 Postscript figure
