We present a new approach to a unified description of the dielectric end-to-end vector and the viscoelastic stress relaxation of entangled homopolymer melts in terms of the tube model. The comparison and quantitative analysis of carefully recorded viscoelastic and dielectric relaxation spectra evidence that the dielectric mode distribution is completely determined by reptation and contour length fluctuations and thus is insensitive to the CR process within experimental accuracy. We show that a joint analysis of viscoelastic and dielectric experiments allows an accurate characterization of pure single chain dynamics. The general validity of our approach is demonstrated on monodisperse polyisoprene (PI) for a wide range of different molecular weights and on poly(butylene oxide) (PBO).