Coherent oscillation is achieved in the geometry of a double-phase conjugate mirror in nominally undoped Sn2P2S6 with 647 nm radiation of a Kr+ laser. The specific temporal dynamics of oscillation with modulated intensity and periodic 0-pi-0-pi variations of the oscillation wave phase is similar to that observed earlier using a semilinear oscillator geometry. The described oscillator ensures submillisecond delay time in the appearance of the phase conjugate wave.