An experimental investigation is presented on oscillations excited by the dynamics of an anode double layer or fireball in a double plasma (DP) machine. The fireball is created by additional ionization processes in front of a small circular anode, which is inserted into the diffusive DP plasma and biased positively. An annular (ring) electrode, usually biased negatively, surrounds the anode. The extension of the ion sheath in front of this ring controls the anode current by varying the effective diameter of the anode during the fireball oscillations. The ring voltage affects not only the oscillation frequency of the anode current but also other characteristics of the instability. Related phenomena consisting of ion-acoustic waves and almost synchronous oscillations of the plasma potential in the chamber where the fireball is formed are also presented.