Abstract: A fixture for holding a vibration sensing transducer near a vibrating machine part, such as a turbine shaft is provided with a probe tube which holds the vibration sensing transducer. The probe tube penetrates a housing of the machine through an opening therein, the probe tube being slidably held by a collet-type holding arrangement. A setting collar is arranged outside of the collet-type arrangement to prevent the probe tube from slipping through the opening in the machine housing when the collet-type holding arrangement is loosened to permit axial mobility of the probe tube. This prevents the vibration sensing transducer from being damaged by contacting a vibrating machine part within the machine housing. In addition, the setting collar permits the vibration sensing system to be calibrated by permitting a feeler gauge to be used to vary the distance between the vibrating machine part and the vibratoon sensing transducer by a predetermined amount.