[en] The feed and control circuit serves for the control of the neutron output of a gas-filled neutron tube, e.g. of the type Amperex 18603, in continuous or pulsed operation. The neutron tube is part of a borehole measuring probe and consists essentially of a target, an ion source, and a replenisher connected with earth, as well as equiptment providing supply voltage. The feed and control circuit contains a constant source of current and an electronic circuit connected with the supply voltage equipment, the replenisher, and the constant source of current. This circuit feeds part of the constant current to the replenisher as replenishing current, corresponding to the target current, this controlling the neutron output. (DG)