Apparatus for use in cathode ray tubes and particularly in multi-gun color television picture tubes and like devices to enable substituting for the active cathodes and associated parts other standby cathodes and parts which will restore the tube to full emissive power and substantially double the life of the tube, said apparatus including a multicathode assembly mounted inside the neck portion of the envelope of the picture tube, said assembly having a plurality of active and standby cathodes and associated heater elements mounted thereon, and means including magnetic means for moving said assembly between alternate positions inside the tube envelope whereby selected ones of the cathodes and their associated heater elements are in operative positions and others are not. The present apparatus enables replacement of the active cathodes without requiring any modification of the tube envelope itself, without requiring removal of the tube from the set in which it is installed, and with little or no modification or adjustment of the set to accommodate the change.