Electron beam spot characteristics can be tuned in each x-ray tube by moving a focusing-ring along a longitudinal-axis of the x-ray tube. The focusing-ring can then be immovably fastened to the x-ray tube.An x-ray source can include an x-ray tube and a focusing-ring. The focusing-ring can at least partially encircle an electron-emitter, a cathode, an evacuated-enclosure, or combinations thereof. The focusing-ring can be located outside of a vacuum of the evacuated enclosure. The focusing-ring can adjust an electron-beam spot on a target material of the x-ray tube when moved along a longitudinal-axis extending linearly from the electron-emitter to the target material.