Electron beam spot characteristics can be tuned in each x-ray tube (10, 30) by moving a focusing-ring (13) along a longitudinal-axis (15) 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 (10, 30) and a focusing-ring (13). The focusing-ring can at least partially encircle an electron-emitter (14), a cathode (12), an evacuated-enclosure (11), 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 (33) on a target material of the x-ray tube when moved along a longitudinal-axis (15) extending linearly from the electron-emitter to the target material.