An electron-beam tube for a scanning electron microscope employs the use of a TF built-up field emission cathode which can be operated from preferably the 100 plane in a substantially continuous mode to provide a stable electron beam having high current density, high resolution, and very high electron optical brightness from a source of very small proportions. The tube, which comprises an evacuated envelope having chambers of different vacuums, is designed to facilitate either quick-change cathode replacement or attendance to the specimen with minimum loss of operating time since the vacuum of the entire tube need not be released. Also, the chamber containing the field emission cathode can be separable from the tube to allow replacement by a new preprocessed cathode in a pre-evacuated chamber. In this case, the mounting means includes a device for puncturing a seal in the cathode chamber to allow the electron beam to pass therethrough.