X-ray tube vs thermionic tube

X-ray tube and thermionic tube both are nouns.

Word	Noun	Adjective	Verb	Adverb
X-ray tube	Yes	No	No	No
thermionic tube	Yes	No	No	No

As nouns, thermionic tube is a hypernym of X-ray tube; that is, thermionic tube is a word with a broader meaning than X-ray tube:

X-ray tube: a vacuum tube containing a metal target onto which a beam of electrons is directed at high energy for the generation of X rays
thermionic tube: electronic device consisting of a system of electrodes arranged in an evacuated glass or metal envelope

Other hypernyms of X-ray tube include electron tube, thermionic vacuum tube, thermionic valve, tube, vacuum tube.

X-ray tube (noun)	thermionic tube (noun)
a vacuum tube containing a metal target onto which a beam of electrons is directed at high energy for the generation of X rays	electronic device consisting of a system of electrodes arranged in an evacuated glass or metal envelope