One can think of 'charge' this way: an excess of surface electrons is a negative (-) charge, while a dearth of surface electrons is a positive (+) charge.
A tube's cathode expresses a negative charge, while the anode(plate) has a positive charge. Thus electrons boiled off a hot cathode are attracted strongly to the anode and accelerate toward it, as though the anode were a sump or 'sink' for the electrons.
This gal gives a pretty decent mini-tutorial on the history and development of the triode.
https://www.youtube.com/watch?v=0Smj2nHo9zA