Every vacuum tube has at least two metal electrodes.
They all have a
and an "anode".
The cathode is heated to a very high temperature so that
electrons, which are quite speedy to begin with,
gain enough extra energy to break away from the cathode's alkaline
earth metal oxide coated surface.
This "thermionic" emission begins as the cathode
starts to glow red. When everthing is hot, a cloud of electrons (negative charge) forms
in the vacuum surrounding the cathode and the tube is ready
to go to work.
A high positive charge, often called the B+, is then applied to the anode or "plate"
of the tube. The plate attracts and collects the free electrons
which are then put to work powering your speakers before they complete