Every vacuum tube has at least two terminals for conducting
electrons. The "cathode" emits electrons and the "anode" collects them.
The cathode is heated to such a high temperature that
electrons in its coating (often an alkaline earth metal oxide)
gain enough energy to break away from the coating.
This "thermionic emission" creates a red glow. Eventually, a cloud of electrons (negative charge)
in the space around the cathode. The tube is then standing
by, ready to go to work.
When you switch the amp out of Standby mode, you connect a surplus of positive charge (often
called the B+) to the tube's anode, which is also called its "plate".
plate attracts the emitted electrons and uses them to power your speakers.
At the same time, more electrons move into the oxide coating, maintaining