a tube is connected first to the filament supply that heats up the cathode so it can emit electrons inside vacuum.
A small offset voltage can be applied to the grid and plates of the tube.
KT88 is a penthode tube connected usually as a tetrode in the tube amps with usage of an extra reflection device called plate. There are 2 plates in penthode totally giving 5 electrodes: Cathode, Anode, Grid, Plate1 and Plate2.
An input signal is usually applied to the grid. That's where you get the MOST voltage gain and the output to the transformer is going to be between anode and ground or Cathode is in this case usually grounded.