Here’s a broken EF40 pentode tube. I dropped it, so took the opportunity to take a peek inside. The construction is intricate and quite beautiful.
|Inside the EF40 tube, glass envelope removed|
|EF40 pentode with screen removed|
|EF40 with getter, mica insulation and plate removed|
|EF40 showing grids heaters and cathode|
Vacuumtubes.net have a good description of how tubes work, and this diagram from shows all the parts: