There are two popular methods for reducing "secondary emission", which is electrons bouncing off the plate and floating back into the gap between cathode and plate, where they then can get caught on the screen grid and cause distortion and excessive grid current that can chew the grids up.
The EL34 is a "true pentode" ("pent-" meaning 5, "-ode" abbreviating "electrode") and has a cathode, three grids, and an anode (aka "plate"). It has a suppressor grid between screen grid and plate, which is tied to ground potential (usually the cathode itself) to repel the electrons that would otherwise float back into the gap. Pentodes usually have a bit more grid current on the screen grids due to poor alignment of the control grid, screen grid, and suppressor grid wires.
The 6CA7 is a "beam tetrode" ("tetr-" meaning 4, and again, "-ode" abbreviating "electrode"), and has a cathode, two grids, beam-forming plates, and an anode (aka "plate"). This construction is shared by the 6V6, 6L6, and the various KT66, KT77, KT88, etc. tubes. It uses the careful alignment of control and screen grids to shoot the electrons in "beams", and add beam-forming plates between screen grid and plate, which are usually internally tied to the cathode, to help keep the beams aimed. Beam Tetrodes have generally lower screen grid current due to the careful alignment of the control grid and screen grid so that the electrons flow in "beams" and the screen grid wires are between the streams of electrons coming from the cathode through the control grid, so they are impacted by far fewer electrons and therefore heat up less and draw less current from the power supply.
Clear as mud?
