Here you can see the heater inside the cathode from which the electrons are emitted and the Anode to which the electrons are attracted. The mica supports hold the electron assembly stable within the glass tube and also supports all the other electrodes. The object of the 'getter' is to burn up all the remaining gases after the glass tube is evacuated. This is responsible for the silvering you can see usually in the top of the tube.If the glass gets cracked you will see this silvering turn to a white powder. The tube is then said to be 'soft'.

This photo shows the internal construction. The cathode is coated with Barium or Strontium oxide which emits electrons readily when the heater, which you see glowing inside the valve, heats the cathode up. As a valve ages this surface deteriorates and less electrons are emitted. We then state the the valve is suffering 'low emission'. The electrons being negatively charged are attracted to the Anode which has a positive voltage, usually around 400 volts. This valve has five electrodes and is thus known as a 'Pentode'. A TRIODE valve has just three electrodes; cathode, grid and anode. The grid (1) is the controlling grid which controls the number of electrodes reaching the anode and it is to this electrode that the 'grid bias' is applied. In the Pentode, the other two grids act as an accelerator and focusing to make the valve more efficient than its triode counterpart.

These are the characteristics of the EL34 and you can see the Transfer Characteristics are very non-linear especially over the initial conducting period. In a push-pull configuration, as the description suggests, one valve pushes the other pulls, and the art of biasing is to bring the valves to an equal point of conduction. It is for this reason that one should always endeavour to used 'Matched' valves, ie valves that have as near identical Transfer Characteristics as possible. Think about it like a see-saw,. you would not try to have a 5 stone person on one side and a 15 stone person on the other and expect the see-saw to balance! The pictures below show the effect on a Clean signal when the biasing is not correctly set up. If an amplifier is used in this state then there is tremendous stress on the valves and they won't last long.