The three main parts of the tube are the cathode, the controll grid, and the plate. These parts control the function of the tube. These parts amplify the signal by controlling the flow of electrons and increasing the magnitude of the input signal. The element connected to the less positive voltage point is the cathode. The element connected to the more positive point is the plate.
Amplification is possible because the output voltage is much greater than that of the input voltage.
The more the plate current per unit grid voltage, the greater the amplification of a given signal. The amplification factor is what makes different types of tubes unique. It defines the maximum amplification for a given type of tube. The first of these small tubes maximizes the input signal and then sends it to the next tube. The second tube in this series trys to amplify the already amplified signal.