In the first part we have learned that the flow of electrons can be controlled by striking the strings.
This flow of electrons inside of the tube can now be controlled by selected resistors.
They control the voltage of the grid between cathode and anode.
Current and Voltage
One distinction that matters in this context is the differentiation between current and voltage.
You can imagine the current as an amount of water running through a garden hose while the voltage pictures the pressure of the water. This means: the higher the water pressure (the voltage) the more water (the current) runs through the hose in a certain period of time.
By adding a resistor you can – in this example – tighten the hose. Depending on how much the resistor tightens the hose you can control the current (amount of water) as well as the voltage (water pressure).
Impedance and Transformers
This kind of resistor is known to be the output-impedance of the tube and is measured in units of Ohms. To protect the speaker this output-impedance has to be send through an output-transformer. Comparable to a pickup the output-transformer is made of copper-coils and is able to reduce the high resistance of the tube so it can be send through the speaker without crashing it.