Here I would like to explain (maybe some kind of inexpertly) try to explain how a tube-amplifier with cathode-bias works:
Every amp has got several preams and powertubes to amplify the guitar-signal.
In the middle of the tube flask you can find a cathode that means an electrical minuspol.
This cathode can be charged with a really small positive voltage. The quiescent current is that kind of current that flows through the circuit when it is not active, is called bias and is (in this example) determined by the choice of the cathode (cathode-bias).
Especially when the cathode in the tube is preheated quite well it becomes able to send out countless electrons (negative charged electrical particles).
The cathode is surrounded by the so-called anode. Musicians know this positive charged pol as the “plate”.
Although the cathode of an amplifier-tube also can be charged lightly positive the positive charge of the anode is so much higher that, in the vacuum of the tube, the electrons would fly unrestrained in the direction of the anode.
To control this firework of electrons you can find a third electric field in the middle between the cathode and the anode, the so-called “grid”.
And this is the point where the lightly positive charge of the cathode gains meaning: the grid is actually uncharged. Through the minimum of positive charge of the cathode the grid appears to be charged negatively.
The grid is situated close to the cathode it keeps the negatively charged electrons in place until one leads a positive charge to the grid. This can be done by using the relatively low charge of the pickups. The guitar-signal of the pickups starts the flow of the electrons from the cathode to the anode. And this flow of electrons amplifies an exact copy of the guitar-sound.
Too much input leads to an overriding of the tube and causes an overdrive.
Depending on the tube that is overdriving we speak of preamp-clipping or power-amp-clipping.
The flow of electrons inside of the tube can be controlled by selected resistors. They control the voltage of the grid between cathode and anode.
Click here for part 2.