There is the six volume textbook answer and the two page answer to how guitar amplifier valves work.
I expect you’d like the shorter version!
To understand the basics of guitar amplifier valves all you need to know is that ‘like charges repel’ and ‘opposite charges attract’. You probably recall that from school. So, negative repels a negative. Positive repels and positive. Negative and positive attract each other. (It’s the same with magnets where like poles repel and unlike poles attract.)
Armed with that fact, you can understand how guitar amplifier valves work.
Here is the explanation.
If you put a torch bulb filament (let’s call it ‘The Heater’) under a thin piece of metal (let’s call it ‘The Cathode’) then as The Cathode heats up, negatively charged electrons will ‘boil off’ by the zillion.
Don’t worry about how/why unless you want a course in quantum physics thrown in for free! Left unattended the negative electrons just all repel each other, bash into HUGE air molecules and generally dissipate.
Let’s encase our heater and cathode in a glass tube and suck all the air out. Guitar amplifier valves have this arrangement of course. The vacuum stops the electrons smacking into these house-sized (as far as the electron is concerned) air molecules.
We still don’t have a guitar amplifier valve though.
So at the other end of the tube we’ll put another thin piece of metal (let’s call it ‘The Anode’) and we will put a really high positive charge on it. In fact we’ll connect it to the HT supply of around +500V. We’ll connect the cathode to ground. Guitar amplifier valves are connected up like this inside your amplifier.
Remember opposite charges attract? Those billions of negative electrons will now rush across the vacuum, attracted by the positive anode. In electronics speak, this is the same as saying ‘a current flows from cathode to anode’.
Guitar amplifier valves rely on this current to work of course. For you interest, in an output valves, this steady state current is called the ‘bias current’.
Guitar Amplifier Valves in Practice
We could leave this set up operating for years and this steady bias current would just flow. We still don’t have a valve though.
At this point I want you imagine putting a loudspeaker between the anode and +500v. In other words ‘in circuit’ with the guitar amplifier valves. In real life the speaker is isolated by an output transformer, not least of all because you don’t want 500V on one of your touchable speaker terminals! But for this explanation let’s ignore that complication.
If you’ve ever tested a speaker by putting (say) a 1.5V battery across the tags you’ll know the speaker cone goes out (or maybe in) and just stays there. That’s because the current flows just one way of course. It’s DC. If you reversed the battery, the cone would move oppositely and stay in the other position.
So in our setup above, with steady state bias current flowing through the valve and through the speaker, the cone would move out (say) and just stay there.
If only we could get it to go in and out rhythmically, we’d have ourselves an amplifier! The guitar amplifier valves in your output stage accomplish this feat.
To accomplish this sneaky trick, here’s what we do. We insert a mesh of metal between the cathode and the anode. Let’s call it ‘The Grid’. It’s a mesh because we want lots of electrons still to be able to get through it.
Guitar amplifier valves arrange for the mesh to be negative with respect to the cathode. Let’s call this ‘negative bias’. Since negative repels a negative, I hope you can see that there is a certain negative grid voltage at which the electrons boiling off the cathode will be turned back by the repulsive action of this negative voltage. They would never reach the anode. When this happens in guitar amplifier valves, the valve current stops flowing (no electrons can battle through the negative grid to get to that temptingly positive anode.) The valve is OFF. Note at this point the speaker cone is just in its middle, resting position since there is no current going through it.
Fine so far?
Let’s make the grid a bit LESS negative than this ‘off’ position so that (say) half the current flows compared to if the grid wasn’t there at all. The speaker cone comes out a little and stays there. This is the correct bias point for this valve. Biasing an amp means no more than adjusting the grid voltage to give this half way idling current. The voltage is around -35V in case you are interested.
In real setups the speaker does not come out and stay out like this as it is buffered by an output transformer.
Okay I expect you’re home and dry now and can understand how guitar amplifier valves work. If the grid voltage goes more positive (say to -30V) then more current will flow and the speaker cone will move out more. If the grid voltage goes more negative (say to -40V) then less current will flow and the speaker cone will move in more.
So all we do is take the AC (not DC!) audio signal from the preamp (think of a sine wave) and superimpose this on top of the fixed negative bias and … the speaker will move in and out accordingly.
Now I’ve simplified a bit (but not much, actually) and you can go into this more deeply if you want, but those are the basics, so now you know!