Eli5: How do vacuum tubes work? Particularly in guitar amps?

Question

How do tubes work? How is a preamp tube different from a power tube? And what components/elements of preamp/power tubes affect the tone of the amp? (example: what makes EL34’s sound different than 6L6’s, or makes 12ax7’s sound different than ef86’s)

In: Technology

Answer 1

A tube is a fairly simple device in principle. Imagine a lightbulb with two separate filaments. If you charge one positive (anode) and one negative (cathode), you might get some electrons to leave the negative terminal, fly across the vacuum, and land on the positive terminal. Current is flowing.

If you put a separate heater near the negative terminal this will work better, since it would be easier for electrons to leave the metal and fly across the vacuum that way. That’s why tubes glow. You’re usually seeing the heater.

Now imagine putting a little bit of a metal screen (like from a screen door) in between the two terminals. If you put a negative voltage on it, the electrons won’t want to leave the negative terminal as much. If you put a positive voltage on it, the electrons will be more likely to do so. Thus the grid voltage can control the cathode/anode current. You’ve made an amplifier! A small voltage change on the grid can control a large current change cathode to anode.

There are a lot of things you can do to change the characteristics of a single tube. Size/shape of the anode/cathode/grid, spacing, heater configuration, etc. You can even have multiple cathodes/anodes/grids. All of these things can change how the tube works over different voltage/current/temperature ranges.

Probably one of the biggest difference between tubes is linearity. An ideal amplifier has the same (change in current)/(change in grid voltage) at all points. But that’s not usually the case, and there can be all kinds of response curves. Similarly, you want the amplifier to work the same across all frequencies, but that’s usually not true either, and different tube configurations will give different results. Those two things will give different sounds to the amplifier circuits the tubes are used in.