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This installment of Dorkolopogous is for all those electric guitar players and audiofiles who know that “tube amps” are far superior to solid-state, but don’t have a fucking clue why. If you just thought, “Hey, sounds like me”, read on.  I’ll try to make things nice and simple so we don’t have to think too much.  

First we have to define a couple of terms, try to keep up.

Electrons - Just think of them as tiny little balls with a negative sign on them for time’s sake.

Current - The quantity of electrons flowing through a wire; it’s like measuring the amount water flowing in the Mississippi, just replace the water with electrons.

Electric Field - Gravity to humans is like Electric Field for electrons. The force of Gravity will pull a human from a high place down to a low place, like when you used to jump off the roof of your parent’s house into the freshly mounded snow bank.
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Figure 1: You and Gravity

Electric field pulls on electrons very much the same way.

Voltage - It’s similar the height of your parent’s roof top. You fell from a high height to a lower height. Likewise, electrons will “fall” from low voltages (negative) to higher voltages (positive).

Conductor - A conductor is usually metal like copper and is like an interstate highway for electrons. Electrons can move about freely inside of conductors.

Whew, that wasn’t too bad. So, now you are on your way to becoming an electrician, and we can start talking tubes. Take a breath and go grab yourself a cigarette and a cup of coffee before reading on.

Ready?

Ok.

So the whole purpose of the tube amplifier is to amplify the signal so it’s big enough for the speakers. Now our audio signal, from some experimental jazz album or a telecaster guitar is going to be in the form of a voltage that changes its height [or potential (po-ten-chal)] very fast, but at this point it’s much too little. We want to find some way to increase signal voltage using our tubes.

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We start by heating a small filament inside of a vacuum tube (so no oxygen molecules will get in the way). As its heat energy increases, the filament begins to eject electrons like fleas off a mange dog.
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Figure 2: mange dog

At this point the electrons scatter in any which direction, which is useless to us for amplification purposes, so let’s apply some of the fun things we just learned and see what we can do.   We’ll apply a positive voltage to a metal plate at the other end of the tube. The induced electric field causes most of those electrons to accelerate towards our plate. The plate is a metal conductor and connected to an outside circuit, so the electrons are sucked up by the plate and sent on their way as alas, we have current proportional to the voltage on the conductor plate.
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Figure 3: Electrons flowing from low voltage to high voltage

The electrons are now flowing from the filament to the conductor plate like our water in the Mississippi. This still doesn’t do much for audio amplification. Hmmmm, but if we stick a metal grid in between the filament and the plate and apply another varying voltage to it, we can vary the force accelerating each electron.

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Figure 4: This is a side view of a tube with the grid between the filament and the collector plate. The dotted lines with arrows show the direction of the electric field and which direction the electrons will want to “fall”. On the left shows a negative grid voltage. On the right shows a positive grid voltage.

As the electric field affecting the electrons varies, we also see a variation in the number of electrons that make it all the way to the collector plate. In the case of the figure on the left in Figure 4 the voltage on the grid is lower than the voltage at the filament, forming a wall pushing all of the electrons back. None of them get through and thus we don’t see a current out of the collector. In the figure on the right, there is a higher voltage on the grid than the filament, so it doesn’t impede the electrons as they flow to the collector plate and we see a large current.

Having trouble with this? Don’t worry we’re almost done. Think back to our river. If you were to stick a hose of running water into the river and point it up stream, the force from the hose would be fighting the flow of the river, just like our grid on the left side of Figure 4. Now take that hose and point it downstream, and all the water just flows nicely in one uniform direction, i.e. the right side of Figure 4.

So how does this give us amplification?

Tubes are generally large and bulky, allowing more electrons for more current, and it doesn’t take a very large voltage on the grid to cause large changes in the number of electrons that make it through to the plate. We can use this much larger current signal to convert back into a large voltage and do any sort of processing we want before sending it off to the speakers.

There you go, we’re done now.  Look at you, all enlightened. Now think about what we’ve learned the next time you pop on those tube amps while you’re waiting for those filaments to warm up, and appreciate their beautiful glow in all of it’s glory.

I denote thee a respectable member of the audio amplifier nerd community. Good Job.
Dorkolopogous: Tubes Can Be Your Friend, Too
By: Ryan Thompson
InDigest