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.
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.
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.
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.
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.
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.
