Peering down inside of a BHK preamp the other day, I was rewarded with the warm rosy glow of its vacuum tubes.
And I was reminded how similar a vacuum tube is to a lightbulb (I know. I am weird).
Both vacuum tubes and lightbulbs have glass envelopes that keep the outside air from getting in. And both vacuum tubes and lightbulbs have filaments that produce both light and heat.
So, where do they differ and why?
A light bulb filament is made of a thin wire, typically tungsten, that is coiled or twisted to maximize its surface area. When an electric current is passed through the filament, the resistance of the wire causes it to heat up to a very high temperature, around 2500°C. At such a high temperature, the tungsten filament glows white-hot and produces visible light.
A vacuum tube filament, on the other hand, is designed to emit electrons rather than light. The material of the filament is often similar to a lightbulb—a slightly modified version based on tungsten—but instead of getting white hot and emitting a bright light, a vacuum tube's filament is cooler, typically around 700-800°C. At this lower temperature, the filament emits a small glow of light, but, more importantly, electrons boil off of its surface in a process called thermionic emission.
With an abundance of electrons coming off of the filament (cathode), we now have the fuel we need to have our vacuum tube work its audio magic. Basically, we need a way to control how many of these generated electrons are put to work, and we need to give them a place to go.
Of the three elements that make up a vacuum tube like the venerable 12AX7—cathode, grid, anode—the grid controls how many of the electrons are being put to work, and the anode (plate) provides the means for attracting them.
Your audio signal is connected up to the grid. The louder the audio signal, the more electrons are encouraged to head to the plate (the tube's output) and we get a bigger signal.
It all starts with that rosy glow from the filament.
