Vacuum Tubes

Part of the reason I worked on restoring this old radio is because I wanted to learn how vacuum tubes work, as well as how radio transmission works.

The simplest vacuum tube is a diode -- a device that allows current to pass only in one direction:

• Green element = the heater. This is a filament that gets hot when current flows through it. Its only purpose is to radiate heat onto the red element = the cathode.
• Red element = the cathode, or the emitter. This is a filament coated in a special substance that can emit electrons when it's heated up.
• Blue element = the anode, the collector, or the plate. This is a flat piece of metal which collects the electrons emitted by the cathode.

The key feature of the diode is that current can only go from red to blue, not the other way. In other words, if the heater is hot, then current can flow from red to blue as shown above. However, if we flip the polarity of the battery (the + would be connected to the red element), no current can  flow.

Notes:

1. The heater circuit operates at low voltage, around 5V.
2. The anode/cathode circuit operates at much higher voltage, frequently above 200V (in my radio, the anode/cathode rail goes up to 750V). This is because, even in a vacuum, the voltage has to be high enough to force electrons to jump across the small gap between the two leads. This high voltage makes older appliances dangerous, they can definitely kill you if you're not careful.
3. The reason tubes are vacuum'ed is because the air molecules get in the way of electrons "jumping" from the cathode to the anode.
4. All vacuum tubes "wear out" in time: the substance that covers the cathode is literally stripped away and eventually stops emitting electrons entirely. This is why vacuum tubes in old appliances had to be replaced every so often (typically measured in years, but it depends on how heavily the device is used).

This is it. All vacuum tubes are variations on this theme. They contain various additional elements that serve to amplify or dampen the flow of current between the cathode and the anode, but the basic principle is the same.

So why is a diode useful? Who cares that we have a device that lets current flow in only one direction?

To answer that question we need to look at the simplest form of radio transmission: Amplitude Modulation, or AM. More on that in the next post.