Valves can be considered as light bulbs with some extra bits added. Inside a light bulb (at least, an old-style one, not a curly or LED one!) there’s a thin metal filament that gets hot and gives off light, and lots of heat.

In a valve, that filament is surrounded by various electrodes, but it is still essentially a lightbulb with benefits.

So, when you turn it on, it starts to glow.

That characteristic glow is one of the defining features of valve electronics. 

In legacy valve equipment, they used to hide the valves away, since they weren’t a particular novelty. Now, we make a feature of them. 

The insides of the valve have to be hot before it will work. So they begin to glow, and then after about 15 or 20 seconds, they will start working.

Sometimes, you will also see a bluish glow. This is different, and only some valves will do it. The blue glow is caused by harmless chemical impurities in the glass envelope reacting with the electric current inside. It doesn’t mean anything is wrong, unless it’s a whitish-purple, which means there’s some unwanted gas inside the tube. Replace it.