We often receive questions about whether their tube has gone bad or not. The question usually comes up because an amp is having problems.
The most common cause of vacuum tube failure is the loss of vacuum.
Tubes are made of glass, which can crack and shatter. Broken glass should be pretty obvious.
Sometimes, however, tubes can have cracks that are hard to see with the naked eye. Such cracks are so small that it only results in a tiny leak. Tubes will often keep working, but eventually, the leak will introduce enough air in the vacuum, and the tube will stop working.
It's easy to tell when a tube loses the vacuum. As air enters the vacuum tube, the getter material will oxidize and turn white. If you see a tube with a white powdery coating inside, the tube has lost the vacuum. You may see the getter partially turning into white dust if the tube hasn't lost enough vacuum, but such a tube will eventually fail.
There are two kinds of vacuum tube noise; white noise and microphonic noise.
The white noise comes from ions bouncing around inside the vacuum. When a new tube is used, these ions get absorbed by the getter material, reducing the noise floor.
A new tube indeed becomes quieter with initial use, then as internal parts outgas and consume the getter material, it slowly becomes noisier. This change is subtle and wouldn't be all that apparent to human ears.
Occasionally, however, enough impurity gets released inside the tube. The tube becomes noticeably noisy in this case. In rare cases, tubes may start making odd sputtering or popping noises. This is due to outgassing ions jumping around the tube and striking electrodes. When this happens, it's best to replace the tube.
Microphony is caused by tubes picking up vibration and shaking their electrodes. All vacuum tubes are microphonic to some extent. Tube manufacturers design tubes to minimize microphony, but a handful of tubes turn microphonic with use. Although it is rare, when a tube becomes microphonic, it's best to replace it as well.
You can read more about micrphonic noise in the Microphonic Noise article.
Loss of Power or Tone
Some power tubes wear out surprisingly fast. A lot depends on the temperature and duration the tubes run, but unlike preamp tubes, the power tubes start degrading as soon as they see heavy usage.
The wear curve of a power tube is not linear. The wear is gradual at first but accelerates over time. When you notice the tube sounds flat or doesn't sound as powerful, it's already lost a lot of the ability to drive the output.
A power tube wears out from slowly losing the getter and becoming gassy, but in some cases, the tube becomes worn out even with the getter in tact. In such a case, the cathode has likely worn out.
The cathode of a vacuum tube is where all magic happens. The metal oxide coating on the cathode gets heated and emit electron. This coating can wear out from heat and use and loses its ability to emit electrons. This can happen independently of getter wear.
There are cases where a perfect vacuum tube fails because of amp issues.
A loose socket is a very common issue that causes a vacuum tube to fail seemingly. A loose socket will make a tube not turn on, not work, or, worst yet, burn up in smoke. Whether your amp is new or vintage, if your tube sockets are loose, you should get them replaced.
A leaking coupling capacitor is also fairly common in vintage amps. When the coupling capacitors start to leak, it's equivalent to setting the bias extremely hot, usually causing the output tubes to go up in smoke.
When tubes fail due to these issues, anything from a blown fuse to a smoking amp can happen. If the problem is catastrophic, the amp will need repair. The component that caused the problem will be replaced, but only after an expensive repair.
You reduce vacuum tube issues by buying from reputable vendors, and in addition, having a good amp tech go over your amp is worth every penny.