Lost Vacuum: As the name implies vacuum tubes are built in such a way that the inside of the bottle (generally glass but in some vintage tubes can be metal), is drawn down to a high level of vacuum. If this vacuum is lost entirely the heater/filament will open and the tube will cease to function. In glass envelope tubes (virtually all modern tubes) a loss of vacuum is indicated by the silver or sometimes mirror like getter flash converting to a white chalky powder.
Noise can occur in many ways. Rustling, crackling, popping, swirling, etc. Tubes are a prime suspect if your amplifier has recently developed excess noise in a variety of ways. Excess hum can also be caused by a bad tube. This is most commonly related to a tube with poor heater-to-cathode insulation and is sometimes called heater-to-cathode leakage. In push/pull amplifiers hum can also occur if the tubes are severely mismatched.
Heater/Filament failures most often occur when a vacuum tube loses its internal vacuum. However it’s also possible for the heater/filament to fail during normal operation. If your tube is cold to the touch it’s a great indicator that the heater/filament has failed. Keep in mind not all tubes expose much of the heater outside of the cathode sleeve. In these tubes the heater will barely emit any light and it may appear “dim” even though it is functioning correctly. This can vary from tube to tube as well because the heater/cathode assembly is typically assembled by hand.
Gas/Leakage generally causes instability in the tube. This can present itself as anything from erratic noise, volume fluctuations, or simply poor overall performance. Gas and leakage currents occur three different ways. As a manufacturing defect, as an “End of Life” symptom, and from misuse or abuse. Gas and Leakage in tubes, how to measure it, and what it means in real world tubes is explain in this article: **Gas/Leakage article**
Intermittent symptoms of any kind are generally quite rare in vacuum tubes. Intermittent problems such as random volume fluctuations and random bursts of noise can occur but it’s not considered a normal/common mode of failure.
Shorts are generally considered a hard-failure and will leave the tube inoperable. Shorts can happen from any one internal electrode to another. In many cases shorts cannot be measured on a cold tube, but will present themselves once the tube is up to operating temp.
Blue and varying colors of glow is most commonly seen in power tubes and some gas rectifiers. Blue glow is almost always acceptable and is not an indicator of performance or health. This type of glow is generally characterized as a blue beam projected against the interior of the glass envelope. This is caused by stray electrons striking the glass envelope and creating a distinctly outlined fluorescence. In some cases this can also be seen within the tube’s plate structure. This is also considered normal. These types of blue glow may pulse with the intensity of the signal passing through the tube while the tube is under heavy loads. With the exception of some rectifier and regulator tubes, Pink and Purple glows indicate a faulty tube.
End Of Life will eventually occur. Historically End Of Life was often defined as a static characteristic (such as plate current) reaching roughly half of its original test value. With this criteria tube life can often exceed 10,000 + hours. Unfortunately in audio equipment we can often hear changes in the tubes performance long before the data-sheets suggested end of life occurs. In HIFI applications we will typically see tubes from the JJ catalog exceed 3,000-4,000 hours of use before any appreciable degradation in sonics occurs. In guitar amplifiers where the tubes can be pushed deep into clipping, mounted right next to speakers, and commonly run under conditions exceeding the max ratings of the tubes (!!), we’ll see about 500-1,000 hours of useable life. If the tube ages gracefully its cathode emission will simply decline over time. The result is reduced headroom, reduced gain, and an overall lethargic sound/dynamic.