The sound of the tube amp was always synonymous with a rock guitar tone. It is said that the sound of the tube amp is "warmer" compared to solid state amps like transistor or mosfet. The saturation of the tube increases gradually with the increase of the volume, this is something that doesn't happen with solid state amps, which by the way weren't even designed to produce distortion. In solid state amps the distortion is simulated with electronic circuits.

This tone, so dear to guitar players, is produced by the tubes, an electrical component invented at the beginning of last century. Between the '40s and '70s, the tube was included in nearly all electronic devices and is now totally obsolete except for in musical instruments and high end audio equipment.

We will not go into technical details of construction of the tube, but will rather try to give a basic overview that can be useful for guitarists and musicians in general. 

If your amp is an all tube amp, and not a hybrid (like Valvestate), you can see that it has two sets of tubes. The first are is made up of small tubes, also known as the "Preamp", while the big ones are the "Power" tubes.

Pre amp tubes

My vintage Marshall head contains 3 pre-amp tubes. The number of these can vary based on the amp design, number of channels etc. Preamp tubes amplify the weak signal coming from the guitar, and serve to shape the tone using the tone controls such as high, mid, bass, presence, etc. 

These tubes also control the gain the distortion of the preamp. The tubes most commonly used is the 12AX7 (as it is called in USA) or the ECC83 (the European name).

Some amps with built in spring reverb may have an additional 12AX7.

Power amp tubes

The purpose of power tubes is to bring dynamic control to the signal. These tubes may also contribute to the saturation and distortion of the signal.

The saturation of the latter creates a thick and dynamic much sought after sound (think of early Van Halen or any album by AC / DC). The most common power tubes are the EL34, 6L6, 5881, KT66.

Usually, 50w amplifiers are equipped with 2 power tubes, while 100w amps usually have 4. Personally I find power tube saturation more pleasant compared to pre amp saturation, which alone produces a more "edgy" sound. It is the combination of both pre amp and power amp tubes distortion that creates the ultimate tone.

An important point: In 100w amp, never remove two valves for half power, unless this is something the amp is designed to (like Mesa Boogie). Also, never use a tube amp without the speaker/cabinet (or a dummy load) connected, to avoid damaging your amp.

Warm / Cold

Tubes operate at very high temperatures, especially the power tubes, for this reason, the amplifiers have a "standby" switch which serves to warm up the amp before operating at full capacity. With a couple of minutes on "standby" the amp will get to the optimum temperature. Skipping this process the tubes will struggle, increasing their consumption and reducing their life span.

After a performance is finished it’s good practice to leave the tubes to cool. When the tubes are hot, they are more fragile and therefore more sensitive to movements or worse, knocks and thumps.

Lifespan

The tubes have a limited lifespan not easily quantifiable. Their lifespan are linked to their usage and some construction characteristics of the amplifiers themselves. Normally the power tubes in an amp played 2-3 times a week can last from 18 to 24 months. Pre amp tubes usually have a double this lifespan.

The tube life varies depending on the style of music too. If you like the saturation of the amplifier and thus always play at 10, the valves will wear out faster.

Although there are several companies on the market that sell tubes like Mesa Boggie, Marshall, Svetlana, etc, the tubes are currently manufactured only in China, Russia and Eastern Europe. 

Companies such as Marshall, Mesa ... they just make careful selection of tubes based on their standards and market them under their brand name.

Balancing

Power tubes are sold in balanced pairs. Balanced means that the tubes have been selected for similar characteristics, i.e. they consume the same amount of current when operating. Most amplifiers have been designed for use with balanced tubes. Tubes of the same type and constructed will always have slightly different characteristics. 

By measuring the current drawn while operating, the tubes are selected and coupled. Using unbalanced tubes may cause accelerated tube consumption.

Symptoms

Power tubes usually have only two states: they either work or not.  A blown fuse is often a warning that it is time for a power tube replacement. A worn or faulty tube will require more current when operating at full capacity, causing electrical overload and blow fuses. 

If instead the amp makes a hissing sound even when there is no instrument connected to the amp, this is the sign of a faulty pre amp tube. Usually the volume of the hiss increases with the increase of the volume. This phenomenon is called microphonic tube. Other symptoms of defective pre amp tube are: lack of volume, lack of bass, lack of high frequencies.

Replacement

It is always good idea to leave tube replacement to a qualified techinician. While we can replace the pre amp tubes by simply swapping the old ones with new ones, the power amp tubes require a setting called bias adjustment (there are also fixed bias amps like Mesa Boogie. In this case you can replace your power tubes like the pre amp ones). 

Be careful to always use the same type of tubes. If your power tubes are EL34, you must only use those. Some manufacturers suggest changing the whole set of tubes when there is sign of backdown. 

As the tubes consume, they draw more current. Putting a new tube between worn ones will result in a faster consumption of the new tube.

Bias

Once replaced, the power tubes, the bias must be adjusted (unless it's a fixed bias amp like Mesa Boogie). The small pre amp tubes do not require the bias adjustment, while for the power tubes it is recommended at each change of set. The biasing is actually the setting of the amount of current that tube draws. This is a task you should not do by yourself unless experienced. 

The amp and tubes are operating at high voltages that can be lethal. The biasing is set to run the amp at optimal condition and helps extend the life of the tubes. Furthermore, the biasing also affects the final tone of your amp.

This was a brief and simple overview regarding the use of tubes in amplifiers from the musicians view. Nowadays modeling technology has made a great step forward in emulating the tube amps we all like, making it easier and more affordable to get the "ultimate tone". 

Nonetheless, some of us will still use our old/new amps when we need it.