Vacuum tubes at one time were the most dominant active electronic components in the majority of instrument amplifiers up until the 1970′s or so. After that time, the semiconductors or (transistors) began to rule supreme in the performance category as well for economic reasons. Reasons include a reduction in weight as well as heat, and a greater reliability factor.

Pair of JJ EL84/6BQ5 Power

Vacuum Tube

The high-end tube instrument amplifiers, however, have gone on to survive but solely as an exception, due to the sound quality.
For the most part, one or more dual triodes are utilized in the preamplifier section to provide enough voltage gain to help offset any losses that may be incurred by the tone controls, and also to drive the power amplifier section.

Often times the output tubes are arrayed in a Class AB push-pull connection that will improve the efficiency but requires an additional triode or dual triode that will split the phase part of the signal. The tubes of the power amplifier stage are nearly always of the pentode or beam tetrode type. (It can also be known as “kinkless tetrodes” which is why you can get the KTxx nomenclature).
Also, some of the high-powered models make use of paralleled pairs of output tubes in push-pull.

Apart from the light negative feedback coming from the secondary end of the output transformer that goes to the driver stage, almost all of the amplifying stages will work in a “raw” open-loop mode. Other designs make use of current feedback via unbypassed cathode resistors.
Most tubes will show “soft clipping” gain non-linearity, adding an input signal that is high enough to overdrive any stage and will most often than not produce a favorable natural distortion.

Tube instrument amplifiers are usually outfitted with lower-grade transformers as well as simpler power regulation circuits than any of the hi-fi amplifiers, not only for the cost-saving aspect but also in the factor of sound creation.

Take a simple power regulation circuit’s output, for example, it tends to sag when there is a sizable load (high output power) and vacuum tubes will usually lose any gain factors with the lower power voltages. What usually happens is a rather compressed sound that could be considered to have a somewhat poor dynamic range in the case of hi-fi amplifiers, but could just as well be desirable as “long sustain” of sounds on a guitar amplifier.

Another thing to consider is that some tube guitar amplifiers will make use of a rectifier tube in place of solid-state diodes just for this reason.
There are a variety of models that contain a “spring reverb” unit that does a good job of simulating the reverberation of an echoic ambient. Reverb units mostly consist of one or more coil springs that are driven by the preamplifier section that uses a transducer driver similar to a loudspeaker at one end and an electro-magnetic pickup and preamplifier stage at the opposite end that picks up any long sustaining spring vibration, which will then be mixed back into the original signal.

Other guitar amplifiers will have tremolo control. An internal oscillator produces a low-frequency signal that has the ability to modulate the input signal’s amplitude as well as the output tubes’ bias, producing a tremolo effect.

Disadvantages of the tube amplifiers compared to solid-state amplifiers include the following. They are somewhat bulkier and heavier due to the iron in power and output transformers. Even though solid-state amplifiers require power transformers, they are nearly always direct-coupled and don’t require output transformers.
Another problem that can arise is that glass tubes are much more fragile and require a higher level of care when the equipment is transported around. Lastly, the performance of the tube amplifier will deteriorate over time until it eventually fails completely.

When tube vacuum is maintained at a high level, it is possible to achieve desired performance as well as life. Tube amplifiers are more prone to picking up mechanical noises (microphonic noise), although any such electro-mechanical feedback coming from the loudspeaker and going to the tubes in combo amplifiers can and will contribute to a very cool creation of sound.
Tube amplifiers benefit from the heater warm-up period prior to the applying of high tension anode voltages. This allows the tube cathodes to operate damage-free and will prolong the life of the tube. It is particularly important for amplifiers with solid-state rectifiers.

Some of the advantages of tube amplifiers over solid-state amplifiers include the following. When compared to semiconductors, tubes contain a fairly low “drift” (of specs) across a very wide range of operating conditions, especially high heat or high power. Semiconductors typically are very heat-sensitive by comparison and this will usually lead to compromises in the design of solid-state amplifiers. If a tube happens to fail, it is replaceable. Solid-state devices can be replaced as well, but it is almost always a much more complicated process.

This can turn out to be a huge problem for the working musician who by comparison when replacing a tube needs simply to look in the back of the tube amp and replace the problematic tube. Tubes can be easily removed and tested while on the other hand transistors can not be.

Also, read: Guitar Amplifier Basics

Tube amplifiers will respond in a different way than transistor amplifiers will if signal levels approach and reach the point of clipping. The transition in a tube-powered amplifier from linear amplification to limiting is not as abrupt as it is in a solid-state unit, therefore resulting in a much less grating type of distortion at the onset of clipping.
Simply, for this reason, many guitarists prefer the sound of an all-tube amplifier, although it will probably be a never-ending debate in the guitar community over which amplifier type possesses the best properties.