Differences between Tubes and Transistors in a Guitar Amplifier

Why are guitar tube amplifiers so desireable when transistors are available at a fraction of the cost? It all has to do with how tubes distort in a way that is different than transistors. Tubes distort uniquely because as the signal emitting from the plate approaches its maxium potential, the tube begins to react less and less to the original input signal.

This results in a type of a compression of the signal and produces a soft clipping. Clipping occurs when the input signal increases, but the maximum power has been reached. Thus the signal becomes cut-off, or “clipped.” Transistors, on the other hand, react exactly the same to the input signals right up to their maximum; then they stop quickly, creating a sharp clipping. These different types of clipping  produce different produce different series of harmonics or overtones.

When the transistor amp clips, it produces more odd-order harmonics and can even sound hollow and dry, whereas tube distortion produces even-ordere harmonics. Tube distortion generally sounds warmer. Various types of transistors and tube distortion are possible, depending on the amp’s design.

In the case of a tube amp, preamp and power amp tubes have different distortion characteristics due to the difference  in both their tubes and their circuit design. For example, relying on a master volume distortion circuit by itself will yield less sensitivity to variations in a player’s touch than if the amp is attenuated – has its volume limited – after its power stage (that is, with a power attenuator). This is due to the contribution of the output transformer to the amp’s sound, as well as difference in sonic qualities between the different power tubes compared to preamp tubes. Leaving some of the distortion to power amp section rather than relying mainly on the preamp section gives a broader range of sensitivity. In addition, the nature of the tube allows the player to vary his touch, producing different tone responses from the amp according to the manner in which he plays.

The are many variables in tube amp designs, and each has its characteristic sounds and quirks. Regardless of what type of amp you use, you find that, like strings on guitar or oil in an automobile, tubes do wear out. Amps are not maintenance-free, and as they age, they undergo changes. The tubes are subjected to wear-and-tear, some of the electronic parts lose their intial properties and pots and jacks get old. Bad tubes can cause premature failure of other parts, such as the output transformer, speaker, and other vital components. If you amp sounds bad, weak, or otherwise not up to par, don’t just hope the problem will go away. Get it fixed!