Musicians and sound engineers alike often comment on the differences between resultant audio when processed by tube amps and transistor-based alternatives. While this issue is a long-running debate, with much support on both sides, it is generally accepted that the sound quality of a tube amp is much fuller and has a lot more body than that of the solid state amplifier. Complaints about transistor amps often include something along the lines of ‘lack of character’, ‘weak mid frequencies’ or ‘hollow-sounding’. But what exactly is it that makes the difference between solid state and tube-based amp tone? The answer ultimately lies in the response of tubes and transistors to different harmonics when amplifying the signal wave. To really understand this, we need to think about headroom, as well as harmonic distortion and linear thresholds.
i. Linear Capacity and Clean Headroom
The basic function of a guitar amp – or any amp for that matter – is to take an audio signal and amplify it (make it louder). A guitar signal is comprised of a lot of different frequencies together which are usually roughly categorized as bass, middle and treble frequencies, and certain styles of music in particular require every frequency to be amplified by the same amount. An amp which does this is said to be operating at linear capacity and amplifies the signal with very minimal coloration of tone. A guitar amp operating in this way is desirable for music which requires bigger volume which still maintains the original clarity of the tone, and doesn’t add any kind of overdrive. Every amplifier nevertheless has a threshold; a point at which the signal is not only amplified but also distorted. This amount of clean space is referred to as ‘headroom’.
Amps with higher wattage have higher power and as a result, a greater amount of headroom: they can get to higher volumes while maintaining a clean tone. Lower powered amps are sometimes desirable for music like rock music, as they provide a natural overdriven tone at more reasonable volumes because of the fact that they have less headroom and a significantly lower distortion threshold.
ii. Harmonic Distortion
When a guitar amplifier is pushed up beyond this natural electronic-acoustic threshold, the signal becomes ‘clipped’ by the natural limitations of the amp itself. This results in the natural overdrive sound which shaped rock history and became an integral part of electric guitar performance. Basically, the clipping of the signal means that rather than looking like the traditional sine wave, the guitar signal looks and behaves a little more like a cross between sine and square wave. The flatter peaks and troughs caused by the clipping mean that harmonics are added to the sound, which were not present previously in the original wave source (which is – in our case – the signal as interpreted by the guitar pickups). The most important thing to the practical guitar player is not however how it looks when represented on an oscilloscope, but rather the effect that clipping has on the audible tone of the guitar amp.
Light clipping (of the kind that vacuum tubes are famous for producing) results in the guitar effect that we refer to as overdrive. With the amp pushed even further than this, the clipping effect becomes more prominent and the amp moves from overdrive into distortion territory. Distortion which occurs as the result of clipping is referred to as harmonic distortion, and it is this kind of distortion which is usually considered desirable for certain heavier genres of music.
iii. Harmonic Distortion With Tube and Solid State Amps
Due to the practical general usage and demands of guitar amplifiers, they deal a lot with transient signals (signals which occur briefly and decay quickly). With regard to transient signals especially, tube amps deal with harmonic distortion in a very different way than solid state amps. Much of the difference between the sound qualities of these two kinds of guitar amp is to do with the presence and dominance of certain harmonics over others. With music and what is generally processed by the human ear as tonally pleasant, the first few harmonics are of most importance, as they tend to play the most significant role in determining and defining the overall colour of the amplified signal. There are three major groupings of these harmonics which are particularly relevant to understanding the differences between transistor and tube tone; ‘lower even’, ‘lower odd’ and ‘high’ harmonics. These groups of harmonics have very different effects on tone, depending upon which ones are more dominant than others.
Lower even harmonics – the 2nd, 4th and 6th – are by far the most desirable. They produce a rich, singing tone which really brings out the full body of the sound. Lower odd harmonics, on the other hand, (the 3rd and 5th) have exactly the opposite effect. They close up the sound, giving that muffled recorded underwater kind of feel. High harmonics – 7th and above – contribute to the edginess and bite of a sound. When considering the different effects of these harmonic groupings on the overall experience of the listener, it becomes evident that in order to give the best possible tone, an amplifier needs to have dominant lower even harmonics, reduced lower odd harmonics, and carefully controlled high harmonics. This would mean that the sound is full, rich, bold and open, yet at the same time not so uncomfortable to the average ear.
Herein lays the fundamental difference between tubes and transistors in the context of the guitar amp. Transistor-based amps naturally give more weight to undesirable harmonics like the 3rd, and therefore when pushed past the threshold into overdrive, the natural drive sound of the amp becomes cold, bodiless and almost sterile. Tubes behave differently and will naturally accentuate the desirable, more musically pleasing lower even harmonics of the distortion.
It is because of this fundamental distinction in the natures of tubes and transistors that tube amps are often praised for tone and still used by the majority of modern professional guitar players the world over.