THE SOUND OF TUBES
At one time it was thought that tube amps would soon become a relic of the past with the emergence of new solid state amplifying technologies in the 1990s.
After all, tube amps are fragile, heavy, noisy, they get hot, they’re inefficient and require regular maintenance. Tube amps are also more expensive to make than solid state amps. There are plenty of reasons why tubes can be thought of as being inferior to transistors but there’s one important reason why tubes are still around after all these years: their sound!
We’ve all heard the difference between tube amps (valves) and solid state amps (transistors) described in subjective terms. Tube amps are typically described as being “warm” while solid state amps are sometimes called “cold” or “harsh”. But what does this all really mean? In this article we will take a technical look at tubes and transistors and find out what it is that makes the tube amplifier sound so favorable.
Distortion - When not being overdriven, like in the clean channel on a guitar amp, transistors can sound very similar to tubes. But what really separates tubes from transistors is how they react when being overdriven to create distortion.
In both transistors and vacuum tubes, distortion is created the same way. The signal from the guitar is “pumped up” by the pre-amp, the first stage in the guitar amplifier. The hotter signal is then used to overdrive the power-amp stage, which creates some form of clipping. The hotter the signal (the higher the wave’s amplitude), the more that signal will be clipped.
The term clipping is probably used because it is a good description of what the resulting waveform looks like after being distorted: the once smooth peaks and valleys of the wave now appear to be clipped or flattened out. What’s really going on here though is the addition of harmonics to the original signal.
To really understand this process you must know what harmonics are (similar to the overtone series in music) and be familiar with the principles of Fourier analysis (how complex waveforms can be broken down into many sine waves of different frequencies). But without getting into all that, just know that when it comes to distortion we are talking about adding harmonics.
In both tube and transistor based amplifiers, odd order harmonics are largely responsible for the distortion effect. The effect is similar to that of the square wave, which can be thought of as a distorted sine wave with only odd order harmonics. This kind of distortion is sometime referred to as soft clipping, since the waveform is clipped very rigidly.
However, tubes exhibit a behavior called duty cycle modulation, that transistors do not when being overdriven. Duty cycle modulation is just a fancy term for the presence of fluctuating even order harmonics. The fluctuation depends on how hard the tube is driven. The even harmonics smooth out the rigid edges of the clipped waveform and produce a “warmer” sounding tone. This is what is typically referred to as soft clipping.
Dynamic Feel – Many players will say they favor the sound of tube amps because of their “feel”, or how the amp reacts to the dynamics of the guitarist’s playing. Much of this characteristic can be attributed to the internal resistance of vacuum tubes.
Resistance, in electrical terms, refers to how easily electrons can flow across an electrical component. Components can have external resistance, measured before and after its placement in the circuit, or internal resistance, measured within the component itself. Tubes have a much higher internal resistance than transistors. In fact, transistors have none.
Because of a tube’s high internal resistance, when a tube is driven really hard it will actually give out less power, especially in the lower frequencies. This is a natural compression effect that actually varies on frequency and reacts differently depending on the dynamics of the guitar player.
This compression is also evident when cranking the amp. The louder the amp is set, the more this compression will be taking place. The result is a distortion sound that’s “smoothed out” and less “punchy” at high levels. Conversely, solid state amps can be turned up very high without any compression. This actually means the amp has more headroom but could be why some players feel that distorted solid state amps sound “harsh” at high levels.
Output Transformers – One of the greatest influences on tone in a tube amp actually has little to do with the tubes. The output transformer is the component responsible for converting the output voltage and impedance (resistance) of the amplifier to the proper level needed to drive the speakers. Solid state amps don’t need to use these because there is no need to convert their output load before connecting them to speakers.
The output transformer is the last stage of the signal flow inside the amp and exhibits its own frequency response and distortion properties, which colors the tone. Some guitarists feel that upgrading a low-quality output transformer to one of higher quality can have a greater effect on the change in tone than using different brands of tubes.
Successful Imitation? – As mentioned before solid state amps are superior to tubes amps in many ways, but before guitarists could embrace them in guitar amplifiers, they had to be made to sound like tubes.
These days some of the best sounding solid state amps are using amp modeling technology. Amp companies such as Line 6, Vox, Marshall, Fender, Roland and Peavey are producing solid state amps that use DSP (digital signal processing) to duplicate the sound of a whole myriad of other classic and modern guitar amps. Some of these amps fall into another category of hybrid amps, which use pre amp tubes in the first amplification stage but also use DSP and transistors in the final power amp stage. This is thought to give the guitarist the best of both worlds: the feel of tubes with all the advantages of transistors.
How well do these amps imitate their all tube counterparts? I can say from personal experience that many of these amp modeling designs come pretty darn close to the real thing. Of course the quality of the amp models differs from amp to amp and in most cases, you get what you pay for.
But for most players, features like tone flexibility, light weight, onboard digital effects and lower cost are worth the trade off of true tube authenticity. On the other hand, if you know you want the sound of a 100 watt Marshall tube amp cranked up through a 4×12 cabinet, then there’s only one sure fire way to get it; use the real thing!
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