Although solid-state technology dominates modern electronics, vacuum tubes are important in guitar amplifiers representing millions of dollars in annual sales.While solid-state amplifiers create a more faithful reproduction of the signal from the guitar's pickups, players tend to prefer the tone of valve amplifers. The human perception of guitar tone is complex and not fully understood: guitar gear designers don't know exactly what engineering measurements are appropriate to evaluating the performance of their creations. A small amount of some kinds of distortion may sound worse to the ear than larger amounts of other kinds, so the only way to evalutate guitar gear is by listening to it.
The preference for tube amps could be due to prior listening experience. The developing guitarist will listen to countless music recordings, most featuring tube amplifiers, and their brain assimilates these audio experiences to form their "ideal" guitar sound. This cannot be the whole story, however, as there is a wide range of musical genres that favour tube-amp tones: from clean rock-n-roll tones to heavily distorted metal genres. Is there something in the tube amp sound that just makes it sound better?
Stephen Bussey and Robert Haigler of CBS-Fender set out to investigate the tube versus solid state sound. In their 1981 paper, Bussey and Haigler describe their experiments to compare the tube and solid state guitar amplifiers. They observed that " the amplifier becomes part of the musical instrument, and is frequently used to radically alter the signal from the guitar. Thus the question of accuracy in the amplification becomes irrelevant, and the choice of one amp over another is left to purely subjective evaluations of the sound quality. Adjectives such as thin, "hollow" or "metallic" have been used to describe transistor amps, while tube amps have been described as "warm", "rounder" or "punchier"."
They compared a Fender Twin reverb tube amplifier with a soild-state amp (consisting of a commercially available power amp with a custom made preamp). This comparison could be criticised, as the solid state amp was not a recognised model, perhaps they should have used something like a Roland Jazz chorus to give a fairer comparison.
First they measured the gain and frequency response of the amps using a spectrum analyzer. The power amplification sections were done first, with each amp operating into a four ohm resistive load. It was soon clear that the response of the tube amp changed dramatically when connected to a four ohm speaker load (see picture). This did not occur with the transistor amp. This effect is caused by the reactive speaker impedance and the output impedance of the amplifiers. The transistor amp had an output impedance of less than a tenth of an ohm, whereas the tube amp has an output impedance of above five ohms. Thus the output of the tube amp increased as the speaker impedance increases. A frequency response difference this large could easily heard and this was one audible difference between the tube and solid state amplifiers.
You can see from the graph the relatively flat frequency response of the solid-state amplifier. This would provide a relatively faithful (Hi-Fi) reproduction of its input signal. The tube amp, however, is operating like a band-pass filter, attenuating frequencies around 100 Hz. Human sensitivity to loudness also follows a similar bandpass function, although it is centered around 1000 Hz.
For the remaining experiements they removed this effect so that it did not mask other, more subtle, differences. They used a third octave equalizer before the transistor power ampts input.This corrected the response differences, but did not alter the output impedance.
They matched the maximum output voltages by playing a guitar through the tube amp and observing the output clipping level on an oscilloscope. The power supply rails on the solid state amp were then adjusted to give identical output clipping levels for each amp.
The tube preamp delivered unclipped signals of around 96 volts peak to peak. The solid state preamp could only give about 27 volts. Thus it was necessary to lower the gain of the solid state preamp so that both preamps required the same input level to be driven into clipping. The gain of the solid state power amp was increased to make up the loss.
It is known that the harmonic distortion of solid-state amps is mostly odd harmonics, whereas tube amps produce both odd and even harmonics. This is shown in the second graph, with the both amps loaded to 5% total harmonic distortion the tube amp has consistently higher output at the even harmonics (except at 8KHz for some reason!).
The next stage was a double blind listening test, using real guitarists. Double-blind means that during the test neither the guitar player or the investigator knew which amplifier the guitarist was playing through.
The guitarist was allowed to switch between either amplifier as much as he liked, until they were convinced whether or not a difference was present. If a difference was heard, the guitarist was asked to explain what they heard, and their preference.
12 guitarists compared the power amp sections of the respective amplifiers. 4 couldn't hear any difference, 8 said they could hear a difference but only one of these players could reliably discriminate between the two amplifiers. A similar result was obtained when the amps were driven to distortion, only two players could reliably discriminate them, (due to a buzzing noise with the tube amp). When comparing the pre-amp sections of the amplifiers, none of the players could tell the difference. The same was found when comparing a complete tube system with a complete solid-state system.
In conclusion this study demonstrated that there are measurable differences between the tones produced by tube and solid state amplifiers. The tube amp has a characteristic gain and frequency response, but if this was controlled for the other differences between the amplifiers were too subtle to be noticed by players. This study demonstrates a useful approach to the evaluation of guitar gear.
Reference: Bussey, W. Stephen, and Robert M. Haigler. “Tubes Versus Transistors in Electric Guitar Amplifiers.” Acoustics, Speech, and Signal Processing, IEEE International Conference on ICASSP '81, Apr. 1981