Excerpts:
Engineers and musicians have long debated the question of tube sound versus transistor sound. Previous attempts to measure this difference have always assumed linear operation of the test amplifier. This conventional method of frequency response, distortion, and noise measurement has shown that no significant difference exists.

This paper, however, points out that amplifiers are often severely overloaded by signal transients (THD 30%). Under this condition there is a major difference in the harmonic distortion components of the amplified signal, with tubes, transistors, and operational amplifiers separating into distinct groups.

As a recording engineer, we became directly involved with the tube sound versus transistor sound controversy as it related to pop recording. The difference became markedly noticeable as more solid-state consoles made their appearance. Of course, there are so many sound problems related to studio acoustics that electronic problems are generally considered the least of one's worries. After acoustically rebuilding several studios, however, we began to question just how much of a role acoustics played.

During one session in a studio notorious for bad sound, we plugged the microphones into Ampex portable mixers instead of the regular console. The change in sound quality was nothing short of incredible. All the acoustic changes we had made in that studio never had brought about the vast improvement in the sound that a single change in electronics had. Over a period of several years, we continued this rather informal investigation of the electronic sound problem. In the past, we have heard many widely varied theories that explain the problem, but no one, however, could actually measure it in meaningful terms.
[Ampex portable mixers back then were all tube.]

Electrical engineers, especially the ones who design recording equipment, can prove that there is no difference in tube or transistor sound. They do this by showing the latest specification sheets and quoting electronic figures, which are visually quite impressive. It is true, according to the parameters being measured, that there is only a marginal difference in the signal quality. But are there some important parameters which are not being measured? One engineer who admits that there might be some marginal difference in the sound, says, "You just have to get used to the nice clean sound of transistors. What you've been listening to on tubes is a lot of distortion."

Of course the question which comes to mind is, what is this distortion and how is it measured? Psychoacoustically, musicians make more objective subjects than engineers. While their terms may not be expressed in standard units, the musician's "by ear" measuring technique seems quite valid.

Consider the possibility that the ear's response may be quite different than an oscilloscope's. "Tube records have more bass. . . . The bass actually sounds an octave lower," says one rock guitarist. A couple of professional studio players have pointed out on numerous occasions that the middle range of tube recordings is very clear, each instrument has presence, even at very low playback levels. Transistor recordings tend to emphasize the sibilants and cymbals, especially at low levels. "Transistor recordings are very clean but they lack the 'air' of a good tube recording." "With tubes there is a space between the instruments even when they play loud... transistors make a lot of buzzing." Two people commented that transistors added a lot of musically unrelated harmonics or white noise, especially on attack transients. This same phenomenon was expressed by another person as a "shattered glass" sound that restricted the dynamics. It was generally agreed that tubes did not have this problem because they overload gently. Finally, according to one record producer, transistor records sound restricted like they're under a blanket. Tube records jump out of the speaker at you. Transistors have highs and lows but there is no punch to the sound."

1) Tube Characteristics:
Fig. 4 shows the distortion components for a typical two stage 12AY7 amplifier. This particular design is quite representative of several single-ended, multistage triode tube amplifiers tested.

The outstanding characteristic is the dominance of the second harmonic followed closely by the third. The fourth harmonic rises 3-4 dB later, running parallel to the third. The fifth, sixth, and seventh remain below 5% out to the 12 dB overload point. These curves seem to be a general characteristic of all the triode amplifiers tested, whether octal, miniature, nuvistor, single-ended, or push-pull. Fig. 5 is the waveform at 12 dB of overload.

The clipping is unsymetrical with a shifted duty cycle. Again this is characteristic of all the triode amplifiers tested. Fig. 6 shows the distortion components for a two-stage single-ended pentode amplifier.

Here the third harmonic is dominant and the second rises about 3 dB later with the same slope. Both the fourth and the fifth are prominent while the sixth and seventh remain under 5%. The waveform at 12 dB over­load (Fig. 7), is similar to the triode, but its duty cycle is not shifted as much.

It is not reasonable to assume that virtually all tube amplifiers can be represented by these two examples. However, the major characteristic of the tube amplifier is the presence of strong second and third harmonics, sometimes in concert with the fourth and fifth, but always much greater in amplitude. Harmonics higher than the fifth are not significant until the overload is beyond 12 dB. These characteristics seem to hold true for wide variations in circuit design parameters. The extreme difference in the tube amplifiers is the interchanging of the position of the second and third harmonics. This effect is not just a characteristic of the pentode, it is common to triodes too.

2) Transistor Characteristics:
Figs. 8 and 10 show the characteristics of two transistor amplifiers. Like the previous figures the curves are representative of all the transistor amplifiers tested.

The distinguishing feature is the strong third harmonic component. All other harmonics are present, but at a much lower amplitude than the third. When the overload reaches a break point, all the higher harmonics begin to rise simultaneously. This point is generally within 3-6 dB of the 1% third harmonic point. The waveforms of these amplifiers (Figs. 9 and 11) are distinctly square wave in form with symmetrical clipping and an almost perfect duty cycle.

Both amplifiers shown have single­ ended inputs and push-pull outputs. However, the circuit designs are radically different.

3) Operational Amplifier Characteristics:
Fig. 12 is a hybrid operational amplifier. The third harmonic rises steeply as the dominant distortion component in a characteristic similar to the transistor. Also rising very strongly from the same point are the fifth and seventh harmonics. All even harmonics are suppressed completely.

The waveform of Fig. 13 is a perfect square wave. As a classification group, operational amplifiers have the most uniform characteristics with almost no deviation from the curves shown in this example.

Relationship of Factors and Findings:
The basic cause of the difference in tube and transistor sound is the weighting of harmonic distortion components in the amplifier's overload region. Transistor amplifiers exhibit a strong component of third harmonic distortion when driven into overload. This harmonic produces a "covered" sound, giving the recording a restricted quality. Alternatively a tube amplifier when over­loaded generates a whole spectrum of harmonics. Particularly strong are the second, third, fourth, and fifth overtones which give a full-bodied "brassy" quality to the sound. The further any amplifier is driven into saturation, the greater the amplitude of the higher harmonics like the seventh, eighth, ninth, etc. These add edge to the sound which the ear translates to loudness information. Overloading an operational amplifier produces such steeply rising edge harmonics that they become objectionable within a 5 dB range. Transistors extend this overload range to about 10 dB and tubes widen it 20 dB or more. Using this basic analysis, the psychoacoustic characteristics stated in the beginning of this paper can be related to the electrical harmonic properties of each type of amplifier.