Tube Preamp - for phono, bass, guitar, mic, stereo - Technical info

Welcome to the page that I did a little over six years ago! On this page, various circumstances of a tube preamp is looken at, for whatever it is used to. As stereo components, as a phono preamp for a turntable, as a stereo preamp in a stereo system, as a preamp in a recording studio for bass, guitar or microphones, or something else . Because not everyone know about tubes, tube sound, distortion, harmonics etc, some things are repeated so that the subject is kept better.

A tube preamp is also called Valve preamp (British english) although tube will be used here. A tube preamp is a unit that preamplifies a weak signal into a level of higher voltage and/or current, and where any overdrive/distortion occurs or is accomplished with one or more vacuum tubes.
Today, when a tube preamp is used, it is mostly for audio purposes ("tube sound") and the info below refers to tube preamps in these applications.

Legendary Marantz model 7 tube preamp introduced in 1958.

Tube mic preamp

Sennheiser e935 dynamic microphoneStudio Projects B1 condenser microphoneArt mp studio tube mic preamp
Microphone vs preamp cost.
Which to choose? Two microphones with different characteristics and good reputation, Sennheiser e935 dynamic microphone vs the somewhat cheaper Studio Projects B1 condenser microphone. The very popular Art tube preamp above costs about a 4th/3rd compared to the microphones.
Sennheiser e935 reviews.
Studio Projects B1 reviews.

In a studio or home studio, this is an essential part of recording. Whether old or new, these can be very expensive but there are indeed priceworthy preamps available too. A tube mic preamp works at a low power so the effect on the audio is not the same as for tube power amps. Instead, input transformers may have a certain effect on the sound in mic preamps.
Tube preamps are not always best for mic recording. Even though tube mic preamps are preferred for its sound in music recording, there are situations when an unaffected sound is wanted and then a clean solid state mic preamp would be a better choice. That is in situations when recordings are to be made for an authentic reproduction.
The most common tube in mic preamps is probably the 12ax7.

Studio preamp usability

Some better or larger tube mic preamps can be used as studio preamps, for instruments and line signals or it can be used as a tube DI.

That very special tone color?

People agree that different tube preamps sound differently. Some have more clarity in the lower frequencies, some have a softer midrange etc. Many tube preamps use input transformers and the characteristics of these transformers have effect on the sound.

Input transformers in mic preamps

Pendulum Audio tube mic/DI preamp
Buyers of the Pendulum Audio tube mic/DI preamp may choose between two different Jensen input transformers to choose between
" 'full' or 'focused' sound".
[Source: www.pendulumaudio.com/MDP-1.html]

Some mic preamps, either solid state or with tubes, have an impedance transformer at the input that colors the tone at least a little. The "warmth" that is commonly associated with vintage or expensive preamplifiers, may (also) come from the tone characteristics in the impedance matching input transformers.
If the impedance matching can be adjusted (like 150 - 600 Ohms), the interaction between the transformer and microphone interaction can be adjusted. Different settings may change the tone a little.

The impact on the sound that input transformers do in tube mic preamps are often described by the manufacturers with various attributes, rounder low end or lower mids and and also mentioned here, the common phrase "silky top end". Or the presence is promised to be improved without sounding boosted. If these words are true or not, may vary. As described in a chapter below, where you can read more about a certain method.
People agree that different tube mic preamps with input transformers sound differently. Some have more clarity in the lower frequencies, some have a softer midrange etc. Since many tube preamps use input transformers, the characteristics of these transformers have effect on the sound.
Some preamps (also the expensive ones, in particular) may have a very little filtering. That is, very little filtering although it makes just a very little "subtle" difference. These filter shapes are usually very soft and not possible to do with an ordinary graphic equalizer, rather with a parametric eq where the Q-value is very low and a very small gain difference, like +/- 0.5dB. This filtering can be done before (and after) the tube stage(s) so that the low level sound is flat but the sound at mid and higher levels has a somewhat modified harmonic figure.

Mic preamp settings

Presonus Bluetube Tube preamp.

Usually there are XLR and tele inputs and a tele output. Many mic preamps also have a balanced XLR output as well.
Common controls are:
- Gain control.
- Some control over the tube "drive" or output volume.
- High pass filter at ca 20-200Hz, either variable or as a switch.
- Inverting phase switch.
- Pad switch. Lowers the input 10 - 20dB.
- 48V fantom power switch. For condenser mics.
- Input impedance control, typically ca 150 - 3kHz.

Presonus Bluetube, rear side.

Some manufacturers may have own functions such as the Art MPA Gold preamp which also have a normal/high plate voltage switch. A high pass filter is useful to avoid low frequency noise or breathing mic sounds. If it is variable it can be adjusted to any signal source.
A phase switch can be used in a live situation with playback if feedback occurs or to minimize poor phase interaction when two microphones are used.

Bass/guitar tube preamp

Ampeg SVPCL is an all tube bass preamp. Tubes are 2 X 12ax7 and 2 X 12au7.
Ampeg SVPCL reviews - HarmonyCentral.com

Tube preamps for bass guitar and electric guitar are very much alike. They are put in tube bass/guitar amps and they are also available as separate tube preamp units.
However there are not as many separate bass/guitar preamps as there are bass/guitar amps, so if it is possible to use only the preamp in a amplifier and take the signal from an preamp output that some amps have. The Ampeg SVP CL to the right is now a discontinued product so if an Ampeg tube preamp is wanted as new, then an Ampeg tube bass head with preamp output has to be chosen.

Preamp equalizer and other control settings

Bass/guitar preamps typically have controls for input gain, tone/equalizer and master/volume. Some that use tubes may also have control over the bias so that the harmonic overtone figure may be changed. Some also have more tone switches. Some have effects loop connections as well. Unlike bass/guitar overdrive/distortion pedals they should be able to do both a clean gain and distorted sound. The tone controls or equalizer range is usually lower on bass preamps.
It is a great advice to use a clean preamp followed by an equalizer before the tube preamp. As explained on this page, using a filter/eq before distortion occurs can change the sound a lot. Any equalizer setting can be opposite-equalized with another equalizer with opposite settings after the tube preamp so that the sound is unchanged at low levels. But with a control over the distortion figure.

A transistor preamp and a power tube amp would probably sound better

Many bass/guitar players are interested in getting a tube sound and hope that a tube preamp which is cheaper than a tube amplifier, would work as well. Using a solid state amplifier with a tube preamp will most likely provide a better sound than an all solid state amplifier. But as explained above, a one tube preamp cannot sound like an all-tube amplifier and it cannot sound like power tubes.
While recording, some producers or bass/guitar players have recorded the clean sound. Even much later, the recording can be played through a "real" tube amp and although first by then, give an even better sound. When recording a bass/guitar, the clean/dry signal should be recorded too so it can be played through a tube amp or other equipment later.

One tube or all-tube preamp?

Many tube bass/guitar preamps are actually hybrid preamps that use one tube, often a 12ax7 with other semiconductor/transistor circuits. The Ampeg preamp to the right is an all-tube preamp and with several tube stages more tube sound is achieved as read about in its reviews.

Tube bass preamp

Fig 3. Frequency curve graph showing the human hearing range. The widest ranges that are somewhat linear, are ca 300Hz - 2kHz+ at lower levels and ca 150Hz to 2kHz at higher levels.

A tube bass preamp is maybe that tube preamp type that has the largest perceived impact on the original sound. This is very interesting and you may wonder why and I tell you!

First we have to study the diagram to the right. Many readers have probably seen this before. The frequency response of a 4-string bass guitar goes down to 40Hz which is one octave below standard guitars. 5- and 6-string basses go even below. Then, the human hearing range is not linear, at lower frequencies the hearing sensitivity falls off rapidly. We can understand this, if we look at the human hearing range diagram. Compare 40Hz and 80Hz which are the lowest frequencies for a four-string bass guitar and an electric guitar respectively. We see at lower levels the hearing difference between 40 and 80Hz is ca 10dB and at the lowest bass frequencies around 15dB. This means that at 40Hz, the effect has to be between ca 10 times more at higher levels and up to 30 times more at lower levels, to be equal 80Hz in perceived loudness.
[Interesting page on this topic: Hearing, physiological and psychological aspects of - Sound, synthesis and audio reproduction ]

Bass distortion effect

Tube overdrive harmonics are heard more on a bass ! How can it be? Because of the non-linear hearing sensivity, the harmonics of the low bass notes are heard relatively stronger than their fundamental frequencies. This circumstance also applies to guitars though somewhat less and that is why overdrive may sound that well on guitars. Since this effect is more apparent on a bass than an electric guitar, there is a way to handle it.
The simple conclusion is this, to get a smoother bass distortion, avoid "growling" in the lowest frequencies without changing the overall tone. First use a clean sounding preamp, then use an equalizer to lower the gain at the 40-50Hz band (or the one being closest) ca 2 - 4dB. Then use your tube preamp or whatever. After that, gain the bass with another equalizer which is preferably the same model as the first. The important thing is that the two equalizers are equally opposite in settings.
An page telling more about bass sound and bass preamps is here.

Tube guitar preamp

Guitar preamps should be able to do a clean gain and a distorted sound. Like better guitar amplifiers, some guitar preamps have a switchable 2-channel design to perform switching between "clean" and higher gain overdrive distortion sound, where each "channel" has its own gain and tone controls.

Tube phono preamp

There is an interest in phono preamps because many music lovers want to transfer their old record collections to mp3 or cdr. Usb phono preamps especially, since they can be easily connected to the computer.
Phono preamps are sometimes called turntable preamps or riaa preamps. Lp-records are recorded with a preemphasized filtering which is deemphasized, or compensated, with a riaa-filter.
The signal from the cartridge is low so shielding in the signal cables and preamp is needed to prevent unwanted noise (shot noise or EMI).

In the text below, you will find some reasons to use a tube phono preamp.

Phono cartridge/stylus

Phono preamps can handle one or two types of phono cartridges, moving magnet (MM), and the moving coil (MC) type. MC-cartridges generally provides better audio but has a lower signal output than MM so it needs more gain.
Two phono cartridge manufacturers, Ortofon and Denon, has developed high-output MC cartridges to fit standard MM phono inputs.
Phono cartridge distortion can occur if the cartridge is tracking too light or heavy.

Input transformers

Some preamps with MC-input have a step-up-transformer that may step up the voltage 10 times or more, so the preamp gain, and also noise, can be lower. A transformer also has an impact on the tone of the sound. There are several transformer manufactures such as Jensen and Lundahl.

Need for another phono preamp?

Most older amplifiers and receivers have an MM phono input only. Some for MC too. Today, only quality stereo equipment have provision for a phono cartridge. If there is no phono input, an external phono preamp is needed. If there is no MC-input, an external phono preamp with an MC-input is needed. Phono preamps are either MM/MC switchable or only for MC or MM.

What distortion is there on old lp-albums

Most lp-albums have good sound and some better. But can it be even better? The production process of old lp-albums could include several analog stages. Except for direct engraved lps from the 50s, a master tape recording was done first. Around 1965 and after that, multitrack recordings were done before the master tape recording. Then the engraving. A saturation distortion on a tape recorder may be symmetrical or asymmetrical depending on its high-frequency AC bias. For a minimum of THD, tape recorders may have been set to a symmetrical tape saturation (odd order distortion). The audio/studio electronics that was used before ca 1970 used tube equipment which further may have brought a odd harmonic distortion to the sound.
Conditions like these may have put too much amount of odd harmonic distortion on an lp-record.

Lps from the 80s

In the mid 80s, productions began to be put on cd, but lp-albums were still made. Digital studio equipment or master recorders were becoming used more after the mid 80s and it led to a change in sound standards. The digital equipment had a wider frequency response than the analog engraving can handle so without balancing two technologies with eachother the sound did not sound well. Digital equipment with a flat frequency response up to ca 20kHz should have be rounded off at recording and/or mastering/engraving. Some lp-albums from this time sounded too dry and "digitalized" and because cds had room for over 70 minutes, many album's length became longer. The only way to put longer recordings on lp-records, was to narrow the track width which resulted in less dynamics and more distortion.

Why a tube preamp can do a difference

Two reasons can be mentioned why some want a tube phono preamp.
First since the cartridge signal is so small it has to be amplified with a high gain. Because class A preamps have no crossover distortion, it may feel more secure to avoid. (Of course, class A can be made with a semiconductor circuit as well).
The other reason is probably much more significant and applies to the fact that there may be too much
odd harmonic distortion on the lp-album, as mentioned in the text earlier.
The phono cartridge further adds some distortion depending on its type and quality. The distortion figure in the signal from the cartridge may have different patterns.

A class A tube phono preamp may with its even harmonic distortion spectrum somewhat flatten out a non-linear harmonic distortion pattern in the signal from the lp-record. If the cartridge is a high-end type, its own distortion won't bother.
If it is possible to adjust the bias in the preamp, and hence vary the non-symmetry and so the harmonic distortion figure, the harmonic figure can be tuned-in for a best sound.

Related links:
One more preamp design - www.tubecad.com
MC pre-preamplifiers - www.tubecad.com

Stereo tube preamp

A stereo tube preamp by ca 1983, the Luxman CL-40.
[More info: Luxman Cl-40 - thevintageknob.org]

A stereo preamp designed with tubes is usually called a stereo tube preamp. A stereo preamp is like a stereo integrated amplifier but without a final amplifier inside.
In contrary to the preamps mentioned above, the common syntax of the term stereo tube preamp instead has the word tube in the middle.
Stereo means two channels. When the term is stereo preamp, it usually refers to (home) stereo systems. It is like an ordinary stereo integrated amplifier (stereo preamplifier + stereo amplifier in one unit) with source selectors, but without an amplifier inside, which has to be added separately either as a stereo amplifier, or as two mono amplifiers. Therefore, a stereo preamplifier has the same design and size as usual amplifiers.

A stereo tube preamp affects the sound just like a tube phono preamp but to all its sources, am/fm tuner/radio, cassette deck, turntable, cd etc. The sounds from an am/fm-tuner and cassette deck (rarely used these days) also have various types of distortion so a stereo tube preamp may improve the total harmonic distortion pattern on theses source's sound. A variable bias especially makes it easier to fine tune the overall sound.

Most common tube preamp types

As implied above, the most common preamps as in consumer and professional equipment, are audio preamps.
These are also available with tubes, as microphone preamps, bass preamps, guitar preamps, phono preamps and stereo preamps.
The longer word preamplifier seems to be used more on naming preamplifiers for professional/studio use and such for use in home stereo systems (stereo preamplifier).
A tube preamp enhances the sound which a solid state (transistor/IC) preamp does not unless specifically designed to. Usually, vacuum tubes are noted for a gradual and smoother overdrive distortion and transistors/diodes for a harder distortion, if overloaded, although solid state/transistor/ic equipment is designed never to exceed any clipping threshold.

Class A, AB and B tube preamps

Tube preamps are mostly designed to operate in class A. Class A tube amps have a very low power efficiency since they always draw current. Therefore they run warmer. This may sound worse than what it is, because a single ligth bulb in a home uses about the same power. With tube preamps though, the power dissipation is low enough to be neglected.
All tube amps and preamps make a gain compression at higher volume. Class A and B tube amplifier have transfer characteristics that towards its signal peaks becomes non-linear.

In Class A (and AB), the nonlinearities of the signal's min/max gain are asymmetric which creates even and odd order harmonics.
The even harmonics are 2nd, 4th, 6th etc, where the 2nd, 4th, 8th etc are octave intervals above fundamental frequencies). Odd harmonics are 3rd, 5th, 7th so their harmonic relationship is more complex.
The second harmonic dominant is, or should be the strongest of all harmonics.

In Class B, the nonlinearities of the signal's min/max gain are symmetric which creates odd order harmonics.
That is 3rd, 5th etc, as intervals above fundamental frequencies).

The reason why class A is associated to "tube sound" and preferred is because the octave harmonics where the 2nd harmonic (one octave up) is the strongest harmonic, making the sound more "musical" etc.
Octave frequency intervals more enrichens any music's harmonic figure and chord(s), rather than altering it. More about that below or read right away about harmonic interval figures here.

Vintage or all-tube preamps vs hybrid preamps

Most older tube preamps are stereo tube preamps for stereo systems. Vintage preamps, like the Marantz 7 to the right, only have tubes so they have more stages and so, the audio is different compared to some modern "tube preamps" with less tubes used per channel, mixed with semiconductor circuits. These preamps should rather be called hybrid preamps.

Of course, all-tube preamps perform gain in a tube-stage but some hybrid tube preamps first have a semiconductor preamp followed by a tube circuit with a gain near unity, so that the tube gets more or less overdriven depending on the signal level from the semiconductor preamp. A hybrid preamp may also have gain in the tube stages while the other signal routing or whatever is performed by semiconductor circuits.

One clear difference to remember is that all-tube preamps generally have more "tube sound" than hybrid preamps, no matter how fancy these hybrids are.

Tube bias cirrent

All tube preamps have a bias dc current at the signal input of the tube circuit, to balance the symmetry of the transfer characteristics. Sometimes the bias can be varied with a on-circuit potentiometer if there is any or it can be put into the circuit.
In a class A amplifier circuit, an adjustable bias may be used to somewhat control the harmonic distortion pattern while it is still asymmetrical. Read more about harmonic distortion below.
Class A circuits are simpler because they are single ended amplifiers with no crossover distortion.
If a class B tube amplifier has a bias control, it can be set to distort asymmetrically and in that way, also get even harmonics. It is rather then becoming a class AB circuit. Since class AB/B circuits are designed to cancel crossover distortion, a change in the bias just change the crossover point which would be better because if the crossover distortion is audible (very rarely) it will be somewhat covered by a low signal that reaches a certain level until it reaches the crossover point.

Tube sound

A tube preamp is used if and when a "tube sound" is desired. A single tube amplifier generates harmonic distortion mostly at the top levels but also near top levels and even a little below, unlike a single semiconductor amplifier which is usually distortion-free and generates a sudden distortion or "clipping", if overloaded.

A full tube sound is produced by an all-tube amplifier, in the tubes and output transformer. In a one per channel-tube preamp as in most mic/phono stereo preamps, the tube sound is only created in one low power tube in each channel.
Because this is a page about tube preamps, and not tube amps, output transformers may be beyond the scope of this page. You may find a few words about tube amp output transformers here.

Well worth to clarify, is that in some bass/guitar players conception, a good tube bass/guitar preamp would create the "real" tube sound, as of "real" tube amps.
And, that some people who get intested in vintage/tube sound, believe that any preamp with one tube per channel may provide the "real" tube sound. Not! It is not that simple!

For audiophiles, tube sound is often defined by presence, warmth and clarity. It makes the sound kind of blend in with itself, making the music more solid both harmonically and dynamically (see tube gain compression below) .
Secondly, for bass/guitar players, a good tube amplifier may provide a well overdriven yet rounded distortion sound.

Soft clipping?

Amplifiers with semiconductors or fets especially, can be designed to perform a tube-like sound or just "soft clipping" at overload or when high load occurs. When clipping approaches, it results in much clearer reproduction and simultaneously protects sensitive loudspeaker tweeters that otherwise could be easily damaged by the high frequency distortion caused by the clipping.
Although "soft clipping" is a great feature on semiconductor amplifiers, the term "tube sound" should only be used on tube products so do not confuse it with terms like smooth or soft clipping etc. Soft clipping once became one of the profiles of NAD amplifiers.

Tube gain compression

As mentioned above, a tube amplifier's transfer characteristics has a linear range that changes into a nonlinear towards the min and max levels. This nonlinearity creates a compression effect since the peak-nonlinearity of the amplifier's gain can be viewed as a gain compression.
The compression is audible as a dynamic compression and one part of the sound. For guitar players, this compression is stronger since tube guitar preamps/amps are often set at very high gain.
The gain compression alters the waveform and so, creates the harmonic distortion.

Tube distortion and THD (total harmonic distortion)

As mentioned a couple of times, the part of tube distortion associated to the class A tube amplifier sound" is a harmonic distortion, though mostly associated with even harmonic distortion. All amplifiers (and preamps) make THD but some (usually transistor amplifiers) have most of odd harmonic distortion (unless designed differently).
The total root-mean-square (rms) harmonic voltage in a signal, as a percentage of the voltage at the fundamental frequency, is called total harmonic distortion (THD).
This distortion is an overlay of harmonic multiples of the fundamental signal that consists of even (2, 4 etc) and odd (3, 5 etc) order harmonics in various figures. Total harmonic distortion can vary from zero percent (no harmonic energy) to theoretically infinite (no fundamental frequency energy).
The THD figure is often used as an expression of the performance of an audio-frequency amplifier. The amplifier is provided with a pure sine-wave input, and the harmonic content is measured using a didtortion analyzer.
All amplifiers also have other distortions, some common are intermodulation distortion (IMD) and transient intermodulation distortion (TIM), also called slew rate distortion. These are related to hifi so they should of course be as low as possible even in tube amps and preamps.
Class AB and B-amplifiers also have crossover distortion.

Different distortion patterns give different tube sounds

Each frequency's perceived loudness of its 2nd harmonic at 70 phon level (phon equal to db at 1kHz) . The curve would be quite the the same for the 3rd harmonic.

Here is an interesting part. Various tube preamps are reviewed differently as we know. Of course, it depends on many things, choice of tubes, transformers etc. Now look at the diagram to the right. It shows how human hearing hears each frequency's 2nd harmonic in the audio spectrum at 70 phon level, where distortion level is electrically the same. It is calculated by comparing the human hearing sensivity by looking at the dB difference at nf - 2nf on a human hearing sensivity diagram as seen below. Experienced audio engineers know this but it is rare to read about the significance of this circumstance.
This is one of the reasons why tube preamps are favored differently depending on the choice of music, since different music has different levels in the frequency spectrum and therefore, this non-linear perception of harmonics sounds doesn't affect different music in the same way.

Tip for the diy tube preamp builder

There is a way to somewhat equalize the non-linear perception of the 2nd harmonic which is the nearest and strongest of harmonics, hence most significant. Invert the curve in the diagram and copy it, but divide its value like 3 to 5 times. Then design a filter that behaves like that, it will lower the bass and the 1-2kHz/6-10 Khz ranges and gain the 300 - 800 and 2.5 - 8kHz. Then take the filter output to a tube stage where the tube distortion occurs. After the tube stage, use a filter which is the opposite of the first filter. At low levels, the frequency response for this application is flat. At higher levels, it will give another sound with a little more flat 2nd (and 3rd) harmonic loudness within the audio spectrum.
To say it simpler, it is really possible to change the sound with opposite filters before and after the stages where tube compression/distortion occurs. If these filters are equally opposite, like pre and deemphasis filters, the sound is flat at lower lever levels but as the signal is increased, distortion will occur at the frequencies that are boosted in a filter before, and lowered after, but there will be relatively more harmonics than without this method. On the opposite, it can make a tube stage cleaner, if mid frequencies are decreased before the tube stages, and re-increased after, there will be relatively less harmonics.
The filters cannot have the same levels as the diagram to the left, because then the bass and the 1-2kHz/6-10 Khz ranges will have no distortion at all and the other frequencies will get too much distortion. But somewhere inbetween, as by dividing with 3 - 5, a clear difference will be heard. It can also be tried out, to vary the values for each frequency band, as long as the two filters are equally opposite.

Harmonic distortion figure vs music intervals

The difference between even and odd harmonics can be explained in terms of music intervals.
In class A (or AB), the first even harmonic intervals are one octave [2:1], two octaves/fifteenth [4:1] and nineteenth [6:1].
In class B, the first odd harmonic intervals are the twelfth [3:1] and the seventeenth [5:1]

Hifi harmonic distortion spectrum figure

However, the THD-values seen in specifications do not tell about the shape of the THD, unless you read qualified reviews as in hifi-magazines. If the THD is very low, the shape have less impact on the sound.
When it comes to hifi, an audio amplifier should have as little THD as possible at the rated output level. Most amplifiers are rated to the amount of root-mean-square power they can deliver to a speaker of a certain impedance (usually 8 ohms) with less than a specified THD (usually 10 percent).
In a professional hifi reference amplifier which are always transistor amplifiers, the distortion spectrum may be not linear but because it is so low it is not audible and therefore, its non-linearity does not matter. Of course, there is nevertheless different views about this among hifi enthusiasts.

Tube harmonic distortion spectrum figure

Tube distortion is another thing than distortion in a reference hifi amplifier where no distortion is wanted. So in tube amps and preamps, the shape of the distortion spectrum is as important as the value of the total distortion.
As a first example, a decaying harmonic distortion spectrum where the 2nd harmonic distortion is 1%, the 4th 0.5% etc, would sound better than a distortion spectrum where both the 2nd harmonic would have the same 0.5% distortion level as the 4th harmonic.
Infact, the first example with a 2nd - 1%, 4th -0.5% distortion figure would still sound better than a 2nd - 0.3%, 4th 0.3% distortion figure.

Conclusion - Hifi/tube hifi/tube sound?

A conclusion is this, that hifi and tube sound is strictly not the same thing. But as long as a tube amplifier has an audible distortion with a decaying series of harmonics, it performs a distortion that keeps more hifi in the sound, than if the distortion spectrum would be non-linear. And of course, this would apply even for semiconductor amplifiers.
Because many "tube preamps" are infact hybrid tube/transistor preamps, the transistor circuits in them have to be as hifi as possible, or the overall tube sound would become less apparent. Therefore, vintage and all-tube preamps typically have more "tube sound".

Tube preamp sound vs Tube amp sound

Until the 60s, tube sound was nothing but tube sound since only tubes were used. But in later times, tubes are not always used as the only active device in audio equipment even though labeled with "Tube ...". When it comes to tube preamps, most of them use hybrid circuits, ie a tube (often 12ax7) with opamp/ic/fet/transistor circuits.
The "tube sound" of these hybrid preamps doesn't have all of the character as all tube amplifier. Infact, a tube amp sound comes mainly from signal over-loaded final power tubes and from core saturation in the output transformers. It all depends on the circuit design, character of the tube(s) and transformer. [Output transformer questions - Geofex.com]

Therefore, a vintage tube amp sound per its best definition usually refers to the saturated overdrive sound of amplifiers, which are at least equipped with final power tubes.
Equipment that should be able to perform this is :
Vintage amplifiers/receivers, radios, bass/guitar amplifiers where the amplifier is equipped with :
- tube(s), at least in the final power section, hence with one or more speaker outputs.
- an output transformer between tube and speaker(s) (outputs).

Some people claim that only tubes with high-voltage provide a genuine tube sound.
Older class B tube amps have unregulated power supplies. When operated at high volume, the voltage to the power supply dips which results in a reduced power output and so, a compression effect, also called sag.

If we get back to amplifiers with power tubes and output transformers, they will deliver a tube amp sound at its relatively high level, even if they are driven by a clean sounding solid state preamp. Also, a "tube amp sound" might be obtained with a medium power tube amplifier, like a watt or so, which can be resistor loaded and tapped on its signal.

So when only the term "tube sound" is used, it may just refer to equipment with low power tubes such as in preamps or other equipment such as tube equalizers or tube compressors. For guitarists, the cranked up sound of a tube amp would only be available from a tube amp.

The overdrive sound

Class A amplifier distortion:
The asymmetric transfer characteristics results in even order harmonic distortion.

The tube sound is a sound with an overdrive distortion and for the most with the use of class A. Tubes are typically providing a smooth overdrive/distortion and in order to obtain asymmetrical distortion with even overtones too, transfer characteristics have to be asymmetrical otherwise only odd order harmonics are produced. Class A amplifier have these characteristics.

Although transistor distortion usually relates to a harder distortion, a transistor preamp can have a design that somewhat emulates a tube sound and also provides a smooth even order distortion.

Class B amplifier distortion
The symmetric transfer characteristics results in odd order harmonic distortion.

Since vacuum tube pre/amps change the sound it may not appeal to everyone or all situations. In all the glory and sometimes even hype of tube preamps, there is an exception where tube preamps should not be used. When an authentic reproduction is required, the recording have to be hifi sound rather than tube sound. Then a distortion-free semiconductor preamp should to be used.
Relating to the typical amplifier circuits by tubes and semiconductors respectively, the overdrive/distortion by class A tube amplifier circuits increases smoothly with higher signal level and will also generate even harmonic distortion.
Semiconductors only generate odd order distortion at overload unless they are designed to clip asymmetrically. The distortion of a class A tube amp has the 2nd, 3rd, 4th etc harmonic over-tone multiples of the fundamental frequencies. Typical semiconductor/transistor distortion occurs suddenly and unpleasantly if overloaded, with odd 3rd, 5th etc order over-tones. However distortion in a transistor/ic preamp should not happen because they are designed to have sufficient dynamic headroom below the clipping threshold, unless there is a soft clipping circuit (switch). If such a soft clipping is designed to perform a gradual clipping asymmetrically, it will create a smooth even order distortion.

A mic preamp with semiconductor circuits can achieve a sound that resembles tube sound if it is designed to clip smoothly as with a tube preamp and/or asymmetrically (to generate even order distortion harmonics) like an class A tube preamp. This can be good to know if a smooth overdrive is wanted but and it doesn't have to be with tube preamp.



Preamp tubes

12ax7 is maybe the most common preamp tube but of course there are a couple of others too, to say the least. A guitarist who claims a remarkable tube distortion with a certain tube is perhaps not usable for someone who wants a tube to a phono preamp, and vice versa.

Conclusion:
For diy builders or others who want to try out different tubes, it is important not to apply good reviews for certain tubes generally, because tubes are used and reviewed for different applications.

Tube manufacturers

Siemens E88CC/ECC88/6922 vacuum tube. High frequency twin triode preamp tube for studio and hifi equipment.
[More info: E88CC]

Some of the most common vacuum tube manufacturers are Tungsol, Telefunken, Mullard, Tungsram, Sovtek, Electro-Harmonix, Siemens, GE, Sylvania, RCA, Amperex (Philips), CBS/Hytron, RFT, Svetlana, Haltron. Westinghouse, Raytheon, Amperex, Western Electric, Bentley Acoustic Corporation, Colomor, GDR, Zaerix.

List of manufacturers.

There are many views about various preamp tubes, 12**7 especially. The vintage RCA Blackplate preamp tubes have a good reputation for giving a "roundness" in tone and Telefunken tubes have a reputation such as they brighten the tone for clarity and therefore more suitable for hifi.

NOS tubes

These are "New old stock" tubes, meaning they are old but have not been used. There are many Nos tube suppliers so it is possible to find tubes with more various characters than those being made today.

Common preamp tubes

Some of the most used preamp tubes are 12AX7, 12AU7, 12AT7.[ECC81/82/83 are the european equivalents of these].
Below is a list with 12-AT/AU/AX-7 - ECC81/2/3 equivalents.

12AY7/6072. Twin triode preamplifier tube used in Fender Tweed amplifiers.
12DW7/7247/ECC832. Dissimilar twin triode preamp tube used in vintage Ampeg amps. This tube is 1/2 12AU7 and 1/2 12AX7.
6AQ8/ECC85. Twin triode preamp tube.
6CG7/6FQ7. Medium-Mu twin triode preamp tube used in vintage Ampeg, Gibson, and Silvertone amps.
6DJ8/6922/E88CC/ECC88/7308. High frequency twin triode preamp tube for studio and hifi equipment. E88CC has lower noise than ECC88.
6SN7/ECC32/CV181. Very low noise, hifi medium-mu octal twin triode preamp tube.
EF86/6267. Audio pentode preamp tube. Used in VOX amps and in tube microphones.

Other popular preamp tubes are: 12AV7/5965, 12BH7
6EU7, 6KG5/6FQ5, 6H30, 6SL7, 5842, 5755/420A, 7199, EF86.

12AT7/ECC81 equivalents

12AT7/ECC81 has a lower gain than 12AX7/ECC83.

ECC801/s Telefunken. Super premium grade tube. The "S" means it is a special selected tube.
12AT7wa/6201. Has extra thick mica wafers to eliminate microphonics.
12AT7WC. Short plate format for extra low microphonics.
6679. Mobile version.

12AU7/ECC82 equivalents

12AU7/ECC82 is a medium-Mu twin triode.

ECC802/S Telefunken. Super premium grade tube. The "S" means it is a special selected tube.
CV0491
5814/6189/12AU7WA. Milspec versions.
5963. Plate voltage max 250V (12AU7 330V).
6680. Mobile version. May handle +/- 20% filament voltage variation.
7316 Amperex.
6189/12AU7WA. Milspec version.
7730 CBS, Raytheon.
5965. Computer-rated 12av7 with twice the gain of a 12au7

12AX7/ECC83 equivalents

Telefunken ECC803/s. Premium version. "S" are special selected tubes.
5751 GE, Sylvania, Raytheon, RCA. Gain factor ca 70 (12AX7 ca 100). Late versions by Philips not as well.
6681. Mobile version. May handle +/- 20% filament voltage variation.
7025. Hifi version of 12AX7.
7058. 13 V filament.
12AD7 Sylvania. Low microphonics and low hum.
12AX7WA. Extra thick mica wafers to eliminate microphonics.
12DF7 Westinghouse. Low noise and low microphonics.
12DT7/12DM7 Raytheon. Low noise and microphonics.
6681.

Tube preamp kits

Some cheap kit examples are found here:
Cheap tube preamp kit k261 - Oatleyelectronics

Model PL Stereo Tube Preamplifier - Tubedepot

Diy tube preamp schematic and circuits

12AU7/ECC82 2-stage cathode follower tube with low output impedance - Preamp schematic - diyaudioprojects.com
This a great preamp schematics and circuits resource. Many different preamp circuits


Updated September 22nd 2011 | September 15th 2015