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Guitar Preamp Tone Explained – by Phil Taylor / Effectrode

Guitar Preamp Tone Explained

by Phil Taylor

For images and entire article:


During the research and development of the ‘Blackbird’ vacuum tube preamp I undertook a thorough investigation into tube preamp circuits in a quest to discover just what factors are important in creating great guitar tone. The following article is a result of those labours . . .

The interplay between your guitar, amp and playing technique are all fundamental to your tone. Your amp responds or reacts in a certain way to the signal from your guitar pickup, the pickup responds to the way you pick the strings and in turn you react to the sound you hear coming from your amp. The whole system is a closed loop with the implication that your guitar and amp will influence your playing in a significant way – the right guitar and amp setup will let your playing soar whilst the wrong setup will hold you back, keeping you well and truely grounded. It’s impossible to over-emphasise the importance of the relationship between gear and musician – it’s certainly worth delving into the science a little and discovering what makes great tone. This article specifically explores tube guitar preamp circuitry – the aim is to demystify the tube sound, give a clear explanation of what’s happening within the preamp, and gain an insight into how guitar legends created their wonderful, classic guitar sounds.

What’s Inside?
Grounded cathode amplifier
Figure 1 – Common cathode tube amplifer.
Your tube amp is made up of a preamp section and power amp section, both of these contribute to the overall tone and feel of the amp. Figure 1 on the left shows a typical tube preamp stage. This type of circuit is called a common cathode gain stage and is found in many classic Fender amps, including the early Deluxe and Twin Reverb ‘Blackface’ and ‘Silverface’ amps. This circuit is a standard building block for amplifier circuits and is used by many other amp manufacturers including Soldano, Boogie, Marshall and Effectrode in their effects pedals. This circuit turns a small voltage into a large voltage – it’s a voltage amplifier. With a 12AX7 tube in there this circuit gives about 30dB of voltage gain, with a 12AU7 expect about 15dB of gain. In a typical vintage amp this gain stage is followed by passive tonestack and then a power stage drive section.

Tube Distortion
Signal amplification
Figure 2 – Amplification of a signal.
Distortion is low in this type of amplifier circuit as the 1K8 cathode resistor sets the bias point right in the middle of the tube’s linear region as shown in Figure 2. Figure 2 shows the ‘transfer function’ or ‘load line’ for this amplifier. The small input voltage (x-axis) controls anode current (y-axis) flowing through the tube to give amplification. This centre biased amplifier section has the maximum threshold of clipping possible (highest headroom), that is where the grid can be fed with the largest possible signal before clipping. Another way of putting it, is to say the input sensitivity of the stage is at its lowest (input sensitivity describes the input voltage required to drive the stage into the non-linear region on the curve and cause clipping). Consequently, this is a very clean sounding amplifier and introduces relatively little distortion into the signal – early guitar amp manufacturers weren’t striving to generate huge amounts of overdrive and sustain with their circuits, just amplify the signal so the guitar could be heard above the drums.

The non-linearity inherent in tubes means that there will always be small amounts of distortion introduced into the signal, even with a clean amp such as the Fender Twin. This is partly explains why a tube amp sounds warmer and has a more musical character than a solid-state amp. The sound of many great Rock ‘n’ Roll players such as Ritchie Valens, Buddy Holly and Chuck Berry were defined by amps with cathode follower preamp stages. Although the preamp section of these amps is relatively clean sounding, the power amp section generates significantly more signal distortion, especially when the amp is cranked up. The distortion is the result of several factors including:

The power amp tubes entering the clipping region
The rectifier tube voltage sagging
The transformer core becoming saturated
The speaker cone resonances and/or break-up
This subject, however, is outside the scope of this article – so let’s get back to the preamp section.

Symmetrical Clipping
Symmetrical tube clipping
Figure 3 – Symmetrical clipping distortion.
This preamp section can be made to distort (in a very pleasant way) if fed with a large enough signal and that’s exactly what guitarists did during the late 60s and early 70s. Players like Alex Lifeson, Jimmy Page and Robin Trower pushed their amps into distortion by placing a boost pedal before their rig (typically a wall of Hi-watt or Marshall tube heads feeding into 4 x 12 cabs!). Because the bias point of the first amp section is roughly in the middle of the load line, the guitar signal is clipped roughly equally on both sides – symmetrical clipping as shown in Figure 3.

Symmetrical clipping produces a mixture of odd and even order harmonics creating the classic rock distortion tone. What you’re hearing when you use a boost pedal is actually your tube amp distorting – that is, a significant amount of the tonal character of the overdrive tone is generated by the amp. Figure 4 shows a frequency analysis of a centre-biased 12AX7 being overdriven. The fundamental is a 440Hz sine wave (‘A’) and strong 3rd, 5th, 7th, etc harmonics are clearly present in the output signal. Even harmonics are also present, however at reduced amplitude in comparison to the odd harmonics. Although centre biased symmetric clipping distortion is beginning to approach a square wave, the rounded edges ensure that brittle-sounding high order harmonics are absent. Incidentally, a true square-wave is made up of a series of only odd harmonics – this is the kind of distortion produced by hard-clipping diode circuits found in opamp based stompboxes such as the Boss DS-1.

Symmetrical tube clipping
Figure 4 – Symmetrical clipping produces a signal with strong odd order harmonics.
Asymmetrical Clipping
Hot biasing
Figure 5 – Hot Biased asymmetrical clipping distortion.
It wasn’t long before designers began experimenting with multi-stage tube preamplifier circuits – pioneers of the art include Alexander Dumble, Randall Smith and Mike Soldano. Their amps contained several tube stages in series (typically four stages) to massively boost the input signal until it went outside the linear region of the tube. Additionally, by manipulating the bias point on the load line they could exercise a degree of control over the input sensitivity in each stage of the amp so that the signal clipped in a more controlled manner. Because the bias point is offset from the centre of the load line, the output signal clips to a greater extent on one half of the cycle than the other – this is asymmetric clipping. Reducing the value of the 1K8 cathode resistor causes more anode current to flow so that the tube runs physically hot. This shifts the bias point to the right-hand end of the load line and the output signal will be clipped on the positive cycle of the output signal – this is called grid current limiting and is shown in Figure 5. A moderate amount of hot biasing produces a smooth, warm and ‘bluesy’ drive, too much and the sound becomes more aggressive and ‘fuzzy’

Cold biasing
Figure 6 – Cold Biased asymmetrical clipping distortion
The bias point can also be shifted to the left-hand end of the load line by increasing the value of the cathode resistor to reduce the current flowing in the anode – this is called cold biasing. This generates cut-off clipping on the negative cycle of the output signal producing a ‘crunchier’, ‘harder-edged’ distortion as shown in Figure 6. Cold biasing is used in many modern triode high gain tube amp designs as it adds rich, harmonic content giving a ‘heavier’ and ‘crunchier’ distortion tone.

Asymmetric clipping generates a signal rich in even and odd order harmonic overtones. The second harmonic is particulary strong (see Figure 7 below), which is almost certainly one of the reasons tube amps sound so musical – the second harmonic is exactly one octave above the fundamental. Both symmetric and asymmetric clipping occur in a multi-stage tube preamp and it will consequently produce a tone that is a complex mix of even and odd order harmonics. Designing an an amp is an art and involves a substantial amount of experimentation in the quest to discover the ideal biasing point, interstage tone-shaping and gain for each stage. The biasing point of each tube stage affects how the stages interact to produce distortion, ultimately defining the core tone of the amp – whether it is glassy clean, warm and bluesy, classic rock or creamy and sustaining.


PDF – Guitar Preamp Tone Explained _ Effectrode

Aristotle on Music

POLITICS BOOK VIII, chap. 5-7  (Aristotle)


With respect to music we have already spoken a little in a doubtful manner upon this subject. It will be proper to go over again more particularly what we then said, which may serve as an introduction to what any other person may choose to offer thereon; for it is no easy matter to distinctly point out what power it has, nor on what accounts one should apply it, whether as an amusement and refreshment, as sleep or wine; as these are nothing serious, but pleasing, and the killers of care, as Euripides says; for which reason they class in the same order and use for the same purpose all these, namely, sleep, wine, and music, to which some add dancing; or shall we rather suppose that music tends to be productive of virtue, having a power, as the gymnastic exercises have to form the body in a certain way, to influence the manners so as to accustom its professors to rejoice rightly? or shall we say, that it is of any service in the conduct of life, and an assistant to prudence? for this also is a third property which has been attributed to it.

Now that boys are not to be instructed in it as play is evident; for those who learn don’t play, for to learn is rather troublesome; neither is it proper to permit boys at their age to enjoy perfect leisure; for to cease to improve is by no means fit for what is as yet imperfect; but it may be thought that the earnest attention of boys in this art is for the sake of that amusement they will enjoy when they come to be men and completely formed; but, if this is the case, why are they themselves to learn it, and not follow the practice of the kings of the Medes and Persians, who enjoy the pleasure of music by hearing others play, and being shown its beauties by them; for of necessity those must be better skilled therein who make this science their particular study and business, than those who have only spent so much time at it as was sufficient just to learn the principles of it. But if this is a reason for a child’s being taught anything, they ought also to learn the art of cookery, but this is absurd.

The same doubt occurs if music has a power of improving the manners; for why should they on this account themselves learn it, and not reap every advantage of regulating the passions or forming a judgment [1339b] on the merits of the performance by hearing others, as the Lacedaemonians; for they, without having ever learnt music, are yet able to judge accurately what is good and what is bad; the same reasoning may be applied if music is supposed to be the amusement of those who live an elegant and easy life, why should they learn themselves, and not rather enjoy the benefit of others’ skill.

Let us here consider what is our belief of the immortal gods in this particular. Now we find the poets never represent Jupiter himself as singing and playing; nay, we ourselves treat the professors of these arts as mean people, and say that no one would practise them but a drunkard or a buffoon. But probably we may consider this subject more at large hereafter.

The first question is, whether music is or is not to make a part of education? and of those three things which have been assigned as its proper employment, which is the right? Is it to instruct, to amuse, or to employ the vacant hours of those who live at rest? or may not all three be properly allotted to it? for it appears to partake of them all; for play is necessary for relaxation, and relaxation pleasant, as it is a medicine for that uneasiness which arises from labour. It is admitted also that a happy life must be an honourable one, and a pleasant one too, since happiness consists in both these; and we all agree that music is one of the most pleasing things, whether alone or accompanied with a voice; as Musseus says, “Music’s the sweetest joy of man;” for which reason it is justly admitted into every company and every happy life, as having the power of inspiring joy. So that from this any one may suppose that it is necessary to instruct young persons in it; for all those pleasures which are harmless are not only conducive to the final end of life, but serve also as relaxations; and, as men are but rarely in the attainment of that final end, they often cease from their labour and apply to amusement, with no further view than to acquire the pleasure attending it. It is therefore useful to enjoy such pleasures as these.

There are some persons who make play and amusement their end, and probably that end has some pleasure annexed to it, but not what should be; but while men seek the one they accept the other for it; because there is some likeness in human actions to the end; for the end is pursued for the sake of nothing else that attends it; but for itself only; and pleasures like these are sought for, not on account of what follows them, but on account of what has gone before them, as labour and grief; for which reason they seek for happiness in these sort of pleasures; and that this is the reason any one may easily perceive.

That music should be pursued, not on this account only, but also as it is very serviceable during the hours of relaxation from labour, probably no [1340a] one doubts; we should also inquire whether besides this use it may not also have another of nobler nature–and we ought not only to partake of the common pleasure arising from it (which all have the sensation of, for music naturally gives pleasure, therefore the use of it is agreeable to all ages and all dispositions); but also to examine if it tends anything to improve our manners and our souls. And this will be easily known if we feel our dispositions any way influenced thereby; and that they are so is evident from many other instances, as well as the music at the Olympic games; and this confessedly fills the soul with enthusiasm; but enthusiasm is an affection of the soul which strongly agitates the disposition. Besides, all those who hear any imitations sympathise therewith; and this when they are conveyed even without rhythm or verse.

Moreover, as music is one of those things which are pleasant, and as virtue itself consists in rightly enjoying, loving, and hating, it is evident that we ought not to learn or accustom ourselves to anything so much as to judge right and rejoice in honourable manners and noble actions. But anger and mildness, courage and modesty, and their contraries, as well as all other dispositions of the mind, are most naturally imitated by music and poetry; which is plain by experience, for when we hear these our very soul is altered; and he who is affected either with joy or grief by the imitation of any objects, is in very nearly the same situation as if he was affected by the objects themselves; thus, if any person is pleased with seeing a statue of any one on no other account but its beauty, it is evident that the sight of the original from whence it was taken would also be pleasing; now it happens in the other senses there is no imitation of manners; that is to say, in the touch and the taste; in the objects of sight, a very little; for these are merely representations of things, and the perceptions which they excite are in a manner common to all. Besides, statues and paintings are not properly imitations of manners, but rather signs and marks which show the body is affected by some passion. However, the difference is not great, yet young men ought not to view the paintings of Pauso, but of Polygnotus, or any other painter or statuary who expresses manners.

But in poetry and music there are imitations of manners; and this is evident, for different harmonies differ from each other so much by nature, that those who hear them are differently affected, and are not in the same disposition of mind when one is performed as when another is; the one, for instance, occasions grief 13406 and contracts the soul, as the mixed Lydian: others soften the mind, and as it were dissolve the heart: others fix it in a firm and settled state, such is the power of the Doric music only; while the Phrygian fills the soul with enthusiasm, as has been well described by those who have written philosophically upon this part of education; for they bring examples of what they advance from the things themselves.

The same holds true with respect to rhythm; some fix the disposition, others occasion a change in it; some act more violently, others more liberally. From what has been said it is evident what an influence music has over the disposition of the mind, and how variously it can fascinate it: and if it can do this, most certainly it is what youth ought to be instructed in. And indeed the learning of music is particularly adapted to their disposition; for at their time of life they do not willingly attend to anything which is not agreeable; but music is naturally one of the most agreeable things; and there seems to be a certain connection between harmony and rhythm; for which reason some wise men held the soul itself to be harmony; others, that it contains it.


We will now determine whether it is proper that children should be taught to sing, and play upon any instrument, which we have before made a matter of doubt. Now, it is well known that it makes a great deal of difference when you would qualify any one in any art, for the person himself to learn the practical part of it; for it is a thing very difficult, if not impossible, for a man to be a good judge of what he himself cannot do. It is also very necessary that children should have some employment which will amuse them; for which reason the rattle of Archytas seems well contrived, which they give children to play with, to prevent their breaking those things which are about the house; for at their age they cannot sit still: this therefore is well adapted to infants, as instruction ought to be their rattle as they grow up; hence it is evident that they should be so taught music as to be able to practise it.

Nor is it difficult to say what is becoming or unbecoming of their age, or to answer the objections which some make to this employment as mean and low. In the first place, it is necessary for them to practise, that they may be judges of the art: for which reason this should be done when they are young; but when they are grown older the practical part may be dropped; while they will still continue judges of what is excellent in the art, and take a proper pleasure therein, from the knowledge they acquired of it in their youth.

As to the censure which some persons throw upon music, as something mean and low, it is not difficult to answer that, if we will but consider how far we propose those who are to be educated so as to become good citizens should be instructed in this art, [1341a] and what music and what rhythms they should be acquainted with; and also what instruments they should play upon; for in these there is probably a difference. Such then is the proper answer to that censure: for it must be admitted, that in some cases nothing can prevent music being attended, to a certain degree, with the bad effects which are ascribed to it; it is therefore clear that the learning of it should never prevent the business of riper years; nor render the body effeminate, and unfit for the business of war or the state; but it should be practised by the young, judged of by the old.

That children may learn music properly, it is necessary that they should not be employed in those parts of it which are the objects of dispute between the masters in that science; nor should they perform such pieces as are wondered at from the difficulty of their execution; and which, from being first exhibited in the public games, are now become a part of education; but let them learn so much of it as to be able to receive proper pleasure from excellent music and rhythms; and not that only which music must make all animals feel, and also slaves and boys, but more. It is therefore plain what instruments they should use; thus, they should never be taught to play upon the flute, or any other instrument which requires great skill, as the harp or the like, but on such as will make them good judges of music, or any other instruction: besides, the flute is not a moral instrument, but rather one that will inflame the passions, and is therefore rather to be used when the soul is to be animated than when instruction is intended.

Let me add also, that there is something therein which is quite contrary to what education requires; as the player on the flute is prevented from speaking: for which reason our forefathers very properly forbade the use of it to youth and freemen, though they themselves at first used it; for when their riches procured them greater leisure, they grew more animated in the cause of virtue; and both before and after the Median war their noble actions so exalted their minds that they attended to every part of education; selecting no one in particular, but endeavouring to collect the whole: for which reason they introduced the flute also, as one of the instruments they were to learn to play on. At Lacedaemon the choregus himself played on the flute; and it was so common at Athens that almost every freeman understood it, as is evident from the tablet which Thrasippus dedicated when he was choregus; but afterwards they rejected it as dangerous; having become better judges of what tended to promote virtue and what did not.

For the same reason many of the ancient instruments were thrown aside, as the dulcimer and the lyre; as also those which were to inspire those who played on them with pleasure, and which required a nice finger and great skill to play well on. What the ancients tell us, by way of fable, of the flute is indeed very rational; namely, that after Minerva had found it, she threw it away: nor are they wrong who say that the goddess disliked it for deforming the face of him who played thereon: not but that it is more probable that she rejected it as the knowledge thereof contributed nothing to the improvement of the mind. Now, we regard Minerva as the inventress of arts and sciences.

As we disapprove of a child’s being taught to understand instruments, and to play like a master (which we would have confined to those who are candidates for the prize in that science; for they play not to improve themselves in virtue, but to please those who hear them, and gratify their importunity); therefore we think the practice of it unfit for freemen; but then it should be confined to those who are paid for doing it; for it usually gives people sordid notions, for the end they have in view is bad: for the impertinent spectator is accustomed to make them change their music; so that the artists who attend to him regulate their bodies according to his motions.


We are now to enter into an inquiry concerning harmony and rhythm; whether all sorts of these are to be employed in education, or whether some peculiar ones are to be selected; and also whether we should give the same directions to those who are engaged in music as part of education, or whether there is something different from these two. Now, as all music consists in melody and rhythm, we ought not to be unacquainted with the power which each of these has in education; and whether we should rather choose music in which melody prevails, or rhythm: but when I consider how many things have been well written upon these subjects, not only by some musicians of the present age, but also by some philosophers who are perfectly skilled in that part of music which belongs to education; we will refer those who desire a very particular knowledge therein to those writers, and shall only treat of it in general terms, without descending to particulars.

Melody is divided by some philosophers, whose notions we approve of, into moral, practical, and that which fills the mind with enthusiasm: they also allot to each of these a particular kind of harmony which naturally corresponds therewith: and we say that music should not be applied to one purpose only, but many; both for instruction and purifying the soul (now I use the word purifying at present without any explanation, but shall speak more at large of it in my Poetics); and, in the third place, as an agreeable manner of spending the time and a relaxation from the uneasiness of the mind. [1342a] It is evident that all harmonies are to be used; but not for all purposes; but the most moral in education: but to please the ear, when others play, the most active and enthusiastic; for that passion which is to be found very strong in some souls is to be met with also in all; but the difference in different persons consists in its being in a less or greater degree, as pity, fear, and enthusiasm also; which latter is so powerful in some as to overpower the soul: and yet we see those persons, by the application of sacred music to soothe their mind, rendered as sedate and composed as if they had employed the art of the physician: and this must necessarily happen to the compassionate, the fearful, and all those who are subdued by their passions: nay, all persons, as far as they are affected with those passions, admit of the same cure, and are restored to tranquillity with pleasure.

In the same manner, all music which has the power of purifying the soul affords a harmless pleasure to man. Such, therefore, should be the harmony and such the music which those who contend with each other in the theatre should exhibit: but as the audience is composed of two sorts of people, the free and the well-instructed, the rude the mean mechanics, and hired servants, and a long collection of the like, there must be some music and some spectacles to please and soothe them; for as their minds are as it were perverted from their natural habits, so also is there an unnatural harmony, and overcharged music which is accommodated to their taste: but what is according to nature gives pleasure to every one, therefore those who are to contend upon the theatre should be allowed to use this species of music. But in education ethic melody and ethic harmony should be used, which is the Doric, as we have already said, or any other which those philosophers who are skilful in that music which is to be employed in education shall approve of. But Socrates, in Plato’s Republic, is very wrong when he [1342b] permits only the Phrygian music to be used as well as the Doric, particularly as amongst other instruments he banishes the flute; for the Phrygian music has the same power in harmony as the flute has amongst the instruments; for they are both pathetic and raise the mind: and this the practice of the poets proves; for in their bacchanal songs, or whenever they describe any violent emotions of the mind, the flute is the instrument they chiefly use: and the Phrygian harmony is most suitable to these subjects.

Now, that the dithyrambic measure is Phrygian is allowed by general consent; and those who are conversant in studies of this sort bring many proofs of it; as, for instance, when Philoxenus endeavoured to compose dithyrambic music for Doric harmony, he naturally fell back again into Phrygian, as being fittest for that purpose; as every one indeed agrees, that the Doric music is most serious, and fittest to inspire courage: and, as we always commend the middle as being between the two extremes, and the Doric has this relation with respect to other harmonies, it is evident that is what the youth ought to be instructed in. There are two things to be taken into consideration, both what is possible and what is proper; every one then should chiefly endeavour to attain those things which contain both these qualities: but this is to be regulated by different times of life; for instance, it is not easy for those who are advanced in years to sing such pieces of music as require very high notes, for nature points out to them those which are gentle and require little strength of voice (for which reason some who are skilful in music justly find fault with Socrates for forbidding the youth to be instructed in gentle harmony; as if, like wine, it would make them drunk, whereas the effect of that is to render men bacchanals, and not make them languid): these therefore are what should employ those who are grown old. Moreover, if there is any harmony which is proper for a child’s age, as being at the same time elegant and instructive, as the Lydian of all others seems chiefly to be-These then are as it were the three boundaries of education, moderation, possibility, and decorum.

From Four or Five Pro DJ’s to Inumerable – How the July 1977 “Summer of Sam” Blackout in NYC contributed mightily to the sudden rise of Hip-Hop


Thousands of looters on Broadway.

KRS One: “I’m bugging out because church-going, upstanding citizens, the ones you know are teachers and doctors and ‘in the community’ – they are now reduced to looters, thieves and rioters.”

When there is no light, man is reduced to animal.



3:10 into video

“You couldn’t just say, I’m gonna be a DJ. You needed equipment… Before that blackout, you know, [NYC] had literally just maybe four or five legitimate DJ crews. The next day – the very next day – there sprung this whole revolution of DJs. That was a huge spark, and a huge contribution to the Hip-Hop culture.”