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I have built this circuit more than a decade ago and found it to be very sputtery and farting. The second transistor always seemed biased way too high so that there was never a clear signal coming through. Despite all transistor swapping I never seemed to get this one working. I was already considering re-using the aluminium case for another project.

It was lying dormant for over a decade (look at the time stamp on the picture), but recently I saw videos of guys using it and was intrigued again. I finally got it working and found some tricks that really make it shine, which I am glad to share with you.

The Axis Fuzz does not have the typical woolly Fuzz Face rasp, it is rather screaming. It has something which I only by exaggeration managed to identify as spurious octave effect, which in small doses adds what makes this unit special.

The term "fuzz" is actually misleading by today´s nomenclature, since it suggests a typical Fuzz Face architecture, which it has not. Both nevertheless have a lot of sonic territory in common.

More than any fuzz I have it is a unit that goes ultimately clean at low guitar volume settings, but it gets beautifully raucous with picking dynamics.

This unit is heavily geared towards treble boost to counteract some problems that no longer exist, so unaltereded, it were for were for my purposes virtually unusable.

Luckily, those problems were very easy to solve. It has become my new favorite fuzz unit - for a while...

It is fact (R.M. himself has said that) that there have been constant modifications to the devices sculpting Jimi´s tone.
Despite the reduced technical potential of the time, he may still have used HP and LP filters and buffers as needed in the studio, even if it may just have been a cable with high capacity for that high-cut or a different capacitor on the input to cut bass that went without mention.

We do not know what was buffered, why some of his Fuzzes worked "the impossible way" before or after a wah and so on.
R.M. surely did not have any preconceptions about things if they worked, however you can rest assured that he does not reveal all tricks. This kind of secretive policy is starting to make me tired, particularly if it is used to fuel hype and sales numbers at the expense of the customer.

On the end of the day, we have to state we do not know what was inside those cast-iron shells. I thus strongly encourage the kind reader to experiment with those things free of prejudice and dogma. Hendrix by the way was one who was highly unorthodox  musically and technically.

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Be as unorthodox as Jimi when it comes to experimentation
with technology and music.

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Listening to the recordings over time it can be taken for granted that there has been some change in the fuzz tone´s timbre.
I do not hear the wholly tone of germanium on later recordings, which coincides with what is being spread - a change to silicon, de facto meaning a change to the Axis Fuzz. I can feel that.

I think we can safely agree that R.M. understood that all Fuzz Faces suffer from low input impedance, and that he knew how to remedy that. We can thus further conclude that the wah into Fuzz Face problem can be (and has obviously been) solved (by simple measures) and obviously without ruining tone; visual and tonal evidence clearly documents that. So this alone would not be a reason for people to adhere to vintage at all cost. In fact it astounds me to see how many people still fall for the age-old Fuzz Face conundrum after so many years of technical elucidation.
It is obvious that Hendrix chose the Axis Fuzz for reasons that were beyond mere technical improvements.

Nobody ever mentions that  a musician / technician of that time permanently had to fight diverse technical circumstances and that devices like the Axis Fuzz (that already were an evolution of predating effects) had to compensate for.

If you use such devices in a contemporary rig (that is no longer plagued by those shortcomings), unaware of the origins of those properties, then you will inevitable end up with a tone that is unrelated to what you expect.

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Knowledge is power. Know thy rig.

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Vintage fuzzes generally benefit from some l.f. tailoring, it thus comes in handy that I have a bass cut switch on the guitar.
Fuzzes clean up tremendously if the bass into them is restricted. Indeed I do not hear a lot of l.f. content in Hendrix´ guitar recordings.

Those myriads of pedals may sound suspiciously like Grandmaster Jimi´s tone but they have to be used with a simulator that emulates a cooking tube amp if used in conjunction with an amp that is running fairly clean and at bedroom levels. In all instances you have to balance the high end somehow, and be sure, this was addressed back then and you have to address that too.

One of the most famous pitfalls is the fact, that you want to hear the full (bass) range predominantly if you are playing alone to avoid thin tone. The ear´s hearing curve at low sound pressure is part of the problem. When playing in a band context, bass quickly gets in the way, so mid range boosters have been invented for a reason.

Bass content early in the line of effects is a different beast, because it overloads the distortion units. Besides that, overdrives tend to create artificial sub-harmonics that enhance bass anyway. Keep that in mind before reaching for any early dial to bring up bass. However, you may make up for this on the amp to your heart´s content.

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Avoid the temptation to dial in bass early down the line of effects.
Compensate on the amp if you sense a lack of bass.

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People complain that their fuzz sounds flat horrible into their rig. Yes it will if you have a bright amp tone to begin with. Early full range (tube) amps were pretty dark sounding before subsequent models were voiced to current taste. And those amps were running full throttle, introducing additional sound shaping effects like distortion and compression.

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Approximating Jimi´s stage sound in your bedroom is eminently possible.
Bass and treble tailored where necessary and a subsequent overdrive mimicking a tube amp at full throttle will get you far.

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One thing we must not forget is cables. Those used to have a horrendous (by today's standards anyway) capacity which killed all treble flat. Hence the urge to regain treble by applying treble boosts.

Both Fuzz Face and Axis Fuzz (momentarily disregarding misbiased units) have been found to sound lousy in certain configurations, and yes, this is down to the before mentioned facts to a great extent. Several musician's forae are full of complaints that the Axis Fuzz sounds unbearably shrill unless followed by a cooking tube amp that masks all it.

This all is totally avoidable. Later versions of the R.M. effects (Voodoo series) have undergone some improvements that address those problems partially.  We will look at that later and resolve the rest in a wash...

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Experience With The Axis Fuzz

I made a few general observations with my unit that I want to share with you:

• This unit is very temperamental. You have to play with the settings to find the sweet spot more than any other unit. It is not plug-and-play so to speak.
• It is by far the most expressive fuzz unit I have.
  
• I have heard people saying that it is problematic with humbuckers. I can comprehend that, since they have more power in the low registers and compression. I have bass cut dials on all of my guitars. Even a small amount of bass cut will make the Axis Fuzz shine. This makes me believe that R.M. has taken subtle measures to keeps Jimi´s bass content low.
• The effect has a strong hint of an octaver. This gets more intense with higher gain and higher bass content.
  
• Use a treble cut afterwards. I recommend to retrofit a BMP-style control as recommended further down here.
  
• Vintage (coily) cables used to have a high capacity - in the order of several nF. It is no coincidence that a muzique.com schematic recommended 4.7nF as input load capacitor (where normally 470pF goes - to filter out RF). Experiment with that. I found it unnecessary with my rig and maybe a load (literally) in other circumstances. See what Björn Juhl has to say about that (in the Reference).
  
• On the other hand, such a cable will make dimed tones sound muffled and lead tones trebly - the contrary of what you want. You do not hear that on the recordings. What else conceals R.M. from us? This clearly speaks for a buffer.
• Emulate a 60ies gear environment to iron out some of its sharpness,  if you want to go the 60ies rock way. I have made a cute write-up in my silicon tone bender article that shows you how to do this.
• Remove excessive treble: the Axis Fuzz has deliberately (why is explained in the above link) some treble boost built into. Remove the 100nF cap on the drive pot and add the 220k / 1nF R/C. Beware: not 220 Ohms!
  Obsolete. The resistor made no difference. Omit.
  
• I tried adding "Miller" caps to the transistors, 47pF, 100pF - no effect, but unnecessary with before mentioned measures.
• Cut treble on your guitar.
  
• The Axis Fuzz is not as benign with (preceding) wahs as a germanium Fuzz Face, probably due to his temperament. The sweep does not get through as cleanly, but there is no technical incompatiblities.
• This device is geared towards treble boost in more than one place. It seems that this was R.M.'s answer to long coiled or other high capacity cables and / or dark British amplifiers of the time. It appears also fairly mid-deficient or at least flat.
  

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Biasing

This circuit is all about biasing, while being less touchy than a fuzz face (where everything interacts with everything).
I had to work myself through a heap of forum threads to find the answer. This is a shortcut for you.

Several sources (re-) quote that an original unit measures the following voltages (Vbat= 9.15V)

2N3906          2N3904
C- 3.20           C- 5.86
B- 3.64           B-  3.25
E- 4.34           E- 2.68

Original hfe values are (allegedly) around 200. Use with discretion.
I started out with the following transistors:

Q1= 2N3906, hfe = 170
Q2= 2N3904, hfe = 200

All voltage readings were way out. The unit sounded horrible.
According to the recommendations (see links at the bottom) I tried the following:

• Biased Q1 by changing the voltage ladder resistors. Helped for Q1 but not Q2. Reverted to original.
  
• I varied collector resistor Q2 - works! set Q2 base to 3.2V, all other values fall in place except collector, still a bit low.
• I used a BC549C (hfe not measured, but certainly higher): improves slightly. Tried other BC549C specimen from same batch, no difference.
• Changed Q2 emitter bypass electrolytic to a smaller value - sound becomes thin.
• RC on the base of Q2 is used, but with a 220k resistor. There is indications that the 220R is a value that has been traded wrongly. See above.
  
• Removed 100nF on the drive pot, tone becomes shrill. Only recommended on very dark amps. This provides an always-on treble boost, independent of the drive pot setting. Probably owing the inclination towards treble boost.
  

The unit now sounds perfect.

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Worthwhile Additions

As mentioned above, the Axis Fuzz is geared towards treble boost. R.M. lateron elegantly introduced some adjustability by adding a BMP (Big Muff Pi) style tone control onto his Voodoo series (amongst other small features I do not consider essential). He calls the knob Fatness. The values applied in those units are comparable to the ones used in the Page-1 unit (see Reference), which lets us safely assume that they are used in all Voodoo series pedals.

Trace from BMP tone stack variant as used in R.M. Voodoo pedals. (click to enlarge). The schematics can be found on the internet: Page-1, or Voodoo Axe (links see Reference) So by the looks this is also part of current Voodoo series models marketed lately by R.M. Note that standard BMP controls suffer from a substantial mid-dip (see Reference), which this one does not. It is practically flat (the third trace). Many options with a single control - thumbs up R.M.! The BMP stack "eats" away some signal, but fortunately the Axis Fuzz has no shortage thereof.

As you see, on the maximum bass setting there is little damping to treble. Accordingly, tone gets "fatter" towards the bass side, without sacrificing treble, hence the name fatness control. According to the yin-yang of tone an increase of bass is perceived  as a decrease of treble. Again we see R.M.'s attempt to fight against treble sucking.

As mentioned earlier, I have removed the 100nF cap in parallel to the drive pot, which is a treble booster par excellance.
With this BMP style control you have about unity gain when the knob is at 9 o'clock (trace #2-3). We can see that there is little treble damping above. Also, the "pivot point" is pretty high compared to a conventional BMP. This al is deliberately chosen so by R.M.

We could revert the tonestack to a more conventional BMP stack style, even without the mid-scoop (see amzfx in the Reference section), but a BMP stack is always just a yin-yang to an extent.

For my taste treble is still a bit high compared to my other effects. A 2.2 nF cap in parallel to the feedback resistor (22 k) makes a treble roll-off at 3.3 kHz, which seems to complement the fatness control nicely.

Unfortunately, this type of control was not invented before some decades after Hendrix, it may have helped them to save a lot of grief. It is very effective and simple. It simply inserts between the output cap and the volume control, so it is an easy,  inexpensive and rewarding retrofit to existing units.

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Reference

amzfx: AMZ Presence Control, http://www.muzique.com/lab/tone3.htm
AMZ-FX Guitar Effects Blog: Buffers Before Fuzz, https://www.muzique.com/news/buffers-before-fuzz/

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