One of my long time favorites is Miles Davis´s Agharta. Pete Cosey does things to a guitar unheared of before and after.
So I was setting out to replicate that choking guitar sound. The upshot is, it is not the sound of a misbiased transistor. Pete used an early guitar synthesizer for this, and it is even difficult to describe what it does to tone, letting alone putting this into technical terms.

During this quest, I built a silicon fuzz face, which I found best sounding when not sputtering or choking, and now the Tonebender, or, as you may want to call it with equal validity, Buzzaround.

Germanium has an alleged sought-after elusive tone, but, to be honest, I do not miss anything with a well made silicon device. And I have been through germanium.

One of the disadvantages of germanium is leakage. Ironically, the device described hereafter needs leakage to work.
It thus cannot be simply transferred to silicon. Moreover, the notorious low gain values and the spread thereof lead to more problems, predominately biasing issues. You thus probably won´t even get a germanium device working in this circuit.

As a matter of fact, they did not even back when these devices were en vogue, so many pedals out there produce nothing but sputtering and gating, letting alone temperature sensitivity.

So biasing is important. I come to believe, that, although some people explicitly want this sort of device for their crazy avantgarde tones and effects, people initially looked for a distortion unit with great sustain and natural decay. This is how I have set up my device. Chances are, with the values given in all the schematics, or, as the case may be, this schematic, you won´t get the job working well. I hope the subsequent explanations will help you, germanium or not.

Copyright Notice: The circuits and concepts depicted here, particularly the artwork, are the sole intellectual property of persons known as Aquataur (Aquataur Music). They are destined for personal use and for educational purposes. Any commercial exploitation thereof is thus at least immoral. Any re-hosting is prohibited.

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Circuit Details, General Thoughts On The Architecture

The Tone Bender Mk.III and the Buzzaround have much in common. The first thought may be, since the Buzzaround has more controls, it must be the successor of the TB, but it is the other way around. The TB came a year later. Maybe they left away some of the questionable controls.

Mark III circuit (click to download PDF file): you are going to need a PDF viewer to view this file.

The Balance Control

Lets look at the Balance control, this changes the bias of Q3. You can adjust the signal for symmetric clipping, or "more to one side or the other". Of course, you change the collector current and thus operating point where the collector resides. This is assuming, there is proper drive on the base. But what does it "balance"?
Yes, and always in situations like that, this alters the texture of the fuzz. As a side effect, it also changes volume, which is why a volume pot was spared.
Huh, plenty of interaction with little result, isn´t it? They eliminated this one with the advent of the TB by using a trimmer to set the bias and be done. I do not know if this control warrants space on the front panel at all. It may well turn out that this is one of the set-and-forget controls, but I leave it there for experimentation.

I stand speechless with what grimness people (particularly those who manufacture devices like that) adhere to things like the Balance pot in their quest for staying retro at all cost, despite obvious flaws.

The Tone Control

The tone control´s sweep must have been quite revolutionary, but today there is better. Aftermarket productions have replaced this by a Big Muff Pi style tone control, which is what I borrowed. Note that I have put the control in a way that at 7 o´clock bass will be maximum, at 17h treble is maxxed. This seems most intuitive for me.

Biasing The Darlington Stage First - By Ear!

The darlington stage is fairly straightforward. In fact so straight, that some designers (Mictester´s Buzzaround Alike) decided to replace it by a op-amp stage entirely. They claim the circuit sounds like the original, but some say no. We´ll come back to this later.

When I got the whole thing going, I naturally looked at the darlington stage first. It kinda worked right away, and with an external resistor pulling up the base I could nicely adjust the collector waveform. Then I proceeded to Q3. Now this has no biasing network and relies, coming from a germanium circuit, on a leaky transistor, which exhibits some parasitic current leaking from base to collector. This pulls the base up - hopefully enough. (Maybe this explains why some of those units worked better than others...).

Now silicon does, for all practical means, not suffer from such defects. We have to pull the base up ourselves. R.G. Keen suggests a reverse biased GE diode from B to C to accomplish this task, but this did not seem to work. Mictester, for his unit uses a pullup at the fuzz pot, which would present a variable working point for Q3 dependent on the fuzz pot setting. Hmm.
GGG and others then use a 1M trim pot between C and B that does the job nicely. Case settled.

But what about the collector? The 27k that was there, was way to big for my silicon device. Sputtering and farting galore. I twiddled the pot and lo and behold - beautiful signal on the output!
Some time later, I hot-pulled Q3 to try a different unit, and - believe it or not - nothing changed!
It turned out that the pot hat been at zero, so full signal from B to C without Q3!

This appears a great method to fine-tune the darlington state first. (The complete bias setting procedure is described on the schematic sheet in great detail.). An oscilloscope is one thing, the ear another.

The darlington stage on its own does not sound bad, strongly reminding of a fuzz face with low fuzz setting. Now we come round full circle, this makes me believe that the first stage does matter tone wise. You can change its character by the procedure described. Great distortion always happens in small chunks here and there.

The initial, intuitive choice of PN 2369A (with gains of ~60) proved right, although with all this gain cascading noise is not necessarily low. However, it will be much lower than GE.

Biasing Q3

Back to Q3. I tried any old GE transistor I found, and it worked, but sounded so-so. There was no big "Ah, germanium sound!".
For biasing of Q3 I removed the 27k resistor and replaced it temporarily with a 10k trim pot. It turned out, that it requires something as small as a kOhm or thereabouts. This finally brings Q3´s collector to a position, where it sounds like a fuzz should sound. Balance still works, about half down, and cuts then out, as expected.
Fine tuning the 1M leak pot changes little beyond 500k. Note that C13 on Q3´s input is not necessary for biasing per se, but using the Buzzaround´s way of setting the fuzz level (with a pot to ground, which yields better control over the amount of signal hitting Q3...), a low setting of the knob would also upset Q3´s bias point, which results in a choked tone. Using the cap leaves its DC bias alone and allows for very small, precise adjustments.

Since I recently obtained a fair batch of 2N2926 (which are listed as replacement for BC108), I tried those. The gain is around 150 and they sound great there. I bought a handful of 2N2222´s that worked equally great, albeit a bit more gainy (180). There was no subjective difference in noise, the biggest source of noise being the darlingtons.
I used a 220pF B-C-cap, to tone down treble just a hair.

Re-visiting The Darlington Stage

I then went back to stage one and played with the collector resistor. A small change can make the sound just a little hairier, a small correction on the base resistor appeared necessary.

So yes, these units need to be tweaked for good tone, question is, what is good tone? And what, if they were not tweaked at all in the old days? What if bias changed with heat? This will leave you with a unit that is largely not functioning as intended, although this has gained some status of its own for a specialized music genre. It is beyond doubt, these units were not made deliberately to sputter and fart (that came later...), they were made for screaming sustain and natural decay.

So, with silicon, once a unit is tweaked, it will remain the same. And it sounds great. Any alleged shrillness can be concealed with a natural overdrive after or a cranked amp. This unit really has a midrange bark that is matchless.

I used to be not particularly fond of such fuzzy devices, until I heard them used by pro´s like Jimmy Page and Tommy Bolin. Those are not gated and sputtery, they have great sustain and decay and a very raw and untamed character to them, which reflects the sixties´ explosive spirit well.

Many amateur´s fuzz videos are sputtery and trebly, which, at best, can be used for a special effect or punk. For that, I do not like Joe Gore´s devices as well, because he trims them towards this niché sound, although he is classy in what he does. This is an application of a fuzz as a special effect and thus a different item. You have to make your mind up if you want that.

Note that on all pro videos made for vintage rock music sound, this is not the case - they sound smooth:

"(...) I have taken the approach of setting bias so that transition from seemingly clean to distortion is gradual and symmetric, which in use means sounds can be set for near clean to full fuzz and gradually so with as low as possible static noises in the decay of a note. The idea is to make a close to linear amplifier circuit that would overload symmetrically and gradually- that’d be a behavior you more expect from a tube amplifier than a fuzz pedal, but then back in the dawn of fuzzdom one of the things musicians wanted replicated with a fuzz pedal was an overdriven amplifier.   [in the 1970’s] (...) a tube sound was suggested to be a square wave with rounded symmetrical edges and further to be dynamically controlled." - Björn Juhl (BJFE), BJF Bone Bender, https://moodysounds.se/bjf-kits/bjf-bone-bender/

Those devices need to be used under 60ies conditions: into a pretty dark amp. The amps at this time, particularly Marshalls,  were all pretty dark sounding from a start (which is why guys like Vox came into play...), which worsens if you push the thing.
A pushed tube amp will always tend to swallow treble. Hendrix used Twin Reverb´s in his studio work (just because they were reliable and were there...), which are inherently bright, but he thumped them...

• Have your fuzz set up smooth
  
  reason as above. This means correct biasing without sputter and spit. If you are deliberately looking for this sound, don´t read on.
  
  
• Run your fuzz into an overdrive
  
  For bedroom levels, a roaring tube amp phases out immediately, but you can fake this with a suitable overdrive after the fuzz. In this case the overdrive sets the overall loudness. There will be little increase in loudness, even if you run your fuzz into the overdrive at full throttle. I have good results with setting the fuzz somewhat louder, that it really pushes the overdrive. Just as it would push a tube amp. 

• Use an overdrive that stays transparent
  
  Although ultimately being a matter of taste, a transparent overdrive alters your tone the least. Beware, many overdrives, that claim to be transparent, are not! There seems to be some huge misconception about this term. Many of those are far from transparent in the sense, that they prefer or push some part of the sonic range. Most of the latter are tube screamer variants, and you would be surprised how many highly touted units belong to this category^[1].
  

• Use an overdrive that becomes darker when pushed
  
  Note that not all overdrives behave like a tube amp when driven into saturation - becoming darker.
  I have many of them stomp boxes that become ringy when pushed - be it diode clippers or discrete. No amount of treble filtering will remove that, since it is a dynamic effect. 

• Use an overdrive that becomes darker when pushed
  
  If you want to adhere to the paradigm of emulating a smoldering tube amp, the overdrive has to be the last instance of sound shaping effects before the amp. Delays and reverbs exempted.

• Balance the loudness levels between overdrive, fuzz and guitar volume control
  
  Here we can borrow from Jimi and the likes. A fuzz is not a contemporary effects device that you stomp upon occasionally during playing. No, it is kind of permanently on. Use the following procedure to get the hang:
  
  
• set fuzz texture: disable the overdrive. Turn on the fuzz. Turn guitar volume fully up. Set fuzz volume to center for the moment. Determine maximum fuzz you want by setting the fuzz´s drive control appropriately
  
• set the overdrive: disable the fuzz. Set the overdrive like in^[2]. Start with low gain.
  
• arm the guitar´s volume: Roll back the guitar´s volume, until your fuzz starts to clean up. Match the fuzz´ "clean" loudness to your "full" effect-off loudness. This way your "fuzz clean" tone is approximately equally loud as your "fuzz off" basic maximum loudness. That´s your pivot point. By ramping up your guitar´s volume you increase fuzz, without ever touching the stompbox itself. This is why you need a well biased fuzz for that.
  Vintage fuzz units will invariably be somewhat misbehaved and produce a lot of treble, but this will be sorted out by the overdrive
  
• turn on the overdrive: once the overdrive is on, it will compress the fuzz´ huge volume step achieved by ramping up the guitar´s volume. The fuzz' volume will this way end up much higher than unity. Ideally the overdrive will limit the fuzz´ temperament and impart its sonic footprint in a benign way.
  
• play with the overdrive´s gain control: if the overdrive´s gain is set low, it will gradually be overdriven by a loud input signal. This way the fuzz' sonic signature is dominant. The higher you make the overdrive's gain, the more dominant is the overdrive's sonic signature (and the higher the compression). Play with that until you find a happy sweet spot.
  

• Use some basic loudness
  
  Whatever, you need a basic loudness. Set you amp to whisper loudness - it will sound shitty. Turn it up somewhat, it will come alive. But if you follow these measures you will be far from deafening loudness.

One unit that I found (and, no, I have no connection to them...) that works brilliantly for this job, is the Wampler Ecstasy (resp. Euphoria). It appears to get darker when you push it.

Many units behave well if you use them within normal conditions, such as a light overdrive conditions. When you whack them with a huge signal, as it happens when you ride the volume fully up, they start to produce a lot of rough artifacts which ruins tone. Sometimes this is desired, but this is not how a tube amp would behave, it would stay smooth.
This way I wore out many overdrives, such as OCD, Nobels, Anderton C-MOS, BJFE, R.O.G. Thunderbird and more.
The Ecstasy is a notable exception from this league. It behaves even with loud signals.
I assign this to its different method of producing compression / distortion by using the OPA and not diodes of sorts as all the others. All units that use this method are promising candidates, like the Crowther Hotcake. Incidentally, both use a LM741 type op-amp (MC1458 is a dual version thereof). You may also try a RAT, a DOD 250 or MXR distortion + but with the clipping diodes defeated if your unit allows for that (clone). Even old Sansamp models worked this way. The LM741 may not be the first candidate for this, try an OP07, or if this is too dark, a TL070. The latter needs an external compensation capacitor of up to 150 pF, you´d need to experiment with that. All of those op-amps are "vintage" specimen that have a limited slew-rate.

In this context I have made particularly poor experience with units that simulate tube amp behavior with j-fets, but those were high gain units. They produced some unpleasant ring. I do not mean j-fet buffers!

But discrete units may work too. Aim for a unit that subjectively gets darker when you push it.

Resist however the temptation to use the Euphoria for anything else^[2]. It is a one trick pony, but this it does well.

So all recordings you hear, inclusive the ones following, are not what is real. It may and it will sound different with your setup and playing. But you can hear its potential.

"All is an illusion" - Ramana Maharishi

Sound Samples

The recordings have been done using the following setup and no further processing:

• Strat with Bill Lawrence (Wilde Pickups) microcoils, neck pickup + bridge pickup, full bass
  
• J-FET buffer directly after the guitar
  
• Mark III: Vol = 10h, fuzz 15h, texture full, tone 9h (bass slightly cut). (Total volume is louder than unity to get the subsequent stage going...)
• Wampler Euphoria Overdrive, setting = natural. Gain = 1. Slight crunch appears when guit. vol = full and heavy playing.This mimics a tube amp on the edge of breakup
  
• Peavey Bandit 112 silver stripe, clean channel, Resonance on, T-Dynamics fully down, volume=9, all others=12
• Rode M3 mic @ 20cm distance
  
• Recording device: Focusrite PC interface into DAW
  

(Names may be copyrighted by the associated copyright holder, no association with any of them)

A few quick and nasty takes. No exercise in timing or else, excuse the amateur playing. Crank you speakers. On my stereo speakers the recordings sound a bit less trebly than in reality.

Since this is sticking in the ear, doing a bit of Zeppelin. Volume swells in the beginning. Note how the fuzz gets nasty. Note also the endless sustain and the perfect decay.

 

A bit of chord work, riding the volume. Great definition at all distortion levels, great searing lead tone. Never brittle and ear piercing.

A little wah in front. Teams great with a wah. Actually my formant filter´s auto wah section.

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