BJF Bone Bender  


Medlem Moderator
Blev medlem: 2 månader sedan
Inlägg: 6
2018-11-29 02:41  
Hi there,
For the Sparkle Face and Bone Bender research projects 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.
Now, back in the 1970’s there were theories of getting the tube amp response with germanium diodes and that due to the bended transfer curve that in a small segment could be likened to the transfer curve of a triode.
Indeed as the amplification is formed by the development of current through the emitter diode, germanium transistors would be able to get close and further since the bandwidth of germanium transistors is low and the raw gain is fairly low ( as compared to silicon transistors), the power bandwidth at full square output would result in slightly softened corners and that symmetrically to a square wave - much like suggested in the 1970’s the most desirable distortion of tube amplifiers would be: and thus a tube sound was suggested to be a square wave with rounded symmetrical edges and further to be dynamically controlled.
Oh yes I have seen this in some tube amps and most of the early tweed amps would do this: and thus transform from seemingly clean to square output with gently rounded corners.
So my approach to fuzz sound with these research projects is to recreate a desirable function albeit one from the dawn of fuzzdom.
Further specifically for the Bone Bender I also wanted to get a sustain of 2 seconds or longer 😉
Most of the early fuzz and benders were crude in design meaning that they could differ greatly from unit to unit and generally the sound would be totally at the mercy of the transistors used.
For this reason I have serviced a number of fuzzes in my days were bias has drifted with the years locking the circuit……
Yes there would be less artifacts in the decay of Bone bender as compared with typical MK1 circuits and that due to some stabilization techniques that I have used to make a slightly cleaner sound but also longer sustain.
In industrial designs with transistors there’s been a number of stabilization networks developed to limit the influence of variance in the transistors used and with this in mind I have taken the approach of trimming each unit for the behavior I desire.
I have for the germanium circuits also used some filtering techniques mainly to limit wide band noise and thus get more fuzz and less noise at high volume levels
Note on Bone Bender is that it also comes with trimmers and those are set for longest a clearest sustain - as desired by Vic Dick ( See fuzz history thread)
However, quirks and artifacts in the decay can easily be set with the trimmers and e.g. by setting Q3 so that it is normally locked and only opens with signal- then you get switching noises from Q3 as it turns on and off.
At my age and having played so many old things and also recalling what I was thinking then about what I’d really want in sound: I really like to get less tape flutter from the tape delay and clearer decay from my fuzzes and…..
But yes I enjoy playing today some old fuzzes that make mostly noise……….the third element of music….
About NTE103A versus AC127:
For the Bone Bender I had first intended NTE103A as I had some in my drawers and so the first units were made with NTE103A.
I’d really wanted to make an NPN version and with parts that not only would be good but that can still be had a reasonable prices.
NPN germanium transistors have always been rare and have mostly been made for Audio complementary power amps or for computer logic.
NTE103 is a replacement part and is a direct replacement of AC127 and thus NTE103A meets all the key data of AC127.
I was actually when doing the first Bone Bender that "the chef can use what parts the chef can use”…and it also struck me with a bit of humor that
the NTE 103A was initially made for repair of older electronic equipment.
It so happened that I swiftly ran out of NTE103A’s and ordering more would have minimum quantity of 50 units and a delivery of 6 weeks…which could be fine although I felt I needed some swifter
and so I saw another supplier that had some AC127’s in stock so I bought those… not sure when more will be available, but they are possible to order from a distributor in France.
However I ran out of the AC127’s too and got some more NTE103A’s...
Sonically I’d say there are smaller difference between the AC127 and NTE103A than there are within the respective groups 🙂 meaning that yes they are all germanium NPN transistors and they are
all over the place in terms of leakage and gain and for these reasons the Bone Benders have various stabilization networks and also trimming points to get best possible performance.
I really wanted to make the Bone Benders as they could have been made in the mid- 60’s but in a way that would be possible today, meaning that yes could easily use old style transistors to get the sounds but
today it would be good if the fuzz can have DC-input standard polarity, be somewhat controlled in terms of drift with temperature and further have a pilot LED and a typical stomp box size for ease of use in 
pedalboards and further have impedance levels that would work well with industrial standards of today.
I’d summarize and say I was equally pleased with the AC127 version as I was with NTE103A version.
I have included below some data sheets and also maybe particular useful to those not digesting data sheets for breakfast but I found some excellent links with explanations of parameters.
I further used NTE103 for the Bone Bender Mark II and for this pre grouped them for some that I could use as pre driver and that would bias where I wanted them directly in the circuit and the remaining ones were
used in the following circuit that hasa bias trimmer to allow setting up performance.
I have included also the BC107,108,109 series just because in the below data sheets
When I first saw fuzzes and first played some fuzzes back in the late 70’s I found most of them limiting to the sounds I wanted and that I had heard on recordings and frankly many of the amps back then also had a less inspiring distortion and these great sounds I could hear were only for the stars - such were the times.

Therefore when making the Tone bender Mark 1 there was something specific I wanted control over in terms of how it plays and thusly it would not sound totally ’vintage’ but have a sound that one out of one hundred fuzzes could have had in a music store in 1966.
Instructions and schematic on that coming soon and for the Bone Benders made for this research I used either AC127 or NTE103A.

This said I can see many shades of fuzz tones and so I also wanted to try the Tone bender Mark II and really that is a circuit similar to a fuzz face albeit with a preamp and it is capable of tremendous sustain- and while making that I was thing about when my mother in law was sent out by my eldest brother in law to find a fuzz wha one with a long tone 
The Bone Bender mark II also uses AC127 or NTE103A.
Some of these have heavy leakage and some less so I sorted those that had high leakage to be used as first stage and thus those bias at about 3V8 to 4V7  right in the middle of making a nice linear amplifier that will saturate symmetrically. Those with lower leakage I used for the composite current input amplifier that follows the input stage and allow biasing with a trimmer.

Here's short tour through the circuit. The first stage in a Mark 1 is a buffer and it was made with a germanium transistors and the first run of Bone Benders were made like this. However as I consider the buffer having little impact on sound while on the look out for ways to reducing noise I decided to try a J-Fet and the adjusted the inputimpedance to be more similar to what could be expected from a germanium transistor and that is seen in the choice of R1 that effectively sets the inputimpedance of the J-Fet as that has an internal impedance of several Mega Ohm's; for a germanium transistor input impedance is roughly calculated as hie+hfe times the emitter load and thus is a little less predictable and varies with setting of Attack control but I set R1 empirically while listening to how volume control on guitar reacted and top treble and decided 300K was about fine.At buffer output comes Attack control that effectively limits gain of circuit by the divider it forms with the input impedance of gain stage 1. Gain stages 1&2 are made similar and both are set up with traditional stabilization networks for a transistor- note though that since germanium transistors have a leakage current from Collector to base known as Icb and that Icb is a variable between the germanium transistors the stabilization network must take this into account ( for silicon transistors of modern kind Ibc is generally so small it can be ignored for all practical purposes- not so with germanium transistors as those can even be turned on into working state by virtue of Ibc) while in the circuit there's a trimmer to each base so linear at very small signals response can be set- how to do this is described above in a Guitarit's Approach to Biasing Fuzzes. There are two caps one for each gain stage between collector and base and those are amplified by the Miller effect so their true value is gain times pF+ ghost Miller cap,that is formed internally to the transistor- Values of those caps were set empirically to limit upper treble just above 'vital' treble as in just above where a loss was perceived. Finally there's a Volume control and the circuit since it uses all NPN and N-channel J-Fet is negatively grounded and the B-tree is supplied with a decoupling capacitor and series filter resistor and also polarity protection and there's an LED connected to show status of fuzz as in activated or not activated and because of this the fuzz can be driven from a standard +9V supply and it is fairly stable to Voltage fluctuations so there is no real benefit from running the circuit from a battery while it certainly can be driven by a battery. However sound will differ with differing Voltage and on some supplies Voltage can be manually set from say 4,5V to 9V and of course at lower voltages sound will saturate sooner.

Difference between the versions are noted in the schematic and notice since there's ample trimming range any typical germanium NPN transistor can be used in the circuit and be biased but biasing must be performed if linear to small signal conditions shall apply.

Tonebenders I have seen several different and only a few with three transistors like the Marshall Supa Fuzz- actually I made last year a variant on the three transistor Tonebender or Supa Fuzz and called that Super Fuzz- which of course may imply something different altogether but anyway this BJF Super Fuzz I made I also made in dlx fashion with octave footswitch and that is beyond this topic but may pop up later.
Some of the Tone Benders I have seen had a smaller output capacitor making a high pass filter at output which is another classic fuzz filtering cutting treble at input and reducing bass at output to give a leading edge to the waveform. in sound it just means a brighter sound with less bass while the filtering that occurs with guitar and fuzz input stays the same.

I am greatly looking forward to hearing thoughts on the Bone Bender 1 design 

Much more to come and in the meanwhile have great fun


Topic Tags
Active Member
Blev medlem: 1 vecka sedan
Inlägg: 8
2019-01-10 16:35  

This IS an extremely "nice" sounding fuzz, very smooth and can do great overdrive tones too.

New Member
Blev medlem: 1 vecka sedan
Inlägg: 1
2019-01-11 17:11  

Have you tried the circuit with more unusual germanium transistors? I have a few old Mullard OC77's that would be interesting to test and individual trimmers sounds like it could make for some interesting fuzz flavors with socketed transistors.

Active Member
Blev medlem: 1 vecka sedan
Inlägg: 8
2019-01-13 08:41  
Postad av: Hypnodrone

Have you tried the circuit with more unusual germanium transistors? I have a few old Mullard OC77's that would be interesting to test and individual trimmers sounds like it could make for some interesting fuzz flavors with socketed transistors.

If you socket them, just test how they sound, probably very good.

Bjorns design is, as I understand it, done to accept a wide "degree" of Ge-transistors.

Medlem Moderator
Blev medlem: 2 månader sedan
Inlägg: 6
2019-01-13 22:34  

Hi There,

Thank you Goran for the kind words.
Right sometimes setting bias slightly off can give very pleasing results after all a fuzz is a nonlinear amplifier and the beauty lies in its overtones.
Personally I have prefered setting certain silicon fuzz circuits like that. I recall one fuzz face made with BC108C’s that did something really interesting with bias set slightly off.
Yes the Bone Bender one can take more or less any NPN germanium transistor.
The OC 77 is a PNP transistor so it would be slightly difficult to adapt to the Bone Bender 1 pcb-that said older types of transistors can be operated backwards as in interchanging the collector and emitter
More reading on transistors
But essentially a transistor is made outof two back to back connected diodes and has a certain symmetry
In general the distance internally between base and emitter is shorter than the distance between base and collector and that is sometimes a way to identify terminals on older transistors where you’d find the base in the middle and the emitter is the leg closest to the middle leg.
Older transistors can sometimes as stated above be operated reversed and still work as amplifiers allthough h-parameters are different.
With low voltage you can certainly try them backwards-note that you may need to change a few values to make the transistor work as amplifier.
Another way is building the circuit on vectorboard or experimental boards and you just reverse the germanium stages so that emitter goes to B+ and collector load to ground and also reverse the base leak of 1M Ohm so that goes from ground to base and then trimmer to B+. Of course outputs are taken from the collectors and now you can still run the unit on a standard supply- you might need to increase the decoupling capacitor
You could also use a regulator say a 7805 for the plus side and that will serve as a very stale point
Remember in AC circuits B+ and ground are the same
Have great fun


Hoppa till verktygsfältet

Vänligen Logga in eller Registrera dig