BJF Bone Bender

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….

 

 

 

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.

 

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

BJ