Tonebender MkIII: circuit analysis

The Tonebender MkIII is finally quite close to a Fuzz Face: a first stage amplifies the signal, which in returns makes the last transistor saturates: here is the fuzz! The main difference is that contrary to the Fuzz Face, the first stage is a 2 transistors amplification stage, and it amplifies the signal enough so you do not need a feedback loop to make it bigger.
Lets see that in details.

Here is the schematic of the Tonebender:
Tonebender MKIII schematic


And as usual, the schematic divided in functional parts:
Tonebender MKIII schematic
So, the first stage amplifies the signal, which then makes the Q3 transistor saturates. Then, there is the tone control stage to remove of bit of trebles and fizziness, and finally a volume output stage. Notice that the circuit is in reverse polarity because germanium PNP transistors are used here.
Lets analyse each par one by one!

The amplification stage
The signal goes through a first C1 coupling capacitor, to avoid any parasitic DC current to go in the circuit. R1 and R2 form a voltage divider to bias the first Q1 transistor.
Tonebender MKIII schematic
You can notice that Q1 and Q2 are placed one after the other. This configuration is called the Darlington configuration, and it allows the signal to be amplified twice! Q1 amplifies it one time, and then Q2 re-amplifies the signal. Thus, you can get very high amount of amplification, and still use low gain germanium transistors! This avoids the very painful job of finding high gain germanium transistors like with the Fuzz Face. A hfe of 60 is enough for each of these transistor, which is a easy-to-find value for germanium transistors.

After this amplification stage, there is again a coupling capacitor (C2), and the Fuzz potentiometer allow to reduce the signal. When you turn the knob, the resistance of the potentiometer will increase and the signal will be "less amplified", and thus, you will get less saturation out of it.


The saturation stage
Now that the signal has been amplified a lot, it will hit the Q3 transistor and make it saturates, creating a beautiful fuzzy distortion that we love.
Tonebender MKIII schematic
The germanium diode is used to prevent issues with rising temperatures. As you may now, germanium is temperature sensitive. When the temperature changes, the characteristics of the diodes change the same way the characteristics of the Q3 transistor, and avoid biasing problem by flowing the excessive current to ground. I am not sure of how it works precisely though so if you have more insights, I would be glad to hear it in the comment section!


Tone section
The tone section helps to remove a bit of the trebles of the Fuzz, and avoid it to sound too gritty. It is a very nice addition that give the fuzz a bit more versatility!
Tonebender MKIII schematic
The tone section is composed of two low pass filter. One cuts all the frequencies below 7kHz (max trebles), and the other at 159Hz. The tone knob mixes these 2 filters. The more you go on the right, the more the 159Hz will be important and your sound will have less trebles.
It is a very simple layout that works quite well!


Output stage
Simple layout that we have already seen in the Fuzz Face circuit analysis.
Tonebender MKIII schematic

A first resistor reduces the signal, and a 100kB pot sends more or less signal to ground, adjusting the final volume of the fuzz!


There it is! I hope that you liked it.
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2 Comment
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hey bro, the diode thing works by "leaking" the same as the q does, so it compensates for the bias change as the temp changes.its a neat trick and works pretty good. do a google search for "brit face" and you'll find an archived page that explains it all.
peace!
PjP

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Thank you! I will have a look into it; I will finally understand how this works :)

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