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:
And as usual, the schematic divided in functional parts:
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.
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.
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!
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.
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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6 Comment
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.
Answerpeace!
PjP
Thank you! I will have a look into it; I will finally understand how this works :)
AnswerBenoit,
AnswerFrom what I have read, the Tone Bender MK III evolved from the Burns Buzzaround. The Fuzz Face came after the Tone Bender MK III and is a very different animal. I've done a fair amount of circuit simulation of the TB3 and its variants and am now in the process of bread-boarding one. I'll provide more info as the testing progresses. I can tell you that the diode does more than just compensate for temperature variation in the 3rd stage. When the voltage on Q2's collector swings toward -9V, current flows out of Q3's base & charges C2. When the voltage on Q2's collector swings toward ground, Q3's emitter-base junction stops conducting and D1 turns on, providing a path for C2 to discharge. If D1 was not there, C2 would charge on every cycle and accumulate more and more DC, upsetting Q3's operating point. One more point: the first stage has enough gain that it too will go into cutoff and saturation when driven hard.
Regards,
Chuck
A cap followed by a diode is a "clamper". A clamper is a simple circuit which sets a bias by charging the cap. AFAIK, if the diode wasnt there, the cap WON'T get charged.
Answer(If the cap wont get charged the base of Q3 is at ground only allowing AC signals above Vbe (0.2V), giving a gated sound.)
With the diode, positive signals get grounded. This means the cap gets lots of negative swing but less of positive swing, which means the cap gets charged to an equilibrum. This equilibrum biases the Q3 base. If the signal is below the diodes treshold, the cap gets discharged again, dropping the bias again. It is a NOISE GATE!
Tone bender mkIII came in town around 1968. The Fuzz Face in 1966. And that one was actually a development of the "interlude" Tone Bender mk1.5 ...
Answer