Why is my Face (not?) working?

[UrielOhringer]

I'm a noob with limited theoretical knowledge. I previously built an Elektra-based distortion from a step-by-step project, but they didn't explain all the why's and wherefore's. So now I want to progress by moving on to building a Silicon Fuzz Face variation and have looked up schematics online, but I run into two questions:

1) The Elektra-variation had a resistor connecting the Collector to Base. When I look up Muff schematics I see something like that happening there too. But Face schematics don't have that. Which is all fine and dandy, but when I try to breadboard it, I can't get output from the first transistor. As soon as I do bridge C and B with a resistor, it works. So... what does that connection do?

2) Why the double resistors between power and the Collector of Transistor 2? I understand that the output of the pedal is the differential between Q2 and Power, and I've read that the 330Ohm resistor is there to reduce output level to prevent overloading the amplifier. But why? If I want less output to prevent overloading my amp/other pedals, can't I just turn down the pedal's volume? I've built it with just one resistor from power and the output being the differential between that and Q2 and it works. (Don't know how it compares, though, I don't actually have a FF.)

[BetterOffShred]

Check out RG keens article on the technology of a,fuzz face.

http://www.geofex.com/article_folders/f ... fftech.htm

Really informative about what makes FF unique.

[crochambeau]

1) The Elektra-variation had a resistor connecting the Collector to Base. When I look up Muff schematics I see something like that happening there too. But Face schematics don't have that. Which is all fine and dandy, but when I try to breadboard it, I can't get output from the first transistor. As soon as I do bridge C and B with a resistor, it works. So... what does that connection do?

I hope you're ready for a somewhat long answer.

The base of a transistor does not start leveraging current through the transistor until it reaches a certain voltage (I can explain why, but it'll make this post overly convoluted). So, when you plug your signal directly into a base it will ignore EVERYTHING below that threshold voltage and only turn the transistor "on" during peaks. The threshold voltage is established between your base and emitter, and is roughly 0.6 volts for silicon (Si) and 0.3 for germanium (Ge).

The resistor between the collector and the base of the transistor in some schematics is a path by which to charge the base of the transistor so it is always warmed up and ready to go. This trickle voltage is referred to as bias.

Now, why was a bias resistor omitted from the Face?

In all likelihood, the Face schematic is based on Ge transistors. Ge tends to be somewhat "leaky", either through the metallurgy of the PN junction (this is the transition point between collector and base inside of the transistor) or through primitive fabrication methods (process contamination, etc.). The point is that a lot of Ge transistors would essentially self bias and that external path of a bias voltage was not needed.

By adding the large value resistor from collector to base, you provided a bias path and turned your transistor on so it'll work. Another method would be to hit it with larger signal, but that will incur higher levels of distortion (gated fuzz), as your signal will allow the transistor to turn off when it dips below the threshold for conduction.

2) Why the double resistors between power and the Collector of Transistor 2? I understand that the output of the pedal is the differential between Q2 and Power, and I've read that the 330Ohm resistor is there to reduce output level to prevent overloading the amplifier. But why? If I want less output to prevent overloading my amp/other pedals, can't I just turn down the pedal's volume? I've built it with just one resistor from power and the output being the differential between that and Q2 and it works. (Don't know how it compares, though, I don't actually have a FF.)

Signal collected from a collector of a transistor (when in a "common emitter" configuration) is generally the result of the load resistor in that circuit. If you visualize for a moment a transistor:

The collector is connected to the power rail through a 10K resistor and the emitter is connected to ground. For this I'm calling the power rail a +9 volt battery

While the transistor is "off" the effective resistance between the collector and emitter is very high, let's call it 1M.

So, you have a voltage divider consisting of a point between 10K and 1M across the power supply. The "output signal" will be resting at very close to 9 volts, but - this is a DC voltage and the capacitor you have coupling this output node to the rest of world does not pass DC so you have nothing.

Now, let's feed in a bias voltage and turn the transistor "on". Quiescent current through a biased transistor will reduced the effective path resistance between collector and emitter. Let's call it 100K now, without signal.

So, we've got our "output" quietly sitting at a DC point around 8 volts.

Let's hit the base with some signal.

As the signal applied to the base of the transistor goes more positive the transistor conducts more readily. This is to say, the effective resistance across the transistor (from collector to emitter) decreases allowing greater conduction of signal across the power rails.

So let's hit the transistor hard enough to reduce its path resistance to 10K at the peak.

If you are familiar with ohms law, you'll see that a point (the output) between a 10K load resistor and 10K worth of transistor will reflect a voltage exactly one half of the power supply, which in this case is 4.5 volts. Meaning your output signal is swinging between the aforementioned 8 volt quiescent and 4.5 volt peak, a signal that is now 3.5 volts in magnitude.

One way to limit that output signal size (if you are so inclined) is to build in another voltage divider in the load resistor, which is where you've wound up looking at two resistors in series on the collector instead of one. Your resulting signal size will be a ratio of the output swing, bringing your signal back into the expected design range of everything else in your chain.

The reason "why" is an engineered decision by the manufacturer to eliminate the possibility of the end user setting things outside nominal levels and laying waste to stuff down the line. As technology has progressed, we see less instance of actual failure as the parts are more robust, but in 1970 you could overstep the boundary of SOA and the hand crafted PN junction of your prize fuzz pedal would deteriorate, and no one wants that.

The example I painted is a gross generalization/simplification, but I hope that sheds some light on it anyway.

[UrielOhringer]

@BetterOffShred: thanks, will read up on it!

@crochambeau: Thanks for taking the time to respond extensively. The first bit I think I've got now, I will T&E a bit on that (using a non-polar cap on my breadboard because I don't have a polar one of low enough value, maybe changing that makes it respond without the feedback loop.

The second one will need some careful re-reading to get my head around, and also some T&E to see how it pans out. Right now the unit seems a bit fizzy - cleans up nicely with the volume pot although a bit trebley.

[eatyourguitar]

I'm a noob with limited theoretical knowledge. I previously built an Elektra-based distortion from a step-by-step project, but they didn't explain all the why's and wherefore's. So now I want to progress by moving on to building a Silicon Fuzz Face variation and have looked up schematics online, but I run into two questions:

1) The Elektra-variation had a resistor connecting the Collector to Base. When I look up Muff schematics I see something like that happening there too. But Face schematics don't have that. Which is all fine and dandy, but when I try to breadboard it, I can't get output from the first transistor. As soon as I do bridge C and B with a resistor, it works. So... what does that connection do?

2) Why the double resistors between power and the Collector of Transistor 2? I understand that the output of the pedal is the differential between Q2 and Power, and I've read that the 330Ohm resistor is there to reduce output level to prevent overloading the amplifier. But why? If I want less output to prevent overloading my amp/other pedals, can't I just turn down the pedal's volume? I've built it with just one resistor from power and the output being the differential between that and Q2 and it works. (Don't know how it compares, though, I don't actually have a FF.)

1) the fuzz face is not a standard transistor gain stage. the fuzz face is a very special circuit that you will never see in any electrical engineering class or book unless you are taking a class on how to build a fuzz face. I can provide some excellent sources for your free education.

https://www.electronics-tutorials.ws/am ... amp_2.html

you can see that in this standard configuration. R1 and R2 create a voltage divider that will bias the base above the transistor shutoff threshold but not so much that we get into saturation region of the transistor. when R2 is deleted we are no longer using a voltage divider but instead we use a resistor 470K or larger such as 2M that will provide just enough current to bias the base in the middle (linear region) where it should be for normal operation of a small signal amplifier. the fuzz face is a completely different circuit. they are not related and the knowledge of building perfect gain stages does not help us to make an audio controlled switch.

http://www.geofex.com/Article_Folders/f ... fftech.htm

TL;DR the fuzz face will switch the battery on and off for every cycle of the waveform. we get a benefit of %50 less power consumption and %100 more battery life assuming that you built a real vintage correct fuzz face without the LED.

2) you are correct so far. the explanation is not a technical explanation but more of convincing argument of why my chicken soup recipe is loved. if you take the output directly from the collector of Q2 then you will not sound like jimi hendrix, you will just sound like you. if we wanted a well designed audio amplifier with a lot of clean gain then we would simply use a different circuit and probably different components as well. there is no law against designing your own guitar pedals but there is also no guarantee that people will like what you make. the best advice I can give you is to experiment until you have found a circuit you like. that way you are guaranteed to make at least one person happy even if you don't make any money.