Fuzz control on a MK1 ToneBender clone

[SweetFx]

Hi all,
I'm relatively green to pedal building and recently built a ToneBender MK1 clone from a schematic. After weeks of troubleshooting I got it working and wow did it not disappoint. The sound is awesome. One thing I did notice is that the fuzz control doesn't seem to be very responsive. Volume works as expected. Maybe it's just me but it sounds like the same amount of fuzz no matter where I set the fuzz potentiometer. When it's completely off (turned all the way to the left) I get almost no sound unless I powerfully strum the guitar and even then it's still super fuzzy but cuts out abruptly. Then from about 9oclock to max fuzz the fuzz sounds the same. I'm assuming it's the way I wired to potentiometer. But I wondered if this was common on vintage fuzz pedals. Or maybe the potentiometer was made to be more of a sustain. Anyways I'm looking for some seeds of wisdom as to what I could do to fix it or just accept that's how the pedal is supposed to be. Anyways thanks in advance!

SweetFx

[Electric Warrior]

The Attack control is basically a bias pot – not really a fuzz control in the modern sense.
Some vintage units only have a narrow sweet spot on the attack pot where the pedal sounds good, between gating and artefacts. Others have a wide usable range. The resistor between Q2B and 9V was tweaked during production and will help control the range. Maybe you should experiment with it. 

What voltage are you getting on Q2C with the attack pot cranked?

[SweetFx]

Thank you for the info! That makes more sense. The voltage you were looking for was at the second transistors collector arm correct? Sorry I'm still learning a lot of the terminology. The voltage at the collector arm of the second transistor with the attack cranked all the way up is -3.42V and all the way down is the full 9.63V. Sorry if that wasn't what you were referring to. The resistors I used at Q2B that is connected to power is a 470K and a 1k8 to the attack pot. Thanks again

SweetFx

[Electric Warrior]

You're welcome 
Yeah, that's the right voltage. Not sure if this will make it better, but you could try getting it closer to 5V with the attack cranked by temporarily putting a resistor in parallel with the 470k and see if that improves the sweep.

[GuitarMatt]

Hi there, (sorry for resurrecting old thread) I'm actually working on building a MK1 right now, I was curious how you know the proper voltages for the transistors? Can either of you supply me with a source? Thanks!

[bushidov]

Hi Matt,

Well, because the Tone Bender MK I is basically a Fuzz Face with some slight changes in resistor and capacitor values, you can still use the Fuzz Face circuit as a guide, shown below:
https://www.electrosmash.com/fuzz-face

Here is an example of the Tone Bender, MK1 schematic, as a reference:


The idea of getting that bias voltage on Q2's collector pin is you want to be center of your highest and lowest possible voltages. So, with a 9V battery on a PNP style Tone Bender or Fuzz Face, you have 0V through -9V. The middle would be -4.5V. If you were making an NPN style Tone Bender or Fuzz Face, similarly, the range would be 0V through +9V, making the middle +4.5V.



If the voltages on the collector pin of Q2 were too high or too low, as shown in the above diagram, the sine wave-form has a chance of being clipped on one side of the sine wave, but not the other, which doesn't sound all that great and effects your general, overall volume. The closer to the center that voltage is on the collector pin of Q2, the least amount of (and ideally, no amount of) that sine wave will ever be clipped more on one side or the other, which to our ears, sounds pleasant.

Sometimes though, due to properties of some transistors (I'm looking at you, Germanium), you may need to fiddle with that 8.2K resistor to get that collector voltage to stay at the right spot. Leakages from the transistors can make this troublesome, as well as temperature swing. I typically put a 20K trim pot in place of that 8.2K resistor and tie the pot pins 1 and 2 together making it a variable 20K resistor. Then I just dial in to 8.2K to start and then start tweaking from there. If you have a guitar plugged into the pedal with an amp, crank the fuzz and vol to max and then start strumming the guitar and let it ring out. Then start to adjust that trim pot. When you get "max volume" on that trim pot, you have the ideal bias, and you will notice the voltage coming out of that collector pin will be close to center voltage.

As a side note, you may want to do this with the collector resistor on Q1 as well. Typically, I use a 100K trim pot there and start at 10K and then dial in from there, like I do above. However, in this case, you are not looking for the same "half-way in-the-middle" voltages, but the "max volume" trick above still applies.

[GuitarMatt]

Thanks! I'm actually using this circuit which is somewhat different from the Vox tonebender. But I think your info is still usable for me.


I have a bunch of 25k trimpots, can you tell me which resistors I should replace with those pots to properly bias? To my knowledge the purpose is to bias the base of a transistor, is that correct? I'll be sure to add a few ohms in series to protect the transistors.

Also on a side note, I was planning on using old PNP AC128 transistors, but I'm getting values for Hfe ranging from 180 to 610. I'm just using a cheap multimeter to measure Hfe, I don't have a proper transistor-analyzing instrument. Am I wasting my time trying to bias a circuit with these transistors? Or is my multimeter just that wildly inaccurate?

[Scruffie]

Hi Matt,

Well, because the Tone Bender MK I is basically a Fuzz Face with some slight changes in resistor and capacitor values, you can still use the Fuzz Face circuit as a guide, shown below:
https://www.electrosmash.com/fuzz-face

Here is an example of the Tone Bender, MK1 schematic, as a reference:


The idea of getting that bias voltage on Q2's collector pin is you want to be center of your highest and lowest possible voltages. So, with a 9V battery on a PNP style Tone Bender or Fuzz Face, you have 0V through -9V. The middle would be -4.5V. If you were making an NPN style Tone Bender or Fuzz Face, similarly, the range would be 0V through +9V, making the middle +4.5V.



If the voltages on the collector pin of Q2 were too high or too low, as shown in the above diagram, the sine wave-form has a chance of being clipped on one side of the sine wave, but not the other, which doesn't sound all that great and effects your general, overall volume. The closer to the center that voltage is on the collector pin of Q2, the least amount of (and ideally, no amount of) that sine wave will ever be clipped more on one side or the other, which to our ears, sounds pleasant.

Sometimes though, due to properties of some transistors (I'm looking at you, Germanium), you may need to fiddle with that 8.2K resistor to get that collector voltage to stay at the right spot. Leakages from the transistors can make this troublesome, as well as temperature swing. I typically put a 20K trim pot in place of that 8.2K resistor and tie the pot pins 1 and 2 together making it a variable 20K resistor. Then I just dial in to 8.2K to start and then start tweaking from there. If you have a guitar plugged into the pedal with an amp, crank the fuzz and vol to max and then start strumming the guitar and let it ring out. Then start to adjust that trim pot. When you get "max volume" on that trim pot, you have the ideal bias, and you will notice the voltage coming out of that collector pin will be close to center voltage.

As a side note, you may want to do this with the collector resistor on Q1 as well. Typically, I use a 100K trim pot there and start at 10K and then dial in from there, like I do above. However, in this case, you are not looking for the same "half-way in-the-middle" voltages, but the "max volume" trick above still applies.

You're thinking of the MKII and biasing the collector in a MKII to 4.5V is not the recipe for a good MKII, originals were biased to 7-8V.

Thanks! I'm actually using this circuit which is somewhat different from the Vox tonebender. But I think your info is still usable for me.


I have a bunch of 25k trimpots, can you tell me which resistors I should replace with those pots to properly bias? To my knowledge the purpose is to bias the base of a transistor, is that correct? I'll be sure to add a few ohms in series to protect the transistors.

Also on a side note, I was planning on using old PNP AC128 transistors, but I'm getting values for Hfe ranging from 180 to 610. I'm just using a cheap multimeter to measure Hfe, I don't have a proper transistor-analyzing instrument. Am I wasting my time trying to bias a circuit with these transistors? Or is my multimeter just that wildly inaccurate?

You can't use a multimeter to measure germanium transistors, it doesn't tell you anything. Go to Geofex and build the germanium transistor tester.

For a MkI, you can throw out the rules for other fuzzes, it wants quite a bit of leakage to work and there's no special gain brackets. You really need to breadboard (or at least socket) the transistors, audio probe each stage and switch transistors out until you find a set you like. Trimmers should only be considered for fine tuning it, not to get transistors working.

[GuitarMatt]

Thanks Scruffie
Yes I've breadboarded it several times now, and each time I end up with chirping bird sounds coming out of my amp, which go away when I send guitar signal through, which is fuzzy, but that signal then quickly degrades into "chirping" noise.

I will check out your Geofex suggestion. I've felt like I'm flying blind here trying to build a very transistor-sensitive circuit with no real ability to test any parameter of the transistors.

By "audio probe each stage" do you mean listen to the collector signal of each transistor through my amp before stacking three together like they are in the circuit?

[Scruffie]

Your chirping sounds like oscillation, I would try adding a small capacitor (1-10nF) from input to ground like in the Tone Bender MkII.

I'm not exactly a MKI expert I should clarify so someone may be able to better advise but yes, when I did mine I built the circuit and only inserted the first transistor in the circuit, probed the collector and went through all the transistors I had until I found the one that sounded best, then put in the second and did the same (first has to stay in of course or it wouldn't get any signal!) and then third.

[GuitarMatt]

I will try your capacitor suggestion. As for Geofex, (Guitar Effects Oriented, I'm assuming) many of their pages don't load, or direct me to a page to "Buy this Domain". Is there another transistor testing device I can buy? I never know if I'm looking at the right thing, I'm still pretty new to this whole thing. I do have an oscilloscope, for what that's worth.

[Scruffie]

http://www.geofex.com/article_folders/ffselect.htm