The Breadboard Chronicles: Universal Tone Bender MKII/Fuzz Face Board

[Big Monk]

I thought I'd dedicate a thread to my upcoming prototype build of the Universal TB/FF board. Having proven the circuit works, and now waiting for the finalized PCB, my plan is to mount the board on top of "The Prototyper" and have flying leads go through the topside pot holes to the breadboard for auditioning collector resistors, base to collector resistors and capacitors.

I'll post up some pictures as I start laying the setup out.

 

[Big Monk]

Ok.

Had some time today so I took one of the throwaway mistake boards I had and set it up to test with my “The Prototyper” breadboard setup as I/O:



I have the following setup with flying leads so I can change components at the breadboard:

1.) Treble Bleed Cap
2.) Input cap
3.) Q1 collector resistor
4.) Q2 collector resistor
5.) Output cap
6.) Emitter cap
7.) coupling cap

I chose to put flying leads on the coupling Cap because it can serve as the input cap as well for a Fuzz Face.

Needs to hook some thing up and add some external test jacks for ,ensuring collector voltages outside the box.

 

[Big Monk]

It turned out to be extremely unwieldy and I’m getting odd behavior on the collectors:



Before I rip it apart, I have to dome some quick troubleshooting. My concern is that it takes incredibly high collector resistor values to get the voltages in range.

Somethings up and I’ll figure it out!

 

[Big Monk]

Let this be a lesson to everyone: not all good ideas are practical! I’ll be breaking this down and simply breadboarding the circuit normally this week. These were throwaway boards anyway because the power rail was messed up on them.

 

[Big Monk]

I’m switching no gears with this thread.

I’ve done a more traditional breadboard build and I’ll do a tutorial on tuning the classic Mk II Tonebender and Fuzz Face.

The idea will be to start with the stock circuits and then tweak them into perfection and document the process.

Pictures and detailed write ups to follow.

 

[Big Monk]

Tuning a Tonebender MK II - Part I

So in light of the Board + I/O debacle of the other day, I went back to more traditional breradboarding:




Setting Up the Circuit
I mostly went from memory but had the Fuzz Central schematics in mind:

Without dedicating too much of the page to a Tonebender MK II history lesson, the OC81Ds had much less leakage than OC75s. Rb1 was typically 10k when using OC75s and 100k when using OC81Ds. Rc2 was generally 47k with OC75s and 100k with OC81Ds.

The General Electric 2N169 ("Flat Hats" or "Top Hats") units I use (and love) are low leakage like the OC81Ds so I started with the Tonebender MK II Professional schematic above but subbed Rc2 for 47k to begin with.

The first thing you want to do with a Tonebender MK II is tweak the collector voltages. In my case, I am also trying to tune the trimmers that will be on my board. I wired up 2 16mm 50kB pots for use in conjunction with Rc1 and Rc2. I left the Rc3 string at stock with 470/8.2 k (more on that in Part 2).

Side Note: The first thing I had to address was the voltage coming out of the power supply. It's a cheap Snark adapter that I keep at my work table. It was putting out 9.75 vDC. I subbed in a 100 ohm resistor between the DC jack and board power and dropped that to 9.22 vDC.

Initial Tuning
Q1 = hFE - 87, Vbe = 176 mV, Iceo = 14 µA
Q1 = hFE - 114, Vbe = 181 mV, Iceo = 28 µA
Q1 = hFE - 112, Vbe = 112 mV, Iceo = 21 µA
Rb1 = 100k
Rc1 = 10k + 50kB (Voltage Range = 9.25 vDC - 9.63 vDC)
Rc2 = 47k + 50kB (Voltage Range = 0.31 vDC - 0.45 vDC)
Rc3a = 470
Rc3b = 8.2k (Voltage Range = 6.95 vDC - 7.86 vDC)

So the first thing I noticed was the very high voltage on Q1. (Grabs soapbox...) In my opinion, I like to bias Q1 lower than is traditionally thopught of for the MK II. Stock units in the past and present sometimes have voltages exceeding 8.2 vDC, but I've found that voltages lower than 8vDC and, more specifically, around 7.7-7.8 vDC help with cleanup when rolling down the volume. That said, since i'll have an internal trimpot in series with RC1, I wanted a range from about 7.7-8.2 vDC.

As you can see, Q3 is most effectively biased by changing Q2's bias. The range i got at the outset is lower than I target. Again, traditionalists will go for something above 8 vDC and vintage units typically biased up around 8.2 vDC. I personally think note bloom, sustain, and general kerrang is increased by lowering the bias for Q3 into the 7.6-7.8 vDC range. This has one major side affect: Noise. We'll tackle the noise issue in Part 2.

Revised Tuning
With all this in mind, I swapped some resistors to get things in line. Rc1 in particular had a few iterations:

Rc1 = 68k + 50kB (Voltage Range = 8.31 vDC - 8.68 vDC)
Rc1 = 100k + 50kB (Voltage Range = 8.05 vDC - 8.42 vDC)
Rc1 = 150k + 50kB (Voltage Range = 7.78 vDC - 8.16 vDC)
Rc2 = 68k + 50kB (Voltage Range = 0.27 vDC - 0.36 vDC)
Rc3a = 470
Rc3b = 8.2k (Voltage Range = 7.02 vDC - 7.94 vDC)

Here you can see the voltages more in line with what I wanted. At this point I swapped out all three transistors from the spares i was tuning with to the ones I'll be using in my personal build on the universal board. They needed a little further tweaking but I quickly settled on the values I'd be using for this final testing iteration:

Q1 = hFE - 99, Vbe = 180 mV, Iceo = 28 µA
Q2 = hFE - 117, Vbe = 174 mV, Iceo = 35 µA
Q3 = hFE - 101, Vbe = 185 mV, Iceo = 15 µA
Rb1 = 100k
Rc1 = 100k + 50kB (Voltage Range = 7.68 vDC - 8.14 vDC)
Rc2 = 82k + 50kB (Voltage Range = 0.24 vDC - 0.29 vDC)
Rc3a = 470
Rc3b = 8.2k (Voltage Range = 7.61 vDC - 8.06 vDC)

Quick Note: As is sometimes required in this circuit, the Q3 I originally picked here was swapped in at Q2. It's higher (relatively speaking) leakage made it a better candidate for that slot.

This is pretty much what the final configuration will be like. What I may do is sub in 100kB pots to increase the range of the internal trimmers but other than that, the resistors are tuned for my build at this point!

Next we'll talk about noise, how to stop it and frequency shaping.

 

[Big Monk]

Tuning a Tonebender MK II - Part II

Noise
One thing I brought up in the last post was that reducing Q3c voltage brings in more noise. I believe, given the additional gain stage the MK II has over other 2 transistor English fuzz units, that the designers chose to have higher collector voltages to utilize some mild gating to quiet down the circuit. Let's not sugar coat this: the MK II is a crazy noisy circuit.

The problem I have found is that I really like the volume boost, sustain, feedback and improved note bloom when the circuit is biased out of the gating range. So, we have to contend with noise if we are going to do that. If this is not desirable for you, then simply dial in the required amount of gating to quiet the hiss and overall noise and call it a day.

If, on the other hand, lower voltages are desirable for you, let's talk how we can squash that. I bring this point up before talking about overall frequency shaping because some of the mitigation strategies for noise have the affect of a high-end cut, so we don't want to commit to input, emitter or output caps just yet.

For starters, I propose shielded wire from both the jacks to the switch AND the switch to the input of the circuit. This is a bit of work to prepare but i believe in it strongly. Second, we should use PCB mount pots when possible. This cuts down on wire runs and points of interference.

Lastly, base to collector capacitors in the picofarad range are almost a must. i personallly install and tune them on all three transistors, usually starting off high for Q1 and matching the value for Q2 and Q3. Start with no B-C caps and then install a 100 pf for Q1 and 82 pf for Q2 and Q3. Listen closely to a.) how much noise is clamped down and b.) how much high-end you lose. For the circuit I just tuned, I started there and ended up at 470 pF for Q1 and 220 pf for Q2 and Q3.

High-end was reduced somewhat but not drastically and noise was cut down considerably. Granted, I'm on a breadboard with tons of stray wires so I was still getting some serious squeal at higher gain settings but that can be ignored because we know that will disappeared when boxed up, wired properly, etc.

Tuning the other Capacitors
There was not much variation in the frequency shaping components in the original Tonebender MK II family:

Input and emitter caps were either 4.7 uf or 10 uf, output caps were either 0.01 uf or 0.015 uf and the treble bleed cap was either 0.01 uf or 0.015 uf.

Some of you may instantly, and intuitively I might add, just venture to remove the treble bleed cap and be done with it. This is a valid route but sometimes the circuit can sound a touch ragged without it. I have had past success without it but in this particular tuning, I chose a smaller value and kept it. Also, let's note that the emitter cap is often a better place to tune than the input cap. The output is helpful for final tweaks in my experience. Let's look at how I started and how I ended up...

Initial Tuning of Frequency Shaping Components
I started with the bog standard 4.7 uf input and emitter caps, 0.01 uf treble bleed cap and 0,01 uf output cap. I had cut some high ened with the larger B-C caps used to clamp down on hiss and general noise, so i was looking to gain some back.

Cin = 4.7 uf
Ce = 4.7 uf
Ctb = 0.01 uf
Cout = 0.01 uf

First thing I did was my tried and true cap combo:

Cin = 10 uf
Ce = 4.7 uf
Ctb = Removed
Cout = 0.047 uf

Revised Tuning of Frequency Shaping Components
This had a few effects that I have seen in the past. For one, the circuit became a bit more muscular but a little farty. Often times, when removing the treble bleed cap and bumping up Cin, the effect is a bit counterintuitive, i.e. the circuit actually seems to cut more and I usually raise the output cap a touch to compensate. That did not happen here and I tried a common "Plan B" of mine:

Cin = 10 uf
Ce = 2.2 uf
Ctb = 0.0047 uf
Cout = 0.01 uf

This did the trick. Lowering Ce and Cout brought back some of the brightness and cut, and adding back the treble bleed cap at half the original value kept things from getting ragged.

I hope this helps!

 

[BuddytheReow]

Phenomenal write up!

 

[Big Monk]

Phenomenal write up!

Thanks!

Makes sense to do these as I am prepping the units. I've waited a long time and tested this circuit pretty extensively. Finally time to commit the good transistors to these and box them up!

Here is the final revision of the Universal TB MK II/Fuzz Face Board:



Final touches include adding the third pad to the input cap spot to allow for film or electro, re-routing the power in to be upstream of the filtering section and adding the collector voltage test pads.

 

[Big Monk]

Up Next.....Germanium Fuzz Face!

 

[Big Monk]

Tuning a Tonebender MK II - Part III

Output Volume
The output volume of the Tonebender MK II family, MK I.V and Fuzz Face is dictated by the voltage divider string comprised of Rc3a and Rc3b. Germanium units tended to use Rc3a = 470 and Rc3b = 8.2k. Silicon Fuzz Faces tended to use Rc3a = 330.

I’ve personally never encountered issues with reaching unity gain with stock values. Some people, however, complain not of unity gain issues, but having to crank the control up very high to reach unity. I’m not sure what the problem with that is, unless people want more range to crank the volume past unity.

So, if we want to change the output volume threshold of these pedals, we need to increase Rc3a, keeping in mind that we also want to lower Rc3b to keep the total resistance (8.67k or 470 + 8.2 k). Many people make the mistake of simply increasing Rc3a, which changes the collector voltage setpoint of Q3 slightly.

An example:

We want to boost the available output of the standard Rc3a/Rc3b voltage divider. We settle on Rc3a = 1.2k. We would then want to change Rc3b = 8.67k - 1.2k = 7.47k. Use a 7.5k resistor and Viola!

On my Universal board, I have a trimmer for the voltage divider AND Rc3b. I have a space for a fixed resistor at the top of the voltage divider trimmer and the other leg of the trimmer feeds the trimmer for Rc3a, whose bottom lug goes to the fixed Rc3b.

An example:

On my board, I want to determine the values for the fixed Rc3a and Rc3b and the corresponding trimmers. I’ll keep Rc3a = 470. The voltage divider trimmer will be 500 ohm. This means I can increase the effective resistance of Rc3a from 470 to 970, giving an appreciable volume boost. As for Rc3b, I’ll need a total effective resistance of 8.67k - 970 = 7.7k. This means I can use a 5k trimmer and a fixed Rc3b = 2.7k.

Hope this helps!

 

[Big Monk]

The Fuzz Face has been breadboarded and i'm just trying to squelch the oscillation before i make any more circuit tweaks.

I'll document my thought on both the Ge and Si versions here in a similar fashion tot he Tonebender writeup above.

 

[Big Monk]

After some tweaking, the Silicon Fuzz Face is oscillation free and ripping:

 

[fig]

Sounds great!

 

[Feral Feline]

Titanicu Gigantium strikes (a chord) again!

Fuzzoundzgud!

 

[Big Monk]

It actually sounds quite good even with the humbuckers. It didn't require as much tweaking as I thought to "de-mud".

Today I'm testing a higher hFE set of transistors in it.

 

[Big Monk]

I'll have a nice write-up on tuning a Silicon Fuzz Face by tomorrow, as I'm quickly approaching boxing this one up. Then we move to the Germanium version!

 

[Big Monk]

So, as promised, I will be doing a full write-up on the tuning of this Silicon Fuzz Face I have been working on but wanted to briefly discuss the transistors I auditioned today.

At the time I first designed the Universal board, I went through my transistors and picked an Amperex A115 at 121 hFE for Q1 and an Amperex A104 at 285 hFE for this project.

These are the units I went with on the breadboard a number of days ago. My benchmark tone I am using is the "Time" solo off of DSotM. What i was noticing is that, while I liked the crunch tones i was getting on the bridge pickup, It was lacking a little bit of the girth and sizzle inherent in those high gain Arbiter Silicon units.

I decided I'd go back to my transistor folio and see what else I had. I settled last night on a BC109B at 286 hFE for Q1 and an Amperex A104 at 405 hFE for Q2. I popped them in, biased them up and waited until this afternoon to test them out at volume.

HOLY COW! You would not think it could make THAT much of a difference but it did. I still have all the tones from the previous set but now have much more surly tones at the same volumes on the guitar controls. at about 8.5 on the bridge pickup volume control, I have much more gain on tap and only need to turn down to about 7.5 or 8 to get the tones of the previous transistor set. I do have to turn down a touch more to get in the "clean" range but that's alright.

Since this will have a 100kA "Pre-Gain" pot, I can also dial it back there for more dirty overdrive tones.