The Last Bear Bender -- a different sort of MKII

[midwayfair]

This is a Tonebender MKII variant, and it was a bit of an adventure.

I mentioned in the Mossy Sloth thread that I had spent a couple months breadboarding every classic fuzz I could find (thus my unusually high number of fuzzes recently). The Tonebender wasn't exactly my favorite, but there were some things I liked about it: I liked that it wasn't woofy, and I liked that the cleanup wasn't a shelf like in fuzz faces, but I wasn't crazy about the need for a leaky germanium in Q1, and it wasn't fun swapping out transistors trying to get to 8V on Q3's collector, which is the correct value by the way, if anyone's planning on building one. There's some frequent misinformation out there about biasing it to 4.5V like a fuzz face, most likely from Fuzz Central, but numerous vintage units were measured and they were all closer to 8V. Electric Warriorhas a bunch of info from what must be a stable of MKIIs. One interesting thing that happens with this sort of biasing is that, compared to the Fuzz Face, it's easier to saturate Q2 and easier to send Q3 into cutoff. I think this is part of the MKII's character/insanity. But the need for leakage (and thus more noise) just to get a working circuit bugged me.

Anyway, I was less interested in duplicating the circuit than just getting close to the sound of it when it came time to actually solder one up. So I breadboarded a "stock" circuit with three germanium devices, and then put a second one on the other side of the breadboard to experiment with. By far the biggest things about the sound of the pedal are the gigantic treble cut at the input (10nF), the midrange push at Q3, and the cutoff frequency of the output, which is much higher than in a fuzz face (that's why it cuts better in a mix).

I realized that a FET could go in Q1 with only one additional part, a source resistor, and there was room on the layout I'd already made. This would give me high input impedance, and flipping back and forth on a breadboard a FETzer valve stage had similar output levels with 2N5457s as Q1 in the germanium, but it was brighter, cleaner, and not as noisy (more low frequency hum but not as hissy). But that's where things got a bit complicated. It turns out that you have to be really specific about which FET goes in there, because if the drain resistor is too small, it ends up bleeding power noise through the output cap, down to ground through the gain control, and then amplifying it in the negative feedback loop of Q2 and Q3! I was completely lost as to how this was happening until RG Keen indicated that I was definitely hearing positive feedback.

So I had to find a FET that could take a larger drain resistor and still bias to ~6V. J201 works, provided the source resistor is also fairly high, but I ended up using 2SK170 from a group buy a while back. It's DGS, so it's really convenient for building right into standard TB topology, as long as you can put in the source resistor. I tinkered around with the drain resistor and found that 5.6K was large enough to prevent oscillation (I also filtered the power more heavily), but ended up with 7.5K on the drain and 1K on the source, getting me to 6V without the output of this stage getting too high.

There was also room for a bass control, so I added one of those, too. As I've said elsewhere, I find them more useful than treble controls, especially for taming things with a humbucker. Here it doesn't noticeably cut much bass until about half way; it's really more of a pre-gain control.

Finally I decided to mess around with Q2 and Q3. In the end I went with a hybrid here, just to get a little extra stability and because it gave me a sound a little different from anything else I already had lying around. Q2 is a black glass germanium with Hfe ~ 59 and leakage about the same. I really liked how it sounded when combined with a low gain silicon (2N3440 from small bear, hfe ~40) in Q3. I guess technically the gain ranges are backwards, but it sounded VERY close to using both silicon, and except that it's slightly brighter, it wasn't radically different from the "stock" MKII set up right next to it on the breadboard. It did kinda mess up getting to the proper bias range on Q3, though.

One final thing I did was to put a bias trimpot. It's probably more common to put it on Q3 (like in the Fuzz Central Mindbender), but you can actually dial in both Q2 and Q3 by but it on Q2's collector instead and leaving the biasing on Q3 fixed. My bias voltages ended up being Q2C ~.5V and Q3C ~6.5V. Before I soldered in Q3, I messed around a bit and I found that I only needed slightly higher gain to reach 8V on Q3, but again, I just went with what I liked the sound of. I might make another later with germanium in both Q2 and Q3.

The final changes I made were to boost the output by doubling the 470R near the output (a common change that wasn't completely needed here) and reducing the output impedance by cutting the volume pot to 10K (and raising the output cap).

I'm fairly pleased with the result. It does still have some crackly distortion on lower gain settings, but that's true of basically every all-transistor fuzz. I'll try to get around to it in my next batch of demos.

Here's the layout if anyone's interested. It has slots on it for building all-germanium, all silicon, or any number of hybrids.
https://dl.dropboxusercontent.com/u/9878279/Jon%20Patton%27s%20layouts/Tonebender%201590A%20Modular%20MKII.pdf

Etch mask:
https://dl.dropboxusercontent.com/u/9878279/Jon%20Patton%27s%20layouts/Tonebender%20etch%20mask.pdf

I didn't redraw the schematic yet; it's basically a stock FETzer valve stage from Runoff Groove (the newer one without the bypass cap) in place of Q1 (well, the 220pF cap is 10nF here) with a cap blend on the output stage, so it shouldn't be too hard for anyone to imagine.

[Arcane Analog]

Nice! Looking forward to hearing the demo.

[peterg]

John

Noob question: What is the socketed blue three legged thingy?

[midwayfair]

John

Noob question: What is the socketed blue three legged thingy?

It's a metal can transistor with heat shrink on it it keep it from shorting on the lid.