WEM Project V Fuzz project.

[digi2t]

Hi all,

Well, I breadboarded, and fiddled with this circuit for the past few weeks. I find that it's a nice, edgey, yet flexible fuzz. It can go from a nice "Satisfaction" type tone, all the way to a really nice Muffy tone. Besides, the original is a real "hen's tooth", and it's got loads of transistors for a fuzz, two factors that really attracted me to this sucker. I can't help but smile everytime I play with this baby.

Here is the schematic;



And I have done a vero for it;



The vero is unverified for the time being, but as always, if anyone spots an error(s), please let me know. I'll be "fine-toothing" it myself, but I believe it should correct. The more eyes the better.

A little video sound demo of the circuit. The volume drop when the Drive and Edge switches are off has been addressed, near the end of this post.  

I decided to go with axial caps for most of them, since I could only find the exact values in axial format. The 320uF, I'm replacing with a 330uF. 320's are real tough to find. It does make for a bigger board, but I'm trying to stay as true as possible here. Especially since these are the values that sound the best on my breadboard. You can go with 2.2uF's, instead of the 2.5's, but I found that it made a difference to my ears. With the 2.5's, I got a tiny bit more of a buzz-saw edge to it.

As for the inductor, you can just jumper here, or use anything that comes in under 10 ohms. The true value of the original is still unknown at this time. Truthfully, I tried several different ones, and it makes no audible difference. A jumper will do, but I'm throwing a TDK EL0606 series inductor here, just for mojo  .

The original circuit used seven BC108B's, and one BC186. I tried 2N3904/2N3906, 2N5088/2N5087, as well as BC108(9)/2N2907 combos. All sound nice, but the MPSA18/2N5087 set up is the one I like the best. In any case, there's gobs of gain, and a ton 'o fuzz. Just plain nasty!  

Whatever trannies you decide to use, biasing is really critical on this circuit. These are the voltages that I found produced the best results, with the trannies I've put into the layout;

Q3 Base to ground = 1.450 v
Q5 Base to ground = 3.400 v
Q7 Base to ground = 5.15 v
Junction point of 1K5 resistors, above Q7 Base = 6.50 v
Junction point of 4R7 resistors = 4.00 v

Set trimmer VR1 to 22K, and trimmer VR2 to 1K, and then tune from there. It's a bit of a balancing act between the two trimmers to get into the zone, but go slow, take frequent readings, and it will come together. Again, the numbers above are only a guideline, you can tweak it to your liking. You can adjust it to get some really crackly fuzz as the note fades with the attack on full, or tune it to be a bit tamer. Let your ears guide you.

One other change I made from the schematic, is the 150K resistor going to the Drive switch. I've reduced this to 47K on the vero. The 150K was just to much of a "muffler" when the Drive and Edge switches were both off. The 47K allows it to maintain a more decent volume level, whereas the 150K really kills the volume.

So, the parts are ordered, and the board is prepped. One last verification of the vero, and it's build time. Stay tuned....

[PRR]

If VR2 is a 5K trim, it should be shunted with 2K or there is a risk that full-up fries the output transistors.

Three 1N400x diodes in series, pointing down, will work quite fine here and no trim.

There's no technical mojo in your inductor, and it can be well over 10 ohms, or short. 13 turns of heavy house-wire, in red, might have more visual mojo.

The 100K above Q4 serves no purpose.

Biasing:
> Junction point of 4R7 resistors = 4.00 v

If this is right, and values are as marked, all your other points will be fine.

The obvious goal is to get the raw output, junction of 4r7 resistors, to half-supply. In the real amp, this maximizes power. In this low-voltage underloaded amp, power is not the issue but it needs to be near-half to work at all, because the Q7 Q8 Q5 string is eating 20% of the supply. I won't argue an offset from 4.5 to 4.0 because this type stage is not symmetrical and does sound different as you come off of half-way; 4.0 is perfectly reasonable.

The halfway/4.0V is set with the trim to the left of Q3 Base. The theoretical value is somewhat over 18K depending on Q3 hFE and leakage. With the specified Q3/Q4 the actual value may be 18K to 22K. I'd consider 15K fixed plus 10K trim. That will trim output through 3.3V to 4.5V. If you are an extremist, do 10K fix plus 25K trim. These options make it "work" right-off and allow very fine bias trim for asymmetric initial clipping.

> there's gobs of gain

Well, sure. It's a whole guitar amp. Also I don't know about the actual WEM, but this (very common) power amplifier plan "always" has audio NFB around it to stabilize and flatten the gain and reduce the distortion. It looks like audio NFB has been lost. Gain and distortion are out of control. I guess all the switchery drops gain back down from outer-space to something you can put with another amp, but I'd still expect it to be nastier than almost any guitar amp.

[digi2t]

Paul, as always, great thanks for your input.

Yes, absolutely, there should be limiting resistors on the trimmers. I must admit to frying a tranny (Q8) initially while ham-fisting the trimmers. That's why I mentioned the "lo 'n slo" approach to adjusting them.


I love the idea of the 13 turns of wire inductor. Mojo galore. Sort of a vero, gallows noose, death row look. Aye matey... 'tis good  Got to visit the electrical workshop tomorrow....

I'll have to see about that 100K, since it is pictured on the original. Not that I don't believe you or anything, I'll just yank it on the bread board and see if anything happens.

Insofar as it being an amp is concerned, Grizzlytone over at the "other forum" mentioned the same thing in his analysis. It's basically ready to hang a speaker on  .

Maybe the worlds first (inadvertant) attempt at a Rockman?

Thanks again Paul. Input that is much appreciated.

[Mark Hammer]

Nice work!

I'm wondering when a schematic of this beast will show up:  http://www.youtube.com/watch?v=AWxS60-U0os

Gotta exist somewhere because someone has cloned it.

[digi2t]

Thanks Mark. I love doing these weird, rare pedals.

Paul is right about the 100K, no audible difference without it. But, for form's sake, I'll leave it in there. It's clearly pictured on the original.

The vero above has been corrected. I went over it, caught some bo-bo's, and everything looks good now. I'll validate it with a build.

That Pep Fuzz sounds amazing. A lot smoother than the Project V. More like a Muff, whereas the Project V is more all out gain mayhem.

Nice work!

I'm wondering when a schematic of this beast will show up:  http://www.youtube.com/watch?v=AWxS60-U0os

Gotta exist somewhere because someone has cloned it.

I think they have it over at "the other forum". Do a search for Rush Pepbox, or Rush Pep Box.

[PRR]

> ham-fisting the trimmers

My fists go well with rye bread, cheese, and mustard.

> ready to hang a speaker on  

The original surely had "power" transistors on the output. TO-92 might survive at 9V but that's hardly enough power output to be worth selling as a speaker-amp.

This is not the same, but very similar.

http://www.el34world.com/charts/Schematics/files/wem/wem_ss_pa_40w.pdf

Both biases (output centering and output-pair idle current) are done different. This rendition has audio NFB via 10K:390K. Note 55V supply. 40363 output devices are selected/similar 2N3055, the workhorse POWER! transistor of the 1970s.

[digi2t]

> ham-fisting the trimmers
My fists go well with rye bread, cheese, and mustard.

Touche!

Thanks for the link as well. Mighty kind.

[DougH]

Nice work. I followed what you were doing at the other forum. I have to say though, holey-moley that's a lot of parts for a fuzz box (!).

Listening to the sound of it, I think the Anderton Comparator Fuzz (aka Optimum Fuzz Adapter) gets you in the same territory with a much simpler and more elegant design. This is my take on it with a couple minor improvements. Running the op amp with no feedback (as a comparator) gives you that wild-outta-control fuzz due to the ridiculous gain. Give it a try on the breadboard sometime. Would be an interesting comparison.

[digi2t]

OK, it's boxed, and verified. Here is the updated vero;



with an updated schematic, including the calculated voltages (big fat THANKS again to Grizzlytone)



and some pics of the final product;









All I can say is... IT'S FREAKIN' LOUD!!! But, I really like it. It's fuzzy, buzzy, rippin', drivin' nasty. It's a fuzz unlike any other, and the EDGE and DRIVE switches allow you to really change the character of the fuzz between Dr. Jeckyll, Mr. Hyde, Mr. Hyde, and Mr. Hyde. I took Paul's suggestion, and I stuck an inductor that I scavanged out of an old radio. Or was it a tape deck... who knows. Anywho, it looks pretty funky in there.

I'll post a video on Tuesday.

Cheers,
Dino

[digi2t]

OK, videos done. I demo'd both single coils and humbuckers in the video.

Enjoy,
Dino

[slacker]

Nice work, that thing sounds amazing  

[digi2t]

Thanks Ian. It was a lot of fun putting it together. I can't say enough for bread boarding, especially for these vague circuits. 

[pinkjimiphoton]

wow....WOW!! nice resurrection dino!!

[digi2t]

wow....WOW!! nice resurrection dino!!

Thanks bro. I had A LOT of help on this one. It's got a sound all it's own. I did change the parallel 10 ohm resistors (5.1 ohms), for the spec'd 4.7 ohm resistors. It made a difference in the decay, in a great way. If you listen to Brian Eno's "Here Come the Warm Jets", it's got that same fizzy decay/overtone as the fuzz in the song. More so now, then when I made the video. I didn't think it would have made such a big difference, but it did.

And the inductor, that's just my own weirdness. There is an inductor here in the original, but absolutely no info, or money shot, on what the exact value is. I tried a few different types, but I never did get any difference in the audio performance, so I winged it with the coil inductor. Reminds me of my youth, ripping open busted AM radios  .

[Mikecave322]

Hi guys, came across this and I'm not sure if anyone could help.... If i wanted to change the DPDT's to a standard "pot" any idea how i would go about doing this. On a bit of a steep learning curve with this and a Warmjet mod I'm trying to do!