Demon Mojo Power Fuzz (lite) - build thread

[LucifersTrip]

Full details, pics & info

Here's a build thread for anyone who wants to burn the paint off the walls...and actually, for me also, since I wanted to duplicate it with standard components.

When I originally breadboarded it, I used the same power transistors you see in my final build, so no "standard" transistors were used. Those power transistors were connected offboard via alligator clips.
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This one's actually pretty damn easy to get to sound killer.

First, start with transistors with hfe in the 75-100 range. I couldn't submit to using modern plastic case transistors, so I dug out some old, partially oxidized RCA40245's. They have hfe's in the 75-120 range.





I chose 75, 75, 100, 90 for Q1, Q2, Q3, Q4 respectively and got pretty close to the voltages on the schematic.
Q1C = 8.46V, Q2/3C = .74V, Q4C = 6.1V

~First step is to adjust the voltage on Q4, which is very sensitive to the collector resistor. At 6.1V it was a bit bassy and not as smooth. A small increase to 5.75K brought the voltage to 5V.

~Secondly, and moving backwards thru the signal path, forget about the 45K limiter on Q2/3C and drop a 100K in there. Hit the strings. Start at 100K and turn it down (decrease K) as low as possible until you hear noise...you'll know when. If you turn past the noise, it'll cut off completely. Even if you don't hear a "noise", it should be easy to hear the most saturated point. For this Q2/3  set, I wound up with a  27K limiter.

~Third, sub a 5M pot for the 3.75M resistor from Q2/3 CB. I know many of you probably don't have one on hand so you may have to do a little extra work, first starting with a 1M pot, then using a 1M resistor in series, then a 2M in series with the 1M pot. Play a power chord. Start at 0K and work up. At 0K you'll have some weak, low volume crap. The transistors will really turn on a short bit later, maybe at 100-200K, but will be  dirty and not saturated. Turn further and you will get some nice saturation with some dirtier fuzz. Turn a little further and it will be bliss...full smooth saturation. I considered a 3rd external pot for this, but no way in hell could I fit it. For this set of Q's, I wound up with 2.4M. Note: once you hit full saturation, there will be little change afterwards. Ie, you could set it at 8M and it will still sound good, but not quite as good as the point where it first saturates. Edit: I did this again with other sets of transistors and that resistor could be as low as 400-500K. There will also be a very slow decrease in bass response (so, listen carefully) as you increase that resistor, so it will come down to taste.

At that point you should have a fuzz monster, but...

~Fourth, there should be no real need to screw around with the bias of Q1 (if you chose one with a similar hfe), but who knows, you may like it better set somewhere else. You will hear differences if you alter the bias there. Sub a 20K for the 5.5K and turn up until you get the hottest sound. That point will be shortly after "turn on". You rarely want it right at the edge of "turn on"or you'll wind up with odd or raspy tones...best to turn a little past.

~Fifth, diode selection. This may be the most difficult part since I chose atypical germanium diodes.... if you look at the schematic, you'll notice numbers in "( )" after the diodes. These are the forward voltages as measured by the peak. "Normal" ge diodes will be in around the .2 - .4V range, but many of the vintage ones I've measured, including the 1N192's I have in my garage/parts sale have much higher forward voltages. I used the 1N192's here, which measure in the .65 - .8V range. You might think that a standard silicon with that same forward voltage would sound the same, but it didn't. So, you may have to experiment...

Feel free to ask any questions or make up your own layout, pcb, etc...

Demo vid

[Arcane Analog]

Nice! I love me some nasty fuzz.

[Solidhex]

Rad. Wondering about the diodes. Out of all the diodes in parallel with the same anode/cathode orientation wouldn't the one with the lowest forward voltage conduct first essentially rendering the others useless?

[LucifersTrip]

Rad. Wondering about the diodes. Out of all the diodes in parallel with the same anode/cathode orientation wouldn't the one with the lowest forward voltage conduct first essentially rendering the others useless?

just did a quick experiment. I chose 3 ge diodes with the exact fwd voltage (according to the Peak, .72V), put 2 in one direction and 1 in the opposite direction...similar to what I got with 5 in one direction and the 6th flipped. fat sound one way, thin the other.

regarding why I used 6 precisely on the turkey project, I have to admit that was just a lot of trial & error a couple months ago. I just kept throwin' em on until I got what I wanted flipped in both directions.

[Solidhex]

Yeah

  You could lose 4 of those diodes leaving the one with the lowest breakthrough voltage and it would sound the same. Even if they test the same the one that has the infinitesimally lower voltage drop will "win" and conduct essentially removing the other diodes from the circuit. Pedal sounds great!

[midwayfair]

Yeah

  You could lose 4 of those diodes leaving the one with the lowest breakthrough voltage and it would sound the same. Even if they test the same the one that has the infinitesimally lower voltage drop will "win" and conduct essentially removing the other diodes from the circuit. Pedal sounds great!

This doesn't appear to be true in practice.

There's a few examples of parallel diodes facing the same way making a difference in sound off the top of my head -- one of my boosters (Fallstaff) uses two parallel diodes both facing away from ground, one germanium and one silicon. Not only does it sound different with any second diode added, but it sounds different depending on what the second diode is (a silicon diode for the second diode sounds different from Germanium, even though according to your description the Germanium diode would always "win" and the silicon should have no effect at all). The sound difference (thin vs. fat depending on orientation) also matches LT's description above.

Another example of parallel diodes is the Madbean Lavache. http://www.madbeanpedals.com/projects/LaVache/docs/LaVache.pdf

Note that if what you described was true, then the Schotkey would override the silicon, sounding the same as having a single Schotkey instead of two diodes. This doesn't happen. The two diodes in parallel sound different than just an opposed silicon + schotkey pair in this circuit.

I think Jack Orman also includes parallel rather than series unopposed diodes in his clipping article.

I'm sure if I looked hard enough I could find more examples.

[LucifersTrip]

You could lose 4 of those diodes leaving the one with the lowest breakthrough voltage and it would sound the same. Even if they test the same the one that has the infinitesimally lower voltage drop will "win" and conduct essentially removing the other diodes from the circuit. Pedal sounds great!

Thanx...Though, I did a further experiment to show that's not true in practice.

I breaded the simplest thing I could that pumped out a loud signal...LM386, .01 input, .001 output, 100K vol pot, pin 1 connected to pin 8.
Then, I pulled 3 x modern 1N34A's with forward voltage all .39V and connected them from the output to ground (down). I know there will miniscule differences between fwd voltages, but that did not matter because:

Configuration / Produced

Diode 1 cathode up - Diode 2 cathode down / Tone 1
Diode 1 cathode up - Diode 2 cathode down, flipped / Tone 1
Diode 1 cathode up - Diode 3 cathode down / Tone 1
Diode 1 cathode up - Diode 3 cathode down, flipped / Tone 1
Diode 2 cathode up - Diode 3 cathode down / Tone 1
Diode 2 cathode up - Diode 3 cathode down, flipped / Tone 1
Diode 1 cathode up - Diode 2 cathode up - Diode 3 cathode down / Tone 2
Diode 1 cathode up - Diode 2 cathode up - Diode 3 cathode down, flipped / Tone 3

...where tone 1 is the trebliest, tone 3 bassier, and tone 2 the bassiest. If what you stated was true, the last two tests would produce tone 1 since it should be equivalent
to either Diode 1 cathode up Diode 3 cathode down or Diode 2 cathode up Diode 3 cathode down.

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edit:

...wish I did this first, since it's a much easier experiment:

Configuration / Produced

Diode 1 cathode down  / Tone 1
Diode 2 cathode down  / Tone 1
Diode 1 cathode down - Diode 2 cathode down / Tone 2

where tone 2 is bassier
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After reading a little further, I think the reason there's a difference is because one diode does not completely "take over". There may be a large mismatch but not a 100% / 0% split.

[Pyr0]

Diodes in parallel will make a slight difference.
Diodes are not switches, they have a gradual turn on - check the I-V characteristics from any diode datasheet.
So shortly after the one with the lowest Vf just starts to conduct and before it is fully forward biased, the next one in parallel may also just  start conducting.

[Solidhex]

Interesting!