This Week on the Breadboard: the BJFE Emerald Green Distortion Machine

jubal81

Well-known member
looks like something similar but slightly different is what Chuck did on his pink-purple (fuchsia fuzz) with the clipping control, could be tried here too I’d think.
Oh yeah, I see that. Cool. Good catch.
 

Chuck D. Bones

Circuit Wizard
If we did that, we'd need to either reduce the gain of the 3rd stage or move the VOLUME control ahead of the 3rd stage. The reason is that putting a resistor in series with the clipping diodes makes the signal larger. Wasn't a problem with the Fuchsia because there are no more gain stages after the clippers. Different story with EGDM because Q3 is tuned to be on the edge of saturation and cutoff with the clipping diodes limiting the signal level. Put a resistor in series with the diodes and now Q3 is driven to cutoff and saturation.
 

Chuck D. Bones

Circuit Wizard
Try the stock build, it's pretty sweet as-is. The only mod I'm considering is trying other JFETs. That would require tweaking the source and/or drain resistors to get the right bias point. It's likely that BJFe/Bearfoot has to screen their 2N5952s for Vp.
 

cooder

Well-known member
Try the stock build, it's pretty sweet as-is. The only mod I'm considering is trying other JFETs. That would require tweaking the source and/or drain resistors to get the right bias point. It's likely that BJFe/Bearfoot has to screen their 2N5952s for Vp.
So how would we need to adjust when using mmbf4393 instead of the 2N5952 Sir Chuck?
Asking for a friend.... ;)
 

Chuck D. Bones

Circuit Wizard
So how would we need to adjust when using mmbf4393 instead of the 2N5952 Sir Chuck?
Asking for a friend.... ;)

Most other JFETs have lower Vp, so you'd need to reduce the source resistors (R4 & R14). On my breadboard using 2N5952s, Q1's drain voltage is 6.90V, Q2's is 5.82V. These are not magic numbers, just ballpark. That's what I got using stock resistor values and it sounds good to me. I'd shoot for 6V on both JFET's. To bring Q3's gain back to the stock value, you'll need to increase R15 so that their parallel resistance is 1.6K.

I'll give it a try using some MPF4393's I have and report back.
 

cooder

Well-known member
Most other JFETs have lower Vp, so you'd need to reduce the source resistors (R4 & R14). On my breadboard using 2N5952s, Q1's drain voltage is 6.90V, Q2's is 5.82V. These are not magic numbers, just ballpark. That's what I got using stock resistor values and it sounds good to me. I'd shoot for 6V on both JFET's. To bring Q3's gain back to the stock value, you'll need to increase R15 so that their parallel resistance is 1.6K.

I'll give it a try using some MPF4393's I have and report back.
Awesome that will be much appreciated! Cheers Chuck!
 

Chuck D. Bones

Circuit Wizard
Here are some preliminary results...
I installed MPF4393s for Q1 & Q3.
To get the bias close to where it was with the 2N5952s, I changed R4 to 10K and R14 to 6.2K. I increased R15 to 2.2K to bring Q3's gain back down a tad. With the MPF4393s, there was a little more gain and a little more volume. Waveforms and harmonic structure were similar, but not quite identical. MMBF4393s should work fine once you dial in the bias.
 

zgrav

Well-known member
The description and comments prompted me to put this together. Very nice overall sound and touch sensitivity with ringing sustain even without much drive dialed in. The treble range is also well defined and the treble pot is a great fit with it. I used the referenced vero layout and also used the recommended pots (two 50KA and two 50KB) since they were in my existing parts inventory. I added three 20K trimmers for the three resistors that Chuck mentioned for bias adjustments. Probably no need to do so for the 2 that go to the Source pins, although it is interesting to trim the bias just a bit. The third trimmer that goes between the cap and ground is pretty cool at tweaking the lower range, and I think leaving that as a trimmer is good. Some folks might even like that as a pot adjustment on the outside since it seems like a good pairing with the Voice pot.
 

zgrav

Well-known member
I have the recommended two 2N5952 and a BC108 in right now. Liked it so much I haven't tried anything else in it yet.
 
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