Chuck D. Bones
Circuit Wizard
Deja Vu.
This Week on the Breadboard: the BJFE Emerald Green Distortion Machine
- Thread starter Chuck D. Bones
- Start date
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.
jubal81
Well-known member
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.
Chuck D. Bones
Circuit Wizard
And maybe adjusting the TREBLE range.
cooder
Well-known member
So how would we need to adjust when using mmbf4393 instead of the 2N5952 Sir Chuck?
Asking for a friend....
fig
Village Idiot
The size of the soldering iron tip?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
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.
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.
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.
Chuck D. Bones
Circuit Wizard
Cool! What did you use for the transistors?
Funny that you call this the lowest gain of the BJFs you list - I've built a few of these for friends and they have been probably the highest gain of all the BJFs I have built! I have been using the layout from the same source as you as a guide... I'll have to have another look because I prefer lower gain.
(Edit) Ok I checked my layouts and it seems that I may have missed a resistor. I've corrected the layout and will try again! Funny thing is that a local store had an actual Bearfoot version of this pedal which sounded a lot like those I have built - It was a higher gain than normal for a BJF design. I'll have to build it again to see if I am really going mad.
(Edit) Ok I checked my layouts and it seems that I may have missed a resistor. I've corrected the layout and will try again! Funny thing is that a local store had an actual Bearfoot version of this pedal which sounded a lot like those I have built - It was a higher gain than normal for a BJF design. I'll have to build it again to see if I am really going mad.
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Chuck D. Bones
Circuit Wizard
Then you haven't built a Pink Purple (Fuchsia). I use the engineering definition of "gain." The maximum undistorted output signal divided by the input signal required to get it there. All controls are dimed unless otherwise specified. The perception of gain is another thing entirely.
Chuck D. Bones
Circuit Wizard
The Pink Purple does both. When FUZZ is below noon, only Q3 and the diodes clip, providing OD-type tones. When FUZZ is above noon, Q2 also clips which gets us into fuzz territory. By the time GAIN is dimed, it's full-on Fuzzy goodness.
Well I built another Emerald Green today with the fix to the layout and yes, it sounds a lot better. 
I really like it. It's odd though because I remember trying one from a shop (I know them there and they let me take things home to try sometimes) and the actual BJF or Bearfoot or whatever it was sounded a lot like the previous Emerald greens I have built. It had plenty of dirt and sounded kinda thin compared to this one. This new one has a lot of lovely midrange with plenty going on in there.
I used a low leakage 2N1308 in Q2 (HFE around 108) and it sounds pretty good. I'm wondering if I could use a 2N3903? I have a few and some are very low gain. If it's a smidge brighter that's ok.
I really like it. It's odd though because I remember trying one from a shop (I know them there and they let me take things home to try sometimes) and the actual BJF or Bearfoot or whatever it was sounded a lot like the previous Emerald greens I have built. It had plenty of dirt and sounded kinda thin compared to this one. This new one has a lot of lovely midrange with plenty going on in there.I used a low leakage 2N1308 in Q2 (HFE around 108) and it sounds pretty good. I'm wondering if I could use a 2N3903? I have a few and some are very low gain. If it's a smidge brighter that's ok.
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