JFET Vulcan build report

[grs149]

I recently breadboarded the JFET version of the Vulcan overdrive and wanted to share my experience. I had been looking for a moderate gain overdrive circuit to compliment my BSIABII. What I got from the Vulcan is an absolute hi-gain monster! A great distortion sound, but a little over-the-top for my current needs, so I set about tweaking the circuit for a less aggressive low-gain sound.

What I came up with was as follows:

-Removed 1uf bypass cap from first stage.
-Changed 1 uf bypass cap in second stage to .33 uf.
-Changed 2M2 resistor between first and second stages to 4M7.
-Removed 470K resistor divider from output and replaced with Big Muff-style tone control (taken straight from BSIABII circuit). This soaked up roughly the same amount of output volume as the divider, and now I have a nice tone control.

The result is a smooth-sounding overdrive that cleans up nicely at lower gain settings, and still sounds pretty heavy and hard-edged when cranked. Thanks to Joe Davisson for sharing this new circuit concept. It's nice to see a new idea after spending far too long fiddling with reverse-biased diode pairs.

Cheers,
gs.

[WGTP]

Joe's stuff is great.  If you want to incorporate the diode pairs into it, try using just 2 stages with diodes to ground after the second one for a variation with a nice sound.  I'm using a Jfet stage, a normal stage and 2 LED's to ground at the end.  Slightly reduces the parts count.  

[grs149]

Thanks for those comments. I had thought about hacking the thing down to just two stages for lower gain, but haven't tried that yet. Also thought of changing J201 FETs to 2N5457s, which are lower gain devices.

gs.

[rodriki1]

If you want to talk about creativity.

That is the guy.

Keep doing changes.

good lucky.

[jmusser]

I know this is not even the same circuit you're in the middle of, but for a medium gain overdrive, mild fuzz, "tubey acting" circuit, it's really hard to beat Joe Davisson's Antiquity fuzz. It's an easy build, and sounds like more what you're looking for. It's nice and sweet, and I was really impressed with it.

[grs149]

Ok, I've been all over the map with this circuit and, for the sake of completeness, thought I'd report back with what I finally decided to build. I have tried all sorts of mods and (surprise, surprise) found that the circuit sounds best when you stick pretty close to the original design. Btw, this refers only to the JFET Vulcan. I haven't built the bipolar version.

The only significant change I would make lies in the source resistor bypass capacitor values. IMHO, the 1uf values specified are too high for any of the three stages, and result in excessive gain and a muddy tone, so I changed to the following:
- 0.1 uf bypass cap for the first stage
-0.33 uf bypass cap for the second stage
-no bypass cap on the third stage (for moderate gain overdrive) OR 0.33 uf bypass cap (for higher gain)
I found that using the above cap values resulted in a tighter, more balanced overdrive tone.

I also changed the 2M2 resistor between stage 1 and 2 to 4M7, and added a Big Muff style tone control in place of the 470K resistive divider before the output volume control.

I experimented with 2N5457s in place of some/all of the J201s and found the resultant tone to be weak and brittle. It seems that you need the high gain of the J201s to make this pedal work.

I also tried building a 2-stage version of the pedal, with a pair of reverse-biased 1N34s shunted to ground after the second stage. This produced a nice compressed tone, which some builders might like. I didn't go with it in the end because I felt that it took away too much of the unique character of this design.

I am liking this pedal a lot, with a nice, raw, open sort of overdrive sound.

Cheers,
Greg Smith