Breadboarding Davisson's JFET Vulcan

[Ben Lyman]

Ya, no scope, no sig gen, no experience. I feel really foolish right now, I can't figure out how to probe a circuit. All the reading of directions is confusing me, maybe later I will search youtube for some demos.
Some .1uF caps "in places" didn't seem to do anything, the only thing that stops the noise is lowering Q2source cap value.
There is some good news though, I cleaned and tightened up my BB plus moved across the room to my tweeds, this reduced the noise a little. I think that's a very good sign, perhaps boxing up is all it needs. In that case, I owe Mr. Davisson an apology for ever doubting his work. However, as great as this circuit sounds, the low gain range is just a little too bassy for my taste, I'm trying for bright jangly cleans up to massive thick metal distortion so I think I will stick with my previous plan and box it up. Of course, in my schematic I get a little less bass in the high gain settings but the tilt tone control restores it back to the thick metal sound that I'm trying for.
Wish me luck and thanks again for all the help!

[GibsonGM]

Cool, boxing usually does have a positive effect on effects, LOL.   

There ARE ways to lower the bassiness at low gain settings, but sometimes you just want something DONE, that I do understand, ha ha!   

[Joe]

I have some ideas to add, but keep in mind it's been a LONG time since I have worked with these circuits.

1. The .1uF caps might be too large. Something like .02uF is probably more appropriate, increase the value of the 1uF bypass caps if it sounds too thin.

2. A 100K gain control might be a better choice than 1M, something to try.

3. Make sure the drain voltages are between 4-5 volts. If not, adjust the 100K resistors. This is pretty important.

I regret I could not provide designs that would "just work", but discrete circuits have many unknowns and I felt that posting the simplest versions would be the best approach. Good luck.

[GibsonGM]

So true, Joe!  The variability of JFETs really makes it near impossible to just 'throw it together'.  Checking the bias as the drain can make a WORLD of difference!  I've had circuits that required vastly different D resistors in order to sound good - it's just how it is.   
Nice circuit

[Ben Lyman]

Thanks Joe, I will try smaller caps next time for sure. I got this one sounding really great right now with just minimal changes to your original design. Mainly just Q2 source cap plus the tone control and extra clipping diodes. It's all laid in the perf now awaiting solder.
And speaking of building another, J201's are kinda hard to find, is there a current production tranny intended to substitute them?
Thanks!

[Gus]

201's are easy to get
http://www.mouser.com/ds/2/149/MMBFJ201-889832.pdf

[Ben Lyman]

Thanks Gus, those are $3 each, best deal I found is SBE for .48¢ each but I was kinda hoping there was something out there for about $5 per bag of 100pcs   

[Gus]

The SMD MMBFJ201 don't cost too much

[Ben Lyman]

so far, so good. It's soldered up and passed the test run, still needs a box. As an experiment I clipped a 1M resistor across my C1M gain pot and it still sounded very nice. Not as clean at the lowest setting but still a very nice slightly crunchy OD. I can hardly wait to revisit this circuit again on my BB and try those other mods Joe suggested above, I really like this thing a lot!
I'm really glad I gave it a try based on a few comments here and there by people who built it in the past. I couldn't find any good youtube demos of it either

[Ben Lyman]

Here's two vids, the first is without the extra clipping diodes and the second is with.
Sorry for the bad playing, I'm trying to play way above my actual level 

[robthequiet]

Hey Ben, nice job! Sounds like you got it set up for a wide range of sounds, especially with the tone and drive controls. Definitely going to refer back to this when I get my duckies in a row for a new build. Have you thought about what would happen if you had one more gain stage, just for future reference?

[Ben Lyman]

Thanks Rob! Another gain stage might be too much... or just right, I dunno but I just ordered a pile of J201's from small bear so maybe I'll have to find out!
The tone control is subtle, just enough to roll off a little of the high end when the gain is maxed, I really like it so far.
I do think it's a little noisy but that is probably my own fault. I paid careful attention to my layout and I think it's pretty safe.
Maybe there's some other tricks that I haven't learned yet about filtering unwanted noise in a high gain circuit?

[Joe]

Four gain stages will likely cause oscillation when boxed up. The reason is that the input and output lines at the switch are close enough together to have a little interaction.

With three stages, there is a small amount of negative-feedback which helps keeps the entire circuit stable. But with four, the extra gain plus positive-feedback can cause oscillation.

I have ideas for improving these kinds of circuits, but have no time to work on them right now.

Good job with your build and thanks for sharing the videos! 

[Ben Lyman]

It's back on the BB and same as before. I tried a lot of experiments but everything leads back to the way I had it. Q2s 1uF cap causes that noise unless I reduce it to 100nF, then the circuit sounds different, loses a little bass but still sounds great. 
I tried all these experiments:

1. The .1uF caps might be too large. Something like .02uF is probably more appropriate, increase the value of the 1uF bypass caps if it sounds too thin.

2. A 100K gain control might be a better choice than 1M, something to try.

3. Make sure the drain voltages are between 4-5 volts. If not, adjust the 100K resistors. This is pretty important.

1. replaced all 100nF's w/22nF's and problem was worse plus it got trebley-tone.
     Upped all 1uF's one at a time and brought back some bass but noise persisted.
     replaced in/out caps back to 100nF's, not much change.

2. lost a lot of nice pretty distortion with A100k pot, although low gain CCW settings were pleasantly crunchy.
     A500k pot is a happy medium for retaining nice high gain, I'm sticking w/1M pot.

3. First thing I did. Q1,2,3 resistors respectively adjusted to 100k, 82k, 82k.
    Q1,2,3 drains measured respectively at +4.52,+4.47, +4.51

In the end, it is right back where I had it and I like it. I do wish I could get the original sound out of it at high gain, with that powerful heavy metal bass tone but still get a nice bright cleaner sound from it at low gain like the way I have it set up now.
Q2s cap has a HUGE impact on the tone of this circuit but I can't go any higher than 100nF if I want to use the gain knob. If making this pedal with no gain knob, set full on high gain distortion all the time, then I suppose the Q2s cap can be upped to 1uF for that really thick and heavy bass tone. Of course then, the first time someone turns their guitar down a bit, the noise will be obvious.

[Joe]

A different (untested) idea for a gain control. This negative-feedback circuit can go between Q2's source and Q1's gate:

Code Select Expand


(res)   (pot)  (cap)
-\/\/\---\/\/\---||--
           ^--

Not sure about the values, but the basic idea is to have almost no feedback at full gain, then (hopefully) the sound will clean up nicely as the gain is dialed down.

(The fixed resistor is there to set a minimum level.)