Little Jim - a Marshall inspired MOSFET distortion pedal design

[jonny.reckless]

I'm so pleased you got it sounding great. There's plenty of opportunity for further improvement.

Let me know if you would like one of my spare PCBs, or if you'd like me to share the Gerbers and NC drill files for the board I built.

[Marcos - Munky]

So, it took me more time than I initially tought, but I finally built it yesterday.


I used 2N7000 for the mosfets and a J201 (legit) for the jfet. Imo there was no noise because of the 2N7000. I mean, it's a high gain circuit with no sshielded wires and outside of a metal enclosure (for now), so it isn't really quiet, but nothing different than any distortion circuit under the same conditions.

This one surely rocks! Good for palm muting, good for chords, good for harmonics and good for classic rock riffs. The tone pot offers a good range of usable sounds. To be honest, it have the exact drive sound I have using just my guitar and my AX84 Hi Octane amp, and I was looking for a pedal which gives me this exact sound, so building this one was a big win. Imo, it have a bit less gain than I was thinking it have (and also almost the same gain as my HO, maybe a bit more), but not a big deal, a simple booster/od can solve it. I'm kinda afraid on showing this one to my friends, because I know one or two of them that'll want to "borrow it to test it for a while but will probably never return it to me" . I'll surely box it and will add it to my pedalboard, just have to decide on the artwork, which is the hard part since I don't have good artwork ideas .

Edited: I don't have a place to host the layout files, but if Jonny or anybody else want the files, just let me know and I'll gadly share them. I should make a google drive folder or something like that to share my layouts, but I end up being too lazy to create a kinda decent looking pdf file .

[kraal]

Hi,

just have to decide on the artwork, which is the hard part since I don't have good artwork ideas .

I have ideas but lack the skills to make them real

I don't have a place to host the layout files.

You can create a Gitlab [1] account. Gitlab is a free git projects repository [2].
You can then benefit from versioning, issues tracking, releases, branching, tags, wiki, etc. People can then fork, contribute, etc.
If you don't want to share some of your repositories, you can make them private (it's free as well)
And you have looots of space (10GB per project.) You only pay if you require more computing time than what is in the free plan (you need it to compile, test code, which would not be the case in your situation)

I prefer to use it from command line [3,4], but there are lots of GUI clients to use with git [5]

Cheers,

[1] https://gitlab.com/
[2] https://git-scm.com/
[3] https://files.jrebel.com/pdf/zt_git_cheat_sheet.pdf
[4] https://git-scm.com/docs
[5] https://git-scm.com/downloads/guis/

[marcelomd]

https://github.com also has free private repositories.

If it's just for sharing, a free Google Drive, Dropbox, One Drive (the one from MS) will do.

[kraal]

https://github.com also has free private repositories.

I didn't mention Github on purpose (after having used it for years). Without going into details useless in this context, from my experience:

Gitlab: 10GB, way to weight issues, roles based access, better documentation
Github: 500MB, no way to weight issues, unacceptable ways of handling customers (it's my own experience)
I do not recommend the other tools in your list as they are not SCMs. If it's for sharing and working with a community, one should use tools made for sharing and for working together, don't you think (fork, release, tag) ?

Best regards

[aron]

Keep it up! I have like 1000+ BS170 that I need to get rid off! hahaha

[marcelomd]

https://github.com also has free private repositories.

I didn't mention Github on purpose (after having used it for years). Without going into details useless in this context, from my experience:

Gitlab: 10GB, way to weight issues, roles based access, better documentation
Github: 500MB, no way to weight issues, unacceptable ways of handling customers (it's my own experience)
I do not recommend the other tools in your list as they are not SCMs. If it's for sharing and working with a community, one should use tools made for sharing and for working together, don't you think (fork, release, tag) ?

Best regards

I agree with you in general. I've worked with most of the "public" SCM providers and all of them leave something to be desired in one way or another. Github is just the most famous and so it's easier to find help/tutorials/tools =)

Full featured version control for our projects would be nice (if a bit overkill), even if not everybody gets over the learning curve.

[Marcos - Munky]

I have ideas but lack the skills to make them real

Well, I also don't have skills, but what are your ideas?

For the "where to host", I didn't used anything before, so I don't know how Gitlab and Github words. My Google account have a Drive I didn't used even once so far, so space won't be a problem, and pdfs takes so little space anyways. It's the "I don't know how to create a half decent pdf file to share and I do some things on Eagle that I can easily understand but it'll looks like a mess for other users" that kept me from uploading the files on Drive. I have to take some time and make a template of a half decent pdf file then the rest should be easy enough, but there are so many things to do and I keep forgeting on doing that.

[jonny.reckless]

Here's a link to my Google drive folder with the schematic, bill of materials, assembly photos, PCB Gerbers and NC drill files:
https://drive.google.com/drive/folders/1nzRh_Ax2i9DMiyzEAqyzJoUVYRm6h0ny?usp=sharing

Marcos I can grant you write access if you email me with your Google account info.

[rutgerv]

With the Little Jim circuit still on my breadboard I've recently started experimenting a little bit with some variations, such as a full tone-stack behind it. Also, I've been wondering if there would be a way to make it more touch-sensitive and/or clean-up more gradually than the original circuit.

I started looking at some other Mosfet-based overdrive, mainly the Box of Rock (ZVex) and its near-copy-with-extra's: Bogner La Grange. Besides both of these pedals having an extra RC-network between gain-stage 1 and 2, I also noticed they differ in two other ways
1) gain/filter structure. Little Jim uses rather high source resistors and bypasses these for high frequencies with a 10u (or 220n) capacitor. The other two circuits use a much lower (100 to 330 ohm) source resistor without HF bypass.
2) Mosfet biasing. Little Jim uses a special part of the power supply that connects through 100K resistors to the gates of the Mosfets. The other two circuits use resistors to ground and to drain (both 1M).

With my limited knowledge of Mosfet circuit, it seems the Zvex-approach to Mosfet biasing has some advantages. First of all, the dual resistor approach seems to be more forgiving to different supply voltages, and the resistors being an factor of 10 larger also means the gate-source capacitance of the mosfets will act up less easily (in some Spice simulations with 2N7000s and the original Little Jim circuit I can actually see it creates a resonance hump right around 2K, though not sure if GS-capacitance in this spice model is realistic).

I'm wondering if the current bias configuration in Little Jim was a deliberate choice, and weather it has effects sound-wise. I'm wondering similarly about the gain-structure and filtering decisions. Was there a specific reason to do it this way? (Or framing it differently...would Zvex have a specific reason for their topology?)

Hoping to learn from you!

Rutger

[Marcos - Munky]

would Zvex have a specific reason for their topology?

Basically, Zvex "created" the SHO, and then just stacked a few stages and it sounded good. That's it.

@Jonny, I'll try to create a half decent pdf file, then I'll send it to you. Then, if you want, I can send you the eagle files (and gerbers, which can be gotten from eagle). Give me some time, I have to put my mind back to where it should be, then I'll do the file. Good news is I'm almost set on the artwork, so it'll be boxed sooner than I was thinking.

[pinkjimiphoton]

nice work as always, sir! salute!

[LightSoundGeometry]

you can still get real Jfets from mouser but they will run you around 4-5 dollars per unit

[jonny.reckless]

I'm wondering if the current bias configuration in Little Jim was a deliberate choice, and weather it has effects sound-wise. I'm wondering similarly about the gain-structure and filtering decisions. Was there a specific reason to do it this way? (Or framing it differently...would Zvex have a specific reason for their topology?)

I'd never seen the box of rock before, it's quite a different design (apart from obviously using BS170 MOSFETs for the gain). How does it sound in comparison to Little Jim? My guess is it is quite a lot darker, with more low end distortion. Higher impedance gate circuits would mean that the Miller effect becomes even more of a problem, killing pretty much all of the treble in the first gain stage, and possibly introducing high Q resonant peaks, so it becomes quite fussy with what you drive it with. I try to drive the MOSFET gates from low impedance sources to minimize this so as to preserve high frequency gain, and get that tasty bright Marshall crunch. All the MOSFET gain pedal designs I have seen just connect the input to a MOSFET common source gain stage, and it really sucks the tone of a passive pickup, especially a high inductance pickup like a Seymour Duncan JB bridge. This is why I used a JFET source follower on the input, and couple each gate from the previous drain directly with a cap and no series resistance. Putting a passive guitar pickup into a MOSFET common gain stage like this will likely roll off from 500 - 800 Hz, and present an effective Miller input capacitance in the nanofarad range. Perhaps that is what the designer intended, but it wasn't my intention here. I chose the 220n source decoupling capacitor on the first gain stage to provide a suitable treble boost to avoid fuzziness and farting out. I did briefly try a Marshall style treble lift between the second and third gain stages. I typically like this with JFET preamps, but with this pedal, I preferred it without.

My suggestion is just to experiment on your own. If you like the sound, then go for it! It's all a matter of personal taste; there's no "correct" way to do things. I voiced Little Jim to my taste, with my guitars, amps and cabs. I didn't try to model any specific amp or pedal, rather just get that bright, crunchy, swirling sound of a classic Marshall cranked. There's lot of ways you could improve it! If you come up with something you're happy with, please post a demo video and a schematic and I'll make one

[jonny.reckless]

you can still get real Jfets from mouser but they will run you around 4-5 dollars per unit

J113s (my favorite JFET for audio preamps) are $0.129 each for 100 off at Mouser. If somebody wanted $5 a JFET I'd be looking to get into the FET supply chain business

[marcelomd]

The Crazy Tubes Ziggy is similar to both Little Jim and Box of Rock in architecture:




I like it.

I also like how easy is to get good sounds out of simple MOSFET gain stages. Mix and match.

[deadastronaut]

smd j201 , 33p each....

just add converter pcb n legs...

https://www.mouser.co.uk/ProductDetail/ON-Semiconductor-Fairchild/MMBFJ201?qs=ljbEvF4DwONpFpdsobl6fQ==&vip=1&gclid=EAIaIQobChMI24_YssP66wIVyentCh2UzwUGEAQYAiABEgJmvfD_BwE

[LightSoundGeometry]

you can still get real Jfets from mouser but they will run you around 4-5 dollars per unit

J113s (my favorite JFET for audio preamps) are $0.129 each for 100 off at Mouser. If somebody wanted $5 a JFET I'd be looking to get into the FET supply chain business

you got me lol , I wasnt specific. the j201's and 2n5457 are around 5 dollars a piece. I have not compared specs between the j201 and j112. I have never messed with a j113 but guess who is adding to mouser cart some fets lol

https://www.mouser.com/datasheet/2/308/MMBFJ113-D-1811481.pdf

[LightSoundGeometry]

smd j201 , 33p each....

just add converter pcb n legs...

https://www.mouser.co.uk/ProductDetail/ON-Semiconductor-Fairchild/MMBFJ201?qs=ljbEvF4DwONpFpdsobl6fQ==&vip=1&gclid=EAIaIQobChMI24_YssP66wIVyentCh2UzwUGEAQYAiABEgJmvfD_BwE

I totally forgot about doing this . I just got back to work a few weeks ago, forgot a lot of the tricks of the trade. I was really sick due to fiberglass poisoning and am in a lawsuit right now. my house is unoccupied and has 30k worth of damages. im alive though and glad to be back on forums and in the shop.

[Marcos - Munky]

It took me some time, but here's my verified 1590A layout: https://drive.google.com/file/d/1M03EY3GVBd35o7Lodkq3S2jJQcjmUzut/view?usp=sharing

Still didn't boxed it yet, but I'm planning to do it as soon as possible. Let me know if the link works. Also Jonny, if you want me to send you the eagle files/gerbers let me know, or if you want to upload the layout on your google drive account feel free to do it.