This Week on the Breadboard: The Nut Job OD

[Chuck D. Bones]

Another OD inspired by HamishR down under in Perth. His earlier versions had four JFETs and a different tone stack. I took one of the later versions, deleted a source-follower stage between the clipping diodes and FMV tone stack, fiddled with the EQ and clipping a bit and here's where I ended up. He used 2SK170GR JFETs. I don't have any of those, but I do have plenty of 2SK117GR. They are close enough to not require too much fiddling to get the bias and gain right. Plays clean with GAIN below 1. We get edge of breakup from around 1 to 4, depending on the signal strength at the input. There is a healthy dose of gain, distortion and compression when GAIN is turned past 6. All of the distortion comes from Q2 and D1-D3. If you're looking for a light OD, you can eliminate C5 & R9. There is usually one more cap in the tone stack, but since I did not need or want DC isolation there, I jumpered the cap that would normally be between R11/C8 & the BASS & TREBLE pots. Q3 provides volume recovery. Its gain is low enough that it runs clean. Mellow harmonics and a smooth note decay.




Knobs (L-R): LEVEL - BASS - MID - TREBLE - GAIN

 

[cooder]

Nice! Nutty complexity and hints of cocoa beans and smooth decay on the palette...

 

[z2amiller]

What the heck, was looking for an excuse to test drive KiCad 10!

(Still not sure if I like it. DRC and zone fills are so much slower. And it's back to doing the thing where it crashes if you run DRC after making too many changes)

 

[Chuck D. Bones]

The 2SK209 datasheet does not show a thru-hole device, only SMD. The 2SKxxx thru-hole devices I'm familiar with all have the gate on the middle pin. I see what you did there with the switch. Not sure it's necessary, but it does no harm.

 

[z2amiller]

Oh yeah sorry I forgot to mention - I will probably build this with SMD for myself so I was looking for ~equivalent JFETs
and the 209s look close enough so I penciled them in on the schematic.
I forgot to change the silkscreen for the through-hole layout but it'd be the two types you mentioned in the OP.
I'll correct that before sending it off to fab.
(And you're right -- I'll also need to check the layout to make sure the G/S/D pinout matches up)

I added the switch since you said you could cut out C5/R9 to back it off. Maybe I should've called it
"LESS"!

 

[bluedmc777]

This looks like another classic!

 

[Chuck D. Bones]

A minor update. I moved D3, changed it to Schottky and added R16. Now it makes that special grind at low gain settings that HamishR likes so much.

 

[temol]

Is it my mistake somewhere on a breadboard or MID adjustment is quite subtle?

 

[Chuck D. Bones]

It's the typical Marshall MID control. Fairly subtle and interacts with the TREBLE knob. It has the most effect when BASS & TREBLE are turned up.

 

[HamishR]

I find the MID control fairly mild - I don't really like it below noon. I'm even planning on trying a version with a fixed mid. Chuck, Would this work?

 

[Chuck D. Bones]

Yes.

 

[HamishR]

And thanks for the updated version. I will obviously have to try that one too!

 

[temol]

It's the typical Marshall MID control. Fairly subtle and interacts with the TREBLE knob. It has the most effect when BASS & TREBLE are turned up.

Resembles the tonestack from the Boogie MK series