This Weekend on the Breadboard - an all FET Tube Bender

[Chuck D. Bones]

I ran across the Tube Bender schematic while browsing my archive and it got me thinking: "Why not do it with FETs?" So I tried it and it works great.

Here's the final version (schematic updated 14-Apr-2020):



Q4 isn't absolutely necessary. It provides gain recovery after the TREBLE network, a lower output impedance and it removes the inversion. Different JFETs will have different gain. PN4393 has more gain than 2N5246. Q2 & Q3 don't need to be matched exactly, but they should be in the ballpark. A little imbalance makes for some 2nd harmonic content. All component values are negotiable. R13 & R14 are part of the TREBLE control network, that's why I made them smaller than usual. Tweak R4 or R5 to get Q1-D around 5V. Tweak R9 or R10 to get Q3-D around 6V. The clipping is soft and the distortion comes on gradually. The compression is pretty mild. Very dynamic and touch-sensitive, very good volume clean-up. With GAIN & BASS turned down, TREBLE & LEVEL turned up, it makes a nice treble booster. Stage 2 (Q2 & Q3) is where the soft clipping happens. Overall gain from input to stage 2 output (Q3 drain) is a little over 50dB, making this a low-medium gain pedal.

 

[mdc]

Just curious - have you AB'd your FET version with the BJT version?

 

[Chuck D. Bones]

Not yet, but I need to do that.

 

[Feral Feline]

REALLY want to try this one out!

 

[cooder]

Tasty description.
I should put that on the list, would be rude not to...?!?

 

[music6000]

 

[Chuck D. Bones]

Niiiiiiiiiiiiiiiice! Got the rivet board done yet? Let's see a gutshot.

 

[giovanni]

I will take all JFET circuits!!!!

 

[Chuck D. Bones]

Just curious - have you AB'd your FET version with the BJT version?

I just tried them and they don't sound all that different, which surprised me a bit. The main difference in tone is due to the BASS control. I deliberately avoided comparing them on the 'scope because I didn't want to prejudice the listening experience. They both sound good are are very quiet. They sound extra good when they're in series and both are active!

 

[mdc]

I've built a few of the BJT versions for pals and it is a really nice low-gain drive with a different character to it. I'm not sure if it's the differential pair that's responsible for it, but it seems to trade off sustain in favor of articulation, even with a lot of distortion happening.

 

[Chuck D. Bones]

It's definitely the diff pair doing that. The first stage doesn't have enough gain to clip unless you drive it with a booster.

 

[mdc]

He's got an updated version coming out soon:

Coggins Audio

Manufacturer of the Cogmeister three-in-one guitar pedal featuring an overdrive and two boosts.

cogginsaudio.co.uk

Adds a variable-low-cut boost in front and a mid boost at the end in addition to whatever further tweaks he's made to the Tube Bender/OPA-101circuit.

 

[Chuck D. Bones]

Cool! Looks like I was not the only one to add variable low-cut up front.

 

[Chuck D. Bones]

Schematic updated.

 

[cooder]

When I had this finally boxed up I had to sift through all the genius threads in the Boneyard here to find what it is based on... so much brilliance here!
So here's my version of the FET-Tube Bender, the Star Struck Overdrive.
Sounds killa! Amazing design work. I can't stop salivating how the updated Tube Bender Deluxe will turn out then....

 

[Chuck D. Bones]

What am I missing here? The outside shots show 4 pots and the inside shot shows 3.

 

[Chuck D. Bones]

OK, so now you have the correct gutshot. What's the Whiteout Fuzz?

 

[cooder]

OK, so now you have the correct gutshot. What's the Whiteout Fuzz?

Coming soon as build report too ... Of course one of your tweaked goodies...