Choosing clipping diodes for Fulltone Bassdrive Mosfet

[swever]

I am building a fulltone bassdrive mosfet using storyboardist's layout: https://effectslayouts.blogspot.com/2016/06/fulltone-fulldrive-2.html

I believe this is the schematic it corresponds to:
https://images.talkbass.com/attachments/bassdrivemosfetschematic-jpg.2005067/
This one shows the switching more correctly, but has some changes in component types and values: https://www.talkbass.com/attachments/fulldrive-jpg.2527959/

The mosfets here only work as clipping diodes here, don't they?

I used sockets in places where the diodes go so that I can try a few different ones to find a combo I like the most (I've found the combination from the layout to sound a bit mushy at high gain settings). How do I go about choosing them? Is there any more scientific method other than just trying different combinations at random?

Here's a list of the diodes I have available:

1n5818
1n34a
1n4001
1N4732
1N4739
1n914b
1n4148
1n60p

д310
д311а
кд522б
bat43
bat52
д9д
д9к

green flat leds
3mm RedLeds
3mm Orange
3mm Red LED
3mm Blue LED
3mm Green LED
3mm Yellow LED
3mm White LED

Any suggestions?

[Fancy Lime]

Hi Alexander,

just try and listen. I have found that theoretical considerations don't always translate well to actual sound. I played with the same sort of thing for the Onkotherium quite a bit. The way the MOSFETS are wired in the schematic, they work as normal silicon diodes because the body diode is foreward oriented. That sounds very much like a 1N914 or 1N4148 to my ears. You can get much softer clipping by arranging the Mosfets with series diodes in the way I did in the Onkotherium. I quite like the sound of that, very dynamic, not squishy.

1n400x series power diodes sound a bit sharp, metallic to me compared to other silicones. Seems to have to do with their turn-on behavior. But the difference is subtle and may in fact just be my imagination.

I find that germaniums or Schottkys in the negative feedback path of an op-amp generally sound to compressed unless you use 2 or more in series. The LEDs will give you the least compression (apart from the MOSFETs in "correct" orientation), most dynamic but also the least distortion. All LEDs except the blue and white ones will sound very similar, blue and white usually have a higher forward drop, making them even louder and less distorted.

Asymmetric clipping often sounds more pleasing than symmetric. Brings out the first octave, when taken to extremes (like one Ge and one blue LED).


Onkotherium schematic:
http://www.diystompboxes.com/smfforum/index.php?topic=118391.msg1101956#msg1101956

Cheers,
Andy

[swever]

Thanks for the reply, Andy!

Yea, that's what I thought. Gotta try different combinations and see how it works. Also, reading your post made me realise that it's not too much of a problem to put a couple of diodes (russian germaniums cause most interest here) in series even on a pcb. I do that with resistors quite often when I dont have the needed value. Will try that and see what I will come up with.

PS All three of your designs (Adrenochrome, Eemu, Onkotherium) are on the first positions of my to-build list! It just that I am progressing VERY slowly with it, thanks to my day job. 

[Fancy Lime]

It just that I am progressing VERY slowly with it, thanks to my day job. 

Same here. I have my first week of holidays since forever starting tomorrow. But I'll be in Lapland so no internet and no soldering. After that I'll finally have some time to work on my autowah.

Have fun playing with the diodes. Well, I know you will 
Andy

[swever]

Have fun in Lapland! I've been in Levi for a couple of weeks a few years ago. It's been such a refreshing experience. Northern paradise it is there. What I loved most is how few people you meet there, lol. And how many deers. 

[Fancy Lime]

Thanks! Jepp, that is mostly the point, holidays from people. Or rater from too many people in one place, aka the city.

Andy