as far as i know its a fulltone full drive 1 or 2 depending on mosfet. guess what ? yOU ARE RIGHT its just a tube screamer a fulltone fulldrive 2 is a tube screamer with just a couple value changes and a boost added on the end.. you could just build a ts808 or ts9 and sub those values. CHECK THIS OUT
http://www.generalguitargadgets.com/pdf/ggg_its8_mods.pdfscroll down to the expensive boutique modifications. you need to change the .047 uf or 47n to 220n .then change the the 4k7 connected before it to 1k and the 51k to a 10k thats it.the mosfet is just clipping as well. two of the legs are shorted together to make a diode for clipping it sounds bigger and better than it reALLY IS .
you could just make a fulltonefulldrive 2 from scratch.ill fix your byoc box for you if you want as well . i love fixing issues lord knows i have created a lot myself. i did a few pcbs and still do on occasion. it just doesn'
t give the same satisfaction for me. although i rely heavily on the talented guys here like Mark Alex Mirosol,Savvas,rockett88,frank nh,moa,javi and others for there ingenious layouts .we will all do our best to help you