Hi am new here and this is my first post.
I have a DOD FX55B and I was being modding it and I was thinking if I change the original op-amp with a 4558 and change some parts in the circuit to convert it to tube screamer. I can get sounds of the tube screamer or not? Thanks
Changing the op-amp wont get you to TS tone..

Compare both the schematics. I have never played one of these units but I imagine it has a lot of gain just looking at the schem. I would remove the second clipping stage and just leave the clipping stage in the feedback loop.

There is lots of similarities if you look close, there are also many differences. Start tinkering.
I have removed the second clipping stage, that was one ofthe first mods. one of the differences between the two is the bypass
Ya both have lame electronic bypass systems....I always true bypass effects like that. I might also remove the buffers.

Compare the two schems...there is lots that can be done.

Its fun to compare the two schems but I really would not worry too much about getting them too similar. I always tweak with my ears... Just keep playing with the circuit till you find something that works for you.
Thanks , but I will keep this pedal at this point ,because is more esay to build one from scratch than mod this and I can add true bypass.