Just throwing out an update on this project... I got everything breadboarded to test out the different clipping options. (Similar to Jimilee, I'm making a version with an 8-way rotary - 7 options for clipping and 1 op-amp). Once I have the boards together, I can more easily record a video, but for now these were my findings... If there's an option floating around out there (and the parts are still available, hehe), Let me know... I'm open!
1. Silicone symmetrical options: I actually tried Schottkeys first... 5817s and BAT41s. 5817s was too quiet - BAT41s sounded better, but ultimately very similar to 4148/914s with a HAIR less volume, so I think I'll save the BAT41 for asymmetrical option. 4148 is a classic for the clippers, so why not? a little drop in volume, but a fuzzier and smoother sound than the straight op-amp.
2. Germanium symmetrical options: 1n34a and D9V. D9Es are impossible to find, and I'm not sure why I couldn't get 1n60p, but I tried out D9V - cool sound, but definitely the quietest of the bunch, and 1n34a sounded similar with better volume, so it wins the germanium slot. The sound was unique - a little more gritty and dark, less fuzz-like - probably good for overdrive if you pull back the distortion level. Probably the most quiet of the bunch still...
3. LED symmetrical: Red, Yellow and Green were ones I liked (in that order), but Red ultimately wins out (though yellow was a close second - might socket this one). It's the perfect mix of all the best parts of the RAT sound - fuzzy tone (but not too fuzzy), great volume, good clarity but retaining the grit that makes it a RAT. (Of note, orange sounded the same as yellow to my ear and blue didn't change the sound at all, and since the forward voltages go red (1.7), orange (2.0), yellow (2.1), green (2.2), THEN blue (3.2) and upward for the other colors, I didn't even try others).
4. Asymmetrical option - silicone/schottkey mix: a 4001/bat41/bat41 combo sounded pretty good here - it's a little similar to the LED clipping, honestly, but it definitely has its own flavor, and I notice more of a difference with leads, which had more grit.
5. Silicone symmetrical #2: 4001s surprised me - I don't know why, but the sound was smooth and had decent volume. Definitely went more into fuzz territory, but felt more controlled and clear.
6. FET symmetrical option #1: I got some BS170s because they were in some of the original RATs - I combined it with 4148 and it produced something unique, so I'm into it. Hard to describe
7. FET symmetrical option #2: 2n7000s sounded pretty good. So did the BS170s, but the 2n7000s had a hair more volume and a bit more grit. Since the BS170s were already used with the 4148s, the 2n7000s win
8. op-amp clipping (e.g. no diodes or FETs): I was surprised by how different this sounded - more raw for sure... almost feels "incomplete" and I definitely prefer the clipping options, but it might be the most distinct of all the options, so it has earned its place. Perhaps great with the distortion rolled off and in overdrive territory.
Another interesting thing - I A/B'd with the Slow Loris from roughly 2012/2013 (clipping options were LED, none, 914, I think) - There are some circuit differences since then and the new version sounds clearer and louder - my testing was done with an OP07cp, but I'll throw a 308 in there for the final. In LED clipping mode, almost no difference in sound, but in op-amp or 914 mode, it was definitely different. Excited to get it going and boxed up!
I tested using the FAT RAT toggle, as well. I found that on neck pickups (single coil or humbuckers), the fat setting muffled things a bit more than I liked, but on bridge single coils it added just the right amount of thicker tone without changing the character of the pedal. This is more useful to me than the Ruetz mod, so I'm forgoing that altogether.
I initially asked about a transistor version of the RAT, but having FAT and clipping options (or knowing what options are out there) has done given me choices to customize the sound for me, so thank you madbean board for leading me that direction! More to follow on this and I'll hopefully do a video soon... and if you have other suggestions, please send them my way!