All right! It's this damned perfboard. Well, really it's my soldering on the perfboard, but if I would've made a board I wouldn't have to do it this way, so I'll blame in on that. I cleaned up my solder connections. It looks like the 12v that I was connecting to that diode/resistor junction didn't have a good connection, and that was the 12v going into the trigger section.
I hear a little bit of a hum or distortion right after I hit the trigger, like vvvvbeeeeeeeeee (with vvvv being the noise, and beeeeee being the tone), but I didn't on the breadboard, so I'll chalk that up to soldering as well.
With that, I guess I'll clean up my solders and call it day.
Thank you so much for your help. If you're by any chance near Chelsea (London, in case there's more than one), I'd be happy to buy you a beer in a few weeks!