Turns out there's a problem with the preamp - I finally got it to cut off after more than an hour of injecting 440Hz, 1000Hz and 2000Hz sine wave, and also playing a guitar through a fuzz with the controls maxed out - I thought it was never going to shut off and that this problem can only be encountered while rehearsing, but the output cut off eventually. I had to attach a big heatsink to the 100W resistor and use a fan to avoid ovearheating and smoke (the transistors' heatsink barely got warm during the process).
I probed the pre-amp output and there was nothing there. To be absolutely sure, I plugged the guitar cord from the preamp out to another amp, and there was no sound.
I'll probe the whole audio path later, but the problem with troubleshooting this thing is that the pattern seems totally random - one day you can jam on the thing with the volume at full for 3 hours without breaking a sweat, the other day it shuts off after 15 minutes. I couldn't get it to shut off again after it happened once today, so I gave up for tonight.
I probed the pre-amp output and there was nothing there. To be absolutely sure, I plugged the guitar cord from the preamp out to another amp, and there was no sound.
I'll probe the whole audio path later, but the problem with troubleshooting this thing is that the pattern seems totally random - one day you can jam on the thing with the volume at full for 3 hours without breaking a sweat, the other day it shuts off after 15 minutes. I couldn't get it to shut off again after it happened once today, so I gave up for tonight.