This is not an uncommon phenomenon. The frequencies which create the note C are approximately 130Hz, 260Hz, 520 Hz, 1kHz, 2kHz & 4kHz. What you're experiencing is possibly a form of wave cancellation at around those frequencies, which are obviously intimately connected. On another day, in a different space, with the same equipment, you could experience the same thing but with a different note! I encounter this problem with my main gig, because there is a spot in the show where I need to generate feedback on a 12th fret G. 9 times out of ten, it takes off sweetly and all is well. But that tenth time, I will have to move around to find the spot where it will take off. One time out of twenty, quite often at outdoor shows or on particularly dead stages, it just won't take off at all, and I just get air, making me look and sound like a total knob end.
I've learnt to check out the sweet spot at soundchecks, and when I get one of those problem stages, I just say a little prayer to Jimi. I've even had the experience of getting nothing at the soundcheck, but purely with the different atmosphere of the gig, it takes off like a rocket.
There'd be a guy in a lab coat somewhere who could give you a much better explanation, but as a guitar player, I'd be calling "voodoo" and chalking it up to "mysteries of the Universe."
Oh, and by the way, here in Australia, it's already your birthday, so Happy Birthday t4d...
Cheers........................wahwah
