Princeton speaker rattling in the breeze

[clyde1]

This one has me baffled. When you pluck the low E string hard, bass and volume up, as the note dies, the speaker will start to oscillate and rattle almost as if you had your finger on the cone. Another amp doesn't make the speaker do it, changing the output tubes seemed to mitigate it somewhat, subbing in a different OPT didn't help, biasing makes no difference. Turning down the bass and/or volume helps (naturally) and halving the size of the feedback resistor helps too. All voltages are normal, no bad caps, everything is in the zone, and oh, subbing filter caps didn't help either. ????

[Enzo]

Whack the end of the chassis with a mallet. Does it respond with any noises? COuld be a resonant vibration thing.

[clyde1]

Thanks Enzo, I gave it a try out of desperation and respect, but with the chassis on the bench, amp cabinet on the floor, it still does it. I think you're on the right track with the resonance thing, perhaps a circuit resonance, or a synergy between this speaker (Jensen P10R reissue) and the amp that causes some sort of positive feedback loop. I guess I'll try another 10, as the amp checks out from top to bottom and my mojo detector is currently broken.
Crap.