FWIW, a few years ago I installed a brand new quartet of Chinese-made 6SN7 small signal tubes in my VAC amp, that had been provided and tested by a good supplier. After listening to beautifully reproduced music for about 2 hours, all of a sudden there was an eruption of horrific static-like noise from one speaker. I believe it was loud enough, and that it had sufficient high frequency content, to destroy the tweeters in many speakers if allowed to continue for a prolonged period. In this case I had the amp shut down within about 4 seconds, with no damage. I subsequently found that one of the 6SN7's had abruptly developed a short.
I of course have never used any of those four tubes since.
FWIW. Regards,
-- Al
I of course have never used any of those four tubes since.
FWIW. Regards,
-- Al