I have not heard of KT120 filament failures as you describe them nor any of the warnings you mentioned. I believe an autobias amplifier that does not have adequate plate voltage or current may experience this issue if it cannot supply adequate voltage to power the tube in the socket. Some amplifiers are set to bias with a standard tube range made for that amp, for example, my amp is optimized for KT88's, however, I can use the KT120 with my amp and when I adjust the bias manually, I only adjust for the tolerance built in for the KT88 the amp was designed for. I can, and I do, when I use a bias measurement tool, raise the ma for the KT120's, not by much though. The stock adjustment for the KT88 is around .40 and with the 120's, I have tried .55-.65 with no problems.
I did experiment using KT120's with a .75ma bias setting and that experiment was a disaster. The tubes turned black and one of the bases of the tubes separated. I have since learned to live with the lower ma settings and the stock range of the KT88's. The KT120's are lovely though. They do have a pronounced deeper bass and an even frequency response. I hope my information helps you some.
Ciao,
Audioquest4life
I did experiment using KT120's with a .75ma bias setting and that experiment was a disaster. The tubes turned black and one of the bases of the tubes separated. I have since learned to live with the lower ma settings and the stock range of the KT88's. The KT120's are lovely though. They do have a pronounced deeper bass and an even frequency response. I hope my information helps you some.
Ciao,
Audioquest4life