KMD GV60D hi/lo switch

[pdf64]

https://groups.google.com/forum/#!topic/rec.audio.tubes/SM3iWw8ExCY
_{Some tube history about 6L6}

http://web.archive.org/web/20050317085639/http://schema.web1000.com/tech.data/6l6.txt
_{6L6 FOREVER By Eric Barbour}

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http://www.audiotubes.com/6l6.htm
https://www.amplifiedparts.com/tech-articles/6l6gc-comparison-current-made-tubes
https://www.tdpri.com/threads/nos-tung-sol-5881-versus-jan-tung-sol-6l6wgb-differences.757632/

A significant oversight made in the discussion in all the above pages is that tube types rated under the design max system (eg 6L6GC, KT66) are directly compared to those rated under the design centre system (eg 6L6G, 5881).

[Helmholtz]

A significant oversight made in the discussion in all the above pages is that tube types rated under the design max system (eg 6L6GC, KT66) are directly compared to those rated under the design centre system (eg 6L6G, 5881).

Agree.
A different rating system won't make tubes stronger. In fact, design max ratings only apply under controlled ("lab") conditions. I tend to think it was just a clever marketing idea transferring the responsibility to the equipment design engineers.