I will never forget the first time I saw metal tubes; it was a pair of 6L6’s in a Bogen amplifier from the early 1950’s. Metal tubes were introduced by RCA in a Technical Bulletin in 1935. The first metals were: 5Z4 rectifier, 6A8 pentagrid converter, 6C5 triode, 6F5 hi-mu triode, 6F6 power pentode, 6H6 dual diode, 6J7 pentode, 6K7 remote cut-off pentode (called a super control pentode in the bulletin). Metal tubes were also the first vacuum tubes to have an octal base.
Most people currently building tube gear don’t like using metal tubes. This is strange because they seem to always like older versus newer. It’s almost as if they think metal tubes as lesser just because you can’t see the filament glow. That’s too bad because the earliest metal tubes are superior in construction and quality compared to the older ST enveloped 4, 5, 6 and 7 pin tubes produced earlier. Sorry folks, the mystique of an ST envelope and nostalgia don’t change that. By 1935 tube manufacturers had learned quite a bit.
The 6F6 is an interesting tube. Unlike the more common 6V6 the 6F6 is not a beam power tube but an actual pentode. It precedes RCA’s 6L6, the first popular beam power tube, by a year.
I haven’t tried it, but you can operate 6L6’s upside-down in Ethylene glycol bath at a plate dissipation considerably above the 19 watt maximum called out in the specifications. Do not do attempt this if you have small children, pets, or a fussy spouse at home.