[Arran]

Ron;
  I'm not familiar with that tube type, 7199, but I can't understand why someone would want to use an adapter unless the socket is mounted in a P.C board. Substitutes listed were a 6U8, 6BL8, and a 6GH8. If one could make a 6GH8 work there are bucket loads of those around, RCA and Brand Z TVs used to use several of them. Since it's used in audio circuitry it's not quite as critical as an oscillator-mixer type tube like a 1L6. It would seem to me that the 7199 was another of those tubes that RCA's tube division developed that was looking for a market, kind of like their 100 ma series string radio tubes, again I don't think I've seen a set that used them, at least in the Canadian market.
Regards
Arran