[Arran]

Yes;
  I forgot to mention that, you would need to rewire the sockets to swap a 7199 tube for a 6GH8 (or any other nine pin triode-pentode substitute, but I think that's a better course of action then using an adapter. It also need not be permanent, wiring changes are easy to change back with point to point wiring if need be. Of course I haven't compared the interelectrode capacities, transconductance, and electrical characteristics of these 7199 substitutes but one or more has to be close. If the 6GH8 is close that would be ideal, early colour TVs used to use fist fulls of them so millions of spares  were also made.
Regards
Arran