I would use them in the modulator. Drive them with a pair of 845's if you can find a good driver transformer like the one used in a Gates BC1-F or Raytheon RA-1000. Be aware that RCA says 849's are good for full power only up to 3 mHz. They would probably work OK on 160 and 80, but you might see reduced output at 7 mHz and higher.
I still kick myself for passing up a half dozen Amperex 849As (good up to 30 mHz) over a few lousy bucks of selling price. The vendor didn't sell them because no-one else was interested, and I was told they eventually got broken when one of the high school kids they hired part-time dropped them on the concrete floor while cleaning up.
The base socket is a ceramic block with clips that resemble fuse clips. There are three, two for the filament and the one in the middle is for the grid.
http://amfone.net/Amforum/index.php?topic=23381.0
