7199 | 6U8 | 6GH8 | 6EA8

The famous 7199 vacuum tube is very rare and expensive due to limited production. It is the ONLY tube that you can use without using an adapter or rewiring your amplifier if you amplifier calls for the 7199. While they can still be had, many choose to rewire their amplifiers to use the common 6U8 or various other substitutes. 

The 6U8, 6GH8, 6KD8, 6678, and 6EA8 all are interchangeable though slightly different. If you are using a 7199 to 6U8 adapter, any of these tubes may work but your mileage may vary.