The Tung-Sol USN-6U8A is one of the best sounding 6U8 available. Made in 1962, this is the perfect 6U8A for replacing the the exceptionally rare and expensive 7199.
Since all worldwide 7199 tube production has ceased and is not expected to return, the availability of this tube has begun to dwindle and subsequently the cost is quickly becoming prohibitive.
Thankfully, there is a solution by implementing a simple wiring change and substituting the plentiful (for now) and otherwise identical sounding NOS 6U8A tube in place of the 7199.
