Korg have been developing a new type of tube that looks like it may have relevance for guitar amps. Its a small, low energy, long life triode that is supposed to retain all the features of tradditional valve.
It may make valve amps cheaper by reducing size of power supplies etc.
Its called the Nutube 6P1.
Here's the link:
http://www.korg.com/us/news/2015/012212/
There are a few other articles doing the rounds, such as:
http://www.tubecad.com/2015/01/blog0318.htm
IMO, could be back-o-the-net.