Mr Trev
Headphoneus Supremus
Gain wise, 12AU7 = lowest, 12AT7 is medium, and 12AX7 is highest. The reason these work with only 6.3 volts is they have a center tapped filament and were designed to run on either 6.3 or 12.6 volts. Can't do that with all 12 volt tubes, regardless of how the adapter is wired. For example a 12SN7 does not have a center tapped filament and was not designed to operate with 6.3 volts so the chances of it working in the Vali are pretty slim. I won't say it can't work because I've never tried it...and probably never will. Too bad, as 12SN7's are way, way cheaper than their 6SN7 brethren.
Haven't been brave enough to try the other 12a?7 tubes… yet. I remember reading something about the 12at7 not being interchangeable with the others on Brent Jesse though. I also think the 12ax7 runs at an even higher plate voltage than the 12au7 so it may not be optimal. Then again 6sn7s are supposed to get a way higher plate voltage than the Vali2 can dream of providing, but they still work fine…
I hear you about the 12sn7. I know a guy with a Project Solstice. He loves the cheaper 12v based tubes (who wouldn't)
What's the deal with those 7n7s you guys keep going on about over on the Lyr3 thread? If I stack enough adapters you figure they'll work in the V2