6SL7 the same as 6SN7 ???

It's true that they are not interchangeable per se, but they are somewhat close (relatively).. both dual triodes, differing mu. However, 12SN7 is not a sub for 6SN7 in any way, (nor 12Sl7/6SL7) without changing the heater voltage. The 12-series tubes will work in 6-series gear. 12volt heater vs. 6 volt heater. Current does not come into play at all. (e.g.: the above info is wrong.)

6SN7 subs: CV1988, 5962 (most of the time), 6SN7 GT, -GTA, -W, -WGTA, -GTB, -WGT. Maybe some others too (depends on gear)

6SL7 subs: 5691, and all the above suffixes (-GTA, etc.)

-Ed

Yes, VT-231 is the military designation for 6SN7s, VT-229 is the designation for 6SL7. Curiously, 6SL7s are about 1/2 to 1/4 the price of equivalent 6SN7s. (at least at the moment)

FWIW,
-Ed