6SL7 the same as 6SN7 ???

Just to add to Ed's thread: Same in only pin configuration and heater and plate voltage - but not the same operating characteristics. 6SL7's have a higher amplification and a lower transconductance than the 6SN7. The tubes are close enough, however, to WORK in each other's sockets if the amp or preamp just happens to be designed within their respective operating characteristics (my amp manufacturer informed me that the 6SL7 can work - but not substituted -in place of the 6SN7 although I haven't tried it). IMO, if your equipment is designed around one or the other, use just that. I doubt that any amp designer would be surprised to learn that another tube than spec'd sounds better in his topology.

Isn't the VT231 also a sub for the 6SN7?

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

thanks for the help guys, i found a pair of 40 year old ken- rad 6sn7 & i was hoping to put them in my cary-805.. guess not.

A VT-231/6SN7 is also a 5692 tube.
A VT-229/6SL7 is also a 5691 tube