NO. Not even close. 6AS7G and 6SN7 are both 6.3 V octal base dual triodes, but that's the end of the similarities. 6AS7 is 2.5 A of heater current, a plate current of 125 am, a Mu of 2, and a transconductance of 7000. The 6SN7 has a heater current of 0.6, a plate current of 10ma, a mu of 20, and a transconductance of 2600. They are not even close. It probably won't blow up, but it won't work well, either, and at a bare minimum would sound really, really anemic.