I have a gig with my band and we need something to use as an amp stand. i was thinking a foldable chair would work the best, help?
Most likely not, if it's a stage amp. You're going to be making a lot of noise (assumably for a long time!) and the vibrations tend to move things about, even things as big as amps on chairs. It'd just be better to lay it down on the floor. If you really want it risen above the ground, try using something more stable, like a table or stool.
As long as it's a good strong chair sitting level on a solid floor then it should be okay. Just check it from time to time to be sure it stays stable. Years ago , before I got my extension cabinet , I used a chair with no problems. And my amp was a little pig , it was a 70's Fender Twin with 2 heavy duty JBL speakers so it was heavy as all get out but the chairs handled it with no problem. It really helps to have the cabinet up where you can hear it. I used to carry around a couple of those plastic milk crates for the places that didn't have folding chairs available.They worked really good too. I would cover the crates with a black cloth before I put the amp up on them so it didn't look too cheesy.Actually you could't even tell what the amp was sitting on.