My Broken Amp help...
 

when i switch on that amp after 20-30 min. as u see that noise coming out.it looks the problem on speaker.need your comments

I would say it's a microphonice tube. Open it up and give the smaller tubes a little tap with the eraser end of a pencil with no guitar plugged in. If it makes a noticable noise through the speaker, it's microphonic.

Yes, I had same kind of noise coming out of my Laney VC30. It only happened on the drive channel when I turned the volume up (volume at 1 it didn't have any noise). I changed one of the pre amp tubes and it helped.

You changed only ONE power tube? Did you Bias afterward? You should never change just one Power Tube, as they are matched in pairs/quads, and this is important to keep you from putting extra stress on one or running one completely cold. Generally, if you change one PT, you change them all, but you definitely change at least one pair of them. (Obiously, if your amp runs on two power tubes, you replace all of them no matter what, but if it runs on 4, you can get away with only changing two on some amps, although I see no reason why you wouldn't just replace all of them)

^ Oops, pre amp tube, not power amp.

EDIT: Fixed.