So last night my venerable AI500 decided to blow. Very sudden; I noticed one of the EL34s was glowing bright orange, then within 30 secs it went purple, then pop! Fair amount of smoke from the back of the amp. I've had an EL34 go bright orange before and have just replaced the quad set of valves, but never anything like this. Any idea of what's gone, aside from the tube, much appreciated. Also any recommendations for a brand of EL34, I might steer clear of Electro-Harmonix from now on...