Electronic and plastic burning smells are bad, right? About 45 minutes into band practice last night we started noticing a burning smell, and identified it as coming from my amp - no smoke, just smell, and the top of the amp didn't feel any more than slightly warm. I also noticed that my tone had changed from my usual, moderately clean, warm tone to a mild distortion (fuzzy) in the higher frequencies. Everything sounded normal until the burning smell started. This has happened twice now (last time we thought it was the 65 Fender Twin used by one of the guitar players). My initial thought was a pre-amp tube going bad. Is there anything I need to know about doing a tube swap myself? I haven't opened up the case yet to take a look for any apparent damage. Any other ideas what might be going on?