When I turned on my Allen Class Act this evening, it was completely dead, no hum, no signal, but the pilot light was on. Noticed v1 was cooked (a Mullard CV4024). So I replaced it with a JJ ecc83s. Played for about 15 minutes and noticed the tone was bass heavy (could be the JJ). Then all of a sudden the amp began emitting a "loud" hum form the speaker, really deep and loud, scared the #^%@$% out of me. So I turned it on to standby and everything quieted down. I checked the tubes and nothing looked out of place. When I turned the standby off the hum returned and I noticed some blue light coming from inside the plates. I changed output tube (from a winged c el34 to a 6l6) but same thing. Any ideas before I open her up and take a peak?