During the 1st song of the last set the other night I got a distorted sound with a Hi pitched whistle noise. 😞 The Super Bassman Was still gutsy & loud. Asap I looked round back at the bias lights & one was orange. Indicating an issue but not a total fail - which I think the lights then go red -orange light means amp has it sussed & is addressing the problem -again, I think. By the next song it was sounding great again. 😎 The bias lights were all green again after that next song . After the set the lights were all green again too. Whaddya think? The amp doing what it's supposed to do. .. a bad tube so the amp adjusted itself? ? That tube is Probly on its way out etc etc. ..? Re buying a new 6550 or pair of. . So do I buy the same type of tube as what's in there or do I go for another, more highly regarded 6550 or pair ??