This is normal for some Mullards and other tubes. I wouldn't worry about it.
Mullards for Preamp?
I have a Cary SLP-74 tube preamp and just replaced the original Chinese tubes with Mullards. When I turn the preamp on to standby the two output Mullards flare up and then go down. Is this normal? They really sound amazing as compared to the ones that came with it. Very warm and lush sound. Just what I wanted. I just need to know if they are supposed to do the flare up thing? Thanks for any help. -Durr