Preamp tube fuzz in note decay

[andy474x]

Hi all. I've been having this problem with one of my amps for a while now - when I set the clean channel up for very mild breakup, I get this unpleasant fuzzy sound when I let a note or chord ring out. It's hard to describe, but after the intensity of the signal dies down enough that the note is just ringing clean, there's still this secondary fuzz/breakup sound rippling through the background. It eventually dies off as the note continues to decay, but its definitely there. I'm hoping someone else has experienced this and knows what im talking about, it's really hard to describe! I've switched out the V1 preamp tube from a JJ to Tung Sol RI 12ax7, and that helped, so I think this might be a tube thing. Wondering if anyone has had this problem and how they fixed, maybe a specific tube type, or something else altogether?

 

[Boogie]

Did swapping the tube just 'help' or did it cure the problem? Which amp? In my experience, odd, sporadic fizz was in the phase inverter (especially if in both channels of a multi-channel amp) if the other preamp tubes passed the pencil tap test. I also had an odd reaction with the 12AT7 going wonkey in the reverb circuit on my Twin, once. Even at low settings it was fizzy.