It may be one of the 12at7's if it's on both channels. The actual preamp tubes are the 12ax7's, but I believe one runs the clean channel and two others the lead, so if you hear the crackle on both, it's probably a driver tube. Turn the reverb off, if there's still a crackle you can rule out the reverb driver. That leaves the effects loop driver, which is the 12at7 nearest the loop jacks, or the phase inverter, which is next to the power tubes. Don't get me wrong, try the 12ax7's as well, as mentioned above... you never know!
When do you hear the crackle? I have been trying to run down a little ripply buzz in the note decay on my SE30. It's very faint, and only at low gain settings, but it's there, and I think we all know how it goes when you hear something in your tone that's a little off, no matter how minor. I switched out the phase inverter to a 12ax7 on mine, which got rid of the buzz, but also made the amp a lot gainier, so I'm playing with some lower gain preamp tubes right now. Have gotten some nice results with 12au7's, but they're actually a bit too low. I'm gonna order some 12ay7's, which I think will do the trick. I will say, changing out the 12at7 phase inverter to an ax7 has really smoothed out the breakup. I liked it before, but the changeup is sweet. I thought phase inverters didn't have much effect on tone, but my recent experiences seem to prove the opposite. I've always hated 12at7's as gain stage tubes, so it makes sense I guess.
Anyways, sorry for the long rant, just putting in my two cents since we have the same amp!
