I have posted here some isolated Nirvana tracks of Krist playing his Gibson Ripper. One thing I've noticed is that at random points, the sound suddenly changes and has a lot more midrange and less low end. At first I thought there might be some weird midrange pedal in use, but then I realised: it probably shifts to the bridge pickup.
But how would he do this without someone standing in front of him fiddling with the rotary switch? (Which I doubt happened.) He probably couldn't do it deliberately because it happens often right when he would be picking the strings, and he wasn't strumming, so it would be pretty hard to accidentally clip the switch. It's also funny how the second Ripper in the studio (a black 1979 model, tuned to D standard) didn't do this, from what I can hear.
Owing to the fact that the (probable) Ripper in question was a 1976 model (15 years old with all original electronics) and was never seen again on stage by August, being replaced by by a 1974 model, I believe that it had a defective neck pickup wire and kept cutting out randomly so was either destroyed or sold off. Just listen for sudden bass-frequency drops in these tracks...
Hear the pickup drop out at times like 0:16, 0:38, 1:47, 1:55...
There's a huge tone change at 1:26. You can even hear it on the record.
0:27, 0:40, 0:58, 2:58
Always possible that he had to punch in and when he did he had his electronics set differently. Or he moved where he was plucking/picking?