I would venture to say that it is a tube. I also have a tube preamp and the "popping" you describe has, in my case, always turned out to be the tube. You may try cleaning the tube pins with an electrical cleaner but more than likely you need a new tube.
solution for "popping sound" from tube amp?
Hi! I have a Sonic Frontiers SFL-1 tube preamp. It has worked perfectly for the last couple of years (time I've owned it), now suddenly tonight there was a "popping" sound coming from my speakers that I traced to the pre. What is the probably cause, and how might I fix it? I understand this pre only has one tube, and I could probably replace it if required, if a tube going bad could be the root of the problem. Where should I buy a replacement tube, and is there a particular type/brand that you recommend? Any help for this tube electronics novice would be much appreciated! Regards, Ben