I always post this ... get your self a couple spares of each tube type. 12AU7 and 6AS7 or 6080 can both be had for pretty cheap. They may not be your tubes of choice but they get you by if you have a tube problem and they allow you to troubleshoot a bad tube. The crackly-pop in one channel can certainly be a tube going south. It can also be the tube pins or socket contacts causing the crackle when you tap on the cabinet. Or, a cold solder joint is a common possibility. Try reseating each tube several times (as Grainger always suggests) which can help clean both pins and socket contacts. But do yourself a favor and get youself a couple spares of each tube. It could be something other than a tube or socket but if it's a tube, you need a replacement so may as well have spares on hand. If not a tube, you can try the "chopstick test", tapping on the solder joints with a wooden chopstick (or small diameter section of wooden dowel or similiar non conductive object) with amp on and while listening with headphones. Careful where you put your hands.
