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This topic is still under construction.
If you suspect that a preamp tube is going or has already gone bad, the most expeditious way to find it is to have a new, known good spare tube to swap in until you find the problem. Just swap in the new, good one and see if the problem is corrected. If you put a new known good one in, and the problem does not change, then the tube you replaced was not the problem - or at least not THAT problem.
Easter-egging (hey! I wonder why they call it that...) with a good tube is the highest likelihood practice to hit a fix quickly.
"OK," I can hear you saying, "but I don't have a spare preamp tube!" Shame on you. All right, maybe there's something we can do. There are usually several preamp tubes of the same kind inside most amps. Try swapping a pair of them, noting which tube is which so you don't lose track of which one you suspect. It should be vanishingly rare for TWO preamp tubes to go bad at once. You'll usually be able to tell if the problem moves around when you move one particular tube, or if it stays the same no matter what you do. If it stays the same, you don't have a tube problem, you have trouble with the circuits around a tube.