Just to see if they're shorted? I have 6 Mesa STR420s. 4 of them came out of my Dual Rec. One of them is bad, though I didn't know it until I took the amp apart and found a burnt screen resistor. I was thinking about trying them in my amp again, but I have no clue which one is the bad one. I don't have access to a tube tester either. Any way I could check them?