I use 5693s as drivers in my 300B amp. Yesterday, I walked in after running the amp for several hours and found a silent channel. I found that one of the 5693s was not hot to the touch, so I assumed a dead heater. I replaced the tube and the amp is fine. The offending tube tests as dead in an emission tester, but I can measure the same DCR on pins 2 and 7 as on a known good tube. What's going on here?