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5693 issue

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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?
 
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Try applying 6.3V and see if you get any heater current, possibly the filament is intermittent open, and when cold has continuity.

I'm assuming the tube simply no longer lit up in the amp?

Or maybe nothing more than a tube pin to socket contact issue, might want to check for that possibility too. (Has happened to me)
 
I've resurrected a few 6AS7G's by doing exactly that, suck out all the old solder, then refill the pins with fresh solder. A bad pin to socket connection is another possibility.

I would also try tapping on the tube while powered up and see if your current meter jumps about, would confirm a broken filament. I've found tubes that have leaked air internally usually pull the normal amount of current, but wont glow and heat up very slowly.
 
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