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?
I understand that the tube is a goner, I'm just trying to understand why I show continuity on the heater the same as in a good tube, but I get no heat. Any ideas?
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'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)
Thanks for the suggestions, I'll try to measure heater current. I can't tell if it's lit, since it's a metal tube. It doesn't work in the amp or the tester, so not a socket problem, and a replacement works fine in both.
Maybe the vacuum has leaked out? Easy to tell if it were a glass tube, but being metal, you can't see the getter turning all white. With air in the way of the electrons, you won't get any heat off the plate. But maybe a little heat off the heater. In any event, toss it.
The tube draws no current with 6.3VAC applied to pins 2 & 7, but still shows about ohms cold resistance. Go figure....
did you test at the socket to verify presence of 6 volts right at the socket?
there are also instances wherein i had to wiggle the tube slightly to make contact...
happened to me lots of times just as i was about to give up....
there are also instances wherein i had to wiggle the tube slightly to make contact...
happened to me lots of times just as i was about to give up....
A slight amount of current is causing the filiment to open as soon as you apply voltage.
Expansion and contraction....
Expansion and contraction....
One last step would be to try resoldering the pins. I've had this happen once or twice with octals, and soldering restored normal filament operation.
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.
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.
on some of the 6c33 tubes i had, some tested as open, what i did was to use a 400 then 1000 grit sand paper and sanded away, after that the tubes worked fine...but this is another story...
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