Shorted tube?

[anonymous]

I received a used output tube recently with claims of being good from a TV-7 tester. None the less, I always check used tubes for continuity/shorts on the pins with a dmm before powering up in my crack w/sb.(I realize this is not an infallible test, but it is what I have at my disposal) All good. Listened for a few hours with no problems... sounds great. Sent said tube off with others to have professionally tested. My tube in question comes back labeled "shorted". Being curious, I rechecked with my dmm and still no shorts found.

I have plenty of other power tubes to use, it's not a matter of plugging it back in. My questions are, what may have happened if it had shorted while in use?(blown fuse, cap, resistor, transformer, driver tube... or worse?) Should I abandon this simple dmm test if it proves unreliable? Do I have a tube that has a mysterious intermittent occurring short considering I logged 6 hours of drive time on it before having it tested? Scratching my head on this one.

Please and thank you.

 

[Paul Birkeland]

What is the tube in question?

 

[anonymous]

Tung Sol 5998

 

[Paul Birkeland]

What a tube tester looks for in terms of a short and what a DMM can measure aren't necessarily the same.  Still, if you listened to the tube it wasn't shorted.  What would happen in your amp depends a lot on which elements are shorted, but you could expect a lot of excess heat and no sound from whichever side is shorted.

 

[Doc B]

A short that comes up on a tube tester can be more of a leakage issue than a hard short. Also, it is possible to tweak the short testing circuit on a TV-7 to be more or less sensitive. In doing that one could end up with a short test that is too sensitive.

All that said, your amp is a great tube tester. If the tube works in the amp (hits the right voltage range, doesn't make bad noises) and you like the sound it passes the test.