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December 06, 2022, 09:42:03 AM

Author Topic: Shorted tube?  (Read 278 times)

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Offline Headphone Hedonist

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Shorted tube?
« on: November 12, 2021, 04:52:21 PM »
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.

 





 

Online Paul Birkeland

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Re: Shorted tube?
« Reply #1 on: November 12, 2021, 06:52:38 PM »
What is the tube in question?
Paul "PB" Birkeland

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Offline Headphone Hedonist

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Re: Shorted tube?
« Reply #2 on: November 12, 2021, 06:57:14 PM »
Tung Sol 5998

Online Paul Birkeland

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Re: Shorted tube?
« Reply #3 on: November 13, 2021, 04:57:21 AM »
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.
Paul "PB" Birkeland

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Online Doc B.

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Re: Shorted tube?
« Reply #4 on: November 13, 2021, 06:37:28 AM »
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.
Dan "Doc B." Schmalle
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