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Short Lifetime for 6NS7 Tubes

I’ve been experiencing a high rate of 6SN7 tubes and their variants becoming microphonic and some going bad way too soon. I buy NOS tubes from only two venders with stellar reputations that are often mentioned here on the forum.
Please, no lectures on the risk of buying NOS; I’ve been buying 12AU7’s and 12AX7’s for my various preamps for years from these dealers and I can only recall sending two tubes back. Anyway, I’ve owned my current preamp for one year and have been rolling in 6SN7’s.
I’ve had 6SN7GT’s and 6SN7GTB’s go microphonic in a few months time, and I’ve had a RCA 5692 "redbase" and a RCA VT-231 go bad in less than a year. This latest tube (VT-231) drove me crazy as I tried to diagnose why my system was lacking bass and detail; surely it couldn’t be a 6 month old tube. I cleaned the connections, changed cables, but it turned out to be the tubes. I don’t know if it is one or the pair that is bad.

You know who these dealers are and the tubes I buy have the test results written on the box and are matched. They offer 30 day returns, but that doesn’t help in my situation. Has anybody else experienced 6SN7’s living a short life or have I just been unlucky?

* a typo in the title, I'm really not a dummy.

128x128lowrider57

Showing 2 responses by almarg

almarg's avatar
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almarg
9,670 posts
 
I had a 1950s Sylvania 6SN7GTB abruptly become severely microphonic a few years ago, in my VAC Renaissance 70/70 MkIII amplifier. It had previously been working well in my system for the better part of a year. I don’t recall the specifics of the sonic character that resulted, but basically the system just didn’t sound as good as it had previously. There was no obvious distortion, though, and nothing to suggest that anything was plainly defective. Eventually I isolated the issue to the microphonic tube by very gently tapping it with a pencil eraser, which produced a loud sound through the speaker in the corresponding channel. Replacing the tube, and the corresponding tube in the other channel (so that the two tubes were properly matched), restored the good sonics I was used to.

I’ve used a couple of other 6SN7GTBs over the years that have been **slightly** microphonic when gently tapped, but have sounded great and have worked reliably over time.

Regards,
-- Al

almarg's avatar
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almarg
9,670 posts
 
Hi Lowrider,

It is apparently far more common for vintage 6SN7s to be microphonic, or to become microphonic, than for many or most other types of small signal tubes that are commonly used in audio equipment. There was a post here a year or two ago by Kevin Deal of Upscale Audio attesting to that with some rather striking statistics, and describing the troubles he has to go through to find and maintain a reasonable supply of non-microphonic 6SN7s.

Also, I’ve sent you a PM on this subject, **not** having anything to do with him or the tubes he supplies.

Best regards,
-- Al

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