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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.

lowrider57
bigkidz
1,784 posts
 
Sounds like you need to see what the voltage to the tubes should be and measure that to see if the preamp is within spec.  If too high, that could be the cause of short tube life.  What controls the tube voltage in that preamp I do not know.  As far as micros' true with the 6SN7 tube but not as problematic as the 26/101D tubes for sure.

Happy Listening. 
mechans
2,999 posts
 
I have always noted that the old NOS 6SN7 GTs have a strong tendency towards microphonia.  The fact that you have been free of it in those tubes for extended periods is exceptional. You may be forced to use current production to get what you want.  BTW is the microphonia heard whilst just playing or do you have to tap on the tubes. If it's the later just don't tap..
ericsch
180 posts
 
I am relatively new to tubes, my integrated uses two 6SN7's. I'm just wondering what does a microphonic preamp tube sound like? Is there a specific type of distortion that occurs? Sorry, this is a newbie question.
    
larryi
1,972 posts
 

Lowrider,

Do the tubes that go microphonic sound bad?  Mechans asked whether it is a problem with the sound or the "tap on the tube" issue and I think that is pretty important distinction.  I run a different set of tubes in my linestage (310A and 311A), but, they too tend to have some that are microphonic when you tap on the glass and some that are not.  For whatever reason, it could be coincidence, when we were auditioning a bunch of tubes, the ones that were slightly microphonic turned out to sound better.

I never actually checked if the 6sn7s I ran in my amp (Audio Note Kageki) were microphonic or not, as far as tapping the tube, mainly because how it performed under that test was irrelevant to me.

almarg's avatar
almarg
8,673 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

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