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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 4 responses by larryi

larryi
3,738 posts
 
Are the tubes going bad everywhere in the amp or linestage or does it tend to be a tube in a particular position in the gear?  If it is the same position, there could be something wrong with the piece of gear causing excessive stress.  Also, what particular brand and model is at issue here?  Some gear simply push tubes, even non-output tubes, hard so that they don't have long lives. 
larryi
3,738 posts
 

Lowrider,

I have not heard anything about whether the Atmasphere preamp is hard on tubes on not.  Other than a problem with the preamp, I am guessing you have had a run of bad luck.  Is there a possibility the voltage from your power company is off?  In a lot of places, voltage seems quite a bit higher than it should be and this makes some gear much more prone to failure.  The maker of the tube amp and preamp that I bought is a custom builder living in Italy.  Although I never specified voltage, he has made gear to match the specific voltage that the user is getting (he winds power transformers) from his utility; this makes a difference as to performance, and I would suspect, reliability.


It's been a while since I've last chased after 6sn7s.  I never ran into highly microphonic versions of NOS 6sn7, although I bought quite a few weak tubes that supposedly tested "like new."  Given the very limited stock of older tubes, the percentage of tubes that are bad must be rising as bad tubes are put back into circulation.  But, I don't know of current manufacturers making anything like a Tungsol roundplate, which is why we keep foolishly searching.

larryi
3,738 posts
 
Having a steady 120 volts is fine for almost any modern gear.  There is some old vintage gear from the 1950's and 1960's that supposedly work better at 110 volts (I know people who use a variac for that), but, mdoern gear should be built to work with 120.  In my area, I have seen spikes above 130, and someone not to far from me measured spikes close to 140.  A friend of mine had a Tron tube amp that failed repeatedly.  It was always the solid state part of the amp (bridge rectifier), and the manufacturer replaced the diodes several times before he finally decided to source beefier diodes for the repair; the manufacturer said he was unaware of how widely the voltage swings in the US and did not account for this.  I read that an ultra expensive Japanese solid state amp made by Technical Brain also constantly failed in service in the US but never in Japan.  It turned out that Japan has rock steady voltages and the manufacturer assumed the same for the US.  It appears that builders in other parts of the world are slow to pick up on something we living in the US are aware of--the US is rapidly slipping into 3rd world status.
larryi
3,738 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.

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