2. tech-talk
  3. 128788-2280

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
testpilot
896 posts
 
Which preamp are you using?  Perhaps it runs the tubes bard like the old Audible Illusion preamps

lowrider57's avatar
lowrider57 OP
5,396 posts
 
I'm using an Atma-Sphere UV-1 preamp which uses 2 6SN7's in it's line stage. It's an OTL and is paired with a SS amp.
Perhaps Ralph will weigh in regarding the biasing of NOS tubes.

@larryi , as far as tubes becoming microphonic, I have read that NOS 6SN7's do have a higher failure rate than other small signal tubes. The stock Chinese tubes, however, are trouble-free. But they don't stay in the preamp for an extended period of time.
I haven't been able to tell yet if a NOS tube is going bad in a particular socket. But, I've had two different tube types go to an early grave.
larryi
3,739 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.

ebm
9,311 posts
 
This is not true all depends on what voltage they run at had them in my preamp with great sound for years.
lowrider57's avatar
lowrider57 OP
5,396 posts
 
@ebm , so you mean high line voltage doesn't matter?

@larryi , I have a new transformer on my street to correct for power swings from old grid and now line voltage is high, but stable at 120V.
Thanks for your take on the availability of vintage tubes. That may be a possibility.
More to discover