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
ebm
9,311 posts
 
If you have a big swing in line voltage this will shorten tube life as well i don't have that problem.

jond's avatar
jond
5,678 posts
 
Perhaps try using a variac to run your preamp off? I was having voltage issues and high rates of tube failure and got a variac for my amp, I run it at 115 and it's resulted in much longer tube life all around. They are easy to find and cheap on ebay.
bigkidz
3,324 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
3,048 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
751 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.
    
More to discover