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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 7 responses by lowrider57

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.
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.
lowrider57's avatar
lowrider57 OP
5,396 posts
 
I have had one microphonic tube that you could hear the effect thru the speakers and when tapping. Right now I have a matched set of 6SN7GTB’s that are microphonic when tapped only. They sound wonderful, and the problem is due to the luck of the draw from the vendor, but microphonic after only a few months is getting expensive.

@rfogel8
For example, a couple positions in the MP-1 require a 6SN7 GTB because it can handle higher plate voltages. Using a 6SN7GT, VT-231 or 5692 which can’t handle the higher voltage will most likely cause an early death.

This is very interesting information. I’ve had a red base RCA 5692 go bad, plus a pair of VT-231 went bad. So far these are my favorite, and are expensive.
I think I will order a pair of new production tubes so that I have a spare set. Any recommendations for non-Chinese tubes with good imaging and smooth highs?
FYI, classical music.


lowrider57's avatar
lowrider57 OP
5,396 posts
 
Concerning 6SN7's, it seems to me that most of the noisy(kind of a spitty sound) and microphonic tubes I've encountered have been very old 6SN7GT's and their equivalents. 
That's a good observation. I'm not going to buy anymore tubes from the 1940s, and 
maybe 6SN7'S from the 50's are a risk. Although, 1950s were the glory days for tubes.







lowrider57's avatar
lowrider57 OP
5,396 posts
 
Thanks @jond , I’m going to ask Ralph about plate voltages and if I need to use 6SN7GTB’s.
Any other recommendations for a modern tube?
lowrider57's avatar
lowrider57 OP
5,396 posts
 
@ericsch ,
When tapping with your fingernail or a pencil, it will sound similar to a bell ringing. You can usually keep those tubes in system with tube dampers on them. Very often when you tap, they will have distortion and a nasty ringing sound thru the speakers. Those tubes are NG.
lowrider57's avatar
lowrider57 OP
5,396 posts
 
Much appreciated, Ralph.
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