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

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.

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

Three pieces of my audio gear run 6SN7's;  Atma-Sphere MP-1 Mk3, MA-1 Mk3's and a Modwright modded CD player, a total of 22 6SN7's. After initial break-in, all 22 stock Chinese tubes in all three pieces were replaced with NOS. Needless to say, large improvements across the board.

The Atma-Sphere gear is almost ten years old and I have yet to replace a tube, not even a power tube. With all those tubes, I also thought it sensible to own a Hickok 533A tube tester and I check things periodically.

I'm not familiar with the UV-1 but I'd definitely check with Ralph. I remember talking to him early on about which tube positions in his equipment would bring the biggest improvements when rolling. He made it a point to say some circuits and some brands of equipment are more demanding on tubes. 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.  

I've never counted but I must have over a hundred NOS triodes around here with a few nice rectifiers thrown in. 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.

The red base RCA 5692 is a perfect example; some have a brown base and are branded Sylvania but were made by RCA. When they're quiet and up to spec, that tube can last beyond 10,000 hours and it can sound wonderful in the right gear. I've never had great luck with the various VT-231's either. They're an old tube and supposed to be a more "rugged" version made for the military. Most of the ones I've heard were microphonic.
The exception, and I'm guessing I was just lucky, is the highly sought after black glass Tung-Sol 6SN7GT sometimes branded VT-231. I found two pairs of them several years ago and was able to hear them in my own equipment before plunking my money down. They still sound and test fine; actually, they sound exceptional.

My advice, call Ralph. I'd also pick up a couple inexpensive RCA GTB's and give them a try. Some prefer the Sylvania's but to my ears they can sound a bit aggressive
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.


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.