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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
rfogel8's avatar
rfogel8
192 posts
 
Ralph, I actually meant to type MP-1 and my notes from a conversation we had some years ago indicate that the pair of 6SN7's between the 12AT7's on the left and the three 6SN7's to the right in each channel are Constant Current Sources and that they should be low noise and GTB's? Did I miss something?

The three in line on the right side of each channel should be Voltage Amplifier in front then a pair of Output/Power tubes to the rear. Rolling that Voltage Amplifier with NOS makes the biggest difference. 

In the MA-1 amps, looking down on the five 6SN7's in each channel, the "direct-coupled driver" is the forward tube nearest the face plate. I do show a GTB in that position and I've found from experience rolling it can bring nice improvements. 

A question, for those of us who have tube testers, do you think running the "life test" stresses the tube and perhaps causes premature failure down the road? From what I understand, when selecting "life test", voltage is reduced by approximately 10% so maybe not?
atmasphere's avatar
atmasphere
11,794 posts
 
In the case of the MP-1, the CCS tubes can be GTs. They only have about 100V per section on them.

The life test won't damage the tube at all.
rfogel8's avatar
rfogel8
192 posts
 
Thanks.
mechans
3,048 posts
 


Confessions of a 6SN7 addict-

I collected a large variety of 6SN7s to serve as drivers in my first foray into modern tube amps (a Consonance M100 + ). In that role they demonstrated their differences some quite distinct and some only nuanced. The stock S tubes and EH tubes however were no match against the old tubes and thus the slippery slope of tube rolling. 

Then  I bought an inexpensive preamp which uses 6SN7s as the main amplifying element. All of a sudden some of those perfectly good tubes sounded precariously micophonic or otherwise noisy.  Some of them were fairly expensive 'precious' old stock GTs.  The tubes were still quite functional in almost all cases but I couldn't subject them to trauma.  The upshot is that microphonia may be a problem in one circuit and not another, in addition,  particularly  in a gain section of a preamp.

rpnz's avatar
rpnz
28 posts
 

I had tubes go microphonic (x2 NOS Black bottle KenRad VT-232 and new Psvane bottle special edition) when i used to tube roll tubes in and out a lot. Since i stopped swapping tubes i have had none go microphonic. Over 4 years now. I am not sure if this helps, just my personal experience. Cary SLP 98P preamp

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