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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
atmasphere's avatar
atmasphere
11,794 posts
 
For example, a couple positions in the MP-1 require a 6SN7 GTB because it can handle higher plate voltages.
Just for the record, that's not the MP-1, its the MA-1 (I'm assuming this is a typo), which like all of our amps has a direct-coupled driver tube which has a fair bit of voltage, just at the upper limits of what a GT is spec'ed for.  Despite being marked as GTs, the generic Chinese 6SN7 has no problem giving extended service life in this location but when people are using NOS tubes we make a point of letting them know that a -GTA or -GTB should be used in this location. 

The UV-1, like all of our preamps (including the MP-1), does not have any high voltages that could threaten a 6SN7.

I can't think of a circuit malfunction that could cause tube to go microphonic! Excess voltage would not do it- that would contribute to a tube's loss of transconductance, perhaps going gassy, but not microphonics. The latter is as far as I can tell, a defect related to manufacture. 

We do run into microphonic 6SN7s when picking tubes for the preamps. We just avoid them and rarely have to warranty 6SN7s on this account despite a 1 year warranty on our tubes. 
lowrider57's avatar
lowrider57 OP
5,417 posts
 
Much appreciated, Ralph.
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.
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