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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
jond's avatar
jond
5,655 posts
 
I haven't tried them myself, my amp has 3 6SN7's upfront, but folks seems to like the Sophia Electric 6SN7 in terms of current production tubes.
lowrider57's avatar
lowrider57 OP
5,393 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
5,393 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.
ericsch
749 posts
 
Thanks, Lowrider57. Very interesting discussion.
 
atmasphere's avatar
atmasphere
11,566 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. 
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