[nnck]

I posted this in the Modwright circle about a week ago but got no response so far, so I want to try here:

What would cause my 6SN7 tubes to do this in my Modwright LS100 preamp?

I had been using a pair of Sophia Electric Grade A 6SN7 for about a year and a half. A few months back, I switched in some NOS tubes just for a change of pace. Recently, I decided to pop in the Sophia Electrics again. When I put the Sophias back in, I swapped the Left-Right positions of the tubes from the last time they were removed, as I often do. When I powered everything on, there was a great deal of noise coming out the speakers. Not just tube rush, but real hiss and white noise. Swapping the positions of the Sophias back to how they were originally the last time I removed them quieted them down to where I am only hearing tube rush (although the tube rush is noticeably louder than what I hear from some of the other tubes, including the NOS tubes and the stock tubes that come with the LS100). I've tried swapping positions Left-Right and back several time with the same result.

What is this? Is one 'side' of one tube bad?

[JakeJ]

You might want to contact Modwright directly for this.

It doesn't make sense that's for sure, unless the circuit topology only uses one of the two triodes in a 6SN7 per channel and switching the sides placed the unused triode into active service.  I kinda doubt that but you never know.

[nnck]

It doesn't make sense that's for sure, unless the circuit topology only uses one of the two triodes in a 6SN7 per channel and switching the sides placed the unused triode into active service.  I kinda doubt that but you never know.

Actually, I do think that only 1 of the 2 triodes of the 6SN7 tubes are used per channel in the LS100. This is part of the reason why I swap the Left-Right 6SN7 tubes in the LS100 from time to time. I found this quote of Dan W. of Modwright in the 6 Moons Audio review:
"About the 6SN7 double triodes, we use just one half per channel. This gives the advantage of extended tube life as you can channel swap the tubes to use the other two halves. Theoretically we could have used a single 6SN7. In the case of our phono board, we do use a single 12AU7 and 12AX7 for two gain stages, again half a tube each per side."

So it would appear that my guess is right that one triode of one of the tubes has gone bad. I suppose there was even a chance that the one triode was bad to begin with- if it was only ever in a single position in the preamp all along, I may have never known. I guess this is more of a question for why a grade A Sophia 6SN7 tube would have one triode go bad. I'm still a bit of a tube newbie, so maybe this happens more often than I was aware.

[JakeJ]

OK, I was wrong.  Happens all the time.

Agreed that a "Grade A" tube should not be faulty, especially if it's not all that old.  Maybe swapping sooner would have identified it and you could have contacted Sophia about it.  Are they still under warranty?

Tubes do go bad but that's part of the game.  Sorry for your luck.

[DaveC113]

Sorry to hear. FWIW have not had great luck with any new production tubes... NOS 6SN7s are a better choice especially, since there are so many good ones at reasonable prices. Even the cheapest 6SN7 sounds pretty good from my testing.

[Beta]

I too have a LS 100.  I have it paired with a KWA 100 SE.  A match made in heaven IMHO.  When I have the volume turned all the way down, I have a slightly audible hum.  Dan has always been very accessible and helpful.  We tried everything to track down my issue to no avail.  Dan recommended I send the preamp up to him to trouble shoot.  I just haven't had the time to do that, so I have put it off for quite a long time.

The post has motivated me to try to tackle this issue again.  Time to pull the top off and swap out both the 6SN7s and the rectifier.  I am running Psvane CV181-Ts and a NOS Mullard.  This will be the last thing I check before I package the unit up.  Thank you for getting me focused on this again.  Best of luck resolving your issue.