Atma-Sphere MA-1 tube rolling

I was most surprised that this was the failed tube, as it was a new tube, with less than 30 hours on it, and it was a modern Tung Sol 6SN7 GTB. . . so it appears that currently Ralph’s suggestion to stay away from the Russian 6SN7’s seems to be rather accurate.

If you look on our website you'll find this warning, which has been there for nearly 20 years now

NOTE REGARDING SOME TUBES
We have seen on-going problems with 6SN7s marked as "Sovtek" and "Electro Harmonics" More recently we have seen similar problems in reissue tubes marked "Tung Sol" and "Mullard". We do not recommend these tubes in our products. While they will not damage our products, neither will they perform optimally or reliably. If you are experiencing a problem and any of these tubes are installed, replace them first before doing anything else.

ralph, any theories or thoughts as to why this is the case?

We suspect that the environmental control is poor- tubes are made by hand and we've heard that they don't use Latex gloves in their assembly. That comes from Eric Barbour back when he was involved with the US Svetlana brand and so may well be completely inaccurate at this point but we see the same problems now that we saw 20 years go when he was there. That's my best guess as this would affect small signal tubes more. A pragmatic individual will simply roll with the punches...

But it sounds like I should stay away from Russian 6SN7's, which would include National, and some modern Tung Sol from Russia?

I can't speak to the National brand- some of that might be production prior to the fall of the Soviet empire and might actually be OK (Russian 6SN7s from the 1960s can be excellent, but pricey and the differences in appearance from the current Sovtek junk is subtle so buyer beware). But IMO the EH, Mullard or TungSol (which are all nearly the same thing) don't hold up to the (advertising) hype.

FWIW though we have seen some of the Gold Lion branded 12AU7s out of Russia be quite good.

Have your customers mentioned to you what changing out the stock 6AS7G tubes for either the Russian 6H13C or the Chinese (I assume ShuGuang) tubes do differently, for better or worse?

Actually the stock tubes **are** either the Russian or Chinese tube (which seem to sound the same; the difference is in how they fail).

Would you suggest just rolling the 6SN7 driver tubes for more adjustability? 

People roll the 6SN7s all the time with nice results. We've found that that Sophia tubes while sounding great at first, seem to have a short life of maybe only 6 months, whereas a regular NOS tube in the same spot can go for years. The one tricky bit is the driver tube which in the MA-1 will be in a different location depending on when the MA-1 was built. If you have a version with reflected left and right hand chassis the driver tube is on the corner facing the power tubes; if a newer chassis the driver tube is the one in the front. This particular tube must have a -GTA or -GTB suffix if you're using an NOS type. Modern 6SN7s like the Shugang 6SN7 or Sovtek are marked 'GT' but are actually GTBs.


If you are forced to use Russian made 6SN7s (which conversely and confusingly from the Russian power tubes are usually a poor choice), they should be screened on a tube tester to show no leakage at all on the leakage test, otherwise you may encounter a buzz. The lifespan of these tubes is also really variable going anywhere from 2 weeks (in which case send it back to where you got it and get it replaced; most tube sellers have a 90 day warranty) to maybe 2 years.

@drbond
My advice is to stay away from the 6080s on these amps! They are not built to the same spec as 6AS7Gs. The 6080 typically has a smaller grid heatsink and since the amp is biased in class A2, the grid can warp at higher power levels, causing the tube to arc- and often taking out multiple tubes at the same time. This won't damage the amp or speaker, but it can result in the user getting gun-shy and thinking there's something wrong with the amp when there isn't!  Some people claim to have had good luck with 6080s but IME they've told me that while dealing in the same breath with failing tubes. 


This advice extends to the 6AS7GA as well as any other variants. Stick to the 6AS7G. Now when dealing with American versions, if the tubes are NOS they will need to be preconditioned for *at least* 4 days and 4 nights in order to have any reliability. Otherwise the cathode coatings flake off and cause the tube to arc and fail. Of all the vintage types the Sylvania made examples hold up the best (and not surprisingly are the hardest to find).


So the Russian '6H13C' is the most reliable IME. If you use the Chinese 6N13 make sure its a military example such as seen with the 'PJ' suffix. The Chinese tube can develop what looks like a smoke ring on the inside of the glass; if you see this discard the tube (because the tube is failing and might arc). The Russian tube does not do that.