I have the S-30. NOS tubes are essential, as they are for my Audio Horizons preamp. Finding the best tubes can take time and money. But once you do there is no going back.
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The American 6AS7Gs can have issues, although we have found that the 6AS7GAs are the ones that are most problematic.
Here is what we know about the American tubes so far:
1) you get best results if your amplifier is a Mk3 or later.
2) If NOS, the tubes should be preconditioned for 4 days and 4 nights. This is done by placing them in the amp and putting it in Standby only for that time. If you apply B+ during that period the preconditioning process is ended.
Preconditioning can double the life of the tubes, help restore a better vacuum by activating the getter, and will also reduce the possibility of premature arcing.
BTW this process applies to all output tubes, assuming they are new/untested or NOS.
I would regard the use of American 6AS7Gs as experimental as you may run into premature arcing, but some customers have had excellent results. I think a lot depends on the tubes, if they are really NOS, that sort of thing. However- stay away from the GA varieties- they won't work.
Morgan Jones, he of "Valve amplifiers" fame, has written a piece on baking tubes that might be of use here. It's a variant on what Atmasphere talks about above, but using an oven to speed up the process of activating the getter flashing to absorb residual gas. Reductions in grid current are measured and found to be quite substantial, so long as you take care not to damage valves with phenolic bases.
Morgan Jones, Baking Valves
Preconditioning can double the life of the tubes, help restore a better vacuum by activating the getter, and will also reduce the possibility of premature arcing.Atmasphere, this is very interesting. All is not just 6as7g variant but ALL power tubes.
If you apply B+ during that period the preconditioning process is ended.I assume this is not done during tube testing/matching?
I'm a new Atmasphere dealer but of the 3 x S30 amps and 2 x M60 amps I've sold I now only use a Russian 6H13C tube with the best sonic results and no arching. As Ralph states preconditioning really helps. On the side of the tube etched in a box on the glass it has the following:
V1 - 823 r.
I bought over 200 in original NOS boxes but unfortunately I've sold all and my source in Russia is trying to procure more. If you can find these they are worth the asking price and in quantities are no more expensive than the 6AS7G tubes.
Just to throw my two cents in and since I started the thread: regarding input tubes , in my atmasphere ma 2 3.3 amps RCA 6sn7 input tubes sound the best to me. I also have a set of silvania 6sn7's. RCA's seem to add a great warmth and smooth vocals ( like wow vocals) , to me the silvania's sound thin in a bad way, sound stage is not as forward also. So there is no doubt that anyone would hear the differences in sound of different nos input tubes, so input tubes do make a real difference.
Even more inportant and able to modify the sound are the tubes in the mp1 preamp. Again I prefer RCA.
The one exception is in the 1st position in the amps I was lucky enough to find a pair of kenrad jan vt 231 black bottle, that are quiet and not microphonic, this one pair of tubes adds the necessary bass to round out my amps with either set of tubes.