Atma-Sphere MA-1 amps

The American 6AS7G and it variants can be problematic due to early arc-over failure. We have had some reports that the tubes hold up better in the MkIIIs and in fact what little testing we have done seems to confirm that. If American tubes are tried in the amp though, it must be regarded as experimental. One recommendation I can make is that the tubes, if NOS, should be preconditioned (tubes placed in amp and run in Standby only) for 72 hours before being used. That seems to reduce arc-over failure with them.

Looking at the various posts on-line regarding NOS 6SN7s, I see lots of references to the various Sylvanias (GT, WGT, GTB) except the 6SN7GTAs. Are the GTAs somehow inferior to the others?

I think the GTAs and GTBs are essentially the same sonically. The older GTs are preferred by many, but can't be used in all the Atma-amp tube positions becuase the can't handle as much current as the GTA/Bs - they seem to be fine in the preamps. I think you hear more about the Bs because thet are more readily avaiable.

I agree with Pubul57. I used GTAs and was very pleased. When I looked to get more, Andy Bouwman at Vintage Tube Services told me there was very little difference in the sound of the GTBs up through the early '70s manufacture. That's what I'm using now in my MA-2s, and I don't notice a difference. From what I hear in the amps, I wouldn't pay a premium for GTAs over early manufacture GTBs (up to early '70s).

The GTs have a bit of special, but as Pubul57 notes, you can only use them where the high plate voltage capacity of the As and Bs is not required. I need to get a quad to try once again.
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This is a very interesting thread. Trcnetmsncom, I looked up the impedance curve of the Avalon Eidelon. It's below 5 ohms (closer most of the time to 3 ohms) between 50Hz and 20kHz, except for a narrow "peak" in the midrange, where it gets up to 7 ohms. This is not an optimal match for any OTL (like the Atma MA2) that does not use negative feedback to achieve a low output impedance. However, the MA2 has such huge power capability that you are able to overcome the impedance matching problem to a large degree. Nevertheless, I wager that if you were to interpose a pair of Paul Speltz's Zero autoformers between the amp and the speakers, the perceived unpleasant treble distortions would disappear. Given the large investment that you have already made in the amps and speakers, the Zeros amount to a trivial additional expense. I don't think one can ever talk about how an amp "sounds" without considering the speaker load.