Atma-Sphere MA-1 amps

Spencer, we've not used 12SX7s at any time, but at one time it was a popular field modification. The 12SX7 has similar specs to the 6SN7 and it is easy to set op the filaments for 12V, so it was a way to get access to an unused tube stock.

Ralph, is there any problem using US made (e.g., GE) NOS 6AS7s with MKIII amps? I thought I read somewhere that they could be a problem with fixed bias amps (I take it the MKIIIs are not).

Please let's not forget that the Chinese-manufactured 6SN7s are actually very nice tubes. I've paid a bit of a premium for Sophia tester "A matched" metal base military pieces from DIY HiFi Supply, but still only $18 apiece.

Matched pairs of Baldwin labeled 1965 vintage Raytheon GTB are being sold by Brent Jessee, a reliable vendor, for $45/pair.

I just took a gamble on four supposedly "matched" quads of Baldwin labeled Sylvanias of late 50s/early 60s vintage with the BIG chrome domes from Ebay at under $15/tube. The photos are quite sharp and show very clean tubes with spotless pins. We'll see how they test on my B&K Model 700...

One needn't pay upwards of $100/pair or even much, much more for quality 6SN7s. Just doesn't make sense.

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?