The 12AT7 is durable and usually used as a phase splitter or driver tube (part of the power section). It is great for most US made amps, especially Fender. Great for Marshall in the driver stage for a cleaner, brighter tone (should be changed every time power tubes are replaced for best performance).
Preamp Tubes
These tubes are used in the first, low-level stages of tube amps. There are many designs that will interchange and each has its own tone, gain, and response. The best example is the 12AX7 range that includes more than 6 options (i.e.: ECC83, 7025, and 5751). These may be interchanged without any modification to the circuit... user friendly, so experiment!
The Groove Tubes Warranty
Preamp tubes: Warranted to be free of burn out, excessive noise, or microphonics for 180 day from date of purchase.
Power tubes: Warranted to be free of burn out for 90 days from date of purchase. If one tube fails, the whole set will be replaced to ensure proper matching.
GT-Solid Rectifiers and Substi-tubes: Warranted for 3 years from date of purchase.