The "W" is a mil-spec designation. The "GT" is the oldest, followed by the "GTA" and "GTB". The earlier are usually the better. However GT and GTA and WGT are all great. The GTB is very good if made during the first half of the production life of the GTB, and they went somewhat downhill toward the end. Still a good tube though. Any GTB, even the worst ones, are alot better than any of the newer stuff, IMO. I'd try to go for the older stuff whenever possible.
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