I just got 2 of these from ebay. they have GT.BRITAIN stamped on them. anyone know anything about these, or where i can find some info?
My guess is that GT marketted that tube as a 6V6 not because it was a crappy tube, but because the only 6V6 in production at that time was that Reflektor black bottle thing that was absolutely horrible. I don't know how many times I'd stick them in a Princeton Reverb or Deluxe Reverb and it would be instant fireworks.
LABELED made in GT Britain, actually made in Russia. Yes, those are the same low end "6L6GCs".
During the "Cold war" (if you're too young to remember, google it) the US did not trade with Soviet block countries. Since tube production in the US was on the downturn in the 1970s, some craft guys in Western Europe would import Soviet tubes, print "safe" countries of origin on them and sell them to US companies.
Nothing is ever 100%. Sounds like you got lucky with your tubes.
The issue with tubes such as those is that they'll test fine in a tube tester (typically with no higher than 150V on the plates) but short out when installed in an amp, like a Twin, with 450V+ on the plates.
Several years ago I did a simple repair on my friends Bassman 100, he had just put 4 new Audio Glassic 6L6 tubes in, they didn't last a second, he turned on the amp, it blew the grid resistor on one socket and the fuse. After testing all 4 tubes 2 were bad. They looked just like the Westinghouse I have except it said Made in Russia. I put in 4 of my Westinghouse and it still workes fine today.
I'm begining to wonder if there was more than one factory making these tubes. Like I said earlier, I only found 2 out 48 that tested poorly and even they worked but I wouldn't use them.
Probably just the luck of the draw. Time is also a factor. Just because smoking doesn't kill you in two days doesn't mean that it won't in the long run