Originally posted by Number 9
It was a BAD tube that tested GOOD. It was the original diamond-bottom Telefunken that came with the MR67. And it is as you say, because it was being tested cold rather than hot.
Now, I had asked about the 6GH8 at the time PRM, because I saw your comment once before. The fellow who sells me my tubes (and I trust him, he's been at it for nearly 40 years and has no alterior motives) told me that they are very unreliable. He had some of those in stock too, so no skin off his back if I had bought one. He said they were designed for TV use, and said they used to fail very frequently. Miserable in fact. He would buy them buy the case once a year because they were the most common part to fail on old RCA televisions.
Ampex (Made in France) NOS 6U8s are supposedly the best but are extremely expensive.