Be assured, it is not a recitfier. It's a power pentode of some kind. More then likely a 6L6G as Kevin said. Whoever told you it was a rectifier was wrong. If it did come from a power supply, it was used as a series pass tube as 6L6's often were. And the "dag" coating on the inside of the glass was common on the older 6L6Gs. I have never seen a rectifier tube with an internal dag coating.... i was told that its a rectifier tube, im building a tube power supply, so i need to be sure.