So the other day I took my single rectifier in to get the power tubes replaced, since the shop does it for free while under warranty. They replaced them and it seems to work fine. I noticed the color code on the new tubes was green, while the old ones were yellow. The manual is not clear on whether the tubes need to be the same color code. It was my impression that the fixed bias only works with one color code, but I don't know for sure. Later I noticed that one tube was glowing very bright compared to the other. This is what happened with the first pair of tubes, except they stopped producing sound all together (except for a very loud hum), but this time the sound seems normal. Anyone know anything about all this?