Hi Ken
That seems to be a tricky one to find but the prices from some suppliers isn’t that bad compared to some wanting the world. What is the application other than a guitar amp? I have 5U4’s crawling out of the woodwork in my hoard but other than that they are usable depending on how heavy your transformer is as the 5U4 has a draw of 3 amps compared to 1.9 on the 5R4 and some mention the 5U4 will end up with a small voltage drop as well. I think the 5U4 was very common in the old B&W TVs if memory serves correct.
