If you check some of the data sheets for power tubes you will often see mounting instructions. Almost any tube that generates a lot of heat should be mounted upright, or sideways under certain conditions
Upside down is still "upright" to the 2A3. With no enclosure around the sides, they should stay pretty cool.
I was watching this pretty carefully - more for smoke pouring out of the top of that table
but the 2A3's run fairly cool - especially out in the open like that. Overall the entire amp ran cool...then again it is winter and the house isn't exactly 90 degree's inside!
One short WTH moment is at where the ground goes to terminal 8 early in the directions but then the pictures show it on terminal 10. It's electrically the same so it doesn't matter but it threw me for a loop for about a minute as I went back to check the directions.
Also voltage check on terminal 5 and 6 are reversed from what I could tell.