Yes...I was impressed by some of the construction...and good to see that someone has put 5KV on 100th triodes....Class A?
That would exceed the rating of the tube by quite a bit, I would expect... fwiw, and imho.
Yes ...You are correct....However, I have run tubes at (much) higher than ratings and had good results in Ham radio setups...I've always wanted to run 250 ths and 100ths at 5KV levels but I never have .....Not enough spares I guess...