My response:
Thanks for the photos. It’s always helpful to get a visual of what we’re talking about.
R1 and R7 are basically in parallel with each other so they should measure around 750 ohms each. And that’s fine if R4 is a few ohms off, it’s still within a it’s 5% tolerance.
You said it sparked at the ground terminal, but can you tell where it sparked to? There was/is some short of short against the ground terminal there….was a strand of wire touching it somehow? It’s important to know why that happened so you can ensure it won’t happen again. Please replace the fuse with a 1 amp / 250V fast blow fuse….nothing bigger!
The voltages you shared from your test points indicate that the tube isn’t installed…is it? If it is, then please tighten
all the tube sockets to ensure all the pins are making good contact.
Keep me posted on your progress.
-Brach