You're doing great here, Prof. But working on the floor sounds harsh. Can you tack a couple chunks of 2x4 on edge on some scrap and put the chassis on a table? Even having it in a cab isn't quite as accessible. 'Live' measurements on that 6V6 socket aren't the easiest for sure. You may, as you suspect, be measuring pin 3 with or instead of pin 4. But you can check the wiring on a cold amp with drained caps using just the continuity setting on your MM, pin 3 to the left side of your 10K dropping resistor, pin 4 to the right. Also make sure pin 3 and pin 4 don't show continuity -- solder drops and wire tails love to snuggle up in those tight quarters. As long as your dropping resistor measures ~10K, you shouldn't worry too much about the actual B+1 to 2 drop, but if they're wired right you'll see a similar drop between 6V6 pin 3 and 4. (OK, dead horse, take a break). If you have a JJ 6V6S sitting around, I use those for initial startup -- you get 14w and if you kill it somehow it isn't $$ to replace. Likewise if you have more than one 5Y3, see which one gives you the lowest B+. And if you have a cheap but reliable modern 12ax7 sitting around, same idea. With all the tubes in, did you re-measure rectifier pins 3 and 5 (equivalent to 4 and 6 due to diodes, as you note)? I may have missed it, but that'd be helpful. Finally, I doubt this has anything to do with your B+, but for safety and sanity, let's double check your AC power wiring. There's a white wire where I expected a black one, but overlapping stuff always makes it hard to trace in photos, and maybe that's a white jumper and not the household neutral? Anyhow, AFAIK, it should go like this, noting that the two PT primaries are interchangeable, but the order of the wiring connections and which one is hot v. neutral is a big deal.