[tym]

Well, my shortcut for fixing the broken grid wire did not prevent the volume drops, as one occurred in less than 30 minutes of operation.

So, I removed the grid cap, cleaned the grid wire stub of oxidation, crimped on a short extension wire, and soldered it. Following a suggestion I read a while ago from Marcc on ARF, I then added a gentle bend to the extended grid wire for strain relief, reattached the grid cap, and soldered.

I'm playing the set now and will see how it performs. If the problem persists, I'll get and swap in another 75 tube...it's the only tube in the set for which I don't have a spare for troubleshooting.

EDIT:

Issue persists.

I then decided to pull all the other tubes with grid caps (36, two 39/44s). Two of them had the same issue as above (tiny grid wire stub barely poking through the hole on the grid cap). The RF 39/44 was good in that it had an 1/8" of grid wire bent over and soldered to the grid cap.

I repaired the 36 and subbed a good tested Philco globe tube in the meantime...issue persists.

I then attempted to repair the IF 39/44, but the evacuation tip came off with the top cap...oops. Oddly, the getter didn't turn white. I subbed in a replacement (milspec VT-49) and play tested for an hour (all the time I had today). So far, so good.

Fingers crossed.