all tubes tested before starting work, but this one
croaked sometime during repairs.

I just picked up on this thread and read it with interest. Sounds like
you're on the right track. That's a nice receiver when working correctly.

One thing I'm beginning to appreciate about fixing old radios & TVs is that
some components which test "good" or seem acceptable at first, may decide to
give out after a few more hours of operation. The tube (or whatever) may
simply have reached the end of its service life, or perhaps it croaked after
running at full power for the first time in decades.

Before I declare victory on any project, I run it on the workbench for some
hours. Much better to find and correct those latent issues now, rather than
later.

Have fun,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html