[RGallahue]

Well, the only other difference between yours and mine is the capacitive coupling across the interstage transformer primary. Looks like all other possibilities are exhausted. I know you are trying to maintain originality but it would be interesting to splice a replacement transformer across there and see if that resolves the issue.

Also, thanks for the link to your blog. Going to pay more attention to that in the future as I could have used a few of those pointers when I was rebuilding mine!

For a replacement of the interstage transformer I used a Hammond 124a which does a fine job driving the output tubes. It is also small enough that it might either fit inside the case or you might be able to bury it topside within the old transformer housing reconstructed around it.

Good luck!