I have the most basic knowledge of Glenn's solid-state designs, but if they are built along similar lines to his valve designs, they will be extremely robust but very basic circuits.
I would suggest trying someone else to look at it. If you are in the NE, you could try Jez Arkless.
I have no idea if it could be the valve, but changing it would have been my very first attempt to fix it. Is the problem definitely with the 7?
Lots of his circuits appear, scattered across the www, but all, or virtually all, that I have seen have been for pre's and have been drawn by owners who drew them out and put them online. They may exist, but I have not heard of a diagram coming from Glenn.