[doug-5er]

Today opened it up.  Pretty clean and all.
Turned it on to confirm the problem was still present, it was.
I thought, since the change happened suddenly, it had to be a contact, so I pulled and re-inserted tbe 83 and 5Y3 tubes. The problem had disappeared after that.
I washed the panels' contacts with alcohol (easy on the 83, not so on 5Y3 but I gave it the best effort) and then applied some Deoxit with cotton swab. In 5Y3 I first scraped the contacts with some sand paper since I could not wash them well).

After it tested the 16's tubes.
Works fine.

Sometimes it's just the little things that drive you Glad you got it working