Besides rattling around in my trailer, the tubes have been subject to other vibration and attitude change over the years. There is a filament inside, and it's possible that the vibrations will have caused one or more of them to detach. One easy way to tell is to measure the filament resistance with an ohmmeter. The 811 has four pins plus the anode cap. The smaller two of the four pins go to the filament.
One of the tubes was actually unseated from the socket, and the other two were partially out, maybe a millimeter or two. To extract them, you need to disconnect the anode cap.
When I did this, the cap came loose and stayed inside the clip for two of them. I suppose that the glue had baked over the years and became brittle, so the only thing holding the cap on was the wire itself. It's not very strong, and the retention force of the spring in the clip was stronger. So it came off, broke the wire, and essentially made my decision for me, to replace the tubes.
I found a set from Tube Depot (naturally!) and they gave me BLINDINGLY FAST service. They matched a set of three and burned them in for a few hours. And then shipped them. This was all done in less than 12 hours from when I placed the order - that is, I ordered them last night and I got an email this morning that says they are shipped. https://www.tubedepot.com
The final chapter of this story is at https://www.99hobbies.com/amp-is-on-the-air
