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No schematic in front of me.... and I won't be where the schematic is until at least tomorrow.
Here's a couple thoughts, though. The plate voltage change on the r-y amp is probably just due to the grid bias changing. In other words it's probably a normal reaction. As for the plate voltage changing on the previous tube, it might be unrelated. It's tough to guess without my schematic. I think the grid bias is the big thing.
The slowly drifting thing is kind of strange. I have had mylar caps fail in a color matrix, but the intermittent was sudden.
The cap is a possibility, though. Is it a purple mylar thing? Ceramic disc? It would be a good explanation for where the extra voltage came from on the grid, as the other end is connected to the plate voltage of the previous tube, and no DC should be getting through there. If the capacitor leaked, it would raise the grid voltage.
The grid resistor might be a better bet. If it changes value with heat, maybe that would do it. Drifting resistors are usually slower. I would lay a soldering iron on it and see if you can make the change. If the resistor looks like it is made out of something that would melt if you did that, then you could lay a soldering iron on the wire next to the body of the resistor. If you lay it on the wire instead of the case, the soldering iron would have to *not* be grounded I think.
My soldering iron is grounded, so I would instead probably hang an ohmmeter across the resistor and heat it up with the set shut off.
What this really sounds like to me is a gassy tube. You get slow grid bias change over time. I read up above that you switched the tubes to eliminate this, right? Are you sure they really got switched?
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