Bad Power Tubes?

[Cap217]

I think my amp has bad el34 but I’m not sure. What I do know is that using my bias probe On first power up I get plate voltages at 470 then they dropped to 435 after a few minutes and have stayed there.
I’m sure there are many things that can be wrong but I am wondering if this is a sign of bad power tubes? I’d start there if so.

 

[Samhain]

You'd know it if it was bad.
Lots can cause a bias to drop 30 mv. And that doesn't mean its bad.
Are you setting the bias @ 70% dissipation ?