I reached out to TAD about what EL34's they'd recommend for an amp that sits around 520V at the plates w/the screens around the same. I don't know the exact screen reading, but it's on an older modded Marshall and I've always heard Marshall screen voltages tend to only be a few volts below the plates. Anyway, they recommended the TAD EL34-STR Redbase. However, the datasheet says 500v at the screens. To be safe I have always bought tubes that say they will handle the same or higher voltage than what my amp puts out, but since they recommended these it got me scratching my head. I guess I don't really know how absolutely critical it is for the screen voltage to be at or lower than those datasheet numbers. Would grabbing these tubes just be asking for early failure or is this in more in the range of "it's close enough"?