Unlike regular triode tubes (that have a plate, a grid and a cathode) power tubes tend to have an extra grid called the screen grid. It's charged with a high positive voltage, and designed to encourage the electrons to fly towards the plate and produce more power. But more often than not, they miss the plate and splatter back onto the screen grid. The screen resistor connects between the screen grid and the source of voltage, to stop it overheating if too many electrons happen to hit it.
"Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"
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