Screen resistor question

[drewl]

So I have a Peavey Classic 30 and notice only 2 of the output tubes have a 100ohm screen grid resistor, Marshall used the same set up in some of their early amps.
So what are the benefits or drawbacks of having the resistor on only 2 of the 4 output tubes?

[Enzo]

Some amps have no screen resistors. When you double up the tubes, they add the resistor to isolate the two tubes. In other words the resistor is not there so much as a typical screen resistor in series with the screen supply. It is more about not connecting the two tubes directly together.