Ive discovered a really good video, it's authored by a teacher so it's no surprise that it's so good. He teaches us in a very teacherly hypnotic, re-assuring voice that plate load resistors have two very important and different functions. 1) the first I already knew about, which is to simply step down the voltage off the b+ so that the plates of the tubes can work at a voltage level they're happy with. 2) the second, ( i knew there was a second function but all i knew was it had something to do with some sort of blocking function.) The gentleman explains with very easy to understand diagrams how the plate load resistor stops the ( now increased ) AC signal current from simply joining the b+ voltage back on the rail. This sounds like a pile of horseshit from me just reading it here, but watch the video and it's as clear as day. As a bonus, he carefully explains with excellent diagrams how tubes ( and amps) work. For those who get real bored and just want to see the money shot 14:33 on the video explains this second AC 'blocking',( moreso, Hindering) function 10:20 on the video , he explains how tubes work in the first place with excellent diagrams. the first function of a plate load resistor is explained from the beginning of the video.