I have seen a number of posts discussing the importance of biasing output tubes to get the correct plate current, and I understand the concept. However, I pulled down the schematic for the T-E Quatra/Hexa heads, and it appears the plate bias is fixed by resistors that are hardwired in. They appear to have set the bias voltage at -42V and it appears fixed. When re-tubing, are you supposed to solder in new bias resistors, re-valued for the plate current? That seems ridiculously complicated. If that were required as SOP, they should have just put in trim pots. What am I missing here? Thanks.