R66, R67, R68, R69 are power tube screen resistors 470Ω, 5W, 10%. They should be flameproof.
R71 and R72 are 100Ω, ½W, 1% resistors. The artificial center tap for the heater. These act like fuses and blow if there is an issue with the tube heater circuit.
I would change them all. Clean up all the char with isopropyl alcohol.
You should be able to obtain a copy of the schematic by contacting firstname.lastname@example.org