Large resistances i.e. b+. dropping resistors or plate resistors (or cathode resistors in a cf) may need to be several watts even though the circuit may only be pulling a few ma overall.
P = E * I
P can be large even if I is small. In tube circuits you often have 10s to 100s of volt across resistors, so E is large, thereby leading to a large dissipated power, P.
You can also look at it this way: P = I^2 * R, so P is large if R is large, even if I is small.
Also note that if you run a resistor at its rated power it'll get screaming hot (250-300 ºC). Many, me included, derate the resistors to prevent these high temperatures. I often run the resistors at 1/5 to 1/3 of their rated power. Then the resistors run at a more reasonable 80-100 ºC. This also improves the reliability of the circuit.
Tom
P can be large even if I is small. In tube circuits you often have 10s to 100s of volt across resistors, so E is large, thereby leading to a large dissipated power, P.
You can also look at it this way: P = I^2 * R, so P is large if R is large, even if I is small.
Also note that if you run a resistor at its rated power it'll get screaming hot (250-300 ºC). Many, me included, derate the resistors to prevent these high temperatures. I often run the resistors at 1/5 to 1/3 of their rated power. Then the resistors run at a more reasonable 80-100 ºC. This also improves the reliability of the circuit.
Tom
All resistors have a maximum voltage rating. In general, small, short and low power resistors cannot be used in tube circuits because of their low voltage ratings.