I've been teaching myself electronics for a while now and am having a hard time understanding something. Voltage drop. When you want to drop voltage from one value down to a lesser value and are choosing the resistor to use, I've often heard it said something like this: "If we know the current and the voltage we want across, then it's just ohms law, IR=V". What I don't understand is how can you know the current because as soon as you place a resistor in the circuit it changes the current. V/R= I. The only thing I can think of is if the current source has such high internal resistance that adding any resistance would have little effect concerning the V/R=I formula. I hope i'm making sense. thanks A good example would be using screen resistors to drop some plate voltage. If I have say 450 B+ and want to drop it to 420, how would I go about sourcing a resistor? Or perhaps a better example would be choosing the right sized cathode resistor on a preamp section to make the cathode some positive value with respect to the grid, thereby making the grid negative with respect to the cathode.