We have the voltage values all day long on this schematic, which was only an example so everyone could see what resistors I was talking about. And I know that on this schematic, we can work backwards to get the current value. But I needed to know the other way around. I'm designing an amp. I know I need a dropping resistor. What value resistor do I use?
That aside, I have actually been able to piece together the answer to my question which was if I'm starting with a blank sheet of paper, how would I calculate the value of those resistors. I know that Fender, Marshall, and all the others didn't just throw darts and voltage and current values, they started from somewhere. Well, turns out the answer is called a load line calculation. I'm still getting my head around it, but you have to go to the data sheet, look at the graph that shows supply voltage vs plate current based on the grid input, decide what voltage and current values you want to use (that's where I'm still a bit hazy) and draw a line between the two. Then from that you can decide what plate voltage you want to use, which then gives you the difference in the supply voltage vs the plate voltage, and since you already know the current, you can figure the load resistor value. And since we now know the supply voltage we want, we know how much we want to drop it from the last voltage on the B+ rail, and using one thing that has been stated, add up all the current from that and all other downline stages.
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