DO you have the RCA tube manual? Or at least find it online. The 6C4 is a small power triode in a 7-pin mini. It has curves in its section. The 12AU7 signal triode has curves, and the 12AX7 signal triode curves are found on the 6AV6 page. Plate voltage and current at various grid voltages. I think it is what you are looking for.
Oh wait, you are looking AT it, not FOR it. Oh well.
The grid voltage controls the current through the tube. Puting a load resistor in series with the tube then will have a controlled voltage across it due to the varying current. I myself don't think in terms of output voltage unless I have a circuit around it. You can take an output from various points. And by bottom resistor, do you mean the cathode resistor? IN a typical gain stage like a 12AX7 uses, the voltage across the cathode resistor also changes - the current through the tube is also the same current through the resistors in the plate and cathode legs. BUt if we have a 100k plate resistor and a 1k cathode resistor, the voltage change across the 1k will only be 1/100 as much as across the plate resistor's 100k.