Well, imagine a circuit where that grid is direct-coupled from the plate of a previous amp; the grid then may be at 100+ Vdc, let's say. Yet, you still might also want it to be 2-3 Vdc negative with respect to its own cathode. Which would mean that it's own cathode, using a cathode resistor of a much higher value, could be brought up to, say, 103 Vdc positive with respect to its grid. And, also, 103 Vdc positive with respect to its heater, if the heater were grounded. In that case, biasing the heater more positive with respect to the circuit ground would be indicated to keep the heater-to-cathode potential within the limit.
Now, in most Voltage Amp circuits, the grid is held at ground potential through the grid resistor. There will be no Voltage drop across the grid resistor because there will be no current flowing through it. So, with respect to ground, the grid is also at ground potential. If you measure between ground and the grid pin on the tube, your result will be "0". By making the cathode positive with respect to ground, with the grid at ground, you create the situation where the potential from grid to cathode is negative. If you use your meter, black lead on the cathode, red lead on the grid, it will read negative Voltage. Remember, the meter only knows the world between its probes. So, if "this" is more positive than "that", then "that" is also more negative than "this", depending on how you are measuring.
The other way to make the grid negative with respect to the cathode, assuming the cathode is grounded, would be to have a separate negative bias Voltage with respect to ground, and apply it to the grid. This is often done in power amps, since power tubes have more current going through them and also usually require the grid to be more negative with respect to the cathode in order to hit a useful operating point. The cathode resistor can start to get unwieldy in size and power consumption to use cathode-bias.
The Voltage on the plate or anode really doesn't figure into this discussion directly, other than it's another operating point. What you will see in your circuit, though, is that the Voltage between plate and ground will be higher than the Voltage between plate and cathode, by the amount of the positive Voltage on the cathode. IOW, if there is +150 Vdc on the plate with respect to ground, and +2.7 Vdc on the cathode with respect to ground, the Voltage from cathode to plate (black on cathode pin, red on plate pin) will be +147.3 Vdc. That is the amount of Voltage said to be being dropped across the tube, cathode to anode, at this particular point.
Note, though, how the curves approximated the drop across the cathode resistor so closely.
The other take-away is to always keep in mind that potential Voltage values are always relative to what you are considering to be the reference point. A Voltage that is +x Volts with respect to ground, may also be -y Volts with respect to some other reference point in the circuit.
Keep asking questions, if I have been unclear about any of this. I've been up a bit longer than I'm used to, and I hope I'm making sense, but I'm not sure I'm a good judge of that!
