From FB comment

In the schematic, find the difference in voltage between the two ends of the resistor. You can find the current through each of the tubes being fed from the unknown resistor by calculating it from the voltage across the tube’s cathode resistor divided by the resistance of the cathode resistor. For instance, if there is 7 volts across the cathode resistor of 1000 ohms, then the current through that tube is 7 mA.

So say there are three tubes being fed from that unknown resistor: 7 mA, 7mA and 6 mA. That adds up to 20 mA. You can calculate the resistance of that unknown resistor by dividing the voltage by the total current in amps. Say it’s 50 volts and 20 mA or 0.02 Amps. 50 / 0.02 = 2500 ohms. Then multiply the voltage by the current to get the power in watts. If it’s 1 watt, give the resistor some safety margin. Instead of a 1 watt resistor, use a 2 watt instead.