You may be OK without the dropping resistor, as long as the voltages you measure at the plates of the tubes are less than their stated maximums, as determined from a tube manual, or by looking up at
http://www.nj7p.org/Tube.php
Add up the plate currents of all the tubes and that's the "I" to use. If your plate voltages are high, figure about how many volts you need to drop and use that for "E." This will give you the resistance. I x E will give you the wattage of the resistor, but go a size or two higher in watts to help it run cool.