The first question that comes to mind is "why would you want to do that?"
However, if you really do want to, here are a few thoughts:
First, a disclaimer: While the information here is believed to be accurate, I accept no responsibility for its accuracy nor any actions you may take or any modifications you perform. Such actions are completely at your own risk, and you should be aware that such modifications could expose you to potentially deadly voltages even after power is disconnected from the unit. Be sure to discharge the filter capacitors after disconnecting power from the unit before working on it.
That said, in order to determine a value for the cathode resistor, you need to know the expected voltage drop across it and the current through it, as well as its power dissipation.
I would expect the voltage across the cathode resistor to be approximately the same as the grid-to-cathode bias that is present for proper operation in fixed bias mode. So measure the grid voltage with respect to ground at idle and use that for a starting value. The schematic shows the voltage at TP17 to be -15.6 volts, so I would expect this to be close to the measured value at the grids.
The cathode current will be approximately equal to the sum of the plate current and the screen currents. The screen current is simple... measure the voltage drop across each screen resistor. (Measure from one end to the other.) They should be about equal. (If not, you probably have a bad tube.) Calculate the current through the resistor by: voltage drop (volts) / Resistance (ohms). The answer is in amperes, and should be a decimal such as 0.05A. Multiply that x4 to get total screen current.
Next to find the plate current: Disconnect power and discharge the filter capacitors. Disconnect the center tap of the output transformer from J16 and connect a milliameter between the center tap lead and the J16 tab it was connected to. (Bridge the break with the milliammeter leads.) Set the meter up to measure milliamps on a high range. Be sure to arrange all leads safely (this is a HIGH VOLTAGE circuit, and your meter probes will have ~400 volts on them) and turn on the power. Adjust the meter range to get the best reading and record it. If the reading is in milliamps, divide by 1000 to convert that to amperes. This is the total plate current for all the tubes.
Shut off the power and discharge the filter caps before removing the meter leads.
To find cathode current, add the total plate current to the total screen current. Leave the result in amperes.
The cathode resistor value in ohms is found by: bias voltage divided by cathode current in amperes. Use this value as a first test. You may wish to experiment with different values around this one for better results.
The cathode resistor power rating must be greater than the idle current power dissipation. Multiply the cathode resistor bias voltage times the cathode current to get the idle power dissipation. Select a resistor with at least twice this power dissipation rating. I would expect it to be 10-15 watts.
Add a bypass capacitor in parallel with it such as C29 in the VC30. At least 220 uF, maybe more.
You will need to add a DPDT (double pole double throw) switch connected so that the fixed bias position makes the same connections as the amplifier currently has, and the other position adds the cathode resistor and switches out the bias supply and grounds the resistor side of jumper J02.
To make the fixed bias adjustable, I would probably change R78 to a series combination of a 10K resistor and a 10K trimpot with center tap connected to one end. Recalculate power dissipations of these resistors as well as R77, as adjusting the pot may cause R77 to overheat. Be sure to use resistors with power ratings at least twice the maximum possible value.
Have fun, but BE CAREFUL!