If you don't want to bother with fixed bias, you could build the amp cathode-biased. It would probably not be as powerful, but it would be simpler. You'd have to buy a few extra 5-watt resistors, probably in the range of 150-300 ohm? Instead of grounding the power tube cathodes, connect them together to the resistor and then ground the resistor. The value of this resistor will control how much wattage your tubes dissipate. So this involves some testing of different values and measuring voltages and calculating dissipation. You won't hear any difference in "loudness", but you just want to keep the tubes in a safe operating range.
Then take the 220k's that connect the power tube grids to the bias supply. Connect them as shown, but instead of connecting their junction to the bias supply, ground it.