The free information society kindly put up the schematic from my H306, see
which might give you a general idea about the type of circuit designs they were using then ie grid leak bias on the pre amp. Before you go replacing lots of perfectly good resistors, try to fault find which stage in the signal chain the noise is coming from, and just change the bad one. Most likely a plate resistor in the pre amp, or something else with lots of dc. Assuming you've swapped in known good tubes?
Hope that helps - Peter