Sunn or whoever worked on the amp previously probably didn't have the right values and used those to "make" the right values. The amp doesn't care...it just sees a 15k and a 33k. You can fudge these around a bit.....these are what set the voltages for the stages they provide the B+ to. The earlier Dynaco transformer Sunns use lower voltages in the amp, so the preamp and phase inverter voltages are lower too because the schematic is the same as far as the dropping resistors. (Those are the ones you are asking about) The later Schumacher amps had higher voltages, and coorespondingly higher voltages for the preamp and phase inverter. In my Sonic 1 that I converted into a 200S (made in 1970 with Schumacher transformers), I duplicated the earlier amps in the preamp and phase inverter by changing the 33k to a higher value, thus making the tubes for those stages run on slightly lower voltages, which reduces the high end and makes the amp have less gain. It doesn't do the Geddy Lee overdrive sound as nice as it used to but thats ok for me as it sounds fantastic otherwise, with more mellow cleans like the earlier amps. Metal Oxide are what should be used for that application, and any other spot in the amp that needs a B+ dropping resistor, as long as the wattage rating for the resistor is adequate. There is no sound mojo for using the original carbon comp in this spot. Just use a single resistor with the value you need and make sure it is at least 2W for those two you mentioned, and you should be just fine.
Greg
