I am currently using EL34s in this amp and want to try 6550 just for fun. How critical is the bias RG resistor value? The schematic shows 2 variants. One variant is for 6550 that uses 150k RG resistors, and the other one is with 220k RG resistors for EL34. My bias resistors are gone, I am using master pot of 2x250k (with 1.8M safety resistors) instead. Besides adjusting the right bias current for 6550s what else should I be aware of? I am pretty sure it will work just fine if I set the proper bias current and the value of 250k pot shouldn't be a big deal. Or not?