The amps I've seen with 572s run a pair of them at around 2250v and ~45ma idling current per tube. This seems to work well in my modded SB-200. It's a directly grounded design with 3 diodes on the filament transformer center tap to put the cathodes at about 2 volts idling. The gain was a bit over 13dB and I lowered that to ~11dB with a 15 ohm resistor feeding the cathodes to better match my exciter. This put the idle plate dissipation at about 100W per tube. It was 130W per tube without bias when I was using a jumper to figure out where I wanted it to idle. 65 watts drive lets it deliver over 700W PEP on SSB and it can sing like that without breaking a sweat. The original SB-200 gets idling bias from pulling relay (and a resistor across it) current through a 33 ohm resistor to drop a couple volts. A couple years ago, I gave my still working Cetrons to a JA who couldn't find good tubes for his FL2100b. A couple Taylor 572bs from RF parts went into mine and the idling current, gain and power output stayed the same. The new tubes were going strong when I put that amp on a shelf last year. I'll use it in the AM stack that is going together next summer. I haven't dealt with an FL2100 but the numbers in the manual look approximately the same as the Heathkit. I like Sean N8VIL's pictures. It looks really nice in there.