I'm 95% certain that it's FW only. No plans to try FWB and find out. Too many stories of hams doing that and burn out. FW is good enough. UTC made a CG series of plate iron. The model numbers ending in W could be run as either FW or FWB (at reduced current). My 3-400 rig plate trans. is a CG303W (or 301W, can't remember which) and I run it with four 3B28s because that's how the power supply was originally built. Specs say 500 ma. FW and 350 ma. FWB. Been running it at 350 ma. Ip for years & no probs.