This issue has been discussed extensively on this and other reflectors in the past.The filament design and construction is the same as the 8877/3CX1500A7 which runs at 5.0v. For pulse service, the filament voltage is cranked up .5 volts.
For use in ham amplifiers, the 3CPX1500A7 should run at the 5.0 volt spec and it will do fine.
73, Dennis W0JX/8