The 3CPX1500 is not a drop in replacement for the 3CX1500. The CPX version require a higher filament voltage and slightly more current. 5.5v at 11.2a for the CPX versus 5.0v at 10.5a for the 3CX1500/8877. Paul PAUL HEWITT WD7S PRODUCTIONS QRO HOMEBREW COMPONENTS http://wd7s.home.att.net