From: ard at p850ug1.demon.co.uk One think to be wary about. There is a capacitor across the filament string, after the capacitor. If the filament string is open-circuit for any reason, then that capacitor will charge to about 90V. It normally withstands that (although it's not rated to do so), but if you then replair the filament strign, there's enough energy in that capacitor to burn out one of the filaments. I always make sure that capacitor is discharged before fiddling with the valves.

-tony