I restored a black 600 for a friend a couple of years ago. It had been his father's and sat in a closet on a shelf for most of its life. It was a beauty. I recapped it, put in a silicon diode and extra resistance ILO the selenium (which I left in place,) and generally cleaned it up. It had the 1L6 and played like a champ. BTW if you're missing a 1L6, an alternate to buying a new one is to use an equivalent tube/adapter offered by Bill Turner at
http://www.dialcover.com/tubes.html
One note about these sets: with the set turned on when operating on AC, never pull out a tube and then reinsert it. The filter caps will build up a charge and can burn out filament(s) on reinsertion.