A recent image of a Tridonic 2' 40w ballast
posted by Kev has recently sparked up quite a lot of interest here on ATL. Not only is this around the size of a 6' or 8' ballast
and hence would take up best part of the free space inside a regular 2' batten (as well as making such a fitting a heavyweight LOL) it seems such ballasts are rather hard to come by these days!
However, as mentioned previuosly, 2' 40w UV tubes of various types are still being used in things like industrial fly zappers, facial tanning units, UV exposure units, industry and healthcare. Since most of these units are multi-tube, I'm guessing the tubes will be running as series pairs on modern equivalents to the old 5' 80w ballast
! Spare ballasts must still be available as spares and it may well be worth hunting down any scrap equipment such as old face tanners that are being binned off and butchering them for gear for those of you who are desparate to light your old 40w two-footers!
Other options are 2 x 20w ballasts wired in paralell as successfully tried by Eliot for single 2' 40w and I would suggest trying an 80w mercury ballast
with 2 in series using S2 or 155/200 type starters. The 5' 80w ballasts were used for twin 2' 40w circuits on both ballast
and starter switch. Upon switch-on, the starter closes and preheats the tube's cathode filaments. Upon opening, the magnetic field within the ballast
collapses, causing a voltage pulse which strikes the tube. This is often repeated twice or more, causing the distinctive blinking at startup.">switchstart and quickstart so this should work ok.