Hello Repsej/Jesper, looking on the web for a new/replacement tube for the one you mentioned
W66ESF002x13 i came to this thread (although i'm a BW member but nevert read the Scandinavian section.Reading it, i think i understand more or less what it's about but replying in Danish i won't try :)...(i'm Dutch).
Anyway, this tube (W66ESF002x13) if it's in your Avant T8111 is already a replacement for the original and an S-flat type, with which also the deflection/EHT board has changed (should have a label S-flat on it).
I'm looking into this matter to see if this (or a replacement) tube is still available from e.g. Philips since B&O charges over 500 euro making it a total 'write-off' for many Avant owners.
Philips is selling several types of widescreen tubes for less than 200 euro these days (which would make fixing a T8111 worthwhile again).
Coincidentially i have the 'original' T8111 and the 'updated' S-flat here and i'm pretty far in finding the differences between the 2 boards.
Obviously, once i know the differences (looks like a few caps and resistors) the original board could be modified easily to S-flat...
Now i'm curious to know what's the story with your Avant (tube again broken ?, since the one mentioned is already a replacement).
The service doc for Avants is not so easy to get, the T8300 is different, i have both docs but it's quite a book....
Luckily the T8111 is still a bit of an 'conventional electronics' tv (has a remarkable resemblance with the later LX/MX) so still fun to work on....