According to the internets.
Another version of the same has a line to the Center as positive.
On Ebay - some one is selling one with this wall wart power supply
Looking it up on Mouser - it is indeed - Center Positive.
(unless he is selling it with a supply that would make it not work)
But I think it is safe to say - Center +
A frendly Head-Caser has kindly lent me four EL156,
So, I made myself some EL156 to EL34 adaptors.
Here a Megatron SS CCD something with the four EL156.
Same friendly fellow has built himself a couple of filament supply for DHT similar to the one in above post.
Just heard that Ken Downie of The Black Dog passed. TBD released a fantastic number of IDM gems in the first half of the 90s. My personal favorite is Raxmus: