TAD 150 Pre Amp transformer died

Hope you're right, but the search has been nothing but discouraging so far.

Here's the reply from Hammond Mfg:
"Sorry but that would be a custom design very expensive to do but before you ask for a cost I need the amperage for each output voltage."

Is that even possible to calculate from a dead transformer ?

Maybe Tube De Force in Canada can assist you.
tubedeforce.com
They do a lot of rebuilding and ressurecting of old amps and improve on the original as well as consult on some very large scale projects.

All the best,
Nonoise

Mercury Magnetics provide transformers for Manley and VTL amps. A custom built 'fresh' transformer should not jeopardise TAD's performance.

Bury it!!