Filament is easy. 12AX7 requires 12VAC @ 150ma to properly heat it. The plates only need 2 to 4ma current so B+ can be developed in several different ways. Another stepup transformer and rectifier, or voltage multiplier rectifier, or ss multivibrator driving a small hi-freq stepup transformer, are just a few easy ways to develop 100VDC.
Open it up and show us some pics and we'll make some guesses.
open it up
Let me use it first
I would like to do some voltage test
This amp reminds me of a joke
The thermos bottle keeps hot water hot and cold water cold
How does it know