I can't figure it out why 10VDC is so dim.
so I was going to use a transistor driven by a digital out to "blink" the 120VAC to control brightness.
tube would be acting like a relay, which has limited life.
Indeed for saturating a mosfet or a vacuum tube, you need to bang it with higher voltage & peak current totally incompatible with arduino TTL outputs drivers
Not entirely true; many FETs have a "logic" version, where saturation is achieved with logic level voltages.
I would be interested to have this reference or P/N of such FETs.
"logic level mosfet" has an advantage compared to "classic mosfet"