I don't think that's feasible with a 2.5V filament because of the operating temp necessary for thoriated filaments. Of course changing operating points, filament voltages and currents, plate dissipation, impedance, mu, transconductance, cathode materials, plate structure, shape, size or pretty much anything else is allowed these days and you can still call it a 300B or 2A3 as long as it has a UX4 base.
« Last Edit: January 10, 2017, 11:27:06 AM by Doc B. »
Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.