WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I suppose you could try using them as rectifiers. But the voltage drop might be a little high, they have filamentary cathodes (you'll need a separate, well insulated winding for these), they gobble 12.5W of filament power each, and require a top cap, exposing HV above the chassis.