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.
There are also the big 9-pin types, 6DN3 for one. A pair of those could supply a half an amp( that's short for Ampere, not part of an amplifier ). It will drop only 14V at 350 mA of plate current, so you could build a 700 mA PS with a pair of these...
Another benefit of TV damper diodes is their extremely high heater to cathode insulation (normally a ceramic sleeve). This means you can use a near-earth 6.3 volt source, though I wouldn't mix it with any small-signal stage heaters.