Amateur radio HF frequency amplifiers using transistors commonly can be powered with around 12 to 14 VDC, or even lower voltages, for QRP transmitters.
Tube amplifiers seem to require DC power supplies outputting several hundred volts.
For HF (3 to 30 MHz) RF amplification, what limits the power output of low-voltage (say under 50 V plate) vacuum tube amplifier circuits?
What kind of vacuum tube circuit designs are likely to maximize RF power output into a 50 Ω load given the constraint of a low voltage supply (well under AC line voltage)? Are QRP or QRPp levels (1 to 5 W) possible?