Low-voltage vacuum tube amplifier design

Question

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?

Answer 1

It's probably possible to get >1W, say using a large tube such as a 6146W far below its power capability. But the heater alone draws almost 8W. Seems a bit wasteful.

Answer 2

Some World War II era handy-talkies used miniature tubes with 12 V batteries and a mechanical inverter to derive 48 V plate voltage. Output was up to 5 W, if I recall.

Answer 3

XY problem?

Making a plate (B+) voltage isn’t such a difficult thing to do using modern power chips. A step-up DC-DC LED string driver can do this for low cost, easily achieving 50V or more. Likewise making a negative grid bias can use an inverting buck topology, simplifying your cathode bias setups.

Answer 4

There have been low voltage tubes for 6.3 V only.

Source

But due to the high impedance of tubes you get only low power output.