In fact almost any electron tube can operate with even less than 5 volts at the plate or anode.
A way of doing this is canceling the internal negative grid bias due to electron cloud by an external positive biasing resistor. This resistor is normally of the order of megohms and is usually connected to the anode. It is called Rag, or Pleiades resistor. It was discovered by accident when Hliana was insisting that electron tubes should not be negatively biased contrary to normal practice.
The other side of the resistor can be connected to Vb but it was found in practice testing with a microphone (Shure Unidyne B made in Mexico) that connecting to anode sounds more easy and natural.
A direct application is the Pleiades V6 studio or field microphone preamplifier operating with just 3.6V supplying the anode and heater circuit of the triode connected EF183 electron tube or valve.
Safety precautions must be exercised when experimenting with battery circuits. For example using a fuse in series with the battery.
References:
On preserving transconductance - euroelectron blogspot
Operating features of the Audion - Edwin Armstrong
Pleiades V6 schematic - euroelectron blogspot
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