My project this winter is to build a single tube transmitter into an Altoids
box. I am sure that has been done before, but I plan to build in the entire
power supply and key.

It should work. Using an old geiger counter tube, I will use a AA battery
for the filament and will first try a 9 volt battery for the plate and
screen. That should be fine. If not, I will use either a 15 volt or 22
volt battery - but the 9 volt is cheaper.

Power will be low, but having a friend a mile away makes the power
requirement modest. CW only.

As a kid, I built a one tube broadcast receiver using an old acorn tube and
15 volt battery for the plate.

Colin K7FM