Acorn Tubes are really small tubes that were designed during WWII for use at UHF frequencies. I got my 1st Acorn Tube when I bought a WWII surplus Radiosonde Transmitter.
Needless to say, long ago, when hams would build things and not just do contests, POTA/SOTA or plain operate, these Acorn tubes found their way into homebrew QRP CW transmitters. Several years ago, I even saw a radio set using an Acorn Tube CW Transmitter complete with a matching Regen Acorn Tube Receiver. What a hoot.
Rex Harper, W1REX, during the virtual 2020 FDIM Buildathon (you know that COVID 19 Crap) used this schematic as the project foundation. I think it got close to a watt output with this lash up.
I didn't participate in that event but did build my version using scrap stuff I had lying around the garage. The tube filaments are wired in series so I could simply supply 12 VDC through a three-terminal regulator.
Since Cycle 25 has been sub-par (a highly touted but major disappointment) and given the MUF yesterday was no higher than 30M then this jewel would be FB on 40M.
Before anyone gets their underwear in a knot about high voltage DC and the need for a special power supply... Think 20 Transistor Radio Batteries in series. The current draw is less than 10Ma so as they say in Italy -- Non è un grosso problema.
Or in plain English -- "Ain't no big deal". I checked Amazon and twenty 9VDC batteries cost less than $15.
An overwhelming resource on simple vacuum tube receivers can be found here thanks to K3UH. You could get lost on his site for weeks at a time.
Band conditions are supposed to be better today so maybe we can test the ZIA. The MUF is better but unsure about the other one.
Them that know can make things go.
73's
Pete N6QW