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Avoid cross posting! I already suggested a multivibrator circuit. In my opinion, a voltage gain of > +1 is needed to design
a relaxation oscillator or multivibrator. It can't work with only one tube (unless you utilize a transformer).
You can make with one tube - resonant circuit and then place saturation circuit.
Is not relaxation oscillator, but it works.
(keep in mind that resonant circuit plays the role of transformer in amplitude tranbsformation ratio)
I reckon it will work, Mullards (tube manufacturers) designed a 3W Hi Fi amp in the 60s with an EF86 running with 28V on its anode and 12V on its screen grid, they called it "starvation conditions".