I like 3A5 so much, for its mellow and brilliant sound through triode. But this time, the application may be most useless one. It's LED blinking. 

It's blinking period can be determined by C-R discharging time constant. Indeed, we can simply regard Grid-Gate, Plate-Drain, and Kathode(3A5 is direct heated tube so it is GND of heater)-Source between vacuum tube and FET. The big difference is heater power requirement and high voltage requirement. Actually "gm" is common in both FET and vacuum tube!