I got a question regarding IV-15 heater voltage in hackaday messaging system. The questioner burns out more than 10 tubes of IV-15 and asked me how I use. The answer may be useful for all (except for vacuum tube expert), and would like to show,

The instance of power on, the resistance of heater is quite low (proportion to temparature)  and if you simply apply the rated voltage, for example 1.0 V, it will burn out. The remedy is quite simple and was a common sense in 60 years ago of vacuum tube era, "insert tens ohm (for example 20 ohm) in series to heater and apply the rated voltage."

Reading battery-driven vacuum tube circuit, especially lower (< 3 V) heater voltage tube circuit on article and books, we can find "R" in series to heater. Especially low voltage heater tube has a low ohmic value for enough current flow by the rated voltage, so they are more fragile than 6V or 12V or higher voltage heater tube. In the vacuum tube, plate and grid are separated and very strong to voltage application (they are separated and basically no current flow if heater is not ready..), BUT in the all of vacuum tube, heater are, naturally, connected and ready to over-current. Actually heater is weak point of vacuum tube.