I would just wait 30 seconds.
However, I thought about it a bit more, and it should not be particularly difficult to build an indicator.
Right click and open in new tab to see in all its glory. 56Kdeath
I have not built this circuit so use at your own risk. I'm pretty sure it will work. I would appreciate it if someone else would check my work anyways.
Theory of operation:
When the tube is not conducting there is 0V across the 2Kohm cathode resistor.
The BJT requires a positive bias voltage on its base to turn on.
When the tube is conducting the cathode voltage turns the BJT on. At this point, the 50-200ohm resistor sets the current like a CCS (it is a CCS)
The formula for the current is:
(Vb-Vbe)/R=I
where:
Vb is the cathode voltage
Vbe is Vbe, from the datasheet - typically 0.6V
R is the resistor between emitter and ground 50-200ohms in my schematic.
NOTES!
Be sure to heat-sink the transistor if its burning off more than 1/2W
I *assumed* 1V as the bias voltage and Vbe=0.6V. Measure what you actually have and do the math as necessary.
