Neon lamp question

[Heemis]

First off, my apologies for a non-stompbox related question, but you guys have been so helpful in the past I was hoping you could clear up this little issue for me!

So I'm working on a Vox AC4 clone using parts from an Akai M8 tape machine. I'm trying to use as many of the original parts as possible, and there's a perfect neon lamp which was originally the "record" indicator.

I'm having a hard time finding out if this lamp will be happy to run off 120Vac as a pilot light for my new build. The lamp itself doesn't have any markings on it. If I'm reading the schematic correctly it's running off a DC voltage through inductor L2 in the original circuit with a 300k resistor in series. Will this lamp be alright to run off 120Vac with a suitable resistor in series, if so, how do I calculate the resistance and wattage? Schematic attached with part in question circled:



Thanks!

[merlinb]

If it's just a bare naked neon lamp then you can use a 47k to 100k 0.5W resistor. Current isn't critical -anything from 0.5mA to a couple of milliamps is OK for a neon.  10mA is probably too much.
A neon drops about 60V under normal use, IIRC, so treat it like a 60V LED. 120Vrms is 170V peak. Minus the 60V dropped across the neon leaves 110V across the resistor. A 47k resistor would give you 110V/47k = 2.3mA peak, which is fine. The average current will be less, of course.

[Heemis]

Thanks for the reply Merlin!  So I should be ok to run it between live and neutral as a power indicator with a 100k resistor in series?

[amptramp]

Don't forget that neon lamps have a higher start voltage than running voltage.  They turn on at 90 - 95 volts and run at 55 - 60 volts.  It should be OK to run it as merlinb has said.

[R.G.]

To elaborate, neon bulbs in common with other gas discharge devices have a negative resistance characteristic. They insulate until there is enough voltage to ionize the gas, at which point current starts to flow, and that current ionizes more gas. The resistance in series with them is what makes them stable enough to both keep glowing and not destroy themselves.

With AC across them, the voltage rises to the breakover point, drops back to where the negative resistance of the bulb and positive resistance of the limiting resistor hold it steady, then extinguish when the AC voltage drops again; this then repeats on the other polarity/half cycle.  With DC, they don't quit conducting until the voltage and current across them drop to where the ionization of the gases starts decreasing, whereupon they turn off.

Neon bulbs can be used to make single-bulb relaxation oscillators because of their negative resistance, among other things. They were once the innards of electronic organs because of this.

[merlinb]

Thanks for the reply Merlin!  So I should be ok to run it between live and neutral as a power indicator with a 100k resistor in series?

Yes. When you buy a neon mains indicator, that's what you get. A neon with a resistor already included.