Low-cost 12VDC to 120VAC inverter; no idea what frequency it produces. You 
would need an oscilloscope to get a true idea what this inverter really 
does.

I prefer to purchase new parts that have datasheets; it might cost me a bit 
of money but having all the details saves you a lot of time. Remember, time 
is money, too.

All of my present nixie supplies are driven from the mains and they are 
NON-isolated. Just use an isolation transformer during debug & testing. I 
also recommend you use a voltage higher than 170V for nixies. Yes, they 
will fire at 170V, but as they age, their firing voltage will increase and 
you may not get reliable ignition. Another reason for using a higher 
voltage is that you will have less variation in the nixie current. Cathode 
current varies with supply-voltage, tube-condition, and even by digit. 
Using a higher voltage and a higher series resistor will reduce the 
variation in cathode current. If your supply is 170V, and the line-voltage 
drops by 6%, you will have a 33% reduction in tube current (brightness). If 
you run at 340VDC, a 6% drop in line voltage produces only a 7% drop in 
tube current.

My latest designs use current-feedback on the cathodes, so it's literally 
constant-current.

Since I'm in the US, our mains are 120V rms. Using a rectifier and 
capacitor gets you 170VDC. I use a doubler to get +340VDC, which is 
perfect. Tubes typically use 2mA, and have about 140V across them when lit, 
so a good starting value for 340VDC is 100K.   R = (Vsupply-Vtube)/Itube. 
Use the recommended current from the datasheet, and measure the tube's 
actual voltage-drop for various digits, and take the average. Better yet, 
do this for all of your tubes.

Nixies are damaged by excess current; higher voltage does not harm the 
nixie as long as you limit the current to a proper value. Using higher 
voltage does waste more energy, though.

Someday I will make a constant-current driver that wastes no energy, but 
I'm too busy with the nixie wristwatch right now (pcb goes out for fab as 
soon as I finish my simulations)

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