Thanks Hero.
Thats the thing, I know 145V is the total, I can't be sure as to how it is distributed across the transistor, resistor and tube.
In the Townsend region of a glow discharge I-V characteristic, just before the normal glow discharge, the current is in the order of 10nA to 1uA for your average switching diode. These diodes has a similar geometry and gas filling as the ones I am working with, so I expect the same currents to occur.
Looking at the BC337, Icbo at 20 Vcb is in the order of 100 nA, but, as expected, increased to uA at higher temperatures.
So I am worried that the maximum Vceo or/and Vcbo will be exceeded under the right conditions IF the tube doesnt strike from the leakage current as you pointed out.
The MPSA42 has a stated Icbo of 0.1uA at Vcb of 200V. This is much better than for say the BC337.
I know the MPSA42 works reliably, but have not tried 337 or 550. In fact, for the 550 Vcbo was found to be 3 times the datasheet value, 50V. How does that vary across devices with different doping levels? I dont know. The 550 has beed used successfuly, but using numerical 'nixie' tubes and not over the commercial temp range.
Seems to me that using a transistor with max Vceo/Vcbo less than 100V is pushing it.
I could set to measure currents in the nA range, not sure if it's worth it. Is it? lol
This is academic, but how about a very large value resistor across the transistor to limit the voltage across it?
*Just saw Mike's reply*
Thanks Mike. Regulating HT supply is not a biggy as it is usually generated with a boost converter using a switching regulator IC. Unless you are looking into powering from the mains direcly or 555-based designs with no feedback loops... This is powered by 12V DC then boosted with a regulator.
Anyway, I think your reply further reinforces the point that relying on voltage drops across other components in this case is in the realm of 'bad design'.
And a mini-rant on how some datasheets have
NO graphs whatsoever
![Angry >:(](https://www.eevblog.com/forum/Smileys/default/angry.gif)
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