Keithley 180 neon identification/cross reference help

[valley001]

Hi all,

I am continuing my irrational fascination with old nixie tube volt metes, this time a Keithley 180 nanovolt meter. 

I have run into a problem with the #1 "overrange" indicator tube.  This is a longer version of a standard neon tube meant to replicate the numeral 1 in place of using a proper nixie tube (cost savings?).  The example in my meter is used up/very dim due to the metallic deposit on the inside of the glass. 

Does anyone know a proper cross reference for this tube?  I had a couple in my inventory from the old bell and howell 2 1/2 digit kit DMM but the element is not long enough to match the height of the nl841 digits.  The used up one is about 1/4 inch longer than the example from the bell and howell meter. 

Keithley lists this as "A261" and keithley part number "PL-42."  My searching has not been fruitful. 

[fzabkar]

I can find 25.4mm x 6.4mm neons on eBay.

[doktor pyta]

For quick and dirty solution You could use LED filament from damaged bulb:

+ proper resistor in series + red or orange heat shrink to get proper color.

[mag_therm]

Surplus Sales Nebraska have 2 parts about that long.
I found that the very old stock NE2's may not  work consistently (flicker, not fire at same voltage).
I found a GE reference somewhere explaining that the units were tempered after manufacture.
I tried that:
Put them, each with 100 kOhm series,  on a variac, slowly up to 120 V AC ( not DC)  for 24 hours.
Check them with 'scope while they are on for turn on-off in each half cycle , and discard the bad ones.
Most of my batch, about 8 of 10, came up good.

[valley001]

Unfortunately it seems the approx 1" tubes are the same length as the replacement I have, the original is about 1 1/8 inch with a correspondingly longer display element.  Seems it may be unobtanium. 

[james_s]

I've seen those long NE-2 style lamps before but I don't think they have been produced since the nixie era. You might try getting in touch with one of the very small number of people who are producing nixie tubes today, or one of the slightly larger number of people making home made vacuum tubes or neon tubes. A linear glow lamp is a heck of a lot simpler to make than a nixie tube.

It seems most of the NE-2 lamps available today are significantly more stubby than even the ones used for indicators back in the day. I have a couple of different Heathkit thermometers that use NE-2 lamps for the indoor and outdoor indicators and when I replaced the worn out lamps in one of them I got lucky and found some suitable used ones in my junkbox. I couldn't find new ones in that length offered anymore, 20-25mm sounds about right from base to the tip of the exhaust tip.

[floobydust]

Neon lamps are in standard bulb sizes.
Info here: https://www.nutsvolts.com/uploads/magazine_downloads/NeonLamp-Information.pdf Dec. '76 Popular Electronics.
Chicago Miniature used to have a full lineup, VCC now has lots now but are losers with no dimensions available. Careful of electrodes that are way shorter than the envelope, or crooked. You'd have to find NOS made in USA I think.
Low firing voltage neon lamps have a radioactive isotope added to the gas.