Hi Dave Try this link it may help you, hpmemoryproject.org/an/pdf/an_911.pdf, if you have problems with it Google HP AN_911.pdf (HP application Note). In the 1960,s it was common for the behaviour of components to be "adjusted" for a specific manufactures requirements, in the case of the humble Neon this included the addition of radio active materials to them to "improve/lower/stabilise" the striking voltage. You will probably have more joy at finding rocking horse manure than replacement Neons the work correctly. I do not know if this is a restoration project or if you wish to use the differential voltmeter, if you wish to use it I would suggest that a replacement chopper be engineered using FET's. 73 George G6HIG