Very nice, would love to be able to do something like this, but I don't have the space or the knowledge..!

Thanks, I would like a better set-up with a dedicated space for this stuff myself.

Really neat! Have you seen a gradual color change in the right tube over time? If air is/was a component of its gas fill, then the discharge composition and pressure would change as oxygen and then nitrogen are lost via physi- and chemisorption at the lamp's wall and electrodes. These mechanisms are quite effective at removing air, including its argon component, at low background pressure (1 mbar and less). I managed to get a very high vacuum in (sealed) discharge tubes to the point of extinguishing the plasma ang getting an electroluminescent excitation of the glass tube only with just a rotary vane pump and a 20 kV 30 mA transformer... that's actually showing the working principle of ionization pumps.

Thanks Max, yes, I have quite often seen this in tubes I have made, but the results unfortunately improve the vacuum until the neon sign transformer can't strike the tube. I have even produced some tubes that start to produce x-rays when driven with higher voltage power supplies. I am obviously sealing off at too low a pressure. The tube did improve, producing the neon discharge, with a few hours of run time. If I notice any sputtering around the electrodes during the burn in then it generally means the pressure is dropping too much and the voltage is too high. This seems to be an escalating issue as the sputtering also appears to be a good gas absorber, and less selective in reactive terms. My vacuum manifold arrangement is a bit leaky so I over compensate when pumping down, back filling and sealing off.