Slightly off topic but still a Neon filled tube: Neva Franck-Hertz tube, help to get it running please!

[Dekatron42]

I've been able to buy a Neon filled Franck-Hertz tube together with its control unit. Everything looks fine, the output voltages from the control unit are correct and the tube seems to work just fine, however I can't get the experiment to work properly.

Is there anyone here on the forum that can help me out?

I have checked all voltages and used this guide: http://www.bastgen.de/schule/physik/13/Material/Franck-Hertz-Neon.pdf

The control unit can be seen here: https://www.uranmaschine.de/87051.Neva_6751_Franck-Hertz-Roehre_mit_Quecksilber/87051.Neva_6756_Franck-Hertz-Betriebsgeraet_hires.jpg

What I am trying to do is specifically the part where you are supposed to get one or more clusters of neon atoms being excited like in this photo: https://www.leybold-shop.com/media/phk/images/versuche/P6243_Schirmbild.jpg

[Yohan Park]

What have you checked to conclude "the tube seems to work just fine"?

Perhaps too obvious but have you checked with a neon tester to see if there's still neon inside?

[Dekatron42]

Sorry for not writing about what I have checked, it was obvious to me.....

I have checked the following:

*. There are no shorts between any pins except the heater connection which has a low resistance (can't remember right now though what it measured at but it consumes the current stated in the datasheet)

*. There is Neon inside the envelope, and from what it looks when tested with a Plasma ball there is a lot of neon, like in a well filled Dekatron

*. The heater works and glows

*. The voltages on the control unit looks ok when tested with a Fluke 87V DMM and with both an old analogue HP 1740A and with a more modern Lecroy WaveAce 2014

/Martin