Not nixie, but close enough :)

[Per Jensen]

Hi group.

I acquired an item from eBay with 7 somewhat used Tung-sol Digivac (1x DT-1719A & 6x DT-1704B) https://www.ebay.com/itm/132994915019

Two of the tubes exhibit a strange phenomena - when i touch the glass it fades out for a moment. In general it’s pretty sensitive when i wave a finger around the glass envelope.

The filament resistance on the “bad” tubes is close to the other tubes as well (a bit under 10 ohms)

I tried swapping the tubes around and the issue follows the tube.

Any idea on what’s going on, and if anything can be done to alleviate it?

Regards,
Per.

[gregebert]

It's almost as though there is a floating grid in the tube. Can you see any physical differences between good and bad tubes ?

There isn't a specific grid pin for the DT1704 VFD; I have some large VFDs that do have a grid, and it uses a surprisingly large current.

[Per Jensen]

The tubes appear complete and intact inside. There is no grid in these tubes. I wonder if evaporated filament on the glass is acting like a grid?

The “bad” tube even have the same date code as all the good ones, so it doesn’t get any more close.

// Per.

[gregebert]

The DT1704 appears to have 2 parallel filaments inside; if one of those is open, it will definitely affect the shape of the electric field and how the electrons get to the segment anodes.

Do the bad tubes have higher filament resistance compared to the good tubes ?

You should be able to visually inspect the entire filament assembly.