I have been testing some valves with my Valve tester. 10 x 6J6 valves were good. 6J6 is a Twin Triode.

1 x 6J6 has no shorts, heater on. Had an initial Plate current, but slowly reduced to zero ... both Triodes had zero Plate current.

Nothing to loose ... so I gently increased the Grid voltage. ie less negative .... beyond the recommended test voltage. Got a small Plate current, but also a purple glow and a noise from the valve .... then Tester cutout triggered. Both Triodes generate a similar result.

I am assuming that this glow and noise indicate a “gassy” tube?? Hence stuffed??

Definitely looks like a loss of vacuum.
Fairly common with high stress TV valves back in the good old BW days.
JJ

Yes, the blue glow is usually an indication of gone gassy.

Nomenclature issue: If you increase the grid voltage you make it more Negative: Making it more negative gradually cuts it off.

If you decrease the grid 1 voltage to the point of it going positive it then becomes a diode, which may present as a short.

If you increase the voltage between heater & cathode, depending on the valve, that may causes a heater cathode "flash over".

A Blue glow (normally dark {Electric} blue) inside the plates OK: Pastel blue, outside the plates: Gassy.

Holding the "Test" button down too long can damage some valves in some testers.

Regarding blue glow;

Plasma!

This is a good sign. It means the valve has a very high vacuum.

Now this is a bit like "Oils ain't oils". Some valves actually have a label saying where the "Plasma" flow is. Note my comment "Electric Blue" is a much darker, toward Navy blue. The gassy blue is more toward a sky or baby blue and the both occur in a different position.

Yep, I'll buy that Marcc.

The blue that comes with very good vacuum is deep violet, and within the plate area.