19ap4 question

[radio nut]

I have a 19ap4 I would like to sell but i am having issues with it. It has strong emissions and no shorts....
On my sencore tester you have to set the tester to gun balance even on a black and white crt and set the tester needle to a line. As far as I understand that is for contrast capabilities. I can not get the needle to move on that setting.
for every other tube I have tested I can set that function which is the step before checking emissions.
Any ideas? It is a rare tube and has good getter and emissions so it would be a shame not to be able to sell it.