simpler explanation for 'grid leakage' (failure) in a tube?

[Jimbilly]

 An amp stopped working last night, the 12ax7 in V1 tested great except for grid leakage.  I swapped in another good used 12ax7 and the amp is back to normal. I read the description of grid leakage at Tung Sol (link below), anyone have a better 'laymans' explanation?  - A little sad to lose a German Siemens.  

 

http://www.tungsol.com/html/faqs3.html#:~:text=Question%3A-,What is grid leakage%3F,other multi-grid vacuum tubes.

[BoogieMKIIA]

The cathode, grid and anode are electrically isolated from each other. Measuring with an ohm meter will show an open circuit. 

During operation, the voltages applied allow a current between the cathode and anode, controlled by the grid voltage. Like a knob on a water "valve" 😆

Contaminates inside the tube can cause current to flow where it is not supposed to and the will not operate correctly or at all.

On an aged tube, material from the cathode will have evaporated (mostly to the grid) and provides a path for current to flow where it shouldn't. Can also be from loose particles that move around or poor vacuum.

Hope this helps.

[hamerican gigolo]

A bad tube is a bad tube. I accidentally fried a Telefunken preamp tube once. Felt bad about it for a couple of days, but all the regret in the world wouldn't change a thing. Had a RCA 12ax7 go bad. It tested strong on a tube tester, but upon observation, half the tube was dead. The other half lit up. I tossed both tubes & went on. 😞