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Tube Tester Mutual Conductance Falling - What it Means?

I just purchased 16 used KT88's and tested all of them in my calibrated Hickok tester.  Eleven of them had stable readings between 4700-5600 umhos.  Five had readings that started up in the 4600-5000 range but fell to the 3900-4200 umhos range before stabilizing (I'm not sure they fully stabilized, but the falling slowed down a lot). 

Can someone tell me what such a falling reading means?  Is the tube weak or unreliable or ????.  Thanks, Peter

peter_s
atmasphere's avatar
atmasphere
11,677 posts
 
The tube is slightly gassy. It will work but likely will not last as long.
peter_s OP
1,320 posts
 
Thanks Ralph! This may be fear-based, but I want to minimize the chance of tubes failing while being played in my amp (afraid of blowing resistors and such).  My amp is a VAC Phi 200.  I know that your amps are relatively immune to tubes blowing, but I don't know that this is the case for the VAC.  Anyhow - I guess my question is: would it be wise to avoid gassy tubes as they may blow sooner?
atmasphere's avatar
atmasphere
11,677 posts
 
Yes.

A gassy tube tends to run hotter. As it does so, it slowly releases oxygen molecules trapped in that shiny deposit on the inside of the glass. As that happens the edges will slowly turn brown, signaling a failure on the horizon.

So if you can see the tube you'll be able to know when its reaching the end of its service life. IOW, I don't see why you can't use them, just keep in mind that they won't last as long. Its not a sure bet that they will damage a resistor- they might just simply fade away.
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