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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
11,794 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.
glassandlight's avatar
glassandlight
19 posts
 

Hi Peter,


What Hickok tester?  One that only measures shorts or one that measures leakage and shorts such as the 539 type KS and RD.

If you tubes fade that fast you should have done a "life test".  Yes, gas can affect a reading on a Hickok, but it will make a tube read higher not lower since 'gas' takes the grid 'more positive' into the negative voltage range ( makes the tube grid less negative and thus more output as if you turned the pot to 10 or 12 bias from 14).  If your tubes fade back 10% from indicated gm in under 5 seconds they are not reliable...period.

The problem with tubes that show such drops is the cathode is worn out or the grid is not able to control the electron flow as it should.

If you Hickok is a model that can show leakage, and ANY tube show leakage under 1 mega ohm, pitch them out.  ANY under 3 meg keep an eye on.  You get below 1 mega ohm and the chances of a instant full short are greatly increased. Low end Hickok testers will not see anything above 250,000 ohms and that is to near a short for any audiophile amp to have to face. You must remember that your amp using KT88 is well over 500vdc and arcing is a real concern even for NEW tubes.

It is not worth it if you blow a screen resistor or even worse damage the output transistor because some element fuses with another.

If you see a reddish plate or screen wires thru the holes, you best turn it amp off and yank those tubes out for good.  Even new tubes should be observed for the first few hours for this sign.

No matter how expensive or sophisticated an amp is, burning second rate, unmatched tubes in it is just plain not worth it.  One blowout on one tube and you can be sending that Amp to be repaired, so there is no bargain there. How do I know, after 40 years in this 'business' I have never had an amp or tube tester returned.


warm regards

Bud

theschlepp's avatar
theschlepp
1 posts
 

Hey Bud,

I hope your still around. I see your avatar is a triplett 3444. I am restoring one and want to know if you have any photos or can tell me about the mods you made to yours. I can see you have a plate current meter, that's something I'd be interested in doing.

My email is theschlepp@gmail.com

Thanks,

Eric

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