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-   -   Please educate me: Tube testing, the life test, and Grid Leakage (http://www.diyaudio.com/forums/tubes-valves/217689-please-educate-me-tube-testing-life-test-grid-leakage.html)

HiFiHarv 10th August 2012 04:16 PM

Please educate me: Tube testing, the life test, and Grid Leakage
 
Been testing a bunch of Vintage EL84's and 7189's lately. Many of these I have used over the years and they show healthy emmissions but fail the life test. As I understand it, the life test lowers the filament voltage to see if the emissions remain constant. Tester is a Solid State Sencore TC162 Mighty mite V11. Oddly, I even have NOS 7189's from reputable sellers that are failing the life test. Didn't have a tester when I bought them 20 plus years ago. Not sure how much faith I should hold in the life test. These tubes sound great and are unobtainable now so I'd really like to use them. Yes, the Sencore is a "toy", but do I see people with real Hickoks, etc. screening tubes with a life test? Or does their "real" testing make it unnecessary. Are these going to fail any day and take out an amp? Some will drop so slowly into the "bad" area on the meter that I wonder if I should not be holding the life test switch on for so long. It is a spring loaded momentary contact switch. I'm no expert at this, can someone advise me on the proper way to use and interperate these tests.
On a related note, I don't understand what grid leakage is. A couple of these NOS Rca 7189's have healthy emissions, don't waiver a bit through the life test, but show slight or more grid leakage. Tester manual sayes they can be used in a pinch, but keep an eye on them and replace when possible. This advice is aimed at TV repairmen though. Remember, these are audio output tubes, so I don't want to jeopardise the amps with tube failure.
So, should I be rejecting anything that fails the life test or shows grid leakage? One tube goes to the bad zone during grid leakage test and then climbs back up to good. What about this one?
Thanks for any education you guys may offer on this subject, Chris

SY 10th August 2012 04:21 PM

Quote:

Not sure how much faith I should hold in the life test.
Little to none. If the tubes perform well in circuit, that's all that really matters.

20to20 10th August 2012 04:56 PM

I use the TC142 which doesn't use the Life button. I just drop the voltage switch 1 notch and that usually tells you when you have a strong tube but in todays world of higher line voltages the filaments get higher than normal voltages. So if your tubes are failing the life test now, they are certainly weaker than the tester is even telling you. That said, the supply to your amp is going to be higher too, so you may get good emmissions for a while even from a weak tube.

The grid leak test is a different matter. If you get much of a bump off the bottom in the grid leak test you could have a tube that will get seriously leaky in the circuit under full power and heat. The best test for that is to get a testing socket and measure any DC on the grid with a K biased tube running in the amp. A "NOS" tube can sometimes show grid leak and this will diminish as the tube is burned in for the first few cycles, so in that case it's best to run a new tube that may show a grid leak, in the amp with grid voltage monitoring.

DF96 10th August 2012 05:17 PM

The 'life' test is actually an 'end of life approaching' test. A valve losing emission is unlikely to damage an amp.

Grid current can cause trouble, although the valve itself is the most likely victim.

Jim McShane 10th August 2012 06:49 PM

Quote:

Originally Posted by HiFiHarv (Post 3121645)
As I understand it, the life test lowers the filament voltage to see if the emissions remain constant.

Not constant, but a drop of "x" amount or percent - the amount of drop depends on the tester (my B&K 747B has a "life test" switch that lowers the heater voltage by 10%). The theory is that a tube approaching the point in its life where the cathode can no longer maintain adequate emission will have more drop off when the heater voltage is reduced. It's a very subjective test, and even after all these years and 1000's of tests I'm not sure I can give you a hard and fast rule of what it good and what isn't.

Quote:

Oddly, I even have NOS 7189's from reputable sellers that are failing the life test. Didn't have a tester when I bought them 20 plus years ago.
After 20 years in the box there is a good chance they are experiencing some cathode poisoning - contamination deposited on the cathode from other internal parts of the tube outgassing, etc. Almost always those tubes will be fine after running for a few hours - or you can use the Morgan Jones "bake the tube" trick, either of which will remove the deposits and return the tube to full health. DON'T TOSS THEM OUT!!

Quote:

Not sure how much faith I should hold in the life test. These tubes sound great and are unobtainable now so I'd really like to use them.
See my comments above...

Quote:

Are these going to fail any day and take out an amp?
If the only test they fail is the life test it is HIGHLY unlikely they will fail catastrophically.

Quote:

On a related note, I don't understand what grid leakage is.
A bit oversimplified - but it is current flowing from the control grid to other elements in the tube that shouldn't be flowing.

Quote:

A couple of these NOS Rca 7189's have healthy emissions, don't waiver a bit through the life test, but show slight or more grid leakage.
Again, running the tubes for a while will often reduce or eliminate any excess grid current. Another thing to watch out for is deposits between the tube pins - this can cause the tube tester to show grid leakage, but it isn't real. Clean the base and pins well and it'll be gone.

Quote:

Tester manual sayes they can be used in a pinch, but keep an eye on them and replace when possible. This advice is aimed at TV repairmen though. Remember, these are audio output tubes, so I don't want to jeopardise the amps with tube failure.
A LOT depends on the circuit. Some tubes with a small amount of grid leakage will perform very well in some circuits and will fail in others.

Quote:

So, should I be rejecting anything that fails the life test
NO! Failing the life test (such as it is) just means the tube has some wear and tear on it. It is not an indication of impending catastrophic failure.

Quote:

or shows grid leakage?
Maybe. See what I wrote above.

Quote:

One tube goes to the bad zone during grid leakage test and then climbs back up to good. What about this one?
Run it or bake it a la Jones - then re-test. I won't be surprised if it passes with flying colors.

As SY said, trying it in circuit is by far the best test - but sometimes you don't have gear around that uses the tube you want to test!

My advice is this:

1. Get the best tester you can afford, a "mutual conductance" tester is far better in many ways than an emission tester.

2. Read the manual carefull and completely.

3. Test as many tubes as you can to gain experience. I'll never forget when I threw away a quad of Amperex EL84s - they showed all shorts on my just purchased TV-7B, my 1st tester. Well - it turns out that's normal with the pinout on Euro EL84s! Don't fall victim like I did!!

I hope this was some help anyway...

HiFiHarv 10th August 2012 07:16 PM

Thanks to all for your comments so far. In particular, thanks Jim Mc. for concisely commenting on all of all of my questions. You have put my mind at ease a bit about still using these great old RCA's. Such a shame to have them sit in a drawer. I've been using Russian stuff while these just sit there. I will not be afraid now to run the grid leaking tubes, but will pull them out for retesting after a few cycles.

HiFiHarv 10th August 2012 08:11 PM

I just got back from googling the Morgan Jones thing. Haden't heard of this before. Now I am wondering if I should be doing this with the Grid Leakage tubes. Would leaving them in the tester under emissions test have a similar effect? I have no idea what the temp of the tube is while under testing, or operating. I'm guessing though that they might get hotter in the Fisher 30A's than in the tester.
Any thoughts?

boywonder 10th August 2012 11:37 PM

Quote:

Originally Posted by HiFiHarv (Post 3121911)
I just got back from googling the Morgan Jones thing. Haden't heard of this before. Now I am wondering if I should be doing this with the Grid Leakage tubes. Would leaving them in the tester under emissions test have a similar effect? I have no idea what the temp of the tube is while under testing, or operating. I'm guessing though that they might get hotter in the Fisher 30A's than in the tester.
Any thoughts?

You could leave them on the tester, or plug them into an amp (if you have one for that particular tube) and let the filaments heat up for awhile. The basic idea is that heating the tube activates the getter which will then eat some of the stray gas molecules flying around; it's these guys that can cause excessive grid current.

My Hickok tester gets really warm when I test tubes for an extended period of time (it's internals are completely sealed up), so it may or may not be a good idea to use your Sencor tester. I suppose that you could just wire up a tube socket for either 6.3V or 12.6V and bake them that way. If you are contemplating putting them in the oven, make sure the wife is out running errands and go easy on the temp dial as it's easy to melt/distort some of the valve bases.

ChrisA 11th August 2012 05:03 AM

Don't put a lot of faith in tube testers. Their best use is to make eBay buyer feel good be saying you tested the tubes you are selling. For example your "life" test may not apply to most guitar amps the run the heaters to high

Uunderhill 7th October 2012 04:38 PM

I would like to understand tube testing - so was going to raise this very topic.
Thanks to everyone who has added useful info.

It seems as a tube is used, its transconductance lowers - is this correct ?

However, I can only find one graph of how gm decays with use - which is in this article on page 173.
http://www.alcatel-lucent.com/bstj/v...tj36-1-163.pdf
It seems gm does not decay linearly with use.


Also, in a signal tube - its pretty clear that each triode should be tested individually.
Matching triodes is probably quite important for differential pairs and phase splitter circuits.


To test a tube, building a circuit to convert a scope into a curve tracer looks a bit much.

However, here is an idea that suddenly dawned on me after looking at a datasheet
Build the class A reference circuit as shown in the datasheet - apply a signal generator - and look at the output on a scope.
Then check the gain of a new reference tube versus the tube that is being tested.
http://www.radiostation.ru/tubes/6SN7.pdf

Would this work ?
.


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