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Old 16th September 2014, 04:16 AM   #1
Keit is offline Keit  Australia
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Default Testing emission without risk

Testing the emission of tubes by the traditional method of connecting all grids to the anode and applying an accelerating voltage, typically 30 VDC or about 30V AC in cheaper tube testers, subjects the tube to a considerable overload and will quickly ruin the emissive surface. For that reason, tube testers usually had a button you had to hold down to do the emission test.

Today, good digital multimeters are readily available at a cheap price, so we can do accurate measurements that were not practical in the pre-transistor days.

Loss of emission, ie reduced cathode electron emissivity as the tube ages, generally involves subtle mechanical and chemical changes. Most of these changes can be expected to increase thermal emissivity. I got to thinking that this will mean that tube filaments will draw higher than normal current at low voltages. The electrical resistance of metals increases with temperature, so higher heat loss lowers resistance.

I have a large number of tubes of various sorts, ranging from good late production NOS to old worn out tubes taken from old radios and TV's.

I have found in testing that the heater current drawn at 25% of rated voltage is consistently higher for low emission tubes, even though the current at rated voltage is normal. This test was done with all grids and the anode left open. Typically, for a tube having emission 50% low (which in most circuits will still work normally), the ratio of heater current at 25% voltage to that at 100% voltage is about 8% higher.

So it seems that tubes can be screened for low emission by checking this ratio, which is completely non-destructive.

Has anyone come across this method before, or have any thoughts on it?

It is not mentioned in any books on tubes and tube testing that I have access to.

Last edited by Keit; 16th September 2014 at 05:51 AM.
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Old 16th September 2014, 05:20 PM   #2
gsmok is offline gsmok  Poland
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Hi,
It seems to me that this emission measurement in a safe manner can be estimated by using impulse method for measuring the anode current, as it is done in the "uTracer" tester. It is both safe for ranges far above maximum ratings of the tube: plate dissipation and anode currents in the range of saturation. Furthermore, these measurements can be made for the real, not reduced anode voltages.
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Old 16th September 2014, 05:36 PM   #3
kevinkr is offline kevinkr  United States
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I can't see how the increase in filament currents at low voltages you mention can equate to low emission in an indirectly heated tube, I think you would have to have measured the tube periodically over the course of its service life to determine whether or not there was an actual correlation or that the tube always exhibited those characteristics. I can think of a variety of other reasons.

Emission IMO is not usually a very useful parameter to test, and on a proper tube tester limited emission should show up in the course of testing transconductance.

I own a uTracer and bad / emission limited tubes are usually obvious as they do not perform comparably to a known good tube.
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Old 16th September 2014, 06:03 PM   #4
TheGimp is offline TheGimp  United States
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I've been using comparative emission to determine how much life is left in a tube.

Test it at nominal heater voltage and note the Gm. Reduce the heater voltage 10% and check it again. Gm varies very little in a new condition tube. It drops significantly in heavily used tubes.
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Old 16th September 2014, 06:11 PM   #5
kevinkr is offline kevinkr  United States
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This is one of the classic ways to perform such a test.
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Old 16th September 2014, 06:38 PM   #6
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Wow, an actual experiment with a correlation detected.
Thanks for doing this. Perhaps someone else can replicate the experiment.
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Old 16th September 2014, 07:25 PM   #7
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by indianajo View Post
Wow, an actual experiment with a correlation detected.
Thanks for doing this. Perhaps someone else can replicate the experiment.
I find this quite interesting too, and that is not something that I would dismiss lightly.
Unfortunately, I have only have very few directly heated tubes in my stock, and I would be unable to contribute to the statistics.
The idea does look plausible: after all, electrons are significant heat carriers in a number of materials (except here it should work in the opposite direction, but things are often more subtle than they look at first sight). Worth investigating, for sure
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Old 16th September 2014, 07:49 PM   #8
kevinkr is offline kevinkr  United States
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I've done it, it works...
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Old 16th September 2014, 10:38 PM   #9
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most tube testers i own have a "life test" on it; my B&K 700, for example. When the switch is activated ( from what i understand) it basically drops the filament voltage down a bit. If the meter reading drops quickly with the lower filament voltage then the tube is older. If it holds steady, it's good.
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Old 17th September 2014, 02:15 AM   #10
Keit is offline Keit  Australia
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Err... Testing emission in the way suggested by gsmok can cause emission layer burns and physical removal of emission layer material in oxide cathodes, thoough it is acceptable in tungsten cathodes (only used in high power transmitter tubes).

The uTracer is intended to trace curves for the purpose of matching tubes and for obtaining detailed information to assist in designing tube application circuits. Pulse testing has long been accepted as the way to do this - it is how the tube manufacturers compiled curves. While you CAN program it to test emission with high voltages, this is not a good idea. Pulse testing can certainly keep anode and screen dissipation within what the anode and screen can handle, but it doesn't stop particle stress on the cathode, nor does it eliminate field emission, which is harmful to oxide cathodes.

Last edited by Keit; 17th September 2014 at 02:21 AM.
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