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Old 11th June 2013, 01:38 PM   #1
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Default Diode performance testing

I’ve come across about 25x 5AS4/5U4GB that have all been used, but I don't know how much although many looked well used. Apart from doing a quick performance test on each, I was wondering if there were any simple tests to gauge wear out.

Two simple functional tests would be to check leakage current at say about -900VDC, and on-voltage at a moderate steady current of say 225mA (about 10W plate dissipation) to check plate to plate difference (and hence an indicator of mains frequency ripple in a typical rectifier configuration).

As I understand it, wear out would likely be a loss of emission capability, but I couldn't find any specific comments in RDH4 or RCA 1962 or Schure 1958. The Tung Sol 6X4WA datasheet sort of indicates low emission as a metric for acceptance by checking the average rectified current in a rectifier circuit.

I anticipate each diode would operate with its typical constant resistance characteristic up to the transient peak current rating (ie. 4.6A), and maybe show wear signs in that emission limit would be reached at lower current levels. A simple repeatable comparative test could then be a test jig with a few hundred volts DC source and a single pulse FET switch with a small series ballast resistor and the diode as a load, and measuring the voltage across the FET/ballast. A short on-pulse of say 10ms is then within design limits, and would not cause too much droop with a few 470uF 400V caps as source.

Any insights would be welcomed as it will take me a week at least to set things up.

Ciao, Tim

Last edited by trobbins; 11th June 2013 at 01:54 PM.
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Old 11th June 2013, 02:01 PM   #2
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I simply use a power supply to force a constant current into each plate and measure the voltage drop from plate to cathode. The VI curve for the 5U4GB and most other rectifiers is published in the data sheet. It is not quite linear enough to be called constant resistance. A good 5U4GB will drop about 40 volts with 200mA into each plate. I tend to test at about twice the maximum spec.

I find that looking for reasonable voltage drop that is equal for each plate gets rid of most sick tubes. I haven't tried a leakage test, but it is probably a good way to catch the gassy tubes that glow purple and blow stuff up.

I haven't done any 5U4's recently, but I screened a big batch of Bendix 5993's at 150 mA (70 mA spec) and the dead ones were obvious. Most had 10 to 12 volts drop per plate but the bad ones had one or both plates above 20 volts.
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Old 11th June 2013, 02:15 PM   #3
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Ta for that insight, which seems to indicate that gross wear out would definitely cause emission reduction well below the transient max level (eg. 1.6A for the 5993).

I wasn't sure if the wear out would vary the 'constant resistance' slope characteristic or whether the slope would remain fairly equivalent but with an emission limiting region occurring at lower and lower peak current levels, or a blend of both.
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Old 11th June 2013, 03:36 PM   #4
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No need. Checking the forward voltage is enough. it really sorts out the bad ones pretty well.

You can test tubes whit a measurement accuracy of say 3% and anyone whit a regulated power supply putting out at least 20 volts can verify that.



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Old 11th June 2013, 05:06 PM   #5
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Quote:
anyone whit a regulated power supply putting out at least 20 volts can verify that.
You need about 50 volts for tubes like the 5U4.

Quote:
Checking the forward voltage is enough. it really sorts out the bad ones pretty well.
Forward voltage testing might not catch a gassy tube since you are not applying enough voltage to ionize the gas present. I have seen 5U4's that glow purple and instantly blow the line fuse. I have not tried to test one since I don't have a sample handy.

Quote:
Ta for that insight, which seems to indicate that gross wear out would definitely cause emission reduction well below the transient max level
I have seen an issue like this in sweep tubes. A big sweep tube can test good in a tube tester, even a Gm tester, but operate poorly in a medium or high power audio amp. The amp may work OK at low power levels but clip or distort when cranked up. It seems that the cathode can no longer support the high peak currents needed for big power (over 1 amp) but work OK at lower power levels. Maybe a similar issue exists in rectifiers but I haven't seen it yet.

The older "drug store" type tube testers used to have a "life" button on them. You pushed the button, which removed heater power, and watched how long it took for the tube's emission to drop into the "bad" zone. There seems to be some correlation between this type of test, and the sick sweep tubes.
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Old 11th June 2013, 10:00 PM   #6
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The current-voltage characteristic at higher currents than typically plotted in a data sheet is of interest for capacitor input filters where the transient peak current rating can become the limiting factor in normal design considerations (eg. using more capacitance, or having to add additional effective series resistance).

I'd anticipate that wear out would start to be noticeable from overheating of the diode, or falling B+ or more sag, or all of the above.

V4lve, I'm more interested in the change in performance and whether that is easily tested than just a go/no go type of test. Sort of akin to testing a batch of unknown KT66's to determine their remaining life.
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Old 12th June 2013, 10:16 AM   #7
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You can't test any valve for remaining life. The best you can do is test whether it still has some life, by dropping the heater voltage a little and seeing how emission stands up. If it passes this test then it still has some life left, but you can't tell how much. If it fails the test then it is already on the way out.
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Old 12th June 2013, 12:35 PM   #8
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It would be interesting to get a feel for impact of diode wear-out, and I guess that starts with better understanding the V-I curve and how that typically changes.

Schure 1958 shows a general characteristic in Fig 2 where the slowly decreasing resistance region (as typically seen in datasheets) changes to emission saturation region.

It may be that the V-I curve trends southwards along the entire length over service life. But it may also be plausible that the more significant trend is the emission saturation region starting at lower current levels.


Link for those interested:
http://www.itermoionici.it/letteratu...rectifiers.pdf
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