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Old 11th Jul 2020, 5:22 pm   #1
thevalveset
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Default Valve testing unexpected high emission?

Hi everyone,
Over the years of restoring vintage wireless sets I have tested many old valves on my AVO VCM.
Every so often one gets a valve that has a great deal more emission than stated in the data.
And occasionally, some valves have an alarmingly high emission and yet still respond to varying the negative grid volts. (so are not just shorted inside)
I am mainly talking about B4 and B5 era valves though have seen this phenomenon in B7G and B9A types too.

My question to the 'experts' is:-
What is actually going on inside these valves that makes them appear to be 'super active' and should one use or 'bin' them?
Thanks in anticipation.
Dave
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Old 11th Jul 2020, 5:32 pm   #2
turretslug
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Default Re: Valve testing unexpected high emission?

Maybe a teensy bit gassy- but not to the point of going into thyratron behaviour? If so I'd be wary of continuing to use them- a signal valve in that state could distort, muck up AGC action or jeopardise an IFT depending on application, a power valve sliding further into gassiness might be a threat to transformers, chokes and rectifiers.
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Old 12th Jul 2020, 7:40 am   #3
Diabolical Artificer
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Default Re: Valve testing unexpected high emission?

I tested an 807 that did this, something like 185mA as opposed to the 34mA it was supposed to test at. The valve looked fine, getter still good, no signs of air ingress, no burning etc. strangely there was no sign of red plating.

Andy.
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Old 12th Jul 2020, 8:15 am   #4
Radio Wrangler
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Default Re: Valve testing unexpected high emission?

Quote:
Originally Posted by Diabolical Artificer View Post
strangely there was no sign of red plating.
It's likely there wasn't much voltage across it.

Something somewhere else was probably getting a bit too warm, though.

David
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Old 12th Jul 2020, 9:28 am   #5
Bazz4CQJ
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Default Re: Valve testing unexpected high emission?

Valves which a have never been used before will sometimes show high emission which "burns off" over a period of ~24 hours. I've seen it happen on valves I had selected to use as "standard valves". I've sometimes used ECC81's, and curiously, the two triodes "aged" somewhat differently over the first 24 hours.

B
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Old 12th Jul 2020, 11:25 am   #6
Craig Sawyers
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Default Re: Valve testing unexpected high emission?

Tektronix used burn-in racks and defined procedures to (a) weed out early failures and (b) to get their characteristics to settle down. They then applied a label to the glass envelope with a Tek part number (but not obscuring the actual valve part number), and put them in a Tek box with the same part number on it.

Craig
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Old 12th Jul 2020, 11:32 am   #7
Hartley118
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Default Re: Valve testing unexpected high emission?

Quote:
Originally Posted by Diabolical Artificer View Post
I tested an 807 that did this, something like 185mA as opposed to the 34mA it was supposed to test at. The valve looked fine, getter still good, no signs of air ingress, no burning etc. strangely there was no sign of red plating.

Andy.
For some reason, 807s tend to behave as lively oscillators on my AVO VCM. That of course can result in very unpredictable readings.

Martin
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Old 13th Jul 2020, 11:18 am   #8
David Simpson
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Default Re: Valve testing unexpected high emission?

Hello Dave, most likely PO - parasitic oscillation. Often happens with power valves & high-slope valves. Heaps of Forum info - see "Search". AVO VCM's, and particularly CT160's, are prone to this phenomenon. Heaps more "Search" info on fitment of Ferrite beads.
Tektronix & other major players might well have used "Burn-in" racks for valves, but this isn't recommended for use with one's valve testers, particularly expensive AVO's.
As for wee ECC type double triodes - 81's & 88's are two a penny, but 83's can be jolly expensive & E88CC's, E288CC can cost a fortune. I wouldn't put then anywhere near "burning-in" apparatus. Also many of them are Vg sensitive - get that wrong & "flash bang wollop" ! This happens with VCM's & other V/T's when folk believe that "Corse & Fine Ia" or other bridge balancing pots control Ia. Wrong, once under "Test", only the Vg(Delta Vg) pot controls Ia, and mA/V(delta Vg) pot to a much lesser degree.

Regards, David
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