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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 15th Mar 2020, 8:35 pm   #1
G4_Pete
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Join Date: Feb 2014
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Default AVO CT 160 GAS test

I was doing a sort out of some EL34,s I was given on a go no go basis using a B&K 606 and it picked out one as no emission and on the Grid Emission/Gas test it was just in the green but there was a reading. Visually it was good with no shorts so out of curiosity I tried it on my AVO CT 160 to see what was going on in more detail. All was ok until I selected "TEST" when the valve flashed blue and tripped the cut out in and out. (Yes, I do have the recommended meter protect).
The simple B&K 606 did not make any fuss and simply indicated problems whereas the AVO CT 160 went into complete overload on the valve and raised my heart rate for a few mins.

Although the getter was good and visually the valve looked perfect with no shorts, I am presuming it had gone soft /gassy? However, on the AVO why might they have put the gas test after the main "TEST" position? You get no warning of a potential problem until the tester goes into overload.

Am I doing this test correctly or have I misinterpreted something here?
Pete
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Old 18th Mar 2020, 5:41 pm   #2
David Simpson
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Default Re: AVO CT 160 GAS test

Pete, Its imperative that you use "Search" & see the multitude of helpful advice going back years. Yep, some of us AVO VCM/CT160 enthusiasts could give you detailed info & advice, but we'd be merely repeating what has been said many times.
Study all the "banging-on" from past posts - particularly those in the recent few days for "HPWOODY"'s CT160 thread posts. Then hopefully you'll avoid something like his unfortunate outcome - phooked -ve Grid Volts Pot.
Not only are CT160's helluva expensive to buy, they are helluva expensive to repair - - the 30uA Meter, RV2 3 segment 10K Pot, & the single big mains transformer are the most expensive at anything in the region of 200 quid each for a genuine AVO replacement.
If you feel in any way uncomfortable with delving into your CT160 - try & find a helpful CT160 owner nearby to mentor you.
A valve flashing blue & the relay buzzing like hell does not bode well.
Regards, David
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Old 20th Mar 2020, 9:21 am   #3
G4_Pete
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Default Re: AVO CT 160 GAS test

David,
Thanks for the advice. I have now found some of yours and others posts and will investigate the protective relay operation and grid pot protection further before I risk damaging the tester.

I did search before posting but was focussing on searching for gassy valves and using the CT160 itself to investigate that phenomenon. Re reading my post I guess my clumsy wording did not convey that message clearly.

I think I will make a safe test rig to examine the failed valve further for no reason other than curiosity.
Pete
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Old 20th Mar 2020, 11:24 am   #4
GrimJosef
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Default Re: AVO CT 160 GAS test

If you're concerned about gas in power valves then a test rig, or a power amp you can try them in, is far-and-away the best way to go. Valve testers will tell you if they're so gassy that it can be measured when they're running with low anode dissipation. But often gas is only released from the metalwork when all of it gets properly hot. That takes quite a few minutes of running close to the anode dissipation limit and almost no valve tester can do that.

Cheers,

GJ
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Old 20th Mar 2020, 12:14 pm   #5
David Simpson
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Default Re: AVO CT 160 GAS test

GJ is spot-on. Rather you phooked a home brew test rig than a mega bucks CT160.
AVO VCM's & CT160's indicate a possible gas problem by measuring minute Grid current, ie. uA Ig. For the safety relay to Buzz - indicates excessive Anode or Screen current. Also, if the Anode seems to glow a bright blue, that means its jolly hot & drawing excessive Ia.
Me - I always advise AVO VCM & CT160 owners to set -ve Vg a bit higher than the book value, thus holding Ia back a bit, & it avoids the meter's needle "whanging" to fsd - - CT160's, and the old 2 Panel Testers have always been prone to this. With CT160's, the coarse & fine Ia controls DO NOT actually control Ia, they just balance the voltage bridge network prior to the mA/V test procedure. Sadly, CT160's, (unlike the MK's 1 to 4 VCM's) do not show Ia on the meter. Only the Vg pot controls Ia(the telephone dial-like mA/V pot also controls a very small amount of Vg & hence a very small amount of Ia).
All very complicated, I know, and also it being done with 50Hz ac pulses makes it even more complicated.


Regards, David
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