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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 16th Mar 2017, 7:05 am   #1
Diabolical Artificer
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Default EL34 odd failure.

Testing some EL34's yesterday to get matched valves I had one on the tester that suddenly stopped working. After a bit of experimentation I found the heater glows but as soon as I apply anode and scr grid voltage the heater grows dim. Turn the anode and scr volts off, it glows again. There are on no inter electrode shorts. Tapping or knocking the valve doesn't change anything, so don't think it's a dodgy connection in the valve.

Andy.
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Old 16th Mar 2017, 9:36 am   #2
Peter.N.
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Default Re: EL34 odd failure.

That is odd, hasn't got air in it has it, even slightly?

Peter
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Old 16th Mar 2017, 9:59 am   #3
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Default Re: EL34 odd failure.

EL34 is quite a beefy valve for mainstream testers (in both heater and HT power), possibly even a "normal" one operates its transformer close to its rating and a rogue or edge-of-spec EL34 is causing a marked droop in secondary output?
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Old 16th Mar 2017, 11:12 am   #4
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Default Re: EL34 odd failure.

What's the anode current at the point when the heater starts to cool?

Martin
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Old 16th Mar 2017, 11:31 am   #5
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Default Re: EL34 odd failure.

You may want to stick a meter on the heater to check the tester isn't dropping the voltage when the anode current is brought up.

David
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Old 17th Mar 2017, 6:54 am   #6
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Default Re: EL34 odd failure.

No air Peter.

My tester's tfmr is beefy but it still stuggles with big OP valve's. Interestingly it can test 807's scr at 250v
anode at 200v, bias -10v Ia 90mA ish but struggled with these EL34's. Mind you the EL34's are brand new the 807's a bit long in the tooth.

Anyhoo, EL34's tested at 250v scr, 100v anode, bias - 10v Ia 110ma ish. I drop or raise the bias 1v to get gm. All the the other EL34's tested this way tested ok.

Anode voltage around 20v, same for the screen when htr starts to cool. I'll do some more testing today but I don't suspect the tester htr supply which can do 3A @ 30v.

Andy.
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Old 17th Mar 2017, 8:41 am   #7
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Default Re: EL34 odd failure.

But measure it anyway.

When something impossible seems to happen and I wind up going round in circles, it usually turns out to be something I'd assumed it couldn't be. Check everything, even do checks which seem unnecessary. That's where the surprises lurk.

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Old 17th Mar 2017, 9:34 am   #8
Phil G4SPZ
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Default Re: EL34 odd failure.

That's excellent advice, David. I have personally wasted a lot of time going round in circles thinking "no, it can't possibly be that", but it was indeed "that"!
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Old 17th Mar 2017, 10:53 am   #9
TrevorG3VLF
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Default Re: EL34 odd failure.

I tried to test a 6146 on an old AVO tester. It blew the trip and blotted out the radio I had on. Obviously oscillating at RF, a better than normal valve.
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Old 17th Mar 2017, 12:07 pm   #10
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Default Re: EL34 odd failure.

Do you have a circuit that you can try that particular EL34 in? That would verify whether the problem lies with the valve or the tester.

I sometimes get unexplained odd results with power valves on my AVO VCM MkIV - particularly with 807s. I guess that instability is the most likely explanation.

Martin
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Old 17th Mar 2017, 3:11 pm   #11
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Default Re: EL34 odd failure.

Trevor may have a point, I remeber the first time I checked some EL34s on my new Sussex they broke into fierce oscillation at some ultra-sonic frequency. Grid stoppers cured that, but the readings were all over the place before I found the culprit.
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Old 20th Mar 2017, 6:10 am   #12
Diabolical Artificer
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Default Re: EL34 odd failure.

Tester has grid stopper's and ferrite beads on all pins, also checked for oscillation.

This valve wasn't working in circuit.

Tested it yesterday, it now has no emission at all with the heaters staying on. There must have been an intermitant short in there somewhere, couldn't see owt, maybe between heater/cathode.

Anyhoo, it's dead. Another valve for the mantle piece, Andy.
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