„New“ Pentode – measuring resistance between grids G2 and G3 – any idea? - diyAudio
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Old 5th January 2008, 01:22 PM   #1
MRupp is offline MRupp  Germany
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Default „New“ Pentode – measuring resistance between grids G2 and G3 – any idea?

I recently bought some NOS EL3010 pentodes from TFK. Out of bad experience I measured them for shorts between the various grids, anode, and cathode, and the capacitances. One tube shows a resistance of ca. 280 Ohms between grids 2 and 3, so not a dead short but not exactly “open” either.

Does anybody have any experience with this type of fault and can the tube be rescued? If this would just be some getter material that found it’s way onto the mica wafer I could try and “burn” this off? Many thanks in advance.

P.S. One tube is completely dead, the welding between the wire coming from the anode cap and the anode structure appears to be broken. I would not have expected this from a special quality tube that is supposed to be shock resistant but there you go.
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Old 5th January 2008, 03:08 PM   #2
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Sorry, but in the tube domain 280 Ohms would be considered a 'short'.....recall when designing tube circuits we have to take into consideration very 'low' amounts of capacitances between Grids, Cathodes,Anodes...so a reading of 280 Ohms is bad!
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Old 5th January 2008, 03:39 PM   #3
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Martin,

Yes, 280 ohms is a short when we are talking about valves. Since it is already useless in its present state, there is little to be lost by sending a little current through there to see if you can burn off whatever is causing that short. It may not work or maybe it will. This, of course, unless your supplier has some return policy.

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Old 5th January 2008, 04:51 PM   #4
bigwill is offline bigwill  United Kingdom
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Or melt off as things wont burn in a vacuum
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Old 5th January 2008, 05:54 PM   #5
MRupp is offline MRupp  Germany
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Thank you all for your replies. I was hoping that this might just be a deposit of getter material that I could burn off but upon further testing, the short sometimes "disappears" when I move the valve then comes back, so this looks like a piece of the screen wire that moves around

I am now trying to get a refund as the tubes were described to be new and "in working condition". Unfortunately the EL3010 is a very rare tube that is hardly ever seen on ebay, so I would indeed prefer to have a working tube.
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Old 5th January 2008, 06:24 PM   #6
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Quote:
Originally posted by bigwill
Or melt off as things wont burn in a vacuum
Sorry for misusing that term. When an incandescent lamp or the heater in a valve fails people commonly say it "burned out". While technically incorrect, people understand the implied meaning. That is what I meant here as well.

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