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Old 28th March 2009, 04:07 AM   #1
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Default Funky Looking 15DQ8s

I scored a bunch of tubes in a recent dollar sale - among the booty is a batch of Raytheon PCL84/15DQ8s made in Hungary. They have a halo getter on top (nothing unusual about that), but also a bunch of mirror-finish deposits on the pentode side that look like misplaced getter action, though there is no getter structure even close. The tubes alll look to be NOS, so this supposedly would have happened some time during manufacture. Insight?
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Old 28th March 2009, 07:16 AM   #2
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"Migrating" getter material usually means that these VTs have been worked pretty hard. That was not unusual for a great many TV tubes, both large signal and small signal as well.. I've seen the same thing with 6V6s that were used in guitar amps.

They might have been cleaned up, but I highly doubt that they're NOS.
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Old 28th March 2009, 07:27 AM   #3
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Maybe I'll post a pic. These are all Raytheons in pretty much unworn but old boxes. The tubes are really clean and look unused. I have tubes (mostly sweep), that have had the snot hammered out of them so that the getter has migrated. These look different - and there's ten of them, looking like peas in a pod, not some lonesome ragtag. It almost looks like the getters were fired really hard while the envelope was still hot, so that some of the barium vapor squeezed down past the mica. Why the extra deposit ended up only on the pentode side of the tube is the mystery. I'll look closer, maybe there's a larger gap between mica and envelope on that side. The extra deposit I'm talking about is dense and shiny, not the brown stuff I've seen in abused tubes.
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Old 28th March 2009, 09:17 PM   #4
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The attached picture shows what I mean - all the Hungarian 15DQ8s I received are like this. The domestic brands I have are made in GB (Mullard?) but don't have this weird getter distribution.
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File Type: jpg 15dq8.jpg (30.1 KB, 286 views)
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Old 28th March 2009, 11:35 PM   #5
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Definitely peculiar, that.
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Old 29th March 2009, 02:31 AM   #6
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Are there holes in the pentode plate behind the spots? The three spots look fairly repeatable between the tubes.

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Old 29th March 2009, 03:57 AM   #7
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Bingo.
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Old 29th March 2009, 04:15 AM   #8
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I can't imagine getter material coming thru the plate. Something is fishy with those tubes I think. If you have a magnifying glass and can peak thru a hole in the plate or around the edge, would be interesting to see whether the whole inside of the pentode is coated too. I would think that would turn it into a plasma arc tube shortly when the plate gets hot. Maybe you have the first positronic prototypes.

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Old 29th March 2009, 04:23 AM   #9
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The getter pretty much covers the holes so you can't see inside. Either I bust one (they were only a back apiece), or get one of my tube testers working and see what goes.
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Old 29th March 2009, 04:33 AM   #10
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Hope that tube tester has a fuse in the B+.

I'm just getting my modded Tek 576 curve tracer going for tube tracing. (I will start a thread on that in a week or so.) Would be interesting to try one of those oddballs on it to see what's up (or down) there. Assuming they don't just fail outright.

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