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Old 2nd November 2012, 01:56 AM   #61
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also, i have noticed on several different brands of amp, that they use a spring loaded keeper ring on the top of output tubes. unfortunately, if these keepers aren't used properly, they can create tiny fractures in the glass. i usually see it right where the spring is attached to the keeper. the coil spring is in contact with the glass, and causes a small cold spot in the glass because the coil spring conducts some heat away from the glass. this eventually causes little spider-web cracks around the spot where the spring touches the glass. every time i repace output tubes that have these keepers, i bend the ears of the keepers out away from the glass. i have seen quite a few tubes get first gassy, or worse, get enough air in them to oxidize the getter coating, as a result of these tiny fractures in the glass.. in case anybody isn't familiar with the keepers i'm talking about, it's a metal ring, about the diameter of the tube, with a pair of tabs that come down for about 1/4 to 1/2 inch along the sides of the tube, where they connect to two springs. the springs are anchored on a ring around the tube socket or two small holes in the chassis. i see them mostly on Line 6 amps these days, but have seen them used on other brands in the past.

as far as shielding and reflecting heat away from the wood, most manufacturers use metal foil glued or stapled to the wood
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Old 2nd November 2012, 05:33 AM   #62
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I would imagine everything would blow up and melt long before the wood actually catches on fire lol
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Old 2nd November 2012, 05:35 AM   #63
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then again not sure maybe like some Wild B+ arcs melting everything then it just burst into flames lol
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Old 2nd November 2012, 05:46 AM   #64
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When I was 15-16 I had an old x100b and I was doing something stupid.....haha think I was trying to bias it through the stand by switch or some **** I had that sucker on and I was probing the contacts or I slipped off or something and I crossed them and BOOOOM haha dude I'm talking sparks flew buddy blew my hot probe apart and caught the nearby ribbon cable on fire lol so yea fires do happen lol that was pretty intense lol that DVOM went to hell after that lol so yea young and dumb lol. Its miracle I'm still alive soo many close calls when I was younger
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Old 2nd November 2012, 05:48 AM   #65
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Haven't read the whole thread but I recently worked on an amp where the original 7591s cleared the wood more or less ok, but the new bottles are bigger and the wood runs a lot hotter. I don't think you can get the original size unless you get NOS tubes. I doubt it will catch on fire, but it's not a good thing either.
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Old 2nd November 2012, 03:19 PM   #66
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Quote:
i have noticed on several different brands of amp, that they use a spring loaded keeper ring on the top of output tubes....i have seen quite a few tubes get first gassy, or worse, get enough air in them to oxidize the getter coating, as a result of these tiny fractures in the glass..
Interesting observation.

A few months ago I was given a dead Park (Marshall) amp. It had a note attached that said "blows fuses". There was a dead 2 amp fuse in the HT fuse socket. I tried a new fuse which quickly evaporated. Upon opening the amp I found 4 Sylvania fat bottled 6CA7's. All had silver getters, but two were obviously thinner than the other two. I tried a second fuse with the back off the amp, and sure enough there was a bright blue flash in one 6CA7 as the fuse blew. Then I noticed that the tube was hot, but had no heater glow. This means air inside. The first time I have seen air in the tube with a silver getter. I popped in 4 matched EH EL34's and the amp works fine.

I still have the Sylvanias, and the 3 other tubes work good in a Tubelab amp. I'll look carefully at the glass in the area where the tube retainers touch the tube.
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Old 2nd November 2012, 06:20 PM   #67
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What does it mean if there's a row of same-brand power tubes, and all but one have silver tops like a glass mirror, but one tube is all clear glass at the top?
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Old 2nd November 2012, 09:28 PM   #68
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All good receiving tubes should have a shiney silver spot somewhere on the inside of the glass. There is a getter ring (it "gets" the air) just under the spot that contains a chemical compound that sucks up all remaining air in the tube when burned creating the silver spot. It is activated after manufacturing by RF energy.

The silver spot will absorb many of the impurities created during the operation of the tube. It will grow fainter and turn grey or brown during this process.

If the spot turns milky white then air has entered the tube due to a leak. I have seen an air leak cause the silver spot to dissapear completely. The tube will get warm, but will not work at all.

It is possible that the one oddball tube has the silver spot somewhere else on the glass. It could have come from a different batch, or even a different manufacturer. All the old tube manufacturers sold each others tubes with their brand on them.

It is possible that the getter ring failed to activate during the gettering process, or the getter ring was left out during manufacturing. I have seen two or three examples of this in 50+ years of tinkering with tubes. The tubes seemed to work OK.
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Old 2nd November 2012, 10:14 PM   #69
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Yes, the silver spot on one of my GE 6v6 went away guess it got air in so the others take over and get loaded down and want to run away sucks man cause it sounded good for awhile my guess it was almost worn out and my voltages stressed it too much who knows. I took it out and popped in a CBS I had mixed with the GE they they seem pretty even except for the screen grid wire is glowing pretty red near the base lol
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Old 2nd November 2012, 10:15 PM   #70
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sounds awesome though its like I have the top end of the CBS and the bottom end of the GE lol
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