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tube flash

post #1 of 19
Thread Starter 
i recently received a pair of amperex 12ax7s for my dared mp5. they sound great, but one of the tubes flashes when i turn the amp on. i had the tube arcing problem with the ld2+ and it definetely isn't as "violent" of a flash, sort of an orange flare, but i'm still concerned. should i contact the seller? is this something i should be worried about? thanks.
post #2 of 19
Some tubes do flash on start up but in my experience it's the European made 12at7 types
post #3 of 19
Yep, perfectly normal for Amprex & Mullards too, mine being 12AU7s for my Foreplay Pre... In fact it is yet another test for their not being knock offs or rebadged imitators....
post #4 of 19
Iv got a Mazda grey plate, 1950's French tube, that does that
has ran fine for about 400 hours, or more
post #5 of 19
Quote:
Originally Posted by phergus_25 View Post
Iv got a Mazda grey plate, 1950's French tube, that does that
has ran fine for about 400 hours, or more
And to expand on this, when we tested them the one that does have the heater "flash" tested better all around than the one that didn't do it. So yeah, certainly nothing to worry about.
post #6 of 19
i think it's trapped oxygen. it effects the durability of the tube in theory.
post #7 of 19
Quote:
Originally Posted by Fitz View Post
And to expand on this, when we tested them the one that does have the heater "flash" tested better all around than the one that didn't do it. So yeah, certainly nothing to worry about.
I had forgotton about taht
thanks
post #8 of 19
Quote:
Originally Posted by cotdt View Post
i think it's trapped oxygen. it effects the durability of the tube in theory.
in theory the oxygen in the tube should collect on the getter flashing and oxidize it. on a plus side, you are correct: once a tube has oxygen in it to the point where it is not reacting with the getter flashing (because the flashing is fully reacted) it has a very short lifespan, usually measurable in moments to meltdown.

why spread these brainstorm ideas when the reality of the situation exists elsewhere (in this thread even) and has been shown to be quite different?
post #9 of 19
Quote:
Originally Posted by nikongod View Post
in theory the oxygen in the tube should collect on the getter flashing and oxidize it. on a plus side, you are correct: once a tube has oxygen in it to the point where it is not reacting with the getter flashing (because the flashing is fully reacted) it has a very short lifespan, usually measurable in moments to meltdown.

why spread these brainstorm ideas when the reality of the situation exists elsewhere (in this thread even) and has been shown to be quite different?
ok then if it's not oxygen, then why does it flash? i suppose i can run my tubes through a spectrometer to find out what it is. who's to say that a small amount of oxygen would cause a meltdown in a small moment? it's not the same as cracking open the tube and just running it that way. and I said "i think" so that people would take it with a grain of salt. you, sir, are against the spirit of interesting discussions! someone has had too much caffeine this morning.
post #10 of 19
Quote:
Originally Posted by cotdt View Post
ok then if it's not oxygen, then why does it flash? i suppose i can run my tubes through a spectrometer to find out what it is. who's to say that a small amount of oxygen would cause a meltdown in a small moment? it's not the same as cracking open the tube and just running it that way. and I said "i think" so that people would take it with a grain of salt. you, sir, are against the spirit of interesting discussions! someone has had too much caffeine this morning.
The "flash" being talked about is the heater filament glowing very brightly upon first powerup, then quickly settling to it's normal glow. It's not the same kind of flash as something reacting or arcing inside the tube. This is normal behavior for a lot of 12AX7s and variants, and like Hi-Finthen said can be an indicator of authenticity for certain ones.
post #11 of 19
Thread Starter 
cool. thanks for the info guys.
post #12 of 19
This glow is normal for some tube makes and types. When the heater element is cold on a tube it has very little resistance. Hence a lot of current can flow. AS it does the element heats up quickly (hence the bright glow) but once it does the resistance climbs and reduces the amount of current passing. Hence the tube settling back down to the normal glow. It is not the same as the "arcing" problem that was seen in the LD2+ where that was actually caused by current passing between the internal structures of the tube (IIRC).

Poor vacuum, ie oxygen in a tube can show up in different ways. If theres enough there, the element will simply burn out. A little bit causes a blue glow in some tubes, but not all. EL84s I think show this sometimes. Confusingly blue glow is also normal in some tubes where its caused by electrons fluorescing at the glass envelope.

I always thought that the getter was only used once, when the tube was made. What I thought was that they made the tube in near vacuum condidtions, but that there was always a bit of air left. So when the finally sealed the tube, heated it and the getter material reacted with the oxygen to bind it and remove it. I could be wrong on this.

This question comes up quite often......



Fran
post #13 of 19
Quote:
Originally Posted by fran View Post
This glow is normal for some tube makes and types. When the heater element is cold on a tube it has very little resistance. Hence a lot of current can flow. AS it does the element heats up quickly (hence the bright glow) but once it does the resistance climbs and reduces the amount of current passing. Hence the tube settling back down to the normal glow. It is not the same as the "arcing" problem that was seen in the LD2+ where that was actually caused by current passing between the internal structures of the tube (IIRC).
that doesn't explain why some tubes flash while other tubes of the same make and model do not. seems to be "normal" in the same way that diseases and illnesses are normal in humans.
post #14 of 19
Quote:
Originally Posted by cotdt View Post
that doesn't explain why some tubes flash while other tubes of the same make and model do not. seems to be "normal" in the same way that diseases and illnesses are normal in humans.
Because they are NOT the same make and model. There is a incredibly huge amount of variation between tubes which are nominally the "same make & model", for instance a Philips 12AX7. The first issue is which year, many of these tubes had decades long production runs and the construction methods, materials, and actual physical construction changed over the years. Now that you have the production date picked out you need to find the factory codes, if any, and look very carefully at the physical structure of the tube. Two Philips tubes of the same year & model could've been made in separate factories, the machinery & construction methods will vary slightly and the tubes won't be exactly the same. One might have two spot welds holding the getter in place while the other has only one, and so on and so forth.

Now if you have tubes from the same production facility made at the same time and the physical structure looks identical, I'd be very surprised if some of them flash while others do not. I have large factory cartons of various tubes, if one of them flashes it's almost guaranteed that the entire lot will flash.

Going back to why they flash. For whatever reason, and there's been a great deal of speculation on this, some tubes were built with a thin spot on the tube filament wires, usually just below where the wires enter the cathode sleeve. When the filament is cold its resistance is very low, and on powerup it sucks down a large current which is forced to flow past the thin spot. The thin spot heats rapidly and glows orange to white for a few moments (this is the flash), after which its resistance and the resistance of the warming filament drops the current to its nominal values, ending the flash.
post #15 of 19
Good answers here and hopefully will put some minds at ease and edify others. It can be good also to put a tube in the oven and heat them to activate the getter as it pulls out more gasses when warm/hot and eliminating these before real voltage is applied to the tube when the real harm can be done if the gasses are present. Yes, some tube experts with many thousands of tubes do this with some tubes they plan on using. You can also use a tube tester to heat the tube by only using the filament/heater and achieve the same result. Even though a tube will work fine without this (NOS) you never know what gasses may have been captured and not allowed to first do some small amount of damage to the inner electral conduction paths.
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