One filament glowing brighter than others - peculiar?

[markmalin]

Has anyone else seen this?  I have a quad of EL84's and when I power the amp on, one of them glows really bright, then the others come on and that one's glow dies back down.  After the other 3 light up they are all even.  I swapped this one with the one next to it and it follows the tube, so it's something with that tube.


Is this is a sign of something amiss with the tube, or just a weird anomaly?  I've never seen this before.


Mark.

[shooter]

I didn't see it on my quad 84's, but this new amp I'm using an old AV7 and it glows noticeably brighter than the 84's or AX7.  filaments are analog so it just might be inherent to that tube, me, I'd probably add another tube to my next order as a cya sorta thing.

[markmalin]

I was going to swap in a new set before sending the amp out, so I should be covered.
Just strange that one tube's filament apparently sucks a bunch of current initially then then when the other 3 light up they balance out.

[eleventeen]

I have certainly seen this on 12AX7s, not so much on power tubes. I have this one 12AX7, a old Mullard that goes positively 100 watt incandescent lightbulb bright when first turned on. It's fairly alarming. Its neighbors wired in parallel are perfectly normal, and one of them is a Mullard so wtf? It follows the tube. Really don't know what to tell you, there are some tubes that do this. I haven't turned it on enough dozens of times to report on whether it shortens tube life. But if it *did*, what could I do about it? Would I stop using the tube? [ans: nothing, no]

[HotBluePlates]

Has anyone else seen this?  I have a quad of EL84's and when I power the amp on, one of them glows really bright, then the others come on and that one's glow dies back down.  ...

I have certainly seen this on 12AX7s ... I have this one 12AX7, a old Mullard that goes positively 100 watt incandescent lightbulb bright when first turned on. It's fairly alarming. ...

I've seen it a bunch of times. Generally happens in certain era tubes from the Philips conglomerate (Mullard, Philips, Amperex, etc.)

Is this is a sign of something amiss with the tube, or just a weird anomaly?  I've never seen this before.

Perfectly normal for these particular tubes. Bottom line, something about the heater's metallurgy in a series of tubes from Philips during a particular era caused them to emit a lot of (pretty bright white) light when they are cold and you switch them on. As they heat up, the light dies down pretty quickly. The particular tubes which do this will always do this, and it won't shorten their life. I've got quite a few Amperex and Mullard tubes which do it, and like Eleventeen said, most are preamp tubes.


I'm not terribly surprised that you might encounter it in an EL84, but I would expect EL34's to exhibit the behavior. That said, the visible portion of the heater and the position of the getter flash are such you might not see a flash which did occur.


Enjoy the tubes and don't worry. They're fine.

[markmalin]

Thanks, HBP.  :)

[stevehoover]

There was a post a few years ago online about "flash on start up" with old Mullard 12AX7's and new also newer JJ El84's.   Something was mentioned about impurities in the vacume and the getter, but that it was ok, and not the sign of a defective tube.   I have examples of both, the Mullard being one of their later premium purple box line that was out of a audiophile stash.  I use that tube as a "base line" to compare other 12AX7's. 
The other tube I have that flashes on start up is a JJ EL84 that lives in a Matchless Lightning clone with a higher than recommend B+.  That set has run for a couple of years of light to medium duty with out a problem.


It was scary when I first noticed it for sure, but the consensus at the time was "run em".

[markmalin]

There was a post a few years ago online about "flash on start up" with old Mullard 12AX7's and new also newer JJ El84's.   Something was mentioned about impurities in the vacume and the getter, but that it was ok, and not the sign of a defective tube.   I have examples of both, the Mullard being one of their later premium purple box line that was out of a audiophile stash.  I use that tube as a "base line" to compare other 12AX7's. 
The other tube I have that flashes on start up is a JJ EL84 that lives in a Matchless Lightning clone with a higher than recommend B+.  That set has run for a couple of years of light to medium duty with out a problem.

It was scary when I first noticed it for sure, but the consensus at the time was "run em".

These are JJ EL84's, so that probably explains it.  Thanks for the input.

[HotBluePlates]

Something was mentioned about impurities in the vacume and the getter, but that it was ok, and not the sign of a defective tube. 

It's really neither of those problems, only a curious property of some Philips heaters.


Note Most European tube companies eventually wound up under the Philips conglomerate. Further, later brands such as JJ and EI were using late Philips manufacturing facilities and/or processes. This is why some of those tubes show the same characteristics.