Tube flash.

[TonyMc]

I've read this is a common occurrence in the smaller, older European tubes, but does it happen in USA built larger tubes?
I bought a couple of the NOS RCA 6AS7G tubes and noticed That one flashes on start up. It's just a real quick small white flash that occurs where the filament is connected at the base. It only happens for a split second.
Other than that the tube works and sounds great.

[InfernoSTi]

My NOS Mullard CV4024 (12AT7) has the same flash...it is normal it turns out.  I had read that it was a British tube signature, not an American or Continental tube trait.  Perhaps the tubes you have are rebranded British made RCAs?  This was a common practice back in the day.  Someone with actual expertise could tell you what to look for....

John

[TonyMc]

This was more like a bright white spark. Not the bright orange/yellow tube flash that I've seen on youtube.
After it happened the tube warmed up fine and sounds OK, but I notice after a few minutes the volume seems to taper off quite a bit. I pulled the tube and replaced it with another 6AS7 I had laying around.
After I pulled the tube I shook it a little and noticed a very small, about the size of a pin head, ball of metal rolling around. So I assume something in the tube arched. Oh well I guess I'll put it in the DOA bin.

[InfernoSTi]

DOA indeed....good move to not use it further!  Sorry to hear you got a bad one in the group: I've had that happen to me, too.

John

[TonyMc]

DOA indeed....good move to not use it further!  Sorry to hear you got a bad one in the group: I've had that happen to me, too.

John

Well John the good news is they're a dime a dozen on the e--bay.

[Laudanum]

Yep, had a bad one myself.  It would flash bright white between the plates on one side at startup.  Almost like an arc.  Static in one channel (left I think) accompanied the flash and then intermitted static in that same channel with quick white flashes.  That 6AS7 found the dumpster quick.

Also just had a bad 12BH7.  No flashing.  The right channel was cutting in and out but I wasnt sure it was that tube and then things were fine after a minute of warm up.  Next time I turned it on, nothing but hum in the right channel and no output at all from Crack.  It was the 12BH7 ... The only GE 12BH7 I have too and it wasnt a bad sounding tube.

[porcupunctis]

Recently had a "lightning flash" experience with a 5U4.  Never seen that before.  It just flashed all over for a bit on start up.  Made me a little nervous, so I swapped it out with one that didn't do that.

Not an expert on this but I suspect this is "sub-optimal".

[Laudanum]

I wanted to mention that it's not all that uncommon to have a small piece of something loose in the tube.  Little chip from one of the mica spacers is pretty common.  My understanding is that something like this is benign and the tube is fine to use.  Loose metal is probably a different matter if it can short the leads to the pins at the bottom of the tube.  Or if it gets wedged in somehwere else maybe. 
I have a couple of 9 pin tubes that have been in use for years that have a little mica chip loose at the bottom.  Maybe the tube experts will chime on on what's ok and what's not when it comes to tubes with loose material inside.

As far as those white arcs are concerned, doesnt sound like it's ever a good thing for both of our experiences. 
The Euro-tube flash at start up is neat though and normal. 
And the OD3 (FPIII non-extended) with it's lavender looking glow is just too cool. 

[Doc B.]

Yeah, glass chips rattling around at the bottom of the envelope are fairly common too, and typically benign.

[TonyMc]

The 6080 that came with the crack has a little rattle in the base, but it runs fine. I can't see what material is causing the rattle, but the 6as7g on the other hand... Is a very small, what looks like, a metal ball rolling around which could explain the white arc I saw. Either way that tube is gonzo.