What causes the brown residue/stain on some tubes?

[Shannon Parks]

The Mullard EL84s are notorious for this discoloring. Just recently noticed some 6P14Ps with it, too. My guess is some contamination internally. Possibly even some type of releaser from manufacture. Anyone know the real scoop?

Shannon

[dhuebert]

I have some 6BQ5s like this, Gar Gillies took one look at them and told me the tubes were shot. I didn't ask why, but I still could...

Don

[EWBrown]

Is this some form of "getter flash" perhaps? Usually this will look silvery or black. The "getter" is used to burn off any remaining atmosphere inside the tube after it is vacuumed out and the glass is sealed. It usually has some metallic oxide of (barium or cesium ?) that gets "fired" by applying a strong contained RF field, the getter "goes off," burns off teh remaining air, and leaves the silver or colored residue inside the glass bulb.

I know some old 6V6s and similat BP tubes also had a grey "smoke" substance covering teh inside of the tube, that was a part of manufacture.

I dunno if this will get thru, my ISP is undergoing yet again another series of electro-spasms...

/ed brown

[Shannon Parks]

It's not the 'gray smoke' or getter flash. It looks like a burned up tube - but some have seen this on NOS Mullard tubes (not Ebay 'may be NOS' har har). I'll have to post a pic up here - the Mullards look like hell. I have no reason to suspect that the 6P14P tubes aren't NOS, too.

Shannon

[erichayes]

Hi All,

Bud Wyatt (Sheffield Labs, The Mastering Lab) came over this afternoon to get some parts and trade war stories, and I pulled out one of my perfectly functional, rotten looking Mullard EL84s and said "Wazzat?"

He said that Mullard used a getter that was not a pure metal, such as lithium, sodium or magnesium; rather, they chose to use a halide compound that, after time, caused the "bleeding" that we have come to know and love.

My experience with Mullard EL84s is that you can kick the snot out of them, and they just keep working. They have a somewhat higher internal resistance than NOS American and new production Sovteks (in my amps the damping factor is 4.5 vs 5.5 for NOS and Sovtek), which probably accounts for their "warm" (some call it "yellow") sound.

Bottom line: Don't judge a Mullard EL84 by its looks. It's a great tube.

Eric in McKinleyville

[Shannon Parks]

Thanks for the great answer! I Googled all over the place yesterday looking for the scoop (AA forums and Usenet archive, too) but got nowhere. Thanks, again.

(pic forthcoming)

Hehe, Don - I bet Gar has a whole drawer of these gems that were just given to him.

Shannon

[Shannon Parks]

Good example, folks.

Ebay link to some cruddy looking Mullards

Shannon