tube changes color

[Waspination]

Ive got two 6L6's and two EL-34's in the outer slots.
When i go from standby to on, my right 6L6 gets a little blue/purple in the bottom of the tube. Not popping, and no huge color change, but noticeable. Is it time to change these tubes?

 

[Scary]

Try and tap on it with chopstick. Look for firework. =) Seems ok from the picture, which is nice btw.. makes me want to jam.

 

[Waspination]

Try and tap on it with chopstick. Look for firework. =) Seems ok from the picture, which is nice btw.. makes me want to jam.

I think i had it on standby there, its when i flip it on. mabe i get a pic of it if it picks up cuz its not dramatic

 

[Waspination]

Tapped it. It makes a slight noise, the others dont so looks like i need a new pair

IF i went from two 6L6/EL34 to all 6L6's... would re biasing need to be done?

 

[212Mavguy]

Consult your owner's manual before deciding to run 6l6's in the el34 sockets. If you stick 6l6's in the el34 sockets they might run too hot unless the sockets that have el34's presently were intended to run 6l6's in the first place. Your manual should be a good guide for that decision.

The bias setting is "meant to be left alone" by the designer(s) of the amp.

 

[94Tremoverb]

"Blue glow" being bad is one of the biggest myths about tube amps. I have no idea how it became so widespread, and I can't begin to imagine how many perfectly good tubes have been trashed just because they glow blue, because of this. It's almost always complete nonsense.

Blue/purple glow just inside the glass is a sign of a *good* tube in good working order. It's caused by minerals in the glass glowing when bombarded by stray electrons from the operational structure of the tube, and means the vacuum is good. Almost all power tubes show a very small amount of it, but some of the tubes with the strongest glow are the best quality US-made 6L6 types from the 1970s and 80s - so it's especially sad that these will be among the most commonly thrown out because they glow blue.

*Some* types of blue glow - inside the plate structure - are a sign of gas in the tube and can mean trouble, but it looks different and you can easily tell when you see it - it's more usually slightly green/blue not purple/blue or royal blue. Even then, many tubes (including most EL34s) normally glow at least a little like this when they're perfectly healthy.

If the tube looks like a blue (or purple) neon bulb, it's probably time to worry though... the last time when I saw that happen it was after the amp had started to sound really bad and about a second before the glass bottle melted on one side and a purple flame shot out of it .

 

[Waspination]

"Blue glow" being bad is one of the biggest myths about tube amps. I have no idea how it became so widespread, and I can't begin to imagine how many perfectly good tubes have been trashed just because they glow blue, because of this. It's almost always complete nonsense.

Blue/purple glow just inside the glass is a sign of a *good* tube in good working order. It's caused by minerals in the glass glowing when bombarded by stray electrons from the operational structure of the tube, and means the vacuum is good. Almost all power tubes show a very small amount of it, but some of the tubes with the strongest glow are the best quality US-made 6L6 types from the 1970s and 80s - so it's especially sad that these will be among the most commonly thrown out because they glow blue.

*Some* types of blue glow - inside the plate structure - are a sign of gas in the tube and can mean trouble, but it looks different and you can easily tell when you see it - it's more usually slightly green/blue not purple/blue or royal blue. Even then, many tubes (including most EL34s) normally glow at least a little like this when they're perfectly healthy.

If the tube looks like a blue (or purple) neon bulb, it's probably time to worry though... the last time when I saw that happen it was after the amp had started to sound really bad and about a second before the glass bottle melted on one side and a purple flame shot out of it .

Well i see the other turn blue inside where all the rods and stuff are. but this one does it on the outside. And it causes noise when tapped, i thought that meant its for sure goin bad. Im gonna call mesa see what they say.

 

[stokes]

"the last time when I saw that happen it was after the amp had started to sound really bad and about a second before the glass bottle melted on one side and a purple flame shot out of it" .And just how much acid did you drop when you saw that?In all my life around tube amps,I have never seen or heard of a tubes glass melting.I've seen a lot of amps blow and catch fire,but never melt a tube.A purple flame????
If the amp doesnt show signs of a bad tube,weak,anemic,lifeless sound,dont worry about it.The glow doesnt mean its bad,and the noise doesnt either.All tubes are microphonic to some degree,some more than others.If the amp sounds good dont sweat it yet.The noisy tube may indeed die before the others,but if it isnt causing noise problems unless you tap it,dont tap it.

 

[fdesalvo]

"and just how much acid did you drop when you saw that?"

OK, I just spit coffee onto my laptop!

 

[94Tremoverb]

I was in a band practice playing through an old Silverface Fender Twin, and I noticed the sound was getting really mushy and squashed - to be fair I was driving it with a fuzz and an octaver, so it took me a while to notice! I looked over at the amp and the wall behind was lit up with an odd purple-ish light - I looked round the back of the amp and saw one of the power tubes glowing bright purple like a neon bulb. It was so cool I didn't do what I should have done - turn it off - but turned the amp round for the rest of the band to see (no top back panel on the amp so it was easy to watch). As I did that, the glass got hot enough to melt on one side, the vacuum pulled it into a little funnel, a hole appeared and a little purple flame about an inch long shot out! Just momentarily. And I really hadn't taken any acid! I have witnesses too .

I've spent all my adult life around tube amps as well, and it's the only time I've seen it - at least to catch it happening, I've seen a dozen or so tubes melted with holes in the glass. It's rare but it can happen if the tube sustains a short for long enough and the fuse doesn't blow. (Usually because someone has put a car fuse in the amp...)

Here is a pair of EL34s that came out of a Marshall with a bias failure:

 

[fdesalvo]

That's nuts! One time I was removing a 12ax7 and it shattered in my hand. Cut up my finger and thumb.

 

[Old BF Shred]

Yup, tubes are even more delicate after they have gotten excessivley hot.

I will use a rag as protection when pulling a tube if it does not come out, initially, very easily (and of course, always if it is hot). Although I have never had one cut me when breaking (just lucky) but it can happen for sure!

I, for one, would be careful tapping too hard on a tube when it is hot. Not only is the glass more delicate in this state, but so are the internals. Hard impact can make an otherwise OK tube arc internally, ensuring the need to replace it.
I speak from my own experience.
I know you have to, in order to test for microphonic condition, but just go kinda easy on it...

Good luck

 

[94Tremoverb]

I deliberately tap *hard* with my knuckle on tubes when they are hot and running, when I'm testing an amp - exactly because this can make them arc. If it does, the tube is not really otherwise OK - it's really faulty already and will quite likely fail when exposed to the vibration from the speakers at high volume (even in a head). You don't want this to happen at a gig - best to get it out of the way on the bench...

Tubes are not *that* fragile, and I want any ones I use in my amps or ones I've repaired to be as thoroughly tested as possible. I'd much rather sacrifice a tube on the bench where I can see it and shut the amp off immediately than have it happen when you might not notice until it takes out some other component as well.

 

[Old BF Shred]

Tubes are not *that* fragile, and I want any ones I use in my amps or ones I've repaired to be as thoroughly tested as possible. I'd much rather sacrifice a tube on the bench where I can see it and shut the amp off immediately than have it happen when you might not notice until it takes out some other component as well.

Good point.....failures on stage SUCK! :evil:

Take care!

 

[Waspination]

Would it be better to get the tubes from mesa
I found this site sells them for less.. but dunno if they are of lesser quality or something
http://www.rainbowguitars.com/accessories/mesa-boogie/6l6-gc-str-440-tubes-matched-pair/750618D/MB

This is the color im seein... best pic i could get of it

 

[94Tremoverb]

Pretty pretty blue glow. I love that color .

That's normal glass glow. The tube may or may not be faulty if it rattles and is microphonic, but it's not because of the blue glow.

 

[Waspination]

Pretty pretty blue glow. I love that color .

That's normal glass glow. The tube may or may not be faulty if it rattles and is microphonic, but it's not because of the blue glow.

THX! I emailed mesa and they said the same pretty much. I did notice the volume at times went down slightly.... then came back up while playing. SO still might have some faulty tube. Im gonna buy 2 pair soon, switch and see if it stops that.

 

[badjohnny]

Interesting. I just saw one of my power tubes look like this in my Express 5:50 the other day. What is weird is that if I switch the left/right tubes, the purple color stays with the socket, not the tube. So is some weird current going on here?

 

[94Tremoverb]

Are you running it in 5W mode? If so, only one power tube is on, the one that's blue.

If it's in 50W mode and one isn't blue, try pulling the one that is blue and see if the amp still works (in 50W). If it doesn't, the tube socket with no blue glow is the cause, most likely because of a failed screen resistor. If it does work it will sound bad, but don't worry about that, it's just a quick way of testing the tube sockets.

Even if both sockets are working, in some amps one tube does glow more strongly than the other, caused by the signal output from the phase inverter not being symetrical - this is a normal consequence of some phase inverter designs, not a fault. I'm not familiar with the Express but older Marshalls are well known for this (the side nearest the preamp tubes glows more).

 

[badjohnny]

Whether in 5w or 50w makes no difference in the glow. The purple side does get much hotter than the other. Amp is at practice space so can't test with one tube out or not yet. Is that safe to do? I have a call in to Mesa, should talk to them soon.