Hot smelling amp and blue tubes?

[tripstan]

So I was jamming for an hour or so on a Mark III and then started smelling something hot. Thought it was x-mas lights but checked my amp anyway. That's when I notice 2 tubes were glowing bright blue. Amp still works fine but when I flip the standby to off the blue goes away then returns when I turn it on to play. Should I expect some tubes to blow soon or what going on?

 

[bananaladonpcp]

If it was another amp I would say you need to bias it. But since it has a fixed bias your tubes are beyond the range that it's fixed for. Time for new power tubes. Blowing a power tube can hurt some components so don't wait. then again sometimes you can blow one and the amp be fine...


-Alex

 

[Chris6870]

6L6's glowing blue is not uncommon and does not mean they are bad.

 

[thunda]

6L6's glowing blue is not uncommon and does not mean they are bad.

+1. Blue tubes are no problem. Russian manufactured power tubes do this a lot. No need to worry. Now, glowing bright red is another problem entirely...

 

[tripstan]

Well it's kind of weird cause it's only 2 tubes one side. It's all 6L6's. And I know blue can be normal but the hot smell kind weirded me out.
One more question. Since its fixed bias can I just pop a quad of 6L6s in it? I was think about using two 34's on the outside cause it can do class A.

 

[steve_k]

You are a little vague on info here about the other two tubes and if they are normal or not glowing at all. This amp will be paired with inners and outers. The problem could be anything from tubes to an output transformer. The fact that something smells like it is burning can lead to all sorts of speculation.

Are these tubes which you have installed? Maybe someone has two duets of different ratings in the left and right pairs. Mesa uses a color matching system. Inners need to be of the same lot and same for the outers. Your fixed bias is inners/outers, not left right.

-Easiest thing to do is take the tubes out. Using the the two tubes which you think are good, put them in the two inside positions. Plug your 8 ohm cab into the 4 ohm slot and turn the amp on and play.

-Remove those tubes and put the two you think may be bad and repeat the above. Any problems there?

-If they all check out, put the other two tubes in the outer sockets and plug you cab back into the 8 ohm socket and play with the amp on Simuclass.

If you have a bias meter, you should use it to check the bias current to the tubes as you test the innners and outers. Fixed bias or not, it will still show a problem. If this is an amp you have just bought, always be skeptical as many men out there don't know what they are doing. I just had to replace an OT in a Mesa Dual Recto that i bought used. It was showing similar symptoms, BTW.

PM me if you need more info.

Steve

 

[bananaladonpcp]

6L6's glowing blue is not uncommon and does not mean they are bad.

+1. Blue tubes are no problem. Russian manufactured power tubes do this a lot. No need to worry. Now, glowing bright red is another problem entirely...

Only 2 of them glowing on a fixed bias amp.... Come on guys....

 

[bananaladonpcp]

Well it's kind of weird cause it's only 2 tubes one side. It's all 6L6's. And I know blue can be normal but the hot smell kind weirded me out.
One more question. Since its fixed bias can I just pop a quad of 6L6s in it? I was think about using two 34's on the outside cause it can do class A.

You should get your tubes from Mesa. Because it's a fixed bias amp you need to get tubes within a specific range. you can get them else where but be sure they are within that range.

 

[Chris6870]

6L6's glowing blue is not uncommon and does not mean they are bad.

+1. Blue tubes are no problem. Russian manufactured power tubes do this a lot. No need to worry. Now, glowing bright red is another problem entirely...

Only 2 of them glowing on a fixed bias amp.... Come on guys....

http://www.thetubestore.com/Blue-Glow

BTW to the banana guy, any good tube supplier can give you the properly spec'd tubes for a Mesa amp.

 

[mojotone]

Well it's kind of weird cause it's only 2 tubes one side. It's all 6L6's. And I know blue can be normal but the hot smell kind weirded me out.
One more question. Since its fixed bias can I just pop a quad of 6L6s in it? I was think about using two 34's on the outside cause it can do class A.

Take the two tubes that DO NOT glow blue, and put them in the outer left and outer right sockets(Class-A positions). Switch the amp to Class-A mode and note if either one of them glows blue. A blue glow is not uncommon, but this experiment may help to rule out any output transformer issue. If all seems well, take the other two 6L6's and install them in the inner pair of sockets. Switch the amp to Simulclass mode and play test it again, taking note of any glowing, smell, etc.... Report back your findings.

 

[Erock]

I've had tubes that had the blue glow, and they worked great for a long time, but they also did it right from the start. It was also that cool fluorescent blue. I would be more cautious if they just started doing this, especially if you are detecting added heat. There is a violet blue glow that looks like it's attaching to the structures inside, when a tube starts getting air from a fracture in the glass. I've had that happen, but it was audible in my case.

 

[tripstan]

Had the amp for 5 yrs and this is the third set I've put in. It's one inner tube and one outter tube on the right back side of the amp that glow blue. I'll try your trouble shooting with tube placement. But if I can order a set of mesa's and not need to bias I'll be doing that. Steve, I do have a basic bias meter but Ive only done readings on my Einstein, which has bias points. I don't see any on the Boogie so I don't think I have the correct tool for it.
Thanks again everyone.

 

[mojotone]

Had the amp for 5 yrs and this is the third set I've put in. It's one inner tube and one outter tube on the right back side of the amp that glow blue. I'll try your trouble shooting with tube placement. But if I can order a set of mesa's and not need to bias I'll be doing that. Steve, I do have a basic bias meter but Ive only done readings on my Einstein, which has bias points. I don't see any on the Boogie so I don't think I have the correct tool for it.
Thanks again everyone.

Pin #5 on the power tube socket to ground will do just fine with a voltmeter.