Mark III odd behaviour: Fuse blows and Power Tube Glows red

[milamatik]

Hey guys,

I hope you can help me out with my problem, thanks in advance for reading

I own a Blue Stripe Simul Class for at about 5 years. Since 2 months I have the first problems after all this great time with the amp. I never changed anything with the tubes so far, as everything was just okay.

I always play Class A Mode.

4 weeks ago the problems started:

1. In a rehearsal the amp just went silent after one song, LED was off. I took out the fuse but it seemed to be okay. So I dismounted the amp from the cabinet. With a multi-meter i found out that there was no power after the fuse, so stupidly I then checked the fuse with the multi-meter and it despite looking fine it was broken after all.

I replaced the fuse and everything was okay again. Perfect sound as usual.


2. Before a gig at the soundcheck the amp went silent again. So I replaced the fuse again and the gig was fine. At the rehearsal room I replaced the outer power tubes (6L6) from mesa with spare tubes from JJ (EL34) and the problem seemed to have gone. Everything worked well.

I also replaced and changed some of the PreAmp-Tubes also with JJ Tubes with no further effects soundwise and the problem seemed to have gone.

3. I had no further problems till the last rehearsal yesterday. The amp started smelling and when I checked, the left tube (first power tube from the left to the right) was glowing red. The jj-tubes are 4 weeks old. After cool down I changed the outer tubes vice versa and checked again: After some seconds with both standby and power on, again the tube ON THE LEFT starts to glow red. So the tube itself as a failure was ruled out. I thought: It gotta have something to do with the left socket!

Then I put in the original Mesa 6L6s in the outer sockets. Same effect: Left / First Tube starts glowing red. In this moment I saw that the Ground Switch was on position B, which must have happened during the last transportation, cause normaly I play on "ground off". I switched to "ground off" and tried again, no effect: Tube goes red.

Then I switched to SIMUL CLASS just for curiosity. And although the amp is on "stand by", there occurs a loud short "PENG", I thought of a high voltage peek. I checked the tubes, all were glowing normally on stand by, so I switched the stand by to "on" and NO TUBE started glowing. Everthing looked just fine.

I thought: Okay, something is wrong with class A Mode. Played around some time through different volumes, everything seemed to be okay in SIMUL CLASS. Then out of curiosity I switched back to CLASS A. And also here NO TUBE started glowing.

I played the whole rehearsal on CLASS A with no further problems, all channels, all volumes, everything just fine as ever.

Now I wonder whats wrong with my amp. First these fuse blows. Now this strange behaviour.

I would appreciate your help on this! And please pardon my bad english, I am from Germany

Thank you!

 

[Authorized Boogie]

Typically a fuse blows when a power tube is shorting. This can be an intermittent short or a severe short. An intermittent short is when the electrons get over-excited and form an electron-bridge between 2 components in the tube. By turning the amp to standby or off, the electrons settle down and the short is no longer happening. Although the amp may continue to function normally, its a sign the tube is bound for failure.

In a severe short, the electrons can over-run the tube and take out a screen resistor. If you smell any burning smell or see any smoke, its most likely due to a burnt screen resistor. If the screen resistor is burnt, current going to the screen of the tube will run out of control and the tube will begin to glow red.

Keep in mind, the fuse is a protection device - and if you are blowing fuses it will most likely be related to a shorting power tube. Before simply replacing the fuse and moving on - be sure to take care of the REASON the fuse blew.

Here is a video on failing fuses and what does it mean when a fuse fails: http://mesaboogie.com/news/2010/06/fuses-why-do-they-fail-what-does-it-mean-and-what-can-you-do/#more-569

Thanks!

 

[milamatik]

Thanks for the reply.

I tried new power tubes in the outer sockets which are the only ones that I use.

Could the inner tubes also cause a fuse to blow even in CLASS A Mode?

Why are different tubes starting to glow red only when put in the LEFT socket? And why, when I put them in the right socket the same tubes won't glow?

Why was this problem solved with switching to SIMUL CLASS Mode?

 

[Authorized Boogie]

It most likely has to do with the current that is going to the tube. If its socket specific, its most likely related to a burnt screen or plate resistor, which is allowing the current to run out of control. I recommend taking the amp to a competent technician, as this can be resolved rather easily. Thanks!

 

[milamatik]

Ok thanks. Just came back from rehearsal and everything was just fine. I'll wait and see. If anything odd happens again, I'll go for the technician..

 

[stokes]

If the problem is indeed related to the socket it is possible there is a problem in the bias circuit.Not a good idea to use the amp with an intermittent problem like this.Could end in serious problems.

 

[megavoice]

Although I don't doubt that your problem is tube or electronics related I'd advise you at any rate to check the power chord and its connector carefully.
Long ago my amp fooled around with fuse-blowing and tubes glowing red 1 time till I discovered accidently the broken power chord nearby the connector switch that caused short circuits from time to time and made the amp dead not knowing for a long time what the problem might be........
After replacing I hadn't these problems any more for decades now................

 

[milamatik]

Okay. Thanks alot for your advice!

I have the problem that I don't know a good technician in this town. But maybe I have to find one now! I just guess it will be a problem because the amp is just running fine now. So what to tell the technician? It's like your car never fails in the garage when there's the technician around

Question:
Do I have to change the Power Tubes in the inner slots from time to time, although they are never in use, cause I only play Class A?

 

[megavoice]

Okay. Thanks alot for your advice!

I have the problem that I don't know a good technician in this town. But maybe I have to find one now! I just guess it will be a problem because the amp is just running fine now. So what to tell the technician? It's like your car never fails in the garage when there's the technician around

Question:
Do I have to change the Power Tubes in the inner slots from time to time, although they are never in use, cause I only play Class A?

That's a very good question I'd like also to be responded.
It's not long ago that we've been told here in simul class amps the inner 6L6 should not be removed like I did now for more that 25 years, as the lifetime for the outer ones would be shortened from this.
But your's is a good question...................

 

[kdorsey]

Okay. Thanks alot for your advice!

I have the problem that I don't know a good technician in this town. But maybe I have to find one now! I just guess it will be a problem because the amp is just running fine now. So what to tell the technician? It's like your car never fails in the garage when there's the technician around

Question:
Do I have to change the Power Tubes in the inner slots from time to time, although they are never in use, cause I only play Class A?

IMO, no. Or rather, in my experience I've not needed to do that...and I play mostly in class A mode too. Even though the other tubes are still energized, they shouldn't see the same "wear". I'm curious to hear a factory response. Another point: my inner power tubes haven't needed replacing for years, and I used to gig it weekly.

 

[nor]

Time for a visit to the tech. You can easily spend more trying to avoid it.

 

[LeXXXa]

Definitely go to a tech.
I had similar issue - one of the outer tubes started glowing bright purple, amp was making cracking noises, etc. All resulted in a blown transformer (really lucky that there was a person capable of rewiring it, otherwise...).
One of the high voltage/current resistors on the output tubes was also dead, which caused imbalance and glowing again when I tried putting new tubes there.
Hope everything turns out to be ok!