Mk2c+ blows fuses but only when on simulclass setting

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yanyan

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My MK2c+ works great on the Class A setting but if I have it on the Simulclass setting it will blow a fuse eventually whether I'm playing though it or it's just sitting there turned on (not in standby ). The fuse won't blow immediately immediately, sometimes after 30mins or so which makes me think it may not be a valve issue. Any idea what this might be about?
Thanks
Wynn
 
You shouldn't rule away the tubes, it might just be that, a powertube that is on it's way to go bad. If you are familiar with troubleshooting open up the chassis and do a measurment to see if the tubes draw unwanted bias voltage. It should be dead steady.

BY ALL MEANS: if you don't know what I am talking about don't do it, there is lethal voltage inside.
 
I have seen this happen on a 60/100 C+, and it was a power tube issue. It would take a little while, but the tube would eventually start to have a slight red-plate condition and take out the fuse. I would start by replacing the inner pair of tubes with a fresh pair of 6L6's. This happened with a brand new set of SED =C= tubes, for what it's worth. Make sure that the tubes that you use are in the proper current-draw range, for sure.
 
It's been an out of spec power tube fault for me too when I had the same issue.

Blowing fuses after a few minutes and redplating.
 
Thanks for the input, I'm a bit more lost now though. I've followed the guide on testing power tubes on the Mesa website. But I think I must have a bigger problem than just a bad tube. I've tested all of the tubes and none seem to cause a particular problem on their own. Just when I think they are ok I'll turn the amp off, then back on Pop fuse gone. Sometimes a new fuse and it'll work again. other times that one will pop straight away. Could this be a sign that it needs recapping? or something else is wrong, I feel an expensive visit to a tech coming on.
 
My DR c+ will eventually blow a fuse if I use 6L6's in the outer sockets. Mike B. told me the only quad of 6L6's I could use safely in my DR are red Mesa 425's. It hasn't blown a fuse since I installed New Sensor Svetlana EL34's and 6L6's.
 
yanyan said:
I've tested all of the tubes and none seem to cause a particular problem on their own.

Tubes work in pairs. How have you tested the tubes individually? I think that you need a fresh quad of power tubes before you go any further with this.
 
Sorry for hijacking this old thread but a mate has a very similar problem with his MkIIC+. The amp pops a fuse after he played it for a while, then switches it off and later switches on again --> fuse blown.

Was there any cure for the issue the user yanyan was describing?

Thanks!
iefes
 

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