Mark V combo blowing fuses when powering it on

[guijs]

Hello,

My MK5 combo started to blow the fuse when powering it on to full power. Haven't tried variac power but probably the same will occur. I didn't get any smoke or bad smell. At first I thought that the first fuse blowed due to the fact that my pedal board was on before I turned on the amp, but same happened with the board or anything else connected.

Possibly a bad tube? My amp have about 1 year old with very little use, only at home in low-mid volume.

 

[jb's 52]

Could be a power tube issue.
I have had a fuse blow at a place with known electrical issues and no surge protector.
If you didn't get smoke or a bad smell, you should consider yourself lucky.
Last time I had a power tube / fuse blowing issue, I swapped out the inner two first and I was good.

 

[domct203]

Hello,

My MK5 combo started to blow the fuse when powering it on to full power. Haven't tried variac power but probably the same will occur. I didn't get any smoke or bad smell. At first I thought that the first fuse blowed due to the fact that my pedal board was on before I turned on the amp, but same happened with the board or anything else connected.

Possibly a bad tube? My amp have about 1 year old with very little use, only at home in low-mid volume.

Good info troubleshooting here, straight from "the farm".

https://www.youtube.com/watch?v=wSl-l9aWDHw

HTH,
Dom

 

[fr0sty]

You can replace the fuse, turn on power (in standby) then go around back. Flip the amp off standby and then quickly off, all the while watching the power and rectifier tubes and see if there is any flashing or other strangeness.

 

[guijs]

Thanks for the video domct203!

Following the video guide, with the rectifier swtich in diodes, I removed the rectifier tube and the power tubes, and the amp turned on without blowing the fuse.

Then, I tried to do the 1 second test starting with the rectifier tube but I forgot to turn the rectifier switch back to tube and I blown my last fuse... I'll buy some more tomorrow and continue the test. But at least is not a circuit problem, it's probably a bad tube.

Last week I took my amp to a studio to play with some friends. It was the first time the amp left my home. I played it loud but always respecting the guides in the manual. I think I did not exceeded bass or high gain settings. But, maybe I did? This could be something related to that? I played with the amp at home for a couple of hours after that, and no problems.

 

[domct203]

Thanks for the video domct203!

Following the video guide, with the rectifier swtich in diodes, I removed the rectifier tube and the power tubes, and the amp turned on without blowing the fuse.

Then, I tried to do the 1 second test starting with the rectifier tube but I forgot to turn the rectifier switch back to tube and I blown my last fuse... I'll buy some more tomorrow and continue the test. But at least is not a circuit problem, it's probably a bad tube.

Last week I took my amp to a studio to play with some friends. It was the first time the amp left my home. I played it loud but always respecting the guides in the manual. I think I did not exceeded bass or high gain settings. But, maybe I did? This could be something related to that? I played with the amp at home for a couple of hours after that, and no problems.

If the fuse blew after you installed the rectifier (5U4GB) tube, you most likely have a bad rectifier tube. The rectifier tube is always in the circuit, the switch only switches in/out the diodes, and that is why you must remove the tube from the amp to troubleshoot.

When you get your new fuses, put all your tubes in EXCEPT the rectifier tube. Make sure you are switched to silicon diode & 45 watts, and fire up the amp. If the fuse does not blow try full power 90 watts and if the fuse holds you have confirmed that the rectifier tube is the issue.

Dom

edit: added power settings

 

[guijs]

Good to know, I'll do that. Thanks!

 

[guijs]

If I end up with only the 5U4GB tube burned, can I replace it for a Electro Harmonix 5U4GB? Or should I use only Mesa original tubes? They are almost impossible to find here in Brazil. I will have to order one from USA and it will take at least 6 weeks, can't wait that long...

 

[domct203]

If I end up with only the 5U4GB tube burned, can I replace it for a Electro Harmonix 5U4GB? Or should I use only Mesa original tubes? They are almost impossible to find here in Brazil. I will have to order one from USA and it will take at least 6 weeks, can't wait that long...

Any good quality 5U4GB will do.

Dom

 

[guijs]

Great. BTW, I have just tested with only the power tubes inserted and everything went ok in 45-90w. So the problem is only at the 5U4GB.

 

[guijs]

Is it possible/safe to Play without the rectifier tube, using only diodes at 45/90w? Does the rectifier tube is used in ch3?

 

[Authorized Boogie]

Absolutely. Just keep it in 45w set to Diode in CH 1 and 2, or 90w.