# Power Amp Keeps Blowing Fuses



## loopy_al (Mar 12, 2010)

Hi, 

This is a long, confusing story, bear with me. 
I have a Mesa Stereo 2:50 power amp, a Marshall JMP-1 Pre that i run through a stereo JCM 800 cabinet with Greenbacks. 
It all began at a show. Montreal ; The power amp died. I later discovered that it had blown the mains fuse. 
I replaced the fuse. Everything seemed ok again. 
Until another recent show when the same thing happened. 

Initially i assumed i had a bad power tube that was causing the Mesa to protect itself. Ordered new tubes. 
Upon some further testing (before i received the new tubes), my preamp crapped out! Definition ; The signal from my pre amp degraded. (Very low signal, lots of noise/hiss) To hear any signal at all i had to crank the volume and gain, and then i heard a faint signal. Now I'm thinking is it possible that the pre amp is causing my power amp to blow a fuse?
Replaced the pre amp tubes in the preamp. Worked! Everything was fine until the next day when i fire it up again, and i have the same issue again. (Degraded signal from the pre) 
Swapped out the 2 pre amp tubes again. Its working again now. 
Thing is I have a show this weekend ! I ran the pre and the power amp for a few hours last night burning in the new power tubes and it is still working as of now. However I am still confused as to what is going on here and i need to know that it wont happen again in the middle of my set this weekend! 

Any suggestions? I am thinking the problem is with the Pre amp, and its beyond my means to test. Anyone else have similar issues with a JMP-1? 

Thanks! 


Al.


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## Wild Bill (May 3, 2006)

loopy_al said:


> Hi,
> 
> This is a long, confusing story, bear with me.
> I have a Mesa Stereo 2:50 power amp, a Marshall JMP-1 Pre that i run through a stereo JCM 800 cabinet with Greenbacks.
> ...


Al, we can all post a lot of msgs to the thread here but one thing is for sure. It's NOT likely to be something simple that someone can give you a tip and you can fix it yourself!

From what you've described, I think you need a good tech. There are all kinds of parts inside your amp. Something besides a tube is likely blown. Someone has to open up your amp, get out some test equipment and start troubleshooting.


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## loopy_al (Mar 12, 2010)

Thanks Bill for your reply. You're right of course, I will certainly need to have it looked at. I was just wondering if anyone had any input to influence my decision about whether to even bring this rig to my show tomorrow night, or to write it off as a bad bet and rent some gear for tomorrow.


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## nonreverb (Sep 19, 2006)

Beware of Murphy's law!


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## loopy_al (Mar 12, 2010)

I know! ! This decision is driving me crazy! I know I should just forget it and not even bring it. . . but dammit! Its all set up the way i want it ! ! Arrghgh! Damn you Marshall! ! !


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## StevieMac (Mar 4, 2006)

[video=youtube;suiheqHtayY]http://www.youtube.com/watch?v=suiheqHtayY[/video]


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## parkhead (Aug 14, 2009)

its sounds like a classic case of two different problems happening at the same time 

You probably had a power tube problem in the power amp blowing fuses ect & also have a problem with your pre amp or patch cables between the units 
always trouble shoot in increasing order of replacement costs 
batteries, patch cables then tubes ect 

check your cables for corrosion, loose tips or broken insulators wiggle the connections at the jacks 

I just had a PA speaker appear to fail ... turns out one jack is sprung it probably took a hit with a 1/4 plugged in 
the second jack partially worked & the locking neutrik connectors worked perfectly 

years ago at one horrible gig I had an amp cutting out every 7 minutes... on the break I ran home and got my spare amp 
five minutes into set 2 the second amp starts to crackle and sputter 
Later I discovered that abrand new patch cable had been stepped on during setup and the insulator at the tip had shattered 
wiggle the cable and the sound cuts out... the assumption "its a new cable" has never been made since 


Our band always carrys a spare traynor bass head 
and a powered wedge monitor 

the bass head can sub for a guitar head or for the bass player if an amp were to fail 
the powered wedge can take a mic input and has a modest amount of power

within 5 minutes we could be up and running entertaining the crowd by subbing in the back up gear and turning down the volume 

I had a buddy reconfigure his huge rack 6 times in one night (he was at home practicing) 
only to discover the next day that one pair of short patches had corroded inside the jacks (they sat in water or beer or somthing) 
and all top end was lost because of the corrosion inside the cables 

To sum up 
dig deep and look at the small stuff 

have a backup plan 

P


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## loopy_al (Mar 12, 2010)

*Thanks - It's the JMP 1*

Thanks Parkhead for the reply. 

So i managed to recreate the problem . . . live on stage of course! lol. (At least it was the last tune of the set) 

So it seems the Jmp-1 pre's signal is degrading seriously and then when it "pops" back in it causes the fuse to blow in my power amp. 
No clue what is going on with the JMP-1. I am going to have a tech look at it as soon as i can. 

I have used a few techs here in Toronto. Any suggestions for who might be the best guy to look at this JMP-1 issue? 
I have heard good things about "Buzzy" or John Fletcher. . . . 

Al.


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## loopy_al (Mar 12, 2010)

Oh, forgot to mention. When i change the channel, that will sometimes restore the full signal. (And blow the fuse in the power amp) 

When i first got the unit (JMP-1), sometimes when i would power it up there would be no signal until i switched a channel. (At first i figured this was a built mute feature of some kind. . now I'm thinking it was a warning!) 

AL.


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## loopy_al (Mar 12, 2010)

*Tech?*

Hi Guys, 

I have tried contacting Buzzy Burak at 416 282 5088, but there is no answer at this number. Do i have the correct number? 
If anyone has contact details for Buzzy or any other *good* tech in the Toronto area who could take care of my JMP-1, I would really appreciate it if you would PM me the info. 
Wild Bill ; I would love to take this to you, but i don't have transport, so getting to Hamilton would be difficult! 
Thank you ! 


Al.


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