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Marshall JTM 60 - Blows fuse


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2/11/2001 6:37 PM
frothingham
Marshall JTM 60 - Blows fuse
My son has a JTM60. He retubed it a month ago. A few days later it didn't work. The repair person said he replaced a transformer or something. A few days later it wouldn't go on again. The repair person said he had to replace a fuse and my son should be sure to warm the amp up before using it. Well, it has failed again. I found a fuse blown inside that appears to be a 6.3A. There is no writing on the board to say what it is supposed to be. I plan to replace it with the same rating. Is he blowing these because of settings? I am wondering if they blown on power off rather than power on? Any ideas?
 
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2/11/2001 6:59 PM
M.C.S.

It is possible that the amp is under biased or there is a mismatch among the power tubes.I don't think that amp would have a 6.3 amp fuse perhaps it's the fuse for the heaters as they are 6.3 volts it would be a lot easier to troubleshoot with a schematic,which I don't have,perhaps you could post asking for a schematic someone may have one.However the amp should be made right by the tech who worked on it as the problem appears not to have been corrected.  
 
M.C.S.
 
2/11/2001 8:50 PM
frothingham

I was able to locate the schematics on the Internet and verified that it is 6.3A and is on the 6.3V heater circuit. The person that worked on it will probably take care of it. I was just curious as to what ight be going on. If it is in the heater circuit, does that indicate an issue with the new power tubes? I put in Peavey EL34A's. That is when the problem started. Thx for the reply...
 
2/11/2001 9:37 PM
will
Sounds to me like a bad tube, I'd try a set of QUALITY (i.e. Svetlana) tubes and see what happens.
 
2/12/2001 5:17 AM
Richie{~}==:::
Could be one of the tubes ..even new may have an intemiten short, or it could also be part of the rectified heater circut.I have seen these bad in quite a few marshalls,the rectifier or caps in the circut could be causing it to blow.I think i'd be trying a different tech.If its blowing the fuse it has a problem.  
 
Richie{~}==:::
 

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