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Post by tubeholic on Sept 9, 2015 4:47:25 GMT -7
Hi everyone.
My 2011 CG has been working like a charm, just a pretty solid dependable amp,apart from the occasional tube replacement along the way. At least until early this week.
I tried to fire it up and it was dead, turned out to be a bad fuse. No biggie, got a set of slow blow 250V 500mA fuses (europe) fired it up, seemed OK so I powered it off. Next time I tried to Power it up fuse was blown. Got new rectifier, new Power tubes, new everything, same problem.
Debugged the amp as per the Docs instructions and it is definetely blowing the fuse on power off. Even with no tubes on the amp.
I've checked the power grid resistors, Power tranny primary, secondaries, output tranny, everything seems OK, in fact it works and sounds great, untill I Power it down, and get a blown fuse... I'm a little lost. I don't know if this could related to the kind of tolerance on the fuses but I couldn't really get the exact "BUSS MDL" fuse variety (composite spiral around solid core) that came with the amp so I got a T250V 500mA (specified on the amps back plate) single solid wire slow blow fuse.
Any help with what could be wrong with my amp?
Thanks!
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Post by DRZ on Sept 9, 2015 6:52:06 GMT -7
Yes, MDL is a time Delay fuse or what is commonly called a Slo-Blo. Your amp needs a 1/2 watt MDL fuse. So if MDL is impossible to get try a 1 watt Fast Blo or AGC type fuse, the surge rating are very close and is safe with a Ghia.
But this leaves the fact that the fuse is blowing on power off and you have addressed all likely causes. One cause that still remains is the fuse holder itself, if it is making a loose internal connection it can arc on power down, blowing the 1/2 watt AGC fuse. So replace the fuse holder with a new Little Fuse holder , simple to do just two wires, install the correct fuse and you should be fine.
Z
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Post by tubeholic on Sept 9, 2015 15:45:00 GMT -7
Thanks Doc. Will check the fuse holder. Already trying to find MDL fuses locally. I'll let you know how that went. The Doc rocks as usual
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Post by simpleton on Sept 9, 2015 20:46:12 GMT -7
I'm tuned in as well. Getting the right fuse is something I take for granted.
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Post by zpilot on Sept 9, 2015 22:30:45 GMT -7
I've been repairing amps a long time and I was scratching my head on this one. Live and learn.
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Post by tubeholic on Sept 10, 2015 14:22:42 GMT -7
Quick update: Yup, using the 1A Fast Blo works.
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stuz
New Member
Posts: 12
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Post by stuz on Feb 6, 2023 19:54:07 GMT -7
I am having similar issues with my Ghia (2010 combo with brake lite fitted)! I'm assuming the fuse goes on power off! I gig this amp once a week and every now and then I get to the gig and the amp is dead! Change the fuse and the amp is fine for a few weeks... I've gone thro 8 fuses so far (1A Slo Blow)... The most recent fuse went just as I started the gig, first time it has blown while the amp was in use.
The rectifier tube is fairly new, I changed it when the first fuse blew, as there was internal arcing in the tube, which was was responsible for blowing the first 2 fuses. I have also watched the tubes when I first power up the amp and all looks good, no arcing or flashing....
The fuses that have blown mostly look like they're ok, apart from one of them which had blackened itself.
I've ordered more fuses (I'm down to my last 2 now!) and, as the Doc suggested, ordered a new fuse holder....
Just wondering if anyone here has any ideas what could be causing this, I want this lovely little amp to be relighable again!
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Post by premiumplus (Dave) on Feb 7, 2023 3:41:48 GMT -7
It sounds like an intermittent connection in the fuse holder, like Doc mentions above. If the fuse isn't held tightly by the tension spring inside the fuse holder, it's going to arc and pull a current surge thru the fuse when the power is shut off.
If you want to get down in the weeds, it's called flyback voltage. When a transformer's power is cut off the magnetic field collapses and there is a huge current built up in the primary core.
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