Fuse Blew on my new to me MC275IV!

[EasyRiderNYC]

Tubes got damaged during shipping? Like shaking a light bulb?

[EasyRiderNYC]

Originally Posted by nsgarch

I would have thought the power tubes (only because the OEM power tubes have a poor reliability record thus far ;-) however, one troubleshooting trick is to fire up the amp without power tubes -- just run the 9-pins all by themselves for an hour or so. You won't hurt the amp (you won't hear any music ) but that should be enough time for one of the 9-pins to exhibit a gassy condition (it sometimes takes up to an hour of operation.) If one of the 9-pins (particularly one of the 12AT7 driver tubes) is getting gassy and 'running away' it could blow a fuse by overdriving it's associated power tube.

So how will you know if one of the 9-pins is gassy without the power tubes in place? Good question! Turn off the room lights, and if a tube is gassy, it will display a very pronounced blue or magenta glow around the plates, especially near the top and bottom of the plates.

Doesn't hurt to check it out -- it happened with one of my McChina tubes

Well I think the tube in V8 is now toast, put on a little electrical show. Is it worth it to do the test now? Should I unhook the speakers, i did not like the crackle I heard.

[EasyRiderNYC]

I just called Stereo Exchange and was told most likely if the fuse blew as well as the tube a circuit might be bad and it will have to go to McIntosh to get fixed. ;(

[gregswaim]

Originally Posted by EasyRiderNYC

Well I think the tube in V8 is now toast, put on a little electrical show. Is it worth it to do the test now? Should I unhook the speakers, i did not like the crackle I heard.

Yes!
Check all of your tubes ASAP. If you've swapped out the factory tubes and replaced them with some other brand, you can reload your factory tubes while getting those other ones checked out.
If one tube is bad I would recommend replacing ALL of your tubes if you can afford it.

[EasyRiderNYC]

Originally Posted by markc2

This sounds exactly like what happened when I blew one of my tubes. Except the fuse went first, then the tube, the first time. Got a new fuse and then the tube and fuse went together. It put on quite a light show, but nothing that was overly "scary". Put in new tube, and fuse, and I have been up and running for over a year now nothing.

Same order here. First fuse. Put in new fuse, then tube and fuse.

[rcarlton]

Happened to me. Turned out it was a bad KT88 in the third position. Replaced fuse, bought a quad of Penta KT88's and have not looked back since.

[EasyRiderNYC]

Originally Posted by rcarlton

Happened to me. Turned out it was a bad KT88 in the third position. Replaced fuse, bought a quad of Penta KT88's and have not looked back since.

I'm hoping for the same remedy.

[nsgarch]

Originally Posted by EasyRiderNYC

Well I think the tube in V8 is now toast, put on a little electrical show. Is it worth it to do the test now? Should I unhook the speakers, i did not like the crackle I heard.

Easyrider, to answer your question (about running the 9-pins alone?) I'd say 'yes' because it would establish whether V-8 failed by itself, or if its failure was caused by a gassy driver tube. I especially recommend doing this in your case because it was the OEM KT88's that, as you've already read here (and you can include me in that group) were given to spontaneous failure. But you were using SED KT88's, which have been fairly reliable, and so there is at least some possibility that it wasn't a bad power tube; and I just think you should rule it out.

You could also temporarily install the OEM power tubes as someone suggested, but again, before you try that, verify the condition of the 12AT7's by running all the 9-pins by themselves. You wouldn't want one of them taking out one of your OEM tubes as well and for the same reason!

[EasyRiderNYC]

Different set of power tubes and music has been making for an hour!

[PHC1]

Enjoy the amp Chuck.