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why do hi-end fuses keep blowing, while std don't?

I've had my McIntosh MC275 for several years running flawlesly. Up until last Nov I was using stock KT88 and rolled small tubes and had a Hi-Fi Tunning fuse without issues.

In Nov-13 I upgraded the KT-88 to Psvane black bottles measuring 60mA plate current. A few power-ons after I rolled the tubes, I turned on the amp to let it warm up, but returned to a blown fuse. I thought a tube might be bad so used a std fuse, but never had a problem again.

Two months ago I bought a new high-end fuse, replaced it, and soon thereafter the same happened: blown fuse. I replaced it with a std fuse again, which is still running.

So I want to upgrade the fuse, but chances are if I use the 2A fuse it will happen again. Yet I don't want to use a higher value fuse. I'm thinking the Psvanes might be drawing significantly more current than the stock KT88 and the Hi-Fi Tunning fuse might have a tighter spec, driving said fuses to fail while the std ones survive. Would you agree?

Suggestions as to how to resolve this?

thanks much!
lewinskih01
yogiboy's avatar
yogiboy
5,703 posts
 
Are you sure that the fuse rating are exactly the same? Some 2 amp are 250 volts and some are 125 volts! Do you really hear a difference with the fuses? If not just go back to the standard ones and avoid the problem!
davidpritchard's avatar
davidpritchard
866 posts
 
My Zana Deux OTL headphone amp had the same problem. I discussed this with Craig Uthus the amp designer who had me go to the next size up in fuse value. I went from a 2.5 to 3 amp rating. My blow fuses ocurred at start up or shortly there after. This ocurred after some tube rolling.
lewinskih01 OP
909 posts
 
Thank you all for the input.

Yogiboy: both are rated 250V. Moreover, the first hi-end fuse that blew had been running for over a year prior to rolling tubes.

Hifiharv: I'm sure I'm not confusing those. Both are slow blow.

Schubert: good idea. Unfortunately my amp is auto-biasing, though.

Mapman and Krell_man: that's probably the most sensible route, and in line with Davidpritchard's experience. With McIntosh I hesitate a bit to request advice because it's such a large company. But will do and will post their answer for others to benefit too.
timrhu
2,779 posts
 

Like Andrew ( Mapman ) suggested, you need to find out if the Hi-Fi Tuning fuses are simply made to a tighter spec.

Chuck
Krell_man

Or maybe made to a looser spec?
mechans
3,048 posts
 
Who made the stock tubes? I think the tube they use now is the Gold Lion (Russian) KT-88. That is a very good tube especially if you like a warm tone. I thought they were fabulous in my Jadis, but it may not be your cup o tea. McIntosh certainly approves them so they shouldn't blow any fuses.
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