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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

Showing 5 responses by lewinskih01

lewinskih01 OP
906 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.
lewinskih01 OP
906 posts
 
I asked McIntosh...

"We do not recommend the ‘high –end’ super fuses… it is impossible for them to improve sound and it may be causing your problem.
But any amp that blows fuses should have all the tubes tested."

So I learnt I'm delusional since I hear an improvement in sound with hi-end fuses, and my tubes must be bad even though they are not blowing up standard fuses.

Unfortunately this manufacturer is not helpful.

Any other ideas?
lewinskih01 OP
906 posts
 
I couldn't find a way to contact HiFi Tuning Fuses so I asked The Cable Company, from whom I bought the fuses:

"I think that you have hit the nail on the head. The HiFi Tuning fuses are EXACT-rated fuses while off-the-shelf fuses are +10% tolerance which means that they would actually be 2.2A out of the box. The PsVane tubes additional current draw appears to work with that window while the HiFi Tuning fuses blow. This WILL happen again with these tubes, so your hesitancy in installing the new fuses was a good thing."

They don't sell any 5x20 slo-blo hi-end fuses in 2.2A, or any other rating between 2A (what I tried) and 2.5A. While the technician thought 2.5A wouldn't be risky, I'm rather reluctant to do that.

BTW, Lacee asked whether upgraded tubes or fuses made the largest impact. Tubes, for sure. But when I had stock KT88 and introduced an upgraded fuse I did hear an improvement. So now I want the cake and eat it :-)
lewinskih01 OP
906 posts
 
I didn’t bother further. What is reflected in the thread is what I did and learnt, so kept the std fuses and upgraded tubes which have been running for 5 years flawlessly.
The amp runs hotter with these tubes than with the original ones which has to translate into higher current draw. Per the Cable Co answer, std fuses have additional 10% tolerance which seems to be enough. I didn’t want to to go up 25% rating in the fuse from 2 to 2.5A as it seemed unsafe for the amp. So I let go of that part of my audio nervosa and lived happily ever after :-)

In the past years I turned into an active system where now the McIntosh is driving mids/treble and sees a lower demand so runs cooler again. Could try hi-fi fuses again but I’m not prepared to risk throwing away money, so I stayed out. Sound is great and I found areas for more significant upgrades to sound than fuses made, so went that route.

Cheers
lewinskih01 OP
906 posts
 
Thanks for the update.
So you keep you amp switch in the on position and turn on/off by turning the isolation transformer? Interesting.
My balanced power conditioner trips if I do that, but will keep it in mind.
Which isolation transformer are you using?
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