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Amp is blowing fuses

cubdog

cubdog

banging through drywall
Help out guys. My Shuguang KT88 amp is blowing fuses at the power receptical. I asked Rachel at GF which fuses this amp uses and she said 8A250V slow blow. The actual blown fuses I have only say F5A on them. I had this issue once before and the tech installed these current blown fuses. Anyway I'm now confused as to which tube is proper. What do you think?
 
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If GF says 8A I'd use 8A. That being said, if you're having repeated problems with fuses blowing it might be time to have the amp checked out in more depth.
 
F5A sounds like 'fast 5 amp' maybe? Get 8A slow blow in there and see if the problem persists.

But be a little careful, don't crank it up high unless you get through a period of quieter listening OK.
 
What is a typical fuse used in a KT88 amp? I forgot to mention GF wasn't 100 per cent sure what fuse this amp uses but thought it was 8A250V and the tech who checked my amp out before install the current F5 fuses. When two "experts" can't agree it's hard to make an informed decision.
 
I don't think I've ever seen a piece of tube gear that used fast blow. I'd say 8A seems order of magnitude correct. The amp is rated for 50W/channel or so, I just rebuilt a pair of 70 watt monoblocks that each spec'd a 5A slow-blow. So you have two channels at 71% of the output. Just by that math you end up with 7.1A. Do a little rounding and there's your 8A.

I've seen techs throw in whatever fuse they happen to have lying around that's close. I've also seen people convinced that if you can get away with a lower fuse rating or a faster fuse it's somehow safer.
 
Is this stereo push-pull KT88 ? 5 amps seems reasonable, but maybe a smidge small in a fast blow. Slow blow would likely do OK though. 8 amps seems excessive to me. My Bogen mono amps will do 90 honest watts and run 3 amp slow-blow fuses. Original was a 3.2 amp slow blow with 4 output tubes, it now runs 2 output tubes for the same power output.


The tell would be when it blows fuses. If its on power-on, then you need a slow blow fuse. If it blows after its been on a while, either the fuse is too small or it has something going wrong inside.
 
Well I'll be. I looked again in a different spot and found it. 4 amp. So do we agree on slow vs. fast blow?
 
Haha, 3 different answers!

Trust what's on the equipment. Go with slow blow, I'll bet a fast 5 was substituted in hopes that it would be close to a slow 4 :dunno:
 
Actually it says 250V, under the fuse, on the fuse holder and 4v on the amp itself. No mention of fast or slow blow anywhere.
 
Considering the behavior of the power supply circuit in a tube amp (short burst of high current), fast blow seems to be totally wrong. I've only ever used slow blow.
 
WOW ! 8 amps is a lot of power. You should see smoke if your blowing an 8 amp fuse. Something is really wrong. Most amps use 4 or 5 amps. I'd disconnect the pwr xmer secondary from the rectifier and try it again & see if the power xmer is shorted.
 
I didn't say that I was using an 8 amp fuse. I wasn't. I am going to try a 4a sb which I now believe to be the correct fuse. Thanks for the help guys.
 
Me personally, I never try to read fuse specs, on a fuse or a fuse holder, without my reading glasses on, otherwise it's a recipe for a smoke filled room. :)
 
cubdog said:
Believe me I got out my largest magnifying glass then found the best light and squinted really hard. And that was just to find my car keys!
Click to expand...

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I learned about blowing fuses as well when I tried just basic 5amp fuses in my M125 Monoblocks (KT88) - I have to use 5 amp slo-blo fuses. Now I know the difference.
 
I've seen hundreds of tube amps in my lifetime and not one with fast blow fuses. I did work on a tweed Fender Pro that had tinfoil in the fuse holder:eek2:
The rule as I was taught is slo blow for tube, fast blow for SS. Can you not find a schematic for it?
 
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