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PostPosted: Sat, Apr 21, 2018 6:54am 
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A friend brought his new-to-him Peavey XXX head over last night and as we were going through it, he pulled the fuse and immediately dropped it. D'oh.

The thing I found odd (after looking the part up on the google box) was that it was a fast blow. I double checked with the fuses in my Mesa and my KSR and both of those were slo-blow.

I downloaded the XXX manual and it only says 5A fuse, no further word on it. It's a 5A 250V fuse by the markings on the old fuse.

Is there any reason it wouldn't be a slo-blow? Did the previous owner just put the wrong fuse in? Does Peavey build their circuit differently (My mesa is 2.5A, the KSR 3A.)

Thanks for any help you can provide.

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PostPosted: Sat, Apr 21, 2018 8:57am 
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I'd just reach out to the guys at PV.

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PostPosted: Sat, Apr 21, 2018 10:59pm 
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250V is too low to be B+. 5A fast blow seems correct for the primary side of the PT.

Let's back up and I'll explain the reasoning as to why they spec slow blow and where it applies.

When you first turn on the amplifier, the capacitors have no charge on the plates, and as such, look like a direct short to ground. Once they begin charging, they look resistive while building charge. When they are fully charged, they act capacitively. In this transition of states of reaching full-charge (called the 5 tau rule fwiw), the mains power transformer will need to supply a surge in current. This sags the B+ voltage until the capacitors are charged and voltage can then build back up and stabilize. On the primary side however, there's only 120V of voltage to play with, so the extra power needed is drawn as current. Because of this voltage and ampere relationship, you'll typically see amplifiers rated at VAs or volt-amperes.

Back on topic, this whole charging process happens fairly quickly, and because of this, there are now two methods of fusing, depending on whether you're on the primary side of the transformer or the secondary side feeding mains B+. You can spec a higher current fuse as a fast flow where you know the surge current will always be below a certain tolerance (primary). For the secondary side of the PT, you can spec a lower amperage slow-blow fuse (rated to the higher voltage) which requires a constant current draw to overheat the fuse and cause it to go open. The slow-blows are immune to short time-dependent inrush currents which usually last a very short time period - enough that the fuse is not damaged.

So based upon this, and the B+ rating being higher than 250V, I believe the 5A fast blow seems to be the correct value for the primary side of the PT in the amplifier.

Hope this helps.

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PostPosted: Mon, Apr 23, 2018 9:11am 
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Thank you!

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