9444A Fuses

Hi Everyone.

I'm having an issue with one of my 9444A power amps.

At every second or third power on, it immediately blows the main fuse. Other than that, functioning fine.

Altec spec 'normal blo' ceramics, which is what I have in it. It has been restrapped for 240V, and I've double checked that is correct.

I suspect inrush current as a possible cause, any thoughts on if 'normal blo' actually refers to a slow blo fuse, as opposed to a fast blo. Interestingly the 9444B manual recommends slow blo fuses.

Thanks
Nat

A 'Rule Of Thumb' for selecting a fuse for an amplifier with a traditional transformer power supply is Slo-Blo rated at 1.333 x I(full load rated current) and pick the nearest value(higher) rated fuse.

A Normal-Blo fuse of the same current rating is more likely to fail at switch on.

Are you loading to 4 ohms? That creates higher draw current.

Are you using good fuses like Buss not cheapos?

If so would suspect a filter cap being marginal.

I'm using the Bussman equivalent to the specified Littelfuse.

This is the mains fuse, nothing connected to input or output of the amp.

mah I tend to agree with you. I'll try slo-blo fuses and see what happens.

Quote:

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Originally Posted by magnarc

This is the mains fuse, nothing connected to input or output of the amp.

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Doesn't matter. Higher output current means higher current draw from the supply. An amp is a system, any change also creates a change somewhere else.

Check all the internal ground(to chassis) points. There are a few, and they're mechanical connections with the potential to get flaky. There's one in particular near the main filter caps that can be loosened externally by the unwitting golden screwdriver.

I am inclined to think(as OG) that something is amiss in your power supply. I would expect a faulty filter cap to also introduce some audible hum to the output. But that is not at all a hard and fast rule. I don't recall if these have a bridge rectifier or a set of diodes arranged as such, but those would also be on my list of prime suspects.

If this problem has occurred following satisfactory previous operation of the amp with the 'same' fuse as in your other 9444As, the fuse is now probably doing its job of protecting the amp from a fault.

The above posts give good guidance for finding the most likely cause of the problem.

Thanks everyone.