Internal fuse along with an external fuse...?

[dude]

I have an Acoustic 370 SS Bass head from the 70's and I'm replacing the power cord.

I noticed an internal fuse alone along with an external fuse. Both are SB 7 amps. The fuses were fine just the cord had a break in it.

I've never seen an internal fuse along with a common external fuse holder. At first I thought someone put it in but looking at the schematic it has the internal fuse.

What's the reason for this? Seems like if that internal fuse blew, it would be a hassle to replace having to open up the head and solder a new one.

If a power problem would happen which fuse would blow first?

Thanks in advance,
al

[Willabe]

If a power problem would happen which fuse would blow first?

Good question.   

This Ampeg bass amp has 2 fuses, but different values, smaller value fuse is external.  (Right click on schemo to rotate)

http://el34world.com/charts/Schematics/files/ampeg/Ampeg_b25b.pdf


                     Brad   

[dude]

Brad,

That's an a tube Ampeg schematic but basically the same as this Acoustic Solid State as far as the power cord.

I have the manual with the exact schematic, it has a 7A slow blow fuse for "both external and internal". The ampeg schematic has the internal at 6 amps and external at 3 amps which would make sense, the external would blow first.

Export wiring on this Acoustic has only one external fuse at 7 Amps SB at 240v but the 120v has the two at the same valve....?

I'll wire it back as the schematic in the manual for 120v but I guess it's pot luck which one goes with a power failure...?

I can see if the internal was a lower value like the Ampeg but their the same on the original 1972 schematic in the manual..?
 
Wired exactly as the Ampeg too. I would think the internal would go first as it's right off the external fuse to the amp. And the schematic has SB on each...? Maybe if the internal was not SB, might make some sense, that would probably go first...?

strange....?

Thanks,
al   

[Willabe]

That's an a tube Ampeg schematic but basically the same as this Acoustic Solid State as far as the power cord.

Yes I know but it's still a double fused power supply (PSU). I was pointing out that Ampeg used 2 different values of fuse which makes more sense to me anyways.

I can see if the internal was a lower value like the Ampeg

Ampeg had the lower value on the outside. This makes sense to me as if someone keeps putting a new fuse in and the problem, what ever it is, gets worse then the internal fuse blows (last line of defense) and a new external fuse won't work any more so it goes to the repair shop.

One of the deep theory guys will have some insight on why Acoustic used 2 fuses of the same value and type.


                     Brad     

[PRR]

Sometimes the inspecting agency (UL, CSA) requires a fuse which the ordinary user can not defeat. We have all seen tin-foil on fuses. With good tin-foil the shorted amp will BURN. An internal fuse of higher but still safe rating *forces* the user to go to a Qualified Repair Expert, who would ask "why is it blowing??" instead of "how can I bodge it?".

Yes, two same-value fuses is odd. So is a 7A fuse for "export" (usually 230V) amplifier. 230V*7A is 1,600 Watt limit, and surely this amplifier is nowhere near that big?

[dude]

Sometimes the inspecting agency (UL, CSA) requires a fuse which the ordinary user can not defeat. We have all seen tin-foil on fuses. With good tin-foil the shorted amp will BURN. An internal fuse of higher but still safe rating *forces* the user to go to a Qualified Repair Expert, who would ask "why is it blowing??" instead of "how can I bodge it?".

Yes, two same-value fuses is odd. So is a 7A fuse for "export" (usually 230V) amplifier. 230V*7A is 1,600 Watt limit, and surely this amplifier is nowhere near that big?

It's for USA, sorry if I mentioned export.  The Amps 1971 "manual" calls for two 7 amp (3AG) and the schematic, ironically posted in the manual, calls for 7Amp SB, both. The SS Bass head puts out 365 watts max @ 2 ohms, so that's a hair over 3 Amps but there's a plug in the back.

I assume it's 7A, 3 for the head protection and 4 for what you plug in the receptacle.  But usually SS state stuff doesn't use SB fuses...? I may be wrong.

al

[Willabe]

Slo-Blo is for the initial inrush current of a few seconds until it stabilizes it's current draw at it's normal/lower draw.

I wonder how big the B+ filter caps are in that amp?


                  Brad   

 

[PRR]

An amp that big could surely use 7A slo-blo.

> usually SS state stuff doesn't use SB fuses...?

The fuse protects the line-cord and the power transformer. Both are essentially the same SS or tube. High surges will happen, and a transformer can take it for many-many seconds.

It is futile to "protect a transistor with a fuse". Experience shows that transistors will gladly give their life so that the fuse will live. Good amps protect transistors other ways.

Having user and hidden fuses the "same" sounds wrong.

[dude]

Slo-Blo is for the initial inrush current of a few seconds until it stabilizes it's current draw at it's normal/lower draw.

I wonder how big the B+ filter caps are in that amp?


                  Brad   

 

They're bigger than a large can of soup, huge. I think three of them. The transformer is the biggest I ever saw, looks like it could weight 10 lbs., it 8 inches long and 5 high and wide.

I agree with PRR, doesn't make sense that the schematic calls for the same value external and internal, I would think the external would a little lower be lower as to go first if a problem, then if someone use the tinfoil trick the internal would go. But in 1971, I guess anything could be game.....?

al