Strange and Lazy repair !!!

[jamesmafyew]

You would just want the slow-blow to be a bit above the maximum normal operating current, and the fast-blow to be a bit above the turn-on surge current. It would depend on the amp, but you could check by watching a mains ammeter during power up.

[bsco]

OK jamesmafyew......that helps me out very much....I will have to pick up one of these clamp on ammeters.......
Cheers

[Leo_Gnardo]

.I will have to pick up one of these clamp on ammeters.......

Clamp on ammeter will be an all around good thing to have in the shop. You can check filament current draws too. The ammeter doesn't care what voltage, it's looking at AC current. Keep it at least a couple inches away from working power transformers though, as the stray field will throw off your measurement.

Besides 70's Ampegs, I've seen interior series fuses in some 90's Fenders. Good thing to have especially on rental gear.

[Tom Phillips]

....I will have to pick up one of these clamp on ammeters...

Just to make sure you know...
Although you will be able to measure the steady state current draw, you will not be able to effectively measure the turn on transit with a common clamp on ammeter. To see the fast turn on current surge you would need something like a higher end current probe that connects to an oscilloscope. Alternatively, you could monitor the voltage across a small series resistance with a scope and calculate the current.

The next step will be to pick a fuse value using the fuse design data. This process is probably overkill compared to just picking values based on existing amp examples and adjusting as necessary if you experience nuisance fuse blowing.

Tom

[bsco]

Thank you Leo and Tom.....I do need a clamp on ammeter so I will be getting one soon....Tom, if you increased the internal fuse say by 25 to 40% and made it a fast blow, then shouldn't that suffice for most applications?? I realize that big powerful amps would be different but the run of the mill amps that have a power fuse of 3 or 4 or even 5 amps should work ok with this?? I'll go and check some schematics just to see what the difference in current ratings are for a given amp.....I am after having a few items that had extremely high current fuses or tin foil so for me, this is certainly worth looking into....and then I can start putting in-line fuses in the equipment....Thanks for the info and tips guys....
Cheers,
Bernie

[bsco]

Thank you Leo and Tom.....I do need a clamp on ammeter so I will be getting one soon....Tom, if you increased the internal fuse say by 25 to 40% and made it a fast blow, then shouldn't that suffice for most applications?? I realize that big powerful amps would be different but the run of the mill amps that have a power fuse of 3 or 4 or even 5 amps should work ok with this?? I'll go and check some schematics just to see what the difference in current ratings are for a given amp.....I am after having a few items that had extremely high current fuses or tin foil so for me, this is certainly worth looking into....and then I can start putting in-line fuses in the equipment....Thanks for the info and tips guys....
Cheers,
Bernie
Sorry guys my computer posted this twice....I tried to delete it but no luck.....

[g1]

The turn on surge can be quite large. It can even depend on where exactly on the 60hz waveform the power switch is engaged. I have a meter that records the max current and it catches what the display isn't even fast enough to show, however I'm not sure if it is fast enough.
The old alesis method of using the end user as a beta tester may be required .

[bsco]

OK. Thanks g-one.......I'll do some more research on this......
Cheers

[bsco]

Checked out a bunch of different amp schematics.....only one fuse used in the primary transformer ckt....the idea that I had in mind was to just install another fuse inside the equipment,(basically right behind the user accessable one), so that if somebody decided to install another fuse that had a higher rating, or if somebody decided to wrap their blown fuse with tin foils, then the internal fuse would blow and no serious damage would occur........so if the unit is rated for a 4A Slo Blo and you installed another fuse internally for say 4.5A Slo Blo or even a 5A slo blo fuse, would that be sufficient for some extra protection from some stupid action by somebody that does not know what he or she is doing?? I only want to prevent the unit in question from being burnt up to the point that the cost of repair would probably not be feasable.....just an idea I have in the back of my mind.......
Cheers

[nickb]

Just to make sure you know...
Although you will be able to measure the steady state current draw, you will not be able to effectively measure the turn on transit with a common clamp on ammeter. To see the fast turn on current surge you would need something like a higher end current probe that connects to an oscilloscope. Alternatively, you could monitor the voltage across a small series resistance with a scope and calculate the current.

The next step will be to pick a fuse value using the fuse design data. This process is probably overkill compared to just picking values based on existing amp examples and adjusting as necessary if you experience nuisance fuse blowing.

Tom

A note of caution that I'm sure is obvious to most but just in case: if you do try this, don't forget an isolation transformer in one or other.

[Tom Phillips]

A note of caution that I'm sure is obvious to most but just in case: if you do try this, don't forget an isolation transformer in one or other.

Very good point.
Or use a two channel scope in the add channels mode with one channel set to inverted. Or use a differential probe. Or the special scope current probe which will be expensive. Overall it's probably not worth trying to do the turn on surge measurement unless you already have the appropriate test equipment and the necessary knowledge/experience.

Checked out a bunch of different amp schematics.....only one fuse used in the primary transformer ckt....the idea that I had in mind was to just install another fuse inside the equipment,(basically right behind the user accessable one)...

Your idea is good and had indeed been done. There are commercial amps that put the larger extra internal fuse in the primary winding. I'll let you know if I can locate an example. You do not want to make the difference in fuse value too small because it's a real pain for the user if the internal fuse blows without a good reason. I don't have any data on the tolerances of the fuse values but you do want to avoid nuisance blowing of the internal fuse. The outside accessible fuse is sized protect the amp against significant damage and to prevent the amp from catching fire. (This didn't always work in the old days) the inside fuse is to prevent catastrophic meltdown & damage which includes the catching on fire. Unfortunately, it's very difficult to make an amp idiot proof.

[Tom Phillips]

Attached is an example of an Ampeg SVT that uses an external 10A Slo-Blo mains fuse with an internal 15A Slo-Blo mains fuse in series. Note the 50% larger internal fuse. This design technique is mostly found in high powered and generally more "modern" equipment.
Tom

SVT-2 PRO AC-Term Brd.pdf

[jamesmafyew]

The Gallien-Krueger 800RB (solid state) had a user-accessible 8A/slow mains fuse, with a 10A/fast internal fuse in the power transformer primary. I've had to remove that 10A fuse from the power transformers of a couple of these things, units that other techs had said "not worth fixing, the PT's fried." I just wired up a 10A/fast in a fuse holder inside the amp so that the functionality was the same, but it could be replaced if it blew again.

[olddawg]

Actually I had somebody drop off a keyboard to me a few weeks back...It came in all taken apart...Apparently the owner did it trying to see if he could fix it....the screws are all different sizes and lengths...no way to really know which goes where.....once I get it working, my plan is to have them come in and see it in working order, then give it back to him and let him re-assemble it himself or charge him the time it is going to take to figure out where all these screws go...

If you have the manual you can identify what screws went where by part number.

[bsco]

Attached is an example of an Ampeg SVT that uses an external 10A Slo-Blo mains fuse with an internal 15A Slo-Blo mains fuse in series. Note the 50% larger internal fuse. This design technique is mostly found in high powered and generally more "modern" equipment.
Tom

[ATTACH]30115[/ATTACH]

Thanks Tom....I had a look at the schematic...I see the mains fuse 10A user accessable.....the second fuse I found was a.5A slo blo....didn't see the 15A fast blo there..
Cheers.....