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impedence problem? or just a bad head?

B

bassplaya matty

Apr 25, 2007
7
0
Dallas, Tx
#1
Hey guys, i've been having a problem with my amp blowing fuses. Im running a carvin pro bass 500 through a 8ohm 2x12 avatar cab. the back of the head says that when it's bridged it's a min of 8ohms. Im bridging it to the speakon connector on the cab. Am i doing something wrong? Or is it a bad head? Im looking at the markbass amps, would these work with my cab?

Also if i wanted to add another cab would it be better to hook it up to my current cab or plug it straight into the amp?
 
#2
Tell us more info and exactly what components you'd replaced (or any changes that were made) when the fuse blowing problem first started.

Did this amp used to blow fuses with your previous cab and or speaker cable?

Does it blow the fuse immediately at power up or after you play awhile?

If it blows the fuse immediately, try first unplugging the cab, and then also the speaker cable from the amp to see if the short goes away. (Its possible your cab or speakton cable is incorrectly wired or internally shorted)

Does it still blow the fuse when you dont bridge the amp?

Lastly, double check that your using the absolute corect factory recommended size and type (fast-blow/slo-blow) fuse. (if your replacement fuses are to low of an amperage or the wrong type, they could be expected to blow even if there's not a problem. If they're to large you run the risk of further amp damage and even a potential fire).

Most cabs have two jacks wired in parallel. Most amps have two output jacks that are also wired in parallel. So on these running two speaker cables from the amp to each cab is the same connection/ohm-load as using one speaker cable to your first cab and then daisy-chaining to the second cab.
 
#5
my amp just kind of started blowing fuses. i borrowed a buddy's head and his blew a fuse so i thought maybe one of my speakers had some kind of short in it. Since it was a homemade cab and kind of cheap i decided to buy a decent cab. My amp is still blowing fuses, so now i dont know what's wrong.

It doesn't blow as soon as i turn the amp on, i have to be playing for a while.

for some reason guitar stores don't carry the fuse i need for this amp. I've tried guitar center, and a couple of local shops. One guy told me that as long as the amps match up the fuse is good. (volts aren't as important) so it's kind of a pain to locate fuses and figure what's wrong, especially when i have to be playing on the amp for a few hours before it blows. Too time consuming for the time i don't have.
 
#7
ModestCargo said:
Fuses? The hardware store. There are slow blow fuses and fast acting fuses, make sure you have the right one - that could be your entire problem right there.

If you turn your amp on (with no speaker attached) and leave it sit for a while does it still blow a fuse?

Have you tried another cable?
Click to expand...

If it blew on your friends head, then now on your EVEN with a different cab, I too would suspect your speaker cable.

Are you in fact using a SPEAKER cable, and not an instrument cable ?
 
#8
bassplaya matty said:
It doesn't blow as soon as i turn the amp on, i have to be playing for a while.

for some reason guitar stores don't carry the fuse i need for this amp.
Click to expand...

If you have the corect fuse, it sounds like maybe its overheating.
Is your amps internal fan still working? (the manual states its supposed to be thermostatically controlled and come on when needed)
Are any of the amps vents being blocked by your rack or other gear?

That seems to be an older amp model, I've heard of thick accumulations of dust inside the amp causing overheating problems too.

Get the corect fuse and If all else fails, you may find some relief by directing most any fans airflow into the back portion of the amp.

ps:
Radio Shack stores stock most fuses...
 
#9
you would think hardware stores would have a fuse. i've checked radio shack, nothing, even fry's electronics does have the exact one.

yes im using a speaker cable.

It could be overheating, although the fan does come on, and none of the vents are blocked. oh and the fan is clean.
 
#11
I have seen this happen in ignition coils in cars. (A transformer stepping in a different direction) The windings get warm and rub in that spot throughout the useful life until they short. Fuse blows, windings cool down and shift back, short goes away. Repeat.

Yes the windings vibrate. That is what the "humming" from the transformer is.

The fuse is transformer protection. NOT thermal protection. The amp should have a protect mode to keep it from self destructing. If your ohms are right and you are not red lining the amp all night, then more than likely you have a problem with your transformer or a cap is shorting and overloading the transformer.
 
#14
Hi.

bassplaya matty said:
it's a 5 amp 120 volt fuse.
Click to expand...


Are You sure? That ~600W that the fuse handles won't be able to "allow" anywhere near to 500W from the FET class AB output section. 5A/240V and 10A/120V would sound more like it. Even my SWR STUDIO220 would have a 4A/120V Slo-Blo fuse in the 120V world.
A different type of output devices I know, but roughly in the ballpark.

If the fuse value is correct, taking the head to a tech would be a wise thing to do.

BTW how hard are You driving the input?
Low-bass distortion eats a lot of power in the amplifying stage.
Also heavy EQing and using the deep switch could sag the power supply and cause the fuse to blow.

Just my 0.02€
Sam
 
#15
A couple more specific questions.


If you replace the fuse, plug everything in, turn it on and start playing, how long before it typically blows a fuse? A minute or less, or after a few songs, a couple days after a new fuse?

Can you confirm that both speakers in that cab are indeed firing? If you've got two 4-ohm speakers paralleled, you could easily have one dead speaker, and your amp is then seeing only a 4-ohm load, overheating your amp and popping the fuse. There's also the case that this has permanently tweaked your output transformer, but we'll get to that later.
 
#16
Bryan316 said:
A couple more specific questions.


If you replace the fuse, plug everything in, turn it on and start playing, how long before it typically blows a fuse? A minute or less, or after a few songs, a couple days after a new fuse?

Can you confirm that both speakers in that cab are indeed firing? If you've got two 4-ohm speakers paralleled, you could easily have one dead speaker, and your amp is then seeing only a 4-ohm load, overheating your amp and popping the fuse. There's also the case that this has permanently tweaked your output transformer, but we'll get to that later.
Click to expand...

If it is 2 - 4 ohm speakers in parallel it will be a 2 ohm load. If it is 2-4 ohm speakers in series it will be 8 ohms, but if in series and one speaker blows, the other speaker won't work either.
 
#18
The service manuals for a few amps I have worked on, (QSC in particular) state to short the outputs with signal applied when you are adjusting the protection circuit with an O scope. Simply gets the amp real hot real fast. I have also had my Crown's outputs shorted on accident and it only went into protect mode.

Yes, running too low of a resistance value will blow a fuse, however, I think it would overheat the amp first.

Try this- Take your ohm meter and ohm out the primaries in your transformer.
- Unplug the AC cord, Click the power switch on, and ohm out the two power prongs.

When the amp gets warmed up, ohm it out again.

Repeat until it gets to fuse popping stage.

The readings should not change by much. If you see massive changes, there is something wrong with the transformer.

(My 1,100 watt Ampeg ohms out at .5 ohms)

It is hard to give advice without looking at the amp and fondling the guts of it. Try the above and tell us your result.
 

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