Melting and burning

[dangerboy]

I'd really appreciate some help diagnosing a fault with one of my SWR cabs (the other one is withdrawn from sale until I'm sure it's not a fault with both).

On Friday night I did a gig with three other bands - I let one band use my amp, and when they did the room slowly filled with a delicate burning smell. When I got on stage I traced this to the input/crossover panel on my SWR 4x10. The panel was hot (VERY!) to the touch, and the knob on the pot that controls the horn level had melted around the pot spindle. The amp sounded fine, but I obviously switched it off and DIed for my set - no point taking the risk!

The amp I was using was a GK 1000RB - into an SWR Goliath II 4x10. The Xover fuse is fine.

Any ideas what happened? Was this the pot burning out (it was definitely the pot that stank of burning when I opened it all up the next day). The caps didn't look burned in any way. Was the amp putting out too high a signal (the cab is 500W and has been used loads of times before, so I can't understand if this is the problem)?

Any ideas? Do I need a new crossover? A new horn pot?

[Thunderhead]

[quote name='dangerboy' post='310331' date='Oct 20 2008, 12:11 PM']I'd really appreciate some help diagnosing a fault with one of my SWR cabs (the other one is withdrawn from sale until I'm sure it's not a fault with both).

On Friday night I did a gig with three other bands - I let one band use my amp, and when they did the room slowly filled with a delicate burning smell. When I got on stage I traced this to the input/crossover panel on my SWR 4x10. The panel was hot (VERY!) to the touch, and the knob on the pot that controls the horn level had melted around the pot spindle. The amp sounded fine, but I obviously switched it off and DIed for my set - no point taking the risk!

The amp I was using was a GK 1000RB - into an SWR Goliath II 4x10. The Xover fuse is fine.

Any ideas what happened? Was this the pot burning out (it was definitely the pot that stank of burning when I opened it all up the next day). The caps didn't look burned in any way. Was the amp putting out too high a signal (the cab is 500W and has been used loads of times before, so I can't understand if this is the problem)?

Any ideas? Do I need a new crossover? A new horn pot?[/quote]
Was the other band using an overdriven/distorted/fuzzed bass sound? Or pushing the amp harder than you do, maybe just with an aggressive picked bass tone where you would use fingers?

Distortion especially, and more aggressive tone to some extent, puts a lot more high frequencies into the output and this is what will have cooked the horn attenuator, since the cab is not capable of taking the full output of the amp. A 1000W amp into a 500W cab is a bad idea unless you're careful with levels - which it sounds like you have been, maybe not even realising you are! - even if it's not the right impedance to extract the maximum power (eg a 1000W/4-ohm amp will still usually put about 700W into 8 ohms). You will usually get away with it without the extra stress of a harder sound though.

If you're lucky it will just be the attenuator pot, and the tweeter will be OK (especially if the horn level was set quite low, so the pot was taking most of the excess power). The crossover caps will be fine, they don't absorb significant amounts of power. If you're very lucky the pot itself will also be fine - they are at least designed to dissipate a fair amount of heat - and the only damage will be the melted knob. Even if the windings of the pot smell a bit cooked it might still be OK, but best to get it looked at.

Edited October 20, 2008 by Thunderhead

[dangerboy]

[quote name='Thunderhead' post='310459' date='Oct 20 2008, 02:31 PM']Was the other band using an overdriven/distorted/fuzzed bass sound? Or pushing the amp harder than you do, maybe just with an aggressive picked bass tone where you would use fingers?

Distortion especially, and more aggressive tone to some extent, puts a lot more high frequencies into the output and this is what will have cooked the horn attenuator, since the cab is not capable of taking the full output of the amp. A 1000W amp into a 500W cab is a bad idea unless you're careful with levels - which it sounds like you have been, maybe not even realising you are! - even if it's not the right impedance to extract the maximum power (eg a 1000W/4-ohm amp will still usually put about 700W into 8 ohms). You will usually get away with it without the extra stress of a harder sound though.

If you're lucky it will just be the attenuator pot, and the tweeter will be OK (especially if the horn level was set quite low, so the pot was taking most of the excess power). The crossover caps will be fine, they don't absorb significant amounts of power. If you're very lucky the pot itself will also be fine - they are at least designed to dissipate a fair amount of heat - and the only damage will be the melted knob. Even if the windings of the pot smell a bit cooked it might still be OK, but best to get it looked at.[/quote]

Cheers for the advice. Weirdly, the other guy uses a much less trebly/attacking tone than I do. Mine is pretty much the most trebly, most-distorted you can get (aluminium-necked bass, bridge single coil, pick, distorting preamp pedal etc).

The amp only puts out 350W into 8 ohms, so it shouldn't have been too high for the cab.

I might replace the attenuator just in case. I'm assuming the horn is OK because its protection fuse is still alright. It must be that the attenuator was taking all the power. I'll check it out (quietly) tonight!

Thanks again.