Very strange amp fault

[ThomBassmonkey]

I noticed today during practice that my cab was lighting up when I played, when I played lower notes or picked gently it doesn't seem as bad, it died as the note died as well so it's definitely volume related.

The actual lighting up is hard to describe, it seems to be around the rubber around the speakers and I could see it through the port, it seems to be around the top left of the cab (as you face the grille, maybe behind that speaker). It's very bright, like having a lightbulb in the back of the cab.

I really don't know what to make of it, the only thing I could think of is that there's some serious sparking going on, but the cab sounds normal and doesn't smell odd, if there was that much sparking that it was lighting up, I assume something would've burned out. I used a different cab for practice then tried it again at the end and it was still doing the same thing.

I'm gonna ring up Polar Audio tomorrow and see if they have any advice, obviously I'm not going to use it before they have a look at it, but thought I'd post here to see if anyone has any idea what could be causing it.

[ahpook]

the light you can see is the protection bulb attached to the tweeter in the cab. it lights up to prevent too high a current reaching the tweeter (as the current increases, the bulb heats up and so its resistance increases)

relax - it's meant to do that

Edited January 4, 2011 by ahpook

[ThomBassmonkey]

That's strange, it's not done that before. I'm using a newish bass though, but I'm sure it's been more cranked at a gig we did before xmas (where it was insanely loud with the same bass).

What could suddenly start it going and what would stop it (I assume turning the tweeter down would do it, any other options)? As cool as it looks, my tuner gives all the flashy visual FX I need.

edit: just looked it up and it could be the problem, but I read on another forum that it shouldn't be an issue with the bi-amping as the tweeter amp only puts out 50w at 8Ohm (apparently GK combos don't even have the bulbs in). I just checked and the cab's definitely set to bi-amp mode.

Edited January 4, 2011 by ThomBassmonkey

[Phil Starr]

I too think it is the speaker protection. You shouldn't see it round the speaker though as it should be airtight and hence light tight, probably.

Anyway one thing that will cause the tweeter to overload is distortion, particularly clipping. Distortion is mainly high frequency energy and your tweeter may be getting a lot more power than it is designed for. The bulb is protecting it but you might like to think about turning down any treble boost or any distortion effects.