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Bass cab sparking during gig!!?


tvickey

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Hi All,

 

So my new band just played our debut show on Saturday, and I took one of my GK 410 RBH cabs.  These things handle 800 Watts 8 Ohms...

 

We were up first at the show, I didn't notice anything unusual while I was playing through our set.  Signal was going into the cab via the 1/4 socket rather than the SpeakOn.

 

The bassists in the bands that came after us all used my cab, most of them with their own heads.  I was around and helped connect things, and we were always using my speaker cable which is really high quality (nice Designacable one with Neutrik jacks). 

 

During the performance of the very last band, I noticed that something was glowing periodically in the upper left side of the rear of the cab.  There was a white / yellowish glow that looked to me like it was coming from sparks.  The glow was so bright, that it even shined through the paper on the speaker cones on the left of the cab.  Everything continued to sound fine... performance went on and the cab never caught fire or anything. 😂

 

Has anyone seen this happen before?  I'll have to pull out a speaker or two and see what is going on back there...  I also wonder if using the 1/4" jack rather than the SpeakOn connector had something to do with it. 

 

I have two of these 410 cabs, have driven them really hard in the past, but never experienced any issues.  BUT on all other occasions I was using the SpeakOn socket.

 

 

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Some PA Speakers (and possibly cabs?) have a bulb in them as a circuit breaker to protect the speakers…

 

I’m sure peavey did this.

 

I might be wrong…it was a long time ago and I’ve met many beers since then…

 

if I’ve imagined this - I’m very sorry.

 

God…I answered that really quickly and now I’m doubting myself.

 

now I’m trapped in an internal monologue…

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Actually @AndyTravis, I think you might be right.  I don't know the inner workings of these GK cabs, but just googling around a bit now folks mention light bulbs acting as a current limiting device in a lot of cab circuits (as simple fuses could just blow).

 

I was reasonably sure that I had driven these really hard in the past, but perhaps I just never looked closely enough to notice the glow.

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Ah good. I’ve just counted my marbles…and they’re all present.

 

If I might push my luck - I’d suggest the bulb might be housed in or around the back plate (with all the input gubbins).

 

if I’m right there - I will be pleasantly chuffed with myself.

 

 

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3 minutes ago, AndyTravis said:

 

 

If I might push my luck - I’d suggest the bulb might be housed in or around the back plate (with all the input gubbins).

 

 

Yeah, that is all situated at the back-left on the RBH cabs... exactly where I saw the glow.

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These are really common, it's a really simple solution to protecting the speakers, mainly the tweeters. The bulb has a very low resistance when cold but as the current increases it heats up and the resistance rises reducing the current to the tweeter. In the quieter sections it cools down and full power is restored. 

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4 hours ago, Phil Starr said:

These are really common, it's a really simple solution to protecting the speakers, mainly the tweeters. The bulb has a very low resistance when cold but as the current increases it heats up and the resistance rises reducing the current to the tweeter. In the quieter sections it cools down and full power is restored. 

Correct. In effect they work like a compressor, taming transient peaks that otherwise might cause the tweeter to fail. In extreme cases of overpowering they'll blow, turning them into fuses, although that's not their intended purpose. Those extreme cases are usually the result of using distortion effects, which increase the high frequency power content well beyond what most bulbs or tweeters can handle.

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I remember when I was a kid an had started mucking about with electronics, I had an idea…

 

I had been given a pair of 1x12 speaker cabs. They were huge, something like 24”x18”x18”. I assume they were PA cabs or something, they weighed a lot, too.

 

Anyway. Each had a port along the front bottom (haha!) of the cab, and I had the bright idea (pun totally intended!) of wiring a painted (by me) car light bulb in series with the speaker. The idea being to give me a kind of disco light effect shining from the port. Surprisingly it worked! But I had no idea, at the time, that I was actually making the cabs better! (Although, they didn’t actually have tweeters).

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Years ago I was playing in a resident band at a large cabaret venue. Whilst playing, a member 

of the audience tried to catch my attention, and thinking he was a drunk wanting a request I

ignored him. He got more animated so I bent down to hear him shout  ‘your PA cabs are on

fire’ !! Turned out the Ohm cabinets had the bulbs fitted and were glowing a lot. 

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Good oportunity here to improve things. You remember those fibre optic lights that were all the rage in the 70s? Well, they just had one light, a disk with lots of colours that just slowly spun and a group of fibre optic elements. If you can find one of those in a charity shop, take the disk out and fit that to your cab - so if you play hard enough you get a colour light show!

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On 21/02/2023 at 01:01, Bunion said:

They’re in ampeg cabs too the stop the horn from blowing, my old 210 is a normal car tail light but I think they’ve updated them now to something more fancy 😄

Probably something like a self resetting Polyswitch which are now used in my Acme cabinets replacing the lamp.

Edited by BassmanPaul
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