SUNN Beta Lead solid state amp no sound

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oceanblood
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SUNN Beta Lead solid state amp no sound

Post by oceanblood » Tue Jul 23, 2013 3:55 pm

Hi everyone.
I've got a Sunn Beta Lead, I've had for almost a decade with no problems. I was playing a show and it just stopped producing sound. Dude in the audience said the volume was decreasing gradually and finally the amp went silent. I checked the fuse and the fuse is good. It powers up, lights come on, but it produces no sound whatsoever, not even a hiss or buzz. Don't have any idea how to diagnose or fix the problem, but I LOVE this amp and don't want to loose it!!! Any ideas?? Thanks!
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Post by oceanblood » Tue Jul 23, 2013 5:00 pm

So weird. I just did some trouble shooting and figured out it was the CAB!! I have two mini cabs loaded with 8 inch Jensens. The two cabs together make 100w @ 4ohms which is the amps rating, but the cabs are completely DEAD now. I plugged in a 15w 8ohm amp into each one. Utter silence.
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Post by Nick Sevilla » Tue Jul 23, 2013 5:11 pm

oceanblood wrote:So weird. I just did some trouble shooting and figured out it was the CAB!! I have two mini cabs loaded with 8 inch Jensens. The two cabs together make 100w @ 4ohms which is the amps rating, but the cabs are completely DEAD now. I plugged in a 15w 8ohm amp into each one. Utter silence.
They might have fuses in between the speakers and the inputs. Check there.
Howling at the neighbors. Hoping they have more mic cables.

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Post by oceanblood » Tue Jul 23, 2013 8:58 pm

CRAZY UPDATE!!!
So inside my mini cabs I had 4 25w 4ohm 8inch speakers wired in series/parallell, 2 jensens and 2 quantums. I tested them and the Jensens were dead. Since the quantums survived I figured I just got some weak ass jensens, so I just slapped in the old quantums I still had. Anyway, cabs seemed to be working fine and we had band practice, after about 20 minutes cabs died again!!! The thing is, I've been using these cabs with this amp and the same quantum speakers for over a year. What's going on? Did something change in my amp it's now killing off speakers??? Anybody know what's going on here?? The amp is rated at 100W and the 4 speakers @ 25W each total 100W. Amp was on 4 the night it killed the Jensens, was on 2 just now at band practice. Both cabs died again.
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Post by oceanblood » Tue Jul 23, 2013 9:08 pm

Just tested the cabs. So, the quantums that were in the cab the night the jensens died, those died today durring band practice. The old ones I had sitting in a box survived band practice. Did the speakers just die a natural death? Or is there some known problem with solid state amps that makes them fry speakers? Someone please respond.
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Post by Snarl 12/8 » Wed Jul 24, 2013 12:25 am

An underpowered amp is going to blow speakers faster than an overpowered one. Is your amp clipping? When you present a low frequency square wave, the amp is pushing out the speaker and holding it there for a bit, and then sucking it in and holding it. Speakers need to be constantly vibrating when electric current is present in order to not overheat. They, essentially act as their own cooling fan for the voice coil. Are the voice coils fried? What do the blown speakers measure on an Ohm meter? I suspect that something has happened to your amp to cut its power and that is causing the speakers to blow. Get your head checked out. It's probably a cheaper issue to fix now than when it dies completely.

BTW - I am not a pro tech (or a pro audio anything), but I learned this underpowered amp thing in college. The hard, stupid way. (Many, many moons ago.)

Maybe something else is going on, but that's my theory.
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Post by oceanblood » Wed Jul 24, 2013 2:20 am

Hey Snarl!!
Thanks for the reply!!
All 4 speakers are rated at 4 ohms. Amp is 4 ohms. They're wired in series/parallel, which makes the total load 4 ohms. What do you mean by underpowered? I'm reading in other forums people are saying the cab should be at least double the wattage. Perhaps that's the problem?
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Post by Jim Williams » Wed Jul 24, 2013 9:10 am

SS amps produce hard edged waveforms when clipping. Tube amps have more soft clipping.

A clipped wave form to a speaker is like smashing it in and out very fast. No speaker can tolerate that abuse for long. It's like throwing your body back and forth against a hard brick wall. You wouldn't last long either.

A 100 watt SS amp will produce about 270 watts of peak to peak power. 25 watt speakers won't like that either. No need for math here.

The last speaker I fried was a crappy used Mitchell from the UK. A little Fender Deluxe Reverb fried it back in 1993.

All Eminence now. No more speaker issues. My 4x12 is rated at 480 watts, can't blow that no matter how hard I try. Plus, they sound better than those crappy Chi-com Celestians and are 1/2 the price too. BTW, made in the USA, if that matters.
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Post by ashcat_lt » Wed Jul 24, 2013 5:50 pm

Or is it more like trying to push the speaker in one direction further than it wants to go? Or is it that the DC current (the flat "tops" of the clipped waves) end up arcing across the voice coil?

You can destroy a speaker with a solid state amp two ways:

1) If the amp is overpowered, but not clipping, it will slam the speaker back and forth too far and too fast for it's physical limits. Cone can rip, can start to tear free of the basket, in extreme cases the voice coil can "jump the gap", but unless it's way the fuck overpowered, you will normally be warned by noticeable and nasty distortion which is pretty easily recognizable.

2) If the amp is underpowered, and you try to get that little bit more volume out of the thing, you end up clipping the power amp section, the "tops" of which look like DC, which kills speakers nearly instantly. You might not have time to hear the distortion before it's all over.

That second one kind of freaks me out because in the early days of my "career" an old soundguy told me that you could check to see if speakers were blown, or check polarity, by connecting a 9V battery to them and watching them jump. I guess maybe the battery is unable to source enough current to destroy the speaker?

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Post by Scodiddly » Wed Jul 24, 2013 6:45 pm

Just as a quick check for something unlikely but possible - measure the amp output with a DC voltmeter. Ideally have a speaker hooked up to sink any parasitic current. You're looking for problems with the amp producing constant DC, which is *not* something speakers will enjoy. If your speakers are all dead, maybe just a light bulb or a big resistor. Make this measurement with no signal present.

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Post by oceanblood » Wed Jul 24, 2013 7:00 pm

woah. You're saying to hook up a light bulb to my amp? What does it mean if it lights up?
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Post by Jim Williams » Thu Jul 25, 2013 7:41 am

It becomes what we used to call back in the 1960's, a 'color organ'.

Very psychedelic if you use a colored bulb while you play.
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Post by Snarl 12/8 » Thu Jul 25, 2013 9:57 am

oceanblood wrote:woah. You're saying to hook up a light bulb to my amp? What does it mean if it lights up?
It means DC electricity is getting straight from the power supply into the speaker outputs without passing "Go" and collecting $200. I like to think of amplifiers as modulated power supplies. You have a really solid, reliable, big source of constant electricity (DC) available. And then you let it go to the speaker in proportion to the input signal. So, if the input is wigglying very fast, but not very far (up and down, positive to negative, i.e. a quiet, high frequency) then you let through just a bit of electricity but in very fast little bursts, or whatever. One of the things that can go wrong is that the supply isn't modulated by the input, so you're just letting the constant electricity through to fry the speaker.
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Post by ashcat_lt » Thu Jul 25, 2013 2:04 pm

I was thinking maybe a DC leakage across the speakers too, but then I thought that the output transformer would necessarily block that. What could go wrong that there is not DC isolation there?

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Post by The Scum » Thu Jul 25, 2013 2:35 pm

What could go wrong that there is not DC isolation there?
Aren't betas solid state?

Ergo, no output transformer.

So leaky coupling caps could indeed put some DC into the speakers, cooking them directly, or at least handicapping AC performance.
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